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For instance, a Wv value of 50 indicates that with the illumination to which the value applies the illumination on the object must be twice as great as would be required with sunlight to obtain this same photographic effect. Since the efficiency in terms of energy consumption is of considerable interest, the values given in Table 2 have been reduced to that basis and are given in Table 3. The luminous flux incident upon unit area (1 sq. cm.) at a meter distance from the source of 1 mean spherical candle power is
4^ ^ 1 (7)
4 X r2 100 2 lumens This is the value of the luminous flux incident upon unit area of the surface on which the illumination is 1.0 meter candles. If the inertia expressed in terms of energy be denoted by the symbol
le
100^
, this is expressed in terms of
lumens seconds
cm. 2
Now, if the efficiency of the source used be C (in — ),
■^ ^ watts
lumen seconds I
the value remains the same when expressed in
watt second ' C
is the efficiency in
watt seconds lumen seconds i (in m.c.s.) 100^ ^
, and
watts seconds
cm 2
(8)
i (in m.c.s.) 100^
X
10^
c
ergs cm. 2
= "-^ (9)
ie is, therefore, the inertia value expressed in ergs consumed at the source per cm^ at the plate. This value is inversely proportional to the photic efficiency of the source when used on that particular plate. The photic efficiency may be obtained, therefore, by taking the reciprocal of ie.
TABLE 3.
Relative Photic Efficiency = We
Photographic Materials
Visual
Efficiency
Lumens
Ortho
Pan
No. Source Watt
Ordinary
chromatic
chromatic
1. Sun , . 150.0
100.0
100.0
100.0
2. Sky ....
.3. Acetylene ....
0.7
"o.'m
"6!2i"
"6'.24
4. Acetylene screened
0.07
0.04
0.040
0.042
5. Pentane . . .
0.45
0.05
0.9
0.13
6. Mercury arc — quartz
40.0
158.0
130.0
99.0
7. Mercury arc— "nultra"
35.0
50.0
47.0
39.0
8. Mercury arc — crown
37.0
79.0
68.0
62.0
9. Carbon arc — ordinary
12.0
10.0
9.0
8.5
10. Carbon arc — white flame
29.0
52.0
45.0
42.0
11. Carbon arc — enclosed
9.0
11.0
11.0
10.0
12. Carbon arc — "Aristo"
12.0
62.0
86.0
60.0
13. Magnetite arc
18.0
12.0
14.0
10.0
14. Carbon glow .
2.44
0.37
0.52
0.68
Carbon glow .
3.16
0.51
0.74
0.95
15. Tungsten vacuum
8.0
1.7
2.2
2.7
Tungsten vacuum
9.9
2.4
3.0
3.50
16. Tungsten-nitrogen
16.6
6.1
6.8
7.7
Tungsten-nitrogen
21.6
8.9
9.8
11.0
17. Tungsten blue bulb
8.9
5.5
5.2
5.6
Tungsten blue bulb
11.0
7.8
7.31
7.9
18. Mercury-vapor .
. 23.0
47.0
54.0
42.0
87