448 FERREE AND RAND— LIGHTING 



When installed on or near the ceiling in rooms of moderate height, 

 the best results seem to be obtained when the opening, the surface 

 of the reflector and the ceiling have as nearly as possible equal bril- 

 liancy. It seems probable that the effect on the eye of the denser 

 reflectors can be very much improved by increasing the depth of the 

 reflector and by other devices that will lower the brilliancy of the 

 opening. In fact the best results we have as yet gotten from any 

 type of reflector have been from a direct opaque reflector of the deep 

 bowl type, modified so as greatly to reduce the brightness of the 

 opening, giving a field of view with the lowest maximum of bril- 

 liancy of any we have as yet been able to obtain in an actual light- 

 ing situation. This reflector, loj/i in. in diameter and ii^^ in. deep, 

 was lined to a depth of 3.7 in. with a mat surface having a reflec- 

 tion coefficient of about 4 per cent. Moreover, a result almost as 

 good as any we have obtained by indirect lighting was gotten by 

 giving this band or lining a reflection coefficient of about 38.5 per 

 cent. In the former case the brightness of the opening taken from 

 the position of the observer's eye was 0.0129 cp. per sq. in., a reduc- 

 tion of 99.8 per cent, in the maximum brilliancy of the opening; and 

 in the latter, 0.1815 cp. per sq. in., a reduction of 96 per cent. In 

 the former the illumination of the room was reduced on the average 

 25 per cent.; and in the latter, 12.4 per cent. Poor results are given 

 by shallow direct reflectors of all densities unless they are installed 

 so high above the working plane as to be almost if not entirely re- 

 moved from the field of view. 



5. We have frequently been asked to fix an upper limit of bright- 

 ness which the eye can stand without any considerable loss in power 

 to sustain clear seeing through a period of work. At present this 

 can be done at best only very approximately ; moreover, the value 

 assigned can not be made independent of the grouping of conditions 

 in which this brightness occurs. For example, a lighting installa- 

 tion which has its highest brightness well within the field of view 

 demands a smaller maximum than one in which these brightnesses 

 are carried outside the zone of most harmful efifects on the eye. 

 That is, higher brightnesses can be tolerated for the totally indirect 

 reflectors, or for direct reflectors installed on the ceiling, than for 

 semi-indirect reflectors in case of which the highest brightnesses. 



