C. D'D. ISELIN 



INTENSITY 



359 



print is the same under mercury arc and un- 

 der tungsten light. The visibihty measure- 

 ments, made on eight subjects for 8-point 

 book type by means of the Luckiesh-Moss 

 Visibihty Meter, averaged 3.8 for the mer- 

 cury arc and 3.7 for the tungsten hght. 

 Other data (18) confirm the above findings. 

 In general, these results indicate that the 

 mercury arc light is no better and no worse 

 than tungsten filament for visual work. 



Fluorescent versus Tungsten Filament Light 



Periodically, since the advent of fluores- 

 cent lighting, questions have been raised 

 concerning possible harmful effects to the 

 organism. Luckiesh and Taylor (21) have 

 evaluated the criticisms of fluorescent light- 

 ing. They present evidence which leads them 

 to conclude that the ultraviolet, infrared, and 

 visible radiant energy coming from the lamps 

 is not injurious to the human organism. 

 This has not quieted either the criticisms of 

 fluorescent light or the complaints of those 

 working under it (12). Apparently fluores- 

 cent is as effective as tungsten filament light 

 for visual efficiency. Luckiesh and Moss 

 (20) show that the two are equally effective 

 in promoting ease of seeing. The change in 

 rate of involuntary blinking after reading for 

 30 minutes was the same for both kinds of 

 illumination. 



Holway and Jameson (12) carried out ex- 

 tensive investigations in which one of the 

 comparisons was between fluorescent and 

 incandescent illumination. The quality of 

 light derived from the fluorescent lamps, no 

 matter what combination of colors was used, 

 was considered both unpleasant and distract- 

 ing to the workers. The quality was 

 described as thin, harsh, and cold. This 

 undesirable quahty was not present in either 

 daylight or incandescent illumination. This 

 psychologically undesirable quality of light 

 from fluorescent lamps was considered a fac- 

 tor detrimental to ease and comfort in read- 

 ing. The authors recommend that incan- 

 descent rather than fluorescent light be used 

 in reading rooms and offices. 



These authors (12) also state that experi- 

 mentation has shown that, after working 

 under direct fluorescent illumination, the re- 

 covery of sensitivity in the night vision ap- 

 paratus is retarded seriously in comparison 

 to being under incandescent hght. It is 

 stated that this effect is caused by ultraviolet 

 radiation from fluorescent light sources. 

 After 30 minutes of dark adaptation, follow- 

 ing an hour under fluorescent light, night 

 vision was only half as good as it was after 

 being under incandescent light. This retar- 

 dation in recovery of night vision can be 

 eliminated when fluorescent light is shielded 

 or used in indirect fixtures. 



Summary 



For threshold seeing, spectral yellow, or 

 yellow light from sodium vapor lamps, is 

 slightly more effective than other artificial 

 illuminants. But these differences are small 

 and relatively inimportant. Apparently, 

 no illuminant is better than diffused day- 

 light. In ordinary situations, where seeing 

 is supraliminal, as in reading print, no arti- 

 ficial illuminant that is suitable in spectral 

 character for general use in lighting is better 

 than any other such illuminant for critical 

 seeing. This is true for daylight, and for 

 light from mercury arcs, tungsten filaments, 

 or fluorescent lamps. There is, however, a 

 psychological objection to fluorescent light 

 because of its "harsh" and "cold" appear- 

 ance. 



Intensity 



In any environment where visual work is 

 to be done, one must decide how much light 

 (i.e., what intensity of illumination) is 

 needed for easy and effective visual discrimi- 

 nation. Various approaches have been em- 

 ployed to arrive at proper light intensities for 

 effective seeing. We will discuss here the re- 

 lation of illumination intensity to (1) visual 

 acuity; (2) size of object to be discriminated; 

 (3) speed of vision; (4) brightness contrast; 

 and (5) efficiency of performance. An ap- 

 praisal will be given after presenting the 

 data in each area. 



