Bauhaus-Universität Weimar

Optic Projection: Principles, Installation and Use of the Magic Lantern, Projection Microscope, Reflecting Lantern, Moving Picture Machine
Gage, Henry Simon and Henry Phelps Gage
[Ch. XIV 
Consider a sphere of one meter radius having this lighted surface at its 
center. The light received by one square meter of the surface of this sphere 
will then be B cos 0 lumens. Only half of the sphere can receive light from 
this opaque surface and the entire light received by this hemisphere will be: 
2tB sin© cos0 d0 = 
sin20 d20 
= vB 
Now if the reflecting surface is perfectly white there wall be no light lost and 
the entire light received by the hemisphere will equal the light incident upon 
the reflecting surface, that is iB = 1/10,000 and B = i/io,ooo*- candle- 
power per square centimeter. In the above example where the incident 
illumination is 48,000 meter candles, the surface considered as a source of 
light will have 48,000 candle power per square centimeter. 
This same formula will apply to the case of opaque projection (§ 274a) where 
it is desired to determine the ratio of the light getting through the objective to 
form the screen image and the light falling on the opaque surface, assuming that 
this opaque surface is perfectly diffusing and perfectly white. In the case of 
the objective, light over a certain zone of the hemisphere is used. If the angle 
which the objective subtends with a point on the object taken as the center is 
called 20, then the angle between the axis and the edge of the objective is 0, 
and the above formula will apply, i. e., the light flux striking the objective 
from one square centimeter is vB/2 (1 — cos 20). Also the total light flux 
reflected from the surface over the entire hemisphere is irB, hence the ratio of 
the light flux striking the objective to form the screen image to the light flux 
received by the reflecting surface is 1—cos 20. This takes no account of losses 
due to reflection and absorption by the objective. 
* l2r


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