INTENSITY 



361 



Size of Object 



The smallest size of detail that can be 

 barely seen under a given set of conditions is 

 the threshold size. The most complete 

 study on the relation between size and illu- 

 mination was done by Weston (33). Eight- 

 een subjects cancelled all the Landolt rings 

 with a given gap orientation on specially pre- 

 pared test sheets. The rings were black, 

 printed on paper with a reflection factor of 

 84 percent. The gap size at 13 inches from 

 the eye subtended (on different sheets) an- 

 gles of 1, 2, 3, 4, 6, and 10 minutes. The de- 

 tail (space between dot and rest of letter i) 

 of 10 point type (ordinary book type) sub- 

 tends an angle at the eye of approximately 3 

 minutes at a distance of 13 inches. Illumi- 

 nation intensities used were 0.16, 0.8, 4, 20, 

 100 and 500 f .c . Both accuracy and speed of 

 discrimination entered into the scores. The 

 results are given in terms of speed (reciprocal 

 of time), which is unfortunate, since this 

 magnifies small differences unduly. The re- 

 sults show (a) that for all sizes of objects 

 there was a rapid increase in performance as 

 the hght was increased from 0.16 to 4.0 f.c, 

 and (b) that for objects of 4 minutes visual 

 angle and larger there was no appreciable in- 

 crease for illuminations above 20 f.c. For 

 small objects (1, 2, and 3 minutes), the im- 

 provement of visual performance is small 

 and probably not significant beyond 50 f.c, 

 contrary to the author's view. It is pointed 

 out that no practical amount of illumination 

 will make small objects (1 minute) as readily 

 discriminated as large objects (6 minutes). 



Luckiesh (14, p. 98) also presents data on 

 the relation between size of object and illu- 

 mination in seeing. As the illumination is in- 

 creased from 1 to 10 to 100 f.c, smaller and 

 smaller sized objects can be discriminated 

 for all degrees of contrast between object and 

 ground. The limits of practical improve- 

 ment undoubtedly lie between 10 and 100 

 f.c Interpolation reveals that the curve of 

 improvement flattens out rapidly for levels 

 above 30 to 40 f.c. 



These data reveal (a) that for large objects 



(4 minutes and above), there is no practical 

 improvement in visual discrimination with 

 intensities above 20 f.c For smaller objects 

 there is improved visual discrimination at 

 higher intensities. The indications are, how- 

 ever, that these changes are significant only 

 up to 40 or 50 f.c As a matter of fact, these 

 data merely represent another way of meas- 

 uring visual acuity. The amount of illumi- 

 nation needed to perceive an object also 

 depends upon the contrast between object 

 and background. This will be treated below 

 (see Brightness Contrast). 



Speed of Vision 



It takes time to see. Quickness of seeing 

 is influenced by level of illumination as well 

 as by size of object to be discriminated and 

 brightness of the background. Where split- 

 second seeing is desirable, the time factor is 

 important. Luckiesh (14, p. 131) reports 

 data which show that the time required to 

 recognize a black test object decreases mark- 

 edly as the brightness-level of the back- 

 ground increases from 1 to 2 to 5 to 10 

 footlamberts. (One footlambert is the 

 brightness of a perfectly reflecting surface 

 illuminated by one f.c.) As the brightness 

 of background is further increased to 20 foot- 

 lamberts, the additional decrease in time is 

 about .005 second, and there is a hke de- 

 crease in going from 20 to 100 footlamberts. 

 Thus, the decrease in time of seeing is rela- 

 tively insignificant for the more intense illu- 

 mination changes. For a small test object, 

 Cobb (5) found no significant decreases in 

 speed of vision for brightnesses above about 

 40 footlamberts when a pattern of parallel 

 dark bars was presented before and after the 

 test object, i.e., a situation comparable to 

 seeing in everyday life where the eye receives 

 stimulation before and after discrimination 

 of an object. The range of illumination 

 used was 1 to 100 footlamberts. With a 

 relatively large test object and similar con- 

 ditions, there was no significant decrease in 

 time for brightnesses above about 18 foot- 

 lamberts. 



