DISTRIBUTION OF ILLUMINATION 



365 



test object was wasted by glare. The effects 

 of such glare can be largely eliminated by 

 increasing the brightness contrast of test ob- 

 ject and its immediate background and by 

 increasing the illumination on the test object. 

 It is better, however, to eliminate the glare 

 source. Ferree and Rand (7) have demon- 

 strated that after reading for four hours 

 under three systems of hghting, the greatest 

 loss of efficiency occurred with the direct, 

 next greatest with the semi-indirect, and 

 least of all with the indirect system. Where 

 several light sources are in the visual field, 

 the greater the number, the greater the loss 

 of visual efficiency. Luckiesh (14, p. 204) 

 shows that nervous muscular tension is much 

 greater when a glare source is in the field of 

 view. 



Specular reflection, as reflection from a 

 mirror or shiny metal, is also a glare source. 

 Reflections from glossy or polished surfaces, 

 when light is not well diffused, produce vis- 

 ual discomfort and may lessen visual effi- 

 ciency. Such surfaces should be ehminated. 

 Luckiesh (14) shows that in the case of 

 printed matter, specular brightness may be 

 greatly reduced by increasing the light in- 

 tensity. A better solution is to use well- 

 diffused illumination. 



Brightness Contrast 



The distribution of brightness in the visual 

 field is an important determinant of seeing 

 conditions. For convenience the visual field 

 may be divided into: (a) the central field 

 containing the visual task and its immediate 

 background such as the page of a book where 

 the printed words being read are seen against 

 a background area; (b) the surroundings of 

 the task extending out 30 degrees from the 

 visual axis; and (c) the peripheral field be- 

 yond the surroundings extending about 70 

 degrees (for our purpose) out from the visual 

 axis. High brightness contrast between the 

 object and its immediate background is desir- 

 able for ease of seeing. Luckiesh and Moss 

 (19) cite material which shows that surround- 

 ings that are either brighter or darker than the 



central field decrease visibihty of the objects 

 to be discriminated. Apparently it is worse 

 for the surroundings to be brighter than to be 

 darker than the central field. Lythgoe (23) 

 also found that dimly lighted or dark sur- 

 roundings reduced visual acuity in compari- 

 son with surroundings of the same brightness 

 as the central field. Luckiesh (15) also pre- 

 sents data which show that brightness of sur- 

 roundings affects accuracy of visually con- 

 trolled movements. Settings of a visually 

 controlled mechanism showed 12 percent 

 greater error when the surroundings were 

 dark than when the same brightness as the 

 task. The greatest effect was for changes 

 within 15 degrees of the visual axis. He (15) 

 also demonstrated that looking alternately 

 at light and dark surfaces where an object 

 must be discriminated leads to rapid reduc- 

 tion in ease of seeing. 



All these data lead to the conclusion that 

 vision is best when surrounds are the same 

 brightness as the central field. Surrounds 

 should not be brighter than the central field. 

 Brightness ratios between the central field 

 and the surrounds should not be more than 

 five to one. Ratios greater than ten to one 

 should be avoided. According to Moon and 

 Spencer (24), this brightness ratio should not 

 exceed three to one for best vision. It is 

 pointed out that one sees best and visual 

 fatigue is reduced to a minimum when the 

 entire field of view is of approximately the 

 same luminosity as that to which the center 

 of the eye is adapted (i.e., as vision is directed 

 to the work surface). 



In parts of the peripheral field outside the 

 surrounds (beyond 30 degrees from the visual 

 axis), the brightness may diminish consider- 

 ably without materially affecting visual dis- 

 crimination or ease of seeing, according to 

 Luckiesh (15). 



Summary 



Glare from bright spots in the field of 

 vision not only produces discomfort but also 

 reduces visual efficiency. Brightness con- 

 trast between areas within the visual field 



