Bioastronautics Data Book

NASA SP-3006

Paul Webb, M. D., editor

Scientific and Technical Information Division

NASA: Washington D. C., 1964

SECTION 17. VISION

17-1. Introduction 308

17-2. Characteristic luminance on earth and in space 310

17-3. Range of natural illumination levels on earth 311

17-4. Visual range in natural light - I. Daylight 312

17-5. Visual range in natural light - II. Starlight 313

17-6. Topography of the eyeball 314

17-7. Plan of the retina 315

17-8. Schematic and optical constants of the eyeball 316

17-9. Central and peripheral vision 317

17-10. Spectral sensitivities of the rods and cones 318

17-11. Pupillary diameter and luminance 319

17-12. Monocular visual fields 320

17-13. Binocular visual field with head and eyes fixed 321

17-14. Binocular visual fields with head and eye movement 322

17-15. Visual acuity and density of rods and cones 323

17-16. Variation in visual acuity with background luminance 324

17-17. Discrimination of movement in the frontal plane 325

17-18. Discrimination of movement in depth 326

17-19. Absolute threshold for light and density of rods and cones 327

17-20. Instantaneous threshold for light 328

17-21. Dark adaptation 329

17-22. Contrast threshold and density of rods and cones 330

17-23. Variation in contrast threshold 331

17-24. Relation between size, luminance, and contrast 332

17-25. Color discrimination 333

17-26. Temporal discrimination 334

17-27. Flash blindness 335

17-28. Visual acuity during ocular pursuit 336

17-29. Visibility of point and flashing sources of light 337

17-30. Environmental influences on vision-I. 338

17-31. Environmental influences on vision-ll. 339

17-32. References 340

Contributor: W. J. White, Douglas Aircraft Corporation

307.

17-1. VISION - INTRODUCTION

Four topics are covered in this short conspectus of visual research. It is intended to provide the designer with information about: (1) the visual environment of man; (2) the major physical and physiological landmarks of the eye; (3) the operating characteristics of the eye; and (4) factors influencing visual efficiency.

The visual environment of man is considered in terms of the luminance of objects within the solar system as viewed from outside the atmosphere or within it (17-2- l7-5). The term luminance (B) describes the luminous intensity of an object per unit area of surface. There are many different measures of luminance; the basic unit used in this section is the lambert ( L) , which is equal to one lumen per cm² or l/p candle per cm². Since the L is an inconveniently large unit, luminance is usually measured in millilamberts (1 mL = L × 10^-3), in microlamberts (1 mL = L × 10^-6), in micromillilamberts (1 mmL = L × 10^-9), or in micromicrolamberts (1 mmL = L × 10^-12). In the English system, the unit analogous to the lambert (lumens/cm²) is the foot-lambert(lumens/ft²). The conversion is 1 ft-L = 1.076 mL. Illuminance (E) is the density of the luminous flux deposited on a surface. A common unit is the foot-candle (ft-c). Other units are the mile-candle, centimeter-candle (phot) and meter-candle (lux). In each case, the term means that the flux is measured in candles per ft², per mi², per cm², or per m². The candle (c) is the unit of luminous intensity (I). The candle is equal to one lumen per unit solid angle (steradian) .One lumen is 0.00147 watt at 5550 angstroms.

Landmarks of the eye are presented next. These include anatomical features of the eye- ball (17-6, 17-7), the dioptrics of the eye (17-8), and visual fields. The latter include a presentation of the anatomical basis of central and peripheral vision (17-9), the spectral sensitivity of the rod-cone receptors (17-10), and changes in diameter of the pupil with alteration in luminance (17-11). Concluding the presentation of characteristics of the eye are a graphic display and a table on monocular and binocular visual fields (17-12 through 17-14).

The third part of the section defines the operating characteristics of the eye of a healthy, well-trained observer in making spatial, light, and temporal discriminations (17-15 through 17-26). Several types of visual acuity are defined and related in terms of visual angle and luminance, and the ability of man to discriminate movement is shown. The remarkable operating characteristics of the normal eye include, in addition to the advantage of two- and three-dimensional response, an absolute threshold of 10^-5 mL; contrast sensitivity of 1%; differential color sensitivity at the sodium line of a wavelength of 1 millimicron; and minimum perceptible acuity of 0.008 minutes of arc.

The relation between instantaneous threshold and the dark adaptation process is presented in graphic form (17-27) for the simple case of light detection. Light discrimination includes the ability of the eye to detect a difference in luminance between a target and its background, and differences in wavelength that are correlated with color vision. The interrelations between target size, background luminance, and luminance difference are presented in 17-28 and 17-29. At the end of the section are four charts (17-30 and 17-31) showing the modification of visual efficiency by conditions such as hypoxia, acceleration, and vibration.

Capacities of the eye are ordinarily measured and defined in terms of thresholds. "Threshold" is defined as a point probability of a target being seen. Any probability of detection may be considered the "threshold," but 50% is a good value in the absence of a reason to select another. The particular value of the probability selected depends on the use that is to be made of the data. In most practical situations a practical certainty is required, and the choice is between 95 and 99%. The general relation between the physical variable–for example, the visual angle–and probability of detection is an ogive function, shown below.

17-1. VISION - INTRODUCTION, continued

The curve may be used for estimating the target characteristic that will yield various probabilities of detection. It can be seen that doubling the visual angle, for example, for 50% probability of detection, should give almost 100% detection if the location of the target is known.

It would be useful if an error term could be attached to every value quoted in this section, but there is no consistent way of deriving such information for most of the data. Values of visual performance constitute a live and ever-changing subject and it is necessary to provide for numerical changes.

This section does not cover factors in the design of information displays--instruments, warning signals, cathode ray tube indicators, and so on. The books listed here can help the reader to find such specialized information, as well as furnishing fuller details on many of the topics that are presented in the section.

R. A. McFarland, Human Factors in Air Transportation [16] ,

W. E. K. Middleton, Vision through the Atmosphere [18] , and

C. T. Morgan, J. S. Cook, A. Chapanis, and M. W. Lund, editors , Human Engineering Guide to Equipment Design [21] .

17-2. CHARACTERISTIC LUMINANCE ON EARTH AND IN SPACE

Luminance in mL
1 × 10^9
	7 × 10^8	Sun	Viewed from outside earth's atmosphere
	4.4 × 10^8	Sun	Viewed from the earth
1 × 10^8
	8 × 10^7	A-Bomb	Fireball 4 miles from point of detonation of an 800 KT weapon.
1 × 10^7
1 × 10^6
1 × 10^5
	1.58 × 10^4	Venus	Assume albedo (r) of 0.59 viewed from outside atmosphere
	9.4 × 10³	Earth	Viewed from space with cloud cover (r=0.8)
1 × 10^4
	6.4 × 10³	Mercury	Viewed from outside atmosphere (r=0.069)
	4.3 × 10³	Earth	Viewed in January from outside atmosphere, no clouds (r = 0.39)
	2.9 × 10³	Jupiter	Viewed from outside atmosphere (r=0.56)
	2 × 10³	Sky	Average sky on clear day
	1.2 × 10³	Moon	Full moon viewed from outside of atmosphere (r = 0.073)
1 × 10³
	9.6 × 10²	Saturn	Viewed from outside atmosphere (r =0.63)
	9 × 10²	Mars	Viewed from outside atmosphere (r=0.15)
	8 × 10²	Moon	Full moon viewed from earth
	5 × 10²	Sky	Average sky on cloudy day
	2.4 × 10²	Uranus	Viewed from outside the earth (r = 0.63)
	1.1 × 10²	Neptune	Viewed from outside atmosphere (r=0.73)
1 × 10²
	2 × 10¹	White paper	in good reading light
	1.6 × 10¹	Movie screen	(indoors)
	1 × 10¹	TV screen
1 × 10¹
	7 × 10º	Pluto	Viewed from outside the atmosphere
1 × 10º
	8 × 10^-1	Snow	Light of full moon
1 × 10^-1
	2 × 10^-2	Lower limit	Useful color vision
1 × 10^-1
	7.5 × 10^-3	Earth	Viewed from outside atmosphere with full moon
	1 × 10^-3	Upper limit	Night vision
1 × 10^-3
1 × 10^-4
	3 × 10^-5	Earth	Viewed from outside atmosphere at night with airglow, starlight, and zodiacal light providing illumination
	1 × 10^-5	Absolute threshold	Dark-adapted human eye, lower limit for night vision
	1 × 10^-5	Sky	Moonless night sky viewed from earth
1 × 10^-5
5 × 10^-6
4 × 10^-6
3 × 10^-6
2 × 10^-6
1 × 10^-6
	1 × 10^-6	Space background	Background luminance formed by starlight, zodiacal and galactic light.

The table is designed to acquaint the reader with the enormous range of luminances encountered by man on earth and in space. The values given are approximate and generally represent limits as indicated by the notes. Albedo (r) is the ratio of the visible light reflected to visible light incident on a surface and is considered to be invariant with wavelength.
17-3. RANGE OF NATURAL ILLUMINATION LEVELS ON EARTH

The graph shows the range of natural illumination on Earth from the sun and the moon, as the values increase from minimum before sun- or moonrise to maximum at the zenith.

Source: Adapted from U. S. Navy data on illumination.

17-4. VISUAL RANGE IN NATURAL LIGHT - I. DAYLIGHT

This graph shows the sighting range (distance in feet) of targets viewed against the sky, background luminance 1000 mL (full daylight) at probability of detection of 95%. Meteorological range is the distance at which apparent brightness contrast is reduced by atmospheric scatter to 2% of inherent contrast between the object and sky. Contrast is the ratio of the luminance difference between target and background and the luminance of the background (D B/B).

A straight line connecting meteorological range and contrast will intersect a family of curves for various target sizes, which are shown as areas (A) in square feet. The visual range is obtained by projecting up or down to the range scale. By selecting meteorological range at its infinity point, the graph may be used to find threefold contrast for any object of a given size at any assigned distance.

The graph does not apply to long narrow targets, but does apply to targets that are not very out-of-round.

Source: Adapted from Middleton [18].

17-5. VISUAL RANGE IN NATURAL LIGHT - II. STARLIGHT

This graph shows the sighting range of circular targets against the sky with a background luminance 0.0001 mL (starlight). The following is an example of the use of the nomogram: Find the range that an object 100 sq ft in area could be seen in starlight when the meteorological range is 150,000 feet and the contrast of the object and sky is 0.8. A straight line across meets the given range and contrast. The range is read off where the line intersects the 100 sq ft curve. Under these conditions a 100 sq ft target will be sighted with a probability of 95% at 1200 feet.

Source: Adapted from Middleton [18].

17-6. TOPOGRAPHY OF THE EYEBALL

The drawing shows general features of the right eye, viewed from the outer side, showing the visual axis passing through the center of the lens to the point of sharpest vision at the fovea, a special area of the retina where cones are concentrated. (Specific dimensions and axes are given in 17-8.) This view shows some of the muscles that move the eyeball in its orbit. The superior rectus muscle elevates the front of the eyeball; the lateral rectus muscle pulls the eye to the right (outward), Inferior and medial rectus muscles, not seen in this drawing, balance or oppose these movements. The superior muscle, passing through a pulley, and the opposing inferior oblique, rotate the eyeball, also moving the front surface up or down and laterally. These movements allow the visual image to be consistently aligned on the retina when the head is moved or tilted. Inside the eyeball (shown in cutaway), the lining of the major cavity behind the lens is the retina, the light-sensitive neural layer that is detailed diagrammatically in 17-7. Arteries and veins to supply this active retinal tissue come through the back of the eyeball with the fibers of the optic nerve, thence to be distributed over the front surface of the retina.

17-7. PLAN OF THE RETINA

Shown here is a schematic diagram of the retina in cross section, with the forward facing surface below on the drawing and the rearward layers above. Notice that light from the lens falls first on the nerve fibers, traversing these and two layers of ganglion cells before reaching the primary light sensitive cells, the rods and cones. There are many more rods than cones, but in one small area of the retina there are cones only. This rod-less area is the fovea, the area of sharpest day vision (see 17-6 and 17-8). Cones are responsible for vision at the higher illumination levels, and for color vision. The rods are able to handle the lower light intensities of twilight and night. Nerve pathways from the different cells cross and interconnect at the ganglion layers, as illustrated.

17-8. SCHEMATIC AND OPTICAL CONSTANTS OF THE EYEBALL

a.

b.

Constant	Eye Area or Measurement
Refractive Index	Cornea	1.37
	Aqueous humor	1.33
	Lens capsule	1.38*
	Outer cortex, lens
	Anterior cortex, lens
	Posterior cortex, lens
	Center, lens	1.41
	Calculated total index	1.41
	Vitreous body	1.33
Radius of curvature, mm	Cornea	7.7
	Anterior surface, lens	9.2-12.2
	Posterior surface, lens	5.4-7.1
Distance from cornea, mm	Post. surface, cornea	1.2
	Ant. surface, lens	3.5
	Post. surface, lens	7.6
	Retina	24.8
Focal distance, mm	Anterior focal length	17.1
		[14.2]**
	Posterior focal length	22.8
		[18.9]
Position of cardinal points measured from corneal surface, mm	1. Focus	-15.7
		[-12.4]
	2. Focus	24.4
		[21.0]
	1. Principal point	1.5
		[1.8]
	2. Principal point	1.9
		[2.1]
	1. Nodal point	7.3
		[6.5]
	2. Nodal point	7.6
		[6.8]
Diameter, mm	Optic disk	2.5
	Macula	1.3
	Fovea	1.5
Depth, mm	Anterior chamber	2.7-4.2

*Cortex of lens and its capsule

**Values in brackets refer to state of maximum accommodation

The diagram and table give dimensions and optical constants of the human eye. Values in brackets shown in the table refer to state of maximum accommodation. The drawing is a cross section of the right eye from above.

The horizontal and vertical diameters of the eyeball are 24.0 and 23.5 mm, respectively. The optic disk, or blind spot, is about 15 degrees to the nasal side of the center of the retina and about 1.5 degrees below the horizontal meridian.

Source: Spector, ed. [27].

17-9. CENTRAL AND PERIPHERAL VISION

Visual receptors and their neural connections are divided into two systems, the rod system and the cone system (17-7). These two systems differ in distribution and function. When the level of illumination is above 0.01 mL, the eye sees both color and detail (photopic vision). When the illumination is less than 0.001 mL, everything is colorless and the eye sees very little detail. The cones in the fovea, because of their smaller diameter (1 micron) in this area, close packing (12,000 in an area 1 degree in diameter), and individual connection to the central nervous system, give the eye detail vision. The basic physiological process of color vision is also mediated by the cones. The graph shows the density of rods and cones from the nasal (inner) to temporal (outer) edge of the retina near the horizontal meridian of the right eye. The rods have a lower absolute threshold (0.00001 mL) than the cones (0.001 mL). (See 17-19.) When the illumination drops below 0.001 mL, as at night, the cones do not respond, the fovea is “blind,” and vision is limited to the periphery and is due entirely to activity of the rods (scotopic vision) .

Source: A. Chapanis, Chapter 1 in: National Research Council [22].

17-10. SPECTRAL SENSITIVITIES OF THE RODS AND CONES

Wavelength		Foveal Cones	Cones	Rods
			~ 8 degrees above fovea
I	365	-5.401	-6.95	-2.042
Violet	405	-3.806	-3.64	0.427
I	436	-2.643	-2.67	1.675
Blue	492	-1.288	-1.25	2.295
Green	546	-1.980	-1.65	2.095
Yellow	578	-1.966	-1.59	1.375
Orange	621	-1.626	-1.27	0.038
I	691	-3.840	-3.43	-3.635
Red	713	-3.048	-4.59	-4.787
I	750	-4.072	-5.68	-5.890

The relative sensitivity of the eye to radiant flux is shown in the table as a function of wavelength. Entries are in log₁₀ of the reciprocal of the threshold, based on an adjustment of the sensitivities of the rods and cones 8° above the fovea to the maximum foveal sensitivity. Measurements were made with a one degree circular test field exposed for 0. 25 seconds.

Source: Spector, ed. [27].

17-11. PUPILLARY DIAMETER AND LUMINANCE

There are a number of factors which affect the size of the pupil. The relationship shown here is diameter and variations in the luminance of a large uniform field. It is not possible at this time to predict the size of the pupil for non-uniform distributions of luminances in the visual field.

Source: IES lighting handbook [13].

17-12 MONOCULAR VISUAL FIELDS

Figure a is a perimetric chart that shows the average monocular visual field for the right eye. Numbers are degrees; the eccentricity angle in degrees is the distance by which a target is displaced from the fovea. The head and eyes are motionless. The nasal field is to the left, and the temporal field to the right of the chart. Visual fields are mapped with a two-degree, achromatic, circular target with a luminance of about 10 mL. Age (after 40 years) tends to narrow field limits. Errors of refraction (except presbyopia) have no significant effect on the size of the form field, but affect the size of color fields.

Source: Ruch and Fulton, eds. [25].

Figure b is a perimetric chart that shows average monocular visual field for both achromatic and chromatic targets for the right eye. The chart shows that the visual field for form is normally the largest; those for blue, yellow, red, and green are successively smaller in the order given. A three-degree red target that is beyond 60° eccentricity will appear colorless; at 20° the target will appear as red. Increasing the brightness of the target or its size will tend to move color zones outward on the chart. Color fields are less stable than is the field for form.

Source: After Boring et al. [4].

17-13. BINOCULAR VISUAL FIELDS WITH HEAD AND EYES FIXED

This diagram shows the normal field of view of a pair of human eyes. The central white portion represents the region seen by both eyes. The gray portions, right and left, represent the regions seen by the right and left eyes, respectively. The cut-off by the brows, cheeks, and nose is shown by the black area. Head and eyes are motionless in this case.

Source: Ruch and Fulton, eds. [25].

17-14. BINOCULAR VISUAL FIELDS WITH HEAD AND EYE MOVEMENT

		HORIZONTAL LIMITS		VERTICAL LIMITS
		Temporal	Nasal
MOVEMENT PERMITTED	TYPE OF FIELD AND FACTORS LIMITING FIELD	Ambinocular Field (each side)	Binocular Field (each side)	Field Angle Up	Field Angle Down
	Range of fixation	60°		45°
Moderate movements of head and eyes, assumed as: Eyes: 15° right or left 15° up or down Head: 45° right or left 30° up or down	Eye deviation (assumed)	15°	15°	15°	15°
	Peripheral field from point of fixation	95°	(45°)	46°	67°
	Net peripheral field from central fixation	110°	60°***	61°	82°
	Head rotation (assumed)	45°	45°	30° *	30° *
	Total peripheral field (from central body line)	165°	105°	91°	112° **
Head fixed. Eyes fixed (central position with respect to head)	Field of peripheral vision (central fixation)	95°	60°	46°	67°
Head fixed Eyes maximum deviation	Limits of eye deviation (= range of fixation)	74°	55°	48°	66°
	Peripheral field (from point of fixation)	91°	Approx (5°)	18°	16°
	Total peripheral field (from central head line)	165°	60° ***	66°	82°
Head maximum movement Eyes fixed (central with respect to head)	Limits of head motion (= range of fixation)	72°	72°	80° *	90° *
	Peripheral field (from point of fixation)	95°	60°	46°	67°
	Total peripheral field (from central body line)	167°	132°	126°	157°**
Maximum movement of head and eyes	Limits of head motion	72°	72°	80°*	90°*
	Maximum eye deviation	74°	55°	48°	66°
	Range of fixation (from central body line)	146°	127°	128°	156° **
	Peripheral field (from point of fixation)	91°	Approx (5° )	18°	16°
	Total peripheral field (from central body line)	237°	132°	146°	172°**

*Estimated by the authors on the basis of a single subject.

**Ignoring obstruction of body (and knees if seated). This obstruction would probably impose a maximum field of 90° (or less, seated) directly downward; however, this would not apply downward to either side.

***This is the maximum possible peripheral field; rotating the eye in the nasal direction will not extend it, because it is limited by the nose and other facial structures rather than by the optical limits of the eye. The figures in parentheses on the line above are calculated values, chosen to give the maximum limit thus indicated.

NOTES

I. All data except as noted are from Hall and Greenbaum [10].

2. The ambinocular field is defined here as the total area that can be seen by either eye; it is not limited to the binocular field, which can be seen by both eyes at once. That is, at the sides, it includes monocular regions visible to the right eye but not to the left, and vice versa.

3. The term binocular is here restricted to the central region that can be seen by both eyes simultaneously (stereoscopic vision). It is bounded by the nasal field-limits of the eyes.

Source: Wulfeck et al. [301].
17-15. VISUAL ACUITY AND DENSITY OF RODS AND CONES

Angular Eccentricity	Population		Visual Angle
	Rods/sq mm	Cones/sq mm	100 mL		0.002 mL
Degrees	thousands		minutes
			mean	range
0.00	0	136.	0.7	(0.5-1.0)	12.5
0.25	0	84.4	0.8	(0.6-1.1)	–
0.50	7.22	57.5	1.0	(0.7-1.3)	–
1.00	34.2	41.3	1.2	(0.8-1.5)	22.2
5.00	88.	19.4	–	–	11.3
6.00	105.	12.1	4.5	(1.5-6.7)	–
10.00	118.	9.13	–	–	15.2
12.00	125.	7.64	6.1	(2.5-10)	–
12.50	126.	7.63	–	–	–
20.00	158.	7.08	10.	(5.0-17)	21.3
30.00	140.	6.52	–	–	31.2
40.00	132.	5.95	27.5	(14-48)	–
50.00	108.	5.79	42.5	(21-72)	–
70.00	80.4	5.47	100.	(47-X*)	–
90.00	57.7	6.84	X*	(126-X*)	–

* Unmeasurably poor acuity.

The relation between the distribution of rods and cones near the horizontal meridian for various angular eccentricities is shown in the table. The last two columns of the table give visual acuity along the horizontal meridian of the temporal retina at different angles from the fovea (zero degrees) for two levels of luminance. At the highest luminance level, the fovea has the best acuity. At six degrees from the fovea, and at 100 mL, an object must be about twice as large to be seen as one in the central area. At the lowest level, acuity is best about five degrees away from the fovea. Scotopic peripheral acuity does not parallel the rod population or the light sensitivity of the retina. At lower luminance levels, visual acuity is fairly constant from 4 degrees to 30 degrees eccentricity.

Sources: After Mandelbaum and Sloan [15] and Spector, ed. [27].
17-16. VARIATION IN VISUAL ACUITY WITH BACKGROUND LUMINANCE

Variations in spatial acuity with background luminance for high contrast targets, considering the natural pupil and binocular vision.

Minimum separable acuity defines the smallest space the eye can see between parts of a target. The relationship shown is for a black Landolt-ring on a white background. For white targets on black backgrounds the relationship between acuity and luminance holds up to about 10 mL, above which acuity decreases because the white parts of the display blur.

Vernier acuity is the minimum lateral displacement necessary for two portions of a line to be perceived as discontinuous. The thickness of the lines is of little importance.

Stereoscopic acuity defines the just perceptible difference in binocular parallax of two objects or points. Parallactic angle is one of the cues used in judging depth. Beyond 2500 feet, one eye does as well as two for perceiving depth.

Minimum perceptible acuity refers to the eye's ability to see small objects against a plain background. It is commonly tested with fine black wires or small spots (either darker or lighter) against illuminated backgrounds. For all practical purposes, these numbers represent the limits of visual acuity. Another type of acuity, not shown in the graph, is minimum visible acuity. This term refers to the detection by the eye of targets that affect the eye only in proportion to target intensity. There is no lower size limit for targets of this kind. For instance, the giant red star Aldebaran (magnitude 1) can be seen even though it subtends an angle of 0.0003 minutes (0.056 sec) of arc at the eye. (The conditions under which these data were obtained were nearly optimal for a given level of illumination. Changes in contrast, retinal location, rapid changes in illumination, and vibration would decrease the resolution capabilities of the eye.)

Sources: Vernier and stereoscopic acuity data from Berry et al. [2]; minimum perceptible acuity data from Hecht et al. [11]; minimum separable acuity curve after Moon and Spencer [20].

17-17. DISCRIMINATION OF MOVEMENT IN THE FRONTAL PLANE

The differential threshold (D w) is the amount that the angular speed of an object moving at right angles to the line of sight must change to be detected as a new speed. Data points shown on the graph are thresholds gathered from eight different experiments, for abrupt changes in speed from w₁ to w₂.

When an object stationary in the visual field (w₁ = 0) is suddenly set in motion, the minimum speed which is perceived as motion ("rate threshold") varies from 1 to 2 minutes of arc per second (0.017 to 0.033 deg/sec).

Threshold for movement in peripheral vision is higher than the threshold in central vision. Effects of illumination and contrast on differential threshold are imperfectly known at this time. The rate threshold is higher at low illumination levels and when no fixed visual reference is available.

Sources: Brown [6] and Graham [8].

17-18. DISCRIMINATION OF MOVEMENT IN DEPTH

a.

b.

Figure a shows successful perception of movement in depth of a luminous target on a black field as a function of change in visual angle (per cent distance traveled) and of luminance. Figure b shows the time required to perceive movement in depth as a function of rate of change of visual angle (target speed). Both curves are for 75% correct responses, where 50% correct would be chance performance, since the target moved both toward and away from the observer, who had to choose the correct direction.

The target was a lamp measuring 3.5 inches in diameter which was moved back and forth on a track from an initial distance of 25 feet. At the initial distance, the lamp subtended a visual angle of 40 minutes of arc. A 2% change in distance, which was detected as movement at the higher luminance levels, represented a 2% change in visual angle, or a change of about 0.8 minutes of arc. The range of target speeds from 1.65 to 13.2 inches per second produced initial changes in visual angle from about .25 minutes of arc to 2 minutes of arc.

Source: Baker and Steedman [1].

17-19. ABSOLUTE THRESHOLD FOR LIGHT AND

DENSITY OF RODS AND CONES

Angular Eccentricity	Population		Absolute Luminance Threshold
	Rods/sq mm	Cones/sq mm	Nasal	Temporal
Degrees	thousands		(Log m m L)
0.00	0	136.	4.87	4.87
0.25	0	84.4	–	–
0.50	7.22	57.5	–	–
1.00	34.2	41.3	–	–
5.00	88.	19.4	–	–
6.00	105.	12.1	4.26	4.22
10.00	118.	9.13	4.09	4.11
12.00	125.	7.64	–	–
12.50	126.	7.63	–	–
20.00	158.	7.08	4.07	4.21
30.00	140.	6.52	4.16	4.14
40.00	132.	5.95	4.26	4.10
50.00	108.	5.79	4.50	4.16
70.00	80.4	5.47	–	4.38
90.00	57.7	6.84	–	5.33

The table shows the relation between the distribution of rods and cones near the horizontal meridian for various angular eccentricities. The last two columns give the absolute sensitivity of the nasal and temporal retina for detecting a one-degree white test light. The data are average threshold values for 101 subjects, ranging in age from 14 to 70 years. The eye is most sensitive to light at about 20 degrees from the fovea. The absolute luminance threshold for the dark-adapted eye is 0.00001 mL. The sensitivity of different parts of the eye to light is much the same at daylight levels of illumination. A micromicrolambert (mmL) = 10^-9 mL.

Sources: After Sloan [26] and Spector, ed. [27].

17-20. INSTANTANEOUS THRESHOLD FOR LIGHT

The luminance that is just visible immediately after the eye has been adapted to a given luminance is called the instantaneous threshold of the eye. The curve is a straight line except at the higher luminances where factors other than adaptation are present. This graph is for a square target that subtends 10 minutes of arc, and assumes that the obsverver is pre-adapted to a given wide field luminance. An observer adapted to a luminance of 1.0 mL can see a 10 minute square target about one hundredth as bright immediately after the pre-adapting field is turned off. Suppose, however, that the observer was exposed to a field luminance of 1000 mL but the target luminance was 0.0001 mL, what can be said about the luminance threshold? The data on dark adaptation (17-21) show that the observer must wait about 14 minutes after entering a dark room before he can see the target light. This graph is for simple light detection and does not permit a prediction of instantaneous visual acuity threshold, which requires discrimination of form.

Source: Nutting [23].

17-21. DARK ADAPTATION

a.

Rods and cones differ in the time factors associated with their activities. The rods are much slower in action than the cones. The two graphs show the time course of the process of dark adaptation. Dark adaptation results in increased ability to see dimly illuminated objects. The time required to become dark adapted following exposure to white light of various intensities and to colored light at 1.1 mL is shown in the graphs. Note that time required to adapt to a given threshold level is shorter when the pre-exposure brightness is less, and when the pre-exposure light is composed of wavelengths in the red portion of the spectrum. Many factors influence the rate of dark adaptation. These include the characteristics of the pre-exposure field (e. g., size) and the physiological status of the person (e. g., hypoxia).

Sources: Haig [9] and Peskin and Bjornstad [24].
b.

17-22. CONTRAST THRESHOLD AND DENSITY OF RODS AND CONES

Angular Eccentricity	Population			Luminance Contrast Threshold
	Rods/ mm²		Cones/mm²
Degrees	thousands			target size - minutes
				1.00	3.60	15.0	120.0
0.00	0	136.		0.539	0.049	0.016	0.008
0.25	0	84.4
0.50	7.22	57.5
1.00	34.2	41.3
5.00	88.	19.4		2.75	0.096	0.024	-
6.00	105.	12.1
10.00	118.	9.13		4.55	0.333	0.557	0.014
12.00	125.	7.64
12.50	126.	7.63		5.73	0.545	0.072	0.015
20.00	158.	7.08
30.00	140.	6.52
40.00	132.	5.95
50.00	108.	5.79
70.00	80.4	5.47
90.00	57.7	6.84

The table shows the relation between the distribution of rods and cones near the horizontal meridian for various angular eccentricities. The last four columns of the table give threshold contrast value (DB/ B, where B is luminance) for various positions in the visual field for four target sizes. Circular targets of positive contrast were presented for 0.33 seconds against a 75 mL field. Exposure duration was selected to simulate the dwell time used in typical search procedures.

Source: After Taylor [28] and Spector, ed. [27].

17-23. VARIATION IN CONTRAST THRESHOLD

Variations in luminance contrast threshold (DB/B, where B is luminance) are shown as functions of background luminance and target size. (Pupil diameter is shown as it varies with background luminance.)

Two relationships are shown very clearly by this graph: When it gets darker, objects must be a lot blacker or lighter than their background to be seen; and, at any level of luminance, small objects must have more contrast in order to be seen than large objects. The discontinuities in the curves at about 0.0003 mL mark the transition from cone (photopic) to rod (scotopic) vision. The curves are representative of data taken with natural pupils and scanning binocular observation of a gray disk on a darker background. The subjects knew where the target would appear on the background, and exposure duration was of the order of 15 seconds.

Luminance contrast is a measure of how much target luminance (B_t) differs from background luminance (B_b). The equation for obtaining contrast is:

CB = (B_t – B_b )/B_b or CB × 100 = %CB

Contrast can vary from zero to 100% for targets darker than their backgrounds, and from zero to infinity for targets brighter than their backgrounds. Most studies of this aspect of vision consider targets brighter than their backgrounds.

Source: Blackwell [3].

17-24. RELATION BETWEEN SIZE, LUMINANCE, AND CONTRAST

Relation between target size and background luminance for targets of various contrasts is shown here. Thresholds in this graph are at the 50% probability of detection. By multiplying the values by 2 (log 0.3), the values may be converted to about the 95% probability of detection. One thing the graph shows is that a reduction in any one factor–background luminance, size, or contrast–may be compensated for by an increase in one or more of the others. Another thing is that the relation between minimum separable acuity and background luminance shown in 17-16 is nearly reproduced for the 100% contrast curve in this graph. In other words, a family of curves could be drawn in 17-16 to show the interaction between visual acuity, contrast, and background luminance. The chief effect of reducing contrast is a shift of the curve upward in the direction of increased target size for 50% probability of resolving parts of a target. The family of curves also shows the discontinuity at about 0.0003 mL that marks the transition from rod to cone vision.

Source: Blackwell [3].

17-25. COLOR DISCRIMINATION

This chart shows variations in hue discrimination through the visible spectrum. Changes in hue discrimination are not equal for equal increments of wavelength (l). With good illumination (10 mL or better) and saturated colors, 128 hues can be distinguished. The difference threshold in the blue-green and yellow portions of the spectrum is of the order of one millimicron (mm). At the red level of the spectrum, the difference must be as great as 20 m m before it is detected.

Source: Wulfeck et al. [30].

17-26. TEMPORAL DISCRIMINATION

The graph shows the relation between critical fusion frequency (CFF) and luminance. The curve defines the boundary between those combinations of target luminance and flicker frequency that are perceived as flickering and those perceived as steady. CFF is the lowest frequency (c/s) of flashing that can be perceived as steady. Luminance is the variable with the greatest influence on CFF. Other variables are target size, color, lengths of the light-dark cycle, brightness of the surround, region of the retina stimulated, and individual differences. The data shown in the graph are based on a two-degree, achromatic stimulus at zero degrees of angular eccentricity.

Source: Hecht and Verrijp [12].

17-27. FLASH BLINDNESS

a.

b.

c.

Flash blindness is the transient loss of vision for objects of low luminous intensity following an exposure to brief but intense general illumination. Figures a and b at left illustrate the times needed to perceive targets requiring two levels of visual acuity--0.26 for the upper curves, and 0.08 for the lower. Short "adapting" flashes were used, having various intensities as indicated by keying of the individual curves. Figure c shows the effect of 0.1 second flashes of much higher intensity on the ability of the eye to detect a large target, which subtended an angle of 17 min and which had a luminance of 0.7 mL. The brighter the flash, the longer was the recovery time.

Sources: Metcalf and Horn [17] and Brown [5].

17-28. VISUAL ACUITY DURING OCULAR PURSUIT

The effects of increased angular velocity of rotation on visual acuity at each of six levels of illumination are shown in the graph. The relationship shown is for a black Landolt-ring on a white background. The data show that visual acuity declines progressively as angular velocity increases, and that acuity is benefited by increasing the illumination on the target.

Source: Miller [19].

17-29. VISIBILITY OF POINT AND FLASHING SOURCES OF LIGHT

a.

b.

c.

A point source is a light that affects the eye only in proportion to its intensity. The curve in figure a shows the maximum angular diameter of a circular source (white) that can be considered as a point source. The region under the curve represents "point sources." Figure b shows the threshold illuminance from a fixed, achromatic point source for about 98% probability of detection as a function of background luminance. The break in the curve represents the transition from rod to cone vision. Figure c shows how intense a flash of light must be in order to be seen at the 50% probability level. Note that very short flashes must be much more intense than long flashes if they are to be seen. The detection of colored lights requires about the same illumination at the eye as detection of white light.

Source: Blackwell [3] and Chapanis et al. [7].

17-30. ENVIRONMENTAL INFLUENCES ON VISION - I.

a.

Figure a shows the reduction of visual sensitivity due to hypoxia as a percentage reduction of the normal threshold for light. The change begins as low as 5000-6000 feet in altitude when supplemental oxygen is not breathed. Reduced visual sensitivity is a measure of decreased ability to see at night.

Source: Adapted from McFarland [16].
b.

Figure b shows binocular visual acuity as a function of acceleration. If a target is to be seen at 7 –Gx, it must be twice the size of the threshold target at 1 G. (Other visual effects of acceleration are presented in Section 3., Acceleration.)

Source: White [29].

17-31. ENVIRONMENTAL INFLUENCES ON VISION - II.

a.

The diagrams show scale reading error and eye movements when viewing vibrating targets, a radial line disk, below, and a horizontal scale, above. The subject's head was fixed, and he was asked to follow the motions of the disk and scale at the frequencies shown, the excursion of the targets being 4.5 min of arc. The subject's eyes moved with the target at frequencies below 0.5 cycles per second, but less and less as the frequency increased. Distortion of the radial lines also increased with frequency. Errors in reading the position of markers on the horizontal scale also increased as the frequency increased.

Source: Jones and Drazin [14].

b.

17-32. VISION- REFERENCES

1. BAKER, CHARLES A. and WILLIAM C. STEEDMAN. Perceived movement in depth as a function of luminance and velocity. Human Factors 3: 166-173, 1961.

2. BERRY, R. N., L. A. RIGGS, and C. P. DUNCAN. The relation of vernier and depth discriminations to field brightness. J. Exp. Psychol. 40: 349-354, 1950.

3. BLACKWELL. H. R. Contrast thresholds of the human eye. J. Opt. Soc. Amer. 36: 624-643, 1946.

4, BORING, E. G. , H. S. LANGFELD, and H. P. WELD. Foundations of psychology. John Wiley & Sons, New York, 1948.

5. BROWN. J. L. Flash blindness. MSVD Report 61-SD-179, General Electric Company, Missile and Space Vehicle Department, Philadelphia, Pa., 1961.

6. BROWN, R. H. Weber ratio for visual discrimination of velocity. Science 131: 1809-1810, 1960.

7. CHAPANIS. A. , W. R. GARNER. and C. T. MORGAN. Applied experimental psychology . John Wiley & Sons, New York, 1949.

8. GRAHAM, C. H. "Visual perception. " Chapter 23, pp. 868-920, in: Stevens, S. S. , editor, Handbook of experimental psychology. John Wiley & Sons, New York, 1951.

9. HAIG. C. The course of rod dark adaptation as influenced by intensity and duration of pre-adapting to light. J. Gen. Physiol. 24: 735-751, 1941.

10. HALL. MARY V. and LEON J. GREENBAUM. The areas of vision and cockpit visibility. Trans. Amer. Acad. Ophthal. Otolaryng., Sept.-Oct. 1950, 74-88.

11. HECHT. S. , S. ROSS. and C. G. MUELLER. The visibility of lines and squares at high brightness. J. Opt. Soc. Amer. 37: 500-507, 1947.

12. HECHT, S. and C.D. VERRIJP. Intermittent stimulation by light. J. Gen. Physiol. 17: 237-282, 1933.

13. I. E. S. LIGHTING HANDBOOK. Third Edition. Illuminating Engineering Society, New York, 1959.

14. JONES. G. MELVILL and D. H. DRAZIN. Oscillatory motion in flight. Flying Personnel Research Committee report no.1168, R. A. F. Institute of Aviation Medicine, Farnborough, Hampshire, England, July 1961.

15. MANDELBAUM, J. and LOUISE L. SLOAN. Peripheral visual acuity. Amer. J. Ophthal. 30: 581-588, 1947.

16. McFARLAND. R. A. Human factors in air transportation. McGraw-Hill Book Company, Inc., New York, 1953.

17. METCALF, R.D. and RICHARD E. HORN. Visual recovery times from high-intensity flashes of light. WADC TR 58-232, Wright Air Development Center, Wright-Patterson Air Force Base, Ohio, October 1958.

18, MIDDLETON, W. E. K. Vision through the atmosphere. University of Toronto Press, Toronto, Canada, 1958.

19. MILLER, J. W. Study of visual acuity during ocular pursuit of moving test objects. II. Effects of direction of movement, relative motion, and illumination. J. Opt. Soc. Amer. 48: 803-808, 1958.

17-32. VISION -REFERENCES, continued

20. MOON, P. and DOMINA E. SPENCER. Visual data applied to lighting design. J. Opt. Soc. Amer. 34: 605-617, 1944.

21. MORGAN, C. T. , J. S. COOK, A. CHAPANIS, and M. W. LUND, editors. Human engineering guide to equipment design. McGraw-Hill Book Company, Inc., New York, 1963.

22. NATIONAL RESEARCH COUNCIL. A survey report on human factors in undersea warfare. Committee on undersea warfare, NRC, Washington, D. C., 1949.

23. NUTTING, P. G. Effects of brightness and contrast in vision. Trans. Illuminating Engr. Soc. 11: 939-946, 1916.

24. PESKIN, J. C. and J. BJORNSTAD. The effect of different wavelengths of light on visual sensitivity. Report MCREXD 694-93A, USAF Air Materiel Command, Wright-Patterson Air Force Base, Dayton, Ohio, 1948.

25. RUCH, T. C. and J. F. FULTON, editors. Medical physiology and biophysics. W. B. Saunders Company, Philadelphia, Pa., 1960.

26. SLOAN, LOUISE L. Rate of dark adaptation and regional threshold gradient of the dark adapted eye: physiological and clinical studies. Amer. J. Ophthal. 30: 705-720, 1947.

27. SPECTOR, W. S., editor. Handbook of biological data. W. B. Saunders Company, Philadelphia, Pa., 1956. (Also issued as WADC TR 56-273, Wright Air Development Center, Wright-Patterson Air Force Base, Ohio, October 1956.

28. TAYLOR, J. H. Contrast thresholds as a function of retinal position and target size for the light adapted eye. SIO REF. 61-10, Scripps Institution of Oceanography, San Diego, Calif., 1961.

29. WHITE, WILLIAM J. Acceleration and vision. WADC TR 58-333. Wright Air Development Center, Wright-Patterson Air Force Base, Ohio, November 1958.

30. WULFECK, JOSEPH W., ALEXANDER WEISZ, MARGARET W. RABEN, and GEORGE O. EMERSON. Vision in military aviation. WADC TR 58-399. Wright Air Development Center, Wright-Patterson Air Force Base, Ohio, November 1958.

UNITS AND CONVERSIONS

ALBEDO:

The per cent of diffused reflection of "white light" for a given surface. (See 17-2).

AMU (amu):

Atomic mass unit (defined as: 16 amu = the atomic mass of the most abundant isotope of oxygen).

ATMOSPHERE (atm):

The pressure exerted by 76 cm mercury with a density of 13.5951 gm/cm³ at 1 g (the standard barometric pressure at sea level).

1 atm = 1.01325 × 10⁶ dynes/cm² = 1033.2 gm/cm² = 760 mm Hg = 14.696 psi

BRITISH THERMAL UNIT (Btu):

1 Btu = 1.0559 × 10¹⁰ ergs = 251.995 gm-cal = 778.77 ft-lbs = 0.25199 kcal

1 Btu/hr = 0.01667 Btu/min

= 0.004199 kcal/min = 0.2932 watt

1 Btu/min = 0.25199 kcal/min = 0.023599 hp = 17.595 watts

1 Btu/ft², hr = 2.7125 kcal/m², hr

BTPS:

Body Temperature (=37 deg C), ambient pressure, and Saturated (water vapor pressure = 47 mm Hg). (See 15-15).

CALORIC EQUIVALENT OF OXYGEN:

One liter of oxygen (STPD) consumed is equivalent to 4.825 kcal of metabolic heat produced, when the R. Q. is 0.82.

CANDLE (c):

The unit of luminous intensity. (See 17-1). 1 candle = 1 lumen/ steradian

CENTIMETER (cm):

1 cm = 0.03280 ft = 0.3937 in = 0.01 m = 10 mm

= 1 × 10⁴ m

(See also Square Centimeter, Cubic Centimeter) .

CENTIMETER-CANDLE (phot): 1 phot = 1 × 10⁴ lux

CENTIMETERS PER SECOND PER SECOND: 1 cm/sec² = 0.0328 ft/sec²

CENTIPOISE: Unit of absolute viscosity. 1 centipoise = 0.01 poise

CLO (clo):

The unit of insulation resistance for clothing. 1 clo = 0.18 ° C m²hr/kcal

= 0.88 ° F ft²hr/Btu

CUBIC CENTIMETER (cc or cm³): 1 cc = 3.531 × 10^-5 ft³ = 0.061023 in³ = 1 × 10-6 m³ = 1000 mm³

= 2.6417 × 10^-4 gal (US fluid) = 0.0338 oz (US fluid)

= 2.113 × 10^-3 pint (US liquid) 1 cc/sec = 0.0021186 ft³/min

CUBIC FOOT:

1 ft³ = 1728 in³

= 28.32 liters = 0.02832 m³

1 ft³/min = 472.0 cc/sec

= 0.4720 liter/ sec

= 62.43 lbs H₂O/min

1 ft³/sec = 1699.3 liter/min

CUBIC INCH:

1 in³ = 5.787 × 10^-4 ft³

= 1.639 × 10^-2 liter = 1.639 × 10^-5 m³

CUBIC METER:

1 m³ = 35.3144 ft³

= 6.1023 × 10⁴ in³ = 999.973 liters

DECIBEL (db):

Used for comparing power levels, acoustical or electrical.

1 db = 10 log₁₀ P/Po where P is the power to be compared to a reference power Po

1 bel = increase in power (P) by a factor of 10

(See also Sound Pressure Level).

DEGREE (angular):

1 deg = 60 minutes

= 0.01745 radian = 3600 seconds

1 deg² = 3.0462 × 10^-2 steradian

DEGREES CENTIGRADE (°C): °C = 5/9 (°F - 32) ; 1 °C = 1.8°F

DEGREES FAHRENHEIT (°F): °F =(9/5×°C)+32; 1°F = 0.556 °C

DEGREES PER SECOND:

1 deg/sec = 0.017453 radian/sec = 0.1667 rpm

DYNE:

1 dyne = 1.0197 × 10^-6 kg = 2.2481 × 10^-6 lb

1 dyne-cm = 1 erg

DYNE-SECOND PER SQUARE CENTIMETER: Unit of viscosity. (See Poise).

DYNE PER SQUARE CENTIMETER:

1 dyne/cm² = 9.8692 × 10^-7 atm = 0.0010197 gm/ cm²

= 4.0148 × 10^-4 in H₂O = 7.5006 × 10^-4 mm Hg = 1.4504 × 10^-5 psi

ELECTRON CHARGE (e):

e = 1.602 × 10^-19 coulomb

ERG:

1 erg = 9.4805 × 10^-11 Btu = 7.3756 × 10^-8 ft-lb = 2.3889 × 10^-11 kcal = 8.8510 × 10^-7 lb-in

FOOT (ft):

1 ft = 30.48 cm = 12 in

= 0.3048 m

(See also Square Foot, Cubic Foot).

FOOT-CANDLE (ft-c):

1 ft-c = 1 lumen/ft²

= 10.764 lumen/m²

FOOT-LAMBERT (ft-L):

1 ft-L = 1.0764 millilamberts

FOOT PER MINUTE:

1 ft/min = 0.3048 m/min

= 0.005080 m/sec = 0.011364 mph

FOOT PER SECOND:

1 ft/sec = 1.0973 km/hr

= 0.5921 knot (per hr) = 0.6818 mph

FOOT-POUND (ft-lb):

1 ft-lb = 0.001285 Btu

= 1.3558 × 10⁷ ergs = 3.2389 × 10^-4 kcal

1 ft-lb/min = 3.0303 × 10^-5 hp = 0.01667 ft-lb/sec = 0.022597 watt

1 ft-1b/sec = 0.001818 hp

= 0.01943 kcal/min = 1.3558 watts

G (g):

The acceleration of gravity (also the acceleration of a vehicle) .

1 g = 32.174 ft/sec²

= 980.665 cm/sec²

G (g):

The unit of force causing displacement of organs and fluids in the body when the body is accelerated, where 1 g = force per unit mass due to acceleration of 1 g. (See 3-1).

GRAM (gm):

1 gm = 0.001 kg = 1000 mg

= 0.03527 oz

= 0.0022046 lb

1 gm/cm³ = 62.428 lbs/ft³

1 gm/hr = 0.540 lb/day

= 0.0003757 lb/min

1 gm/liter = 0.062427 lb/ft³

1 gm/cm² = 9.6784 × 10^-4 atm = 980.665 dynes/ cm² = 0.9356 mm Hg = 0.014223 psi

1 gm/m², hr = 2.78 × 10^-5 gm/cm², sec = 0.7448 lb/ft², hr

GRAM-CALORIE (gm-cal):

1 gm-cal = 3.0874 ft-lbs = 0.001 kcal

HEMATOCRIT:

The height of the column of red blood cells in a tube of whole blood which has settled or has been centrifuged to separate cells from plasma. Usually expressed in per cent.

HORSEPOWER (hp):

1 hp = 3.300 × 10⁴ ft-lbs/min = 550 ft-lbs/ sec

= 10.688 kcal/min = 745.7 watts

INCH (in):

1 in = 2.540 cm = 0.0833 ft = 25.40 mm

(See also Cubic Inch, Square Inch)

INCH OF WATER (in H₂O)

1 in H₂O = 0.002458 atm

(at 4° C) = 2490.82 dynes/ cm² = 0.0361 psi

= 1.868 mm Hg

JOULE:

1 joule = 1 watt-sec

KILOGRAM (kg) :

1 kg = 1000 gm = 2.205 lb

= 32.1507 oz

KILOGRAM-CALORIE (kcal or large Calorie): 1 kcal = 3.9683 Btu

= 4.186 × 10¹⁰ ergs = 1000 gm-cal = 3087 ft-lbs

1 kcal/hr = 0.0661 Btu/min = 0.857 ft-lbs/sec = 0.1667 kcal/min = 1.161 watts

1 kcal/m² hr = 0.3687 Btu/ft² hr

1 kcal/min = 3.9685 Btu/min

= 51.457 ft-lbs/sec = 0.093557 hp = 69.767 watts

KILOGRAM -CENTIMETER SQUARED: 1 kg-cm² = 0.34171b-in²

KILOGRAM-METER PER SECOND: 1 kg-m/sec = 7.2330 ft-lb/sec = 9.80665 watts

KILOMETERS PER HOUR: 1 km/hr = 0.9113 ft/sec = 0.5396 knot = 0.6214 mph

KNOT (nautical mile): 1 knot = 1.689 ft/ sec = 1.853 km/hr = 1.1516 mph

LAMBERT (L):

Unit of surface brightness. (See 17-1). 1 L = 0.3183 c/cm² = 2.0536 c/in² = 1 lumen/cm²

LITER (1):

1 liter = 0.03531 ft³ ; 61.02 in³ = 1000 ml

1 liter/min = 5.886 × 10^-4 ft³/sec

1 liter/sec = 2.12 ft³/min

LUMEN:

1 lumen = 0.001496 watt

= 0.07958 spherical candle power

1 lumen/ft² = 1 ft-c = 10.7641umen/m²

LUX:

(See Meter-Candle).

METER (m):

1 m = 100 cm = 3.281 ft ; 39.37 in

(See also Cubic Meter).

METER-CANDLE (lux): 1 lux = 1 lumen/m² = 0.092903 ft-c

METER PER SECOND:

1 m/sec = 3.281 ft/sec = 3.600 km/hr = 2.2369 mph

MICRON (µ or mu) :

1 µ = 10^-6 meter

= 3.937 × 10^-5 in = 0.001 mm

MIL:

1 mil = 0.001 in

= 0.0254 mm = 25.40 m

MILES PER HOUR (mph): 1 mph = 88 ft/min

= 1.4667 ft/sec = 1.6093 km/hr = 0.8684 knot

MILLIGRAM (mg):

1 mg = 0.001 gm = 3.5274 oz

= 2.2046 × 10^-6 lb

1 mg/m3 = 6.243 × 10^-4 lb/ft³

MILLILAMBERT (mL):

1 mL = 0.929 lumen/ft²

(perfectly diffused light)

MILLILITER (ml):

1 ml = 1.000028 cc = 0.061025 in³ = 0.001 liter

= 0.0338 oz (US. fluid)

MILLILITERS PER HOUR:

1 ml/hr = 0.06102 in³/hr

MILLIMETER (mm):

1 mm = 0.10 cm

= 0.03937 in = 1000 µ

(See also Square Millimeter).

MILLIMETER OF MERCURY (mm Hg): 1 mm Hg = 0.0013158 atm

(at 0° C) = 1333.22 dyne/ cm² = 1.3595 gm/cm² = 0.019337 psi = 0.535 in H₂O

MILLlREM:

1 millirem = 10^-3 rem

(roentgen equivalent man) (See 8-6).

MILLISECONDS (msec): 1 msec = 0.001 sec

OUNCE (oz):

1 oz = 28.3495 gm = 0.0625 lb

OXYGEN SATURATION:

The ratio of the volume of oxygen (at STP) in a given unit volume of blood, to the maximum volume of O₂ that can be absorbed by that unit volume of blood at high partial pressures of O₂ (e. g. 760 mm Hg); usually expressed in per cent.

PARTS PER MILLION (ppm):

1 ppm = 1.0 mg/liter of H₂O

= 8.345 lbs/million gallons

PHON:

1 phon unit = SPL of a 1000 cycle/ sec tone (See 16-5).

PHOT:

(See Centimeter Candle).

POISE:

Unit of viscosity.

1 poise = 1 dyne/sec, cm² = 1 gm/cm, sec

= 0.067196 lb/ft, sec

POUND (lb);

1 lb = 453.5924 gm = 0.45359 kg = 16 oz

1 lb/day = 18.89 gm/hr

1 lb/hr = 0.7559 gm/min = 10.886 kg/day

POUND-INCH:

1 lb-in = 1.1298 × 10⁶ dyne/cm

POUND-INCH SQUARED:

Unit of moment of inertia.

1 lb-in² = 2.9264 kg-cm²

POUND OF WATER PER MINUTE:

1 lb H₂O/min = 0.01603 ft³/min

= 2.670 × 10^-4/ft³/sec

POUND PER CUBIC FOOT:

1 lb/ft³ = 0.01602 gm/cm³

POUNDS PER SQUARE INCH (psi): 1 psi = 0.06805 atm

= 6.8947 × 10⁴ dyne/cm² = 70.307 gm/cm = 51.715 mm Hg = 27.7 in H₂O

POUNDS PER SQUARE INCH ABSOLUTE (psia): Absolute pressure, where 0 psia = vacuum

POUND WEIGHT:

1 lb wt = 4.4482 × 10⁵ dynes = 453.59 gm wt

RAD (rad):

Radiation absorbed dose. (See 8-6).

RADIAN (rad):

1 radian = 1/(2 p) circumference or revolution (0.15915)

= 57.296 deg

1 radian/sec = 57.296 deg/sec = 9.549 rpm

1 radian/sec² = 572.96 rpm²

RBE:

Relative biological effectiveness. (See 8-6).

RELATIVE HUMIDITY:

The ratio of the quantity of water vapor in an atmosphere to the quantity which would

saturate at the existing temperature. Also the ratio of pressure of water vapor to saturation pressure at that temperature.

REM:

Roentgen equivalent man. (See 8-6).

RESPIRATORY QUOTIENT (R. Q.):

The ratio of the rate of production of carbon dioxide (volume at STP per unit time) to the rate of uptake of oxygen (volume at STP per unit time).

REVOLUTIONS PER MINUTE (rpm): 1 rpm = 6 deg/sec

= 0.10472 radian/sec

1 rpm² = 0.001745 radian/sec²

ROENTGEN (r):

1 r = ionization by X or gamma-rays producing 1 electrostatic unit of charge in 1 cm³ of air (STP) = 83.0 ergs/gm

ROOT MEAN SQUARE (rms):

Square root of the mean of the squares of a set of numbers.

SONE:

Related to phon logarithmically. (See 16-4, 16-5.)

SOUND PRESSURE LEVEL (SPL):

SPL is sound pressure related logarithmically to a reference level of pressure (Po) , which by convention is 0.0002 dynes/cm². The defining equation is:

SPL = 20 log₁₀ p/Po in decibels

(See 16-3 for nomogram to convert sound pressure to SPL).

SQUARE CENTIMETER:

1 cm² = 1.076 × 10^-3 ft² = 0.1550 in² = 100 mm²

SQUARE FOOT:

1 ft² = 929.0 cm² = 144 in²

SQUARE INCH:

1 in² = 6.4516 cm² = 0.006944 ft²

= 645.1626 mm²

SQUARE MILLIMETER: 1 mm² = 0.01 cm²

= 0.001550 in²

STANDARD DEVIATION (S. D.)

The square root of the average of the squares of deviation from the mean. Also called root mean square deviation. Same as Standard Error.

STERADIAN:

1/(4 p) solid angle around a point.

1 steradian = 3282.8063 deg² = 0.07958 sphere

STPD (Standard Temperature and Pressure, Dry): 0° C, 760 mm Hg, water vapor pressure = 0. (See 10-1).

WATT:

1 watt = 1 joule/ sec

= 1 × 10⁷ erg/ sec = 0.7376 ft-lb/sec = 0.001341 hp

= 0.01432 kcal/min