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FRESNEL LENS
Absorption Coefficient 1.3% per cm.
Distanceof
Loss in % by ab
Sum
Loss in zone
center of zone from opt. axis
Loss in % by reflection on surface
sorption and length of path in lens in (mm)
of losses
%
I
2
(10. 0)
7-1
4-3
4-1
1.2 (10. 0)
9-7
I . I
22.0
4.8
4-5
1 .2 (lo.o)
10.5
2.8
29.8
6.0
4-5
1.2 (10. 0)
II. 7
3-3
37.6
9.0
4-5
1 .2
14-7
5-0
Total Loss Absorption Coefficient 3.9% per cm.
12.2%
Distanceof center of
zone from opt. axis
Loss
in % by reflection on surface
Loss in % by absorption and length of path in lens in (mm)
Sum
of
losses
%
Loss in zone
I
2
(10. 0)
7-1
4-3
3-9
3-8
(10. 0)
12.0
1-4
22.0
4.8
4-3
3-S (lo.o)
12.8
3-4
29.8
6.0
4-3
3-7 (lo.o)
14.0
3-9
37-1
9.0
4-3
3.6
16.9
5-8
Total Loss 14.5%
Loss of light on a concave silvered glass mirror as used behind the light source is caused
(a) By the reflection of the in and out going ray on the outside surface, which amounts to about 8%,
(b) By reflection on the silver film, which varies considerably with the quality of silvering, 10% being an average figure, and
(c) By the absorption in the glass which amounts to from 1% to 4% for a mirror of 5 mm. thickness.
These figures add up to a total loss of about 20% on the mirror. If photometric measurements show in general a greater loss up to 50% in commercial mirrors, the reason may be found in most cases in the poor silvering and bad conditions of the surfaces.
In the case of a double condenser about 73% to 80% of the light contained in the angle 2u reaches the image of the source after having passed through the condenser. Of the 80% which in the best case is reflected by the spherical mirror, 20% to 27% are lost by passing the condenser so that the total amount which reaches the image of the source equals in the best case 80% -j 64%, or 1.44 times the amount contained in the angle 2u. If the higher figure for the loss in the condenser is taken, the total is reduced to 73% + 58% or 1.3 1.
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