CH. XII] 



BRIGHTNESS OF SCREENS 



463 



FIG. 249. DISTRIBUTION OF APPARENT BRIGHTNESS WITH DIFFERENT 

 SCREENS WHEN VIEWED FROM DIFFERENT DIRECTIONS. 



On the sides and curved surface are given the degrees of inclination of the 

 reflected light. 



The numbers along the central radius indicate the relative brightness of each 

 screen, magnesium oxide being used as the standard and called 100%. 



A Magnesium oxide screen. It gives the standard brightness of 1 00% and 

 reflects nearly equally throughout the entire 180 degrees. 



B White cardboard. 



C Screen with aluminum bronze facing. This gives 3.2 times the bright- 

 ness of magnesium oxide in the center, but it falls off rapidly at the sides. 



E Mirror screen. This gives 7. 1 times the brightness of magnesium oxide 

 in the center. 



It is to be noted in general that the mirror screens, (C. E.) give great inten- 

 sity when seen near the center, and that this intense light is restricted to an 

 angle of about 25 degrees. Farther to the side the light falls off rapidly, 

 being in marked contrast with the white screens (A B). 



