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We can now apply the various fundamental relationships
we have discussed thus far, to the evaluation
of the photographic reflecting power of colored
objects. To summarize, the relationships upon
which photographic reflecting power depends are:
(a) The spectral reflection characteristics o{ the
colored object, curvc R.
(b) The spectral distribution of energy in the
illuminant, curve J.
(c) The spectral transmission of the photographic
objective, Curve C.
-
(d) The spectral sensitivity of the photographic
material, curve A.
These various functions are shown by the curves
in I'ig. 14. The {ormal expression for the photographic
reflecting power Rp' of the colored object is'
' Arl rcrRrd,
Rp:
l"- z.rc11,
J.
In this expression the symbols A r, J r etc.,indicate
that these values apply to a specific wave-length r.
For correct orthochromatic reproduction of a
group of colored objects, it is necessary that the photographic
reflecting powers, Ro, shall be equivalent
to the visual reflecting powers, R". It is necessary,
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wavELENGTH (mF)
FIGURE
Spectrophotometric functions involved in the evaluation of
visual reflection factor.
therefore, to evaluate also visual reflecting power;
and the {undamental relationships already discussed
provide the necessary information for this operation.
They are shown graphically in Fig. 15. To summarize,
these relationships are:
(a) The spectral sensitivity of the retina (visibility
function, Fig. 7), curve V.
(b) The spectral distribution of energy in the
illuminant, curve J.
(c) The reflection characteristic of the object,
curve R. The evaluation of visual reflecting power
may be expressed in a manner analogous to that giving
the photographic reflecting power, and is given
by the following equation.
XV
R,/
J./
I J rVrRrd
J"
f(''- _--
I r-
J rVtdr
Jo
While the use of the relationships discussed in the
previous pages are ideal from the theoretical point
of view for the determination o{ visual and photographic
reflection factors, many difficulties are encountered
in their application to practical problems.
The experimental uncertainties involved in the determination
of the various functions are sometimes
so large that the computation of reflection factors
by the methods indicated are subject to appreciable
error. It is, therefore, frequently more expeditious
to use a direct method of measuring the visual and
photographic reflection factors o{ the colored objects
in question. However, a thorough knowledge of
these relationships is absolutely essential to a complete
understanding of hozu and zuhy colored objects
are rendered as they are by a specified photographic
technique.
A more direct method of measuring photographic
reflection factors has already been discussed
in a previous paper2. The color samples in question
are photographed along with a series of grays ranging
from black to white. A typical panel is shown
in Fig. 16.x The horizontal row designated as A,
B, C, etc., are neutral grays having reflecting powers
ranging between 2 and, 85 per cent. These values
were measured by the well-known photometric
method. The visual reflection factors of the colored
panels can be determined with relatively high precision
by means of the flicker photometer method
which is the universally used method for comparing
visual intensities in the presence of color difierences.
The negative densities corresponding to the various
gray samples are measured by means of, a suitable
densitometer, and, by plotting these values as or-
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DENSITY (SPECULAR)
N
o
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FIGUR,E
XVII
Characteristic log exposure curve of photographic material
iliuittati.g method of deriving photographic reflection factor'
- ---l
Jot.", Loyd A. The Photographic
-Reflecting
Fower of'Colored Objects. Trans. Soc. Mot' Pict'
Ene. XI. No. 31. 564-581. !927'
-*
S.. iilustration on page 44.
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