33° 



Tables 339 and 340 

 THE SENSITIVITY OF THE EYE 



Definitions: A meter-candle is the intensity of illumination due to a standard candle at a meter distance. The 

 millilambert (o.ooi lambert) measures the brightness of a perfectly diffusing (according to Lambert's cosine law) 

 surface diffusing .001 lumen/cm^. A brightness of 10 meter-candles equals 1 millilambert. 0.001 ml corresponds 

 roughly to night exteriors, 0.1, to night interiors, 10 ml to daylight interiors and 1000, to daylight exteriors. A bright- 

 ness of 100,000 meter-candles is about that of a horizontal plane for summer day with sun in zenith, 500, on a cloudy 

 day, 4, 1st magnitude stars just visible. 0.2, full moon in zenith, .001, by starlight; in winter the intensity at noon may 

 drop about 3. 



TABLE 339. — Spectral Variation of Sensitiveness as a Function of Intensity 



Radiation is easily visible to most eyes from 0.3.50 ll (violet) to 0.770 ll (red). At low intensities near threshold 

 values (gray, rod vision) the maximum of spectral sensibility lies near 0.503 ll (green) for 90% of all persons. At higher 

 intensiti 33, after the establishment of cone vision, the max. shifts as far as 0.560 fx. See Table 346 for more accurate 

 values of sensitiveness after this shift has been accomplished. The ratio of optical sensation to the intensity of energy 

 increases with increasing energy more rapidly for the red than for the shorter wave-lengths (Purkinje phenomenon); 

 i.e., a red light of equal intensity to the eye with a green one will appear darker as the intensities are equally lowered. 

 This phenomenon disappears above a certain intensity (above 10 millilamberts). Table due to Nutting, Bulletin 

 Bureau of Standards. 



The intensity is given for the spectrum at 0.535^1 (green). 



Intensity 

 (meter-candles) = 

 Ratio to preceding step 



.00225 

 9-38 



• 0360 

 16 



■ 575 

 16 



36.9 



147.6 



S90.4 



4 



Wave-length, X. 



Sensitiveness. 



0.430/ii 

 0.450 

 0.470 

 0.490 

 0.50S 

 0.520 

 0535 

 o.555 

 0.57S 

 0590 

 0.605 

 0.625 

 0.650 

 0.670 

 X , maximum sensitiveness 



0.081 



0.33 



0.63 



0.96 



1. 00 



0.88 



0.61 



o. 26 



0.074 



0.025 



0.008 



0.004 



0.000 



0.000 



0.503 



0.093 

 0.30 

 0-59 

 (0.89) 

 1 .00 

 0.86 

 0.62 

 0.30 

 o. 102 



0.034 

 0.012 

 0.004 

 0.000 

 o. 000 



0.504 



o. 127 

 o. 29 



0.54 



(0.76) 



1 .00 



0.86 

 0.63 

 0.34 



0.122 



0.054 



0.024 



O.OII 



0.003 



O.OOI 



0.504 



0.128 

 031 

 0.58 

 (0.89) 



1 .00 



0.94 



o. 72 

 0.41 



0.168 



0.091 

 0.056 

 0.027 

 0.007 

 0.002 



0.508 



o. 114 



0.23 



0.51 



(0.83) 



0.99 

 0.99 



0.91 



0.62 



(0.39) 

 o. 27 



0.173 



0.098 

 0.025 

 0.007 

 0.513 



o. 114 



0.175 



o. 29 

 0.50 



(0.76) 

 (0.85) 

 (0.98) 



0.84 



(0.63) 



0.49 

 0.3s 



o. 20 

 0.060 

 0.017 

 0.530 



0.45 

 0.66 

 0.8s 

 0.98 

 0.93 



(0.76) 



0.61 



(0.45) 



o. 27 



0.085 



0.025 



0.541 



TABLE 340. — Threshold Sensibility as Related to Field Brightness 



The eye perceives with ease and comfort a billion-fold range of intensities. The following data were obtained with 

 the eye fully adapted to the sensitizing field, B, the field flashed off, and immediately the intensity, T, of a test spot 

 (angular size at eye about 5°) adjusted to be just visible. This table gives a measure of the brightness, T, necessary 

 to just pick up objects when the eye is adapted to a brightness, B. Intensities are indicated log intensities in milli- 

 lamberts. Blanchard, Physical Review, n, p. 81, 1918. 



LogB 



/ Log T, white. 

 I T/B 



Log T, blue 



Log T, green . . . 



Log T, yellow. , 



Log T, red 



+3.0 



+0.28 



.0019 



SMITHSONIAN TABLES. 



