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ON THE THEORY OF TONE REPRODUCTION 143
In Table I is given a schematic outline showing the various steps in the computation of the final result of a given tone reproduction problem. It will be noted that the process is divided into ten distinct steps, represented by the Roman numerals I to X. The necessary factors required at each step are designated by the corresponding arabic numerals. In the parentheses, designated by the letters A, B, C, etc., are statements either verbal or symbolic of the transformation factors which convert the factor of one step into that of the next succeeding step. At the starting point, I, it will be noted that the "subjective object" is taken, by which is meant the subjective impression of brightness and brightness difference excited in the mind of the observer. This impression, it is obvious, is that which it is desirable to reproduce rather than the actual physical brightness values which are measured by the usual physical methods. In practice, however, it is more convenient to take as the actual starting point of the operation the measured brightness values, B«, of the object and to evaluate the subjective relation between Bo (II) and Bso (I) relative to that between B™T (IX) and B»* (X) after having obtained the necessary values of Bmr.
Table I.
I. The Subjective object (SO) .
1. Subjective brightness of object (Bso) .
A. (Adaptation level of observer when viewing object, Ao, B*0 = f (Ao).)
II. Object (O).
2. Brightness (Bo).
B. (The constant of the image forming system, Kx.)
III. Image on the X (negative) material.
3. Illumination (/x) = Bo. K*.
C. (Exposure time, tx.)
IV. Exposure on the X (negative) material.
4. Exposure Ex = (Ix.tx) .
D. (Development, fixing, washing, drying, intensification, reduction, etc. Specified by Dn = f (Ex) .)
V. Negative (Ar) .
5. Density, D„, or transmission, Tn.
E. (Illumination, /„.)
VI. Image on the Y (positive) material.
6. Illumination, / = (Tn./n).
F. (Exposure (printing) time, ty.) VII. Exposure on the Y (positive) material.
7. Exposure Ey zz (Iy.ty) .
G. (Development, washing, fixing, toning, etc. Specified by Dp = f(Ey).)
VIII. Positive (P).
8. Reflecting power, Rv.
H. (Incident illumination. /,,.) IX. The material reproduction (MR) .
9. Brightness (B,„r) = (RV.IP) .
I. (Adaptation level of observer. Amr.B = f (Amr) .) X. The Subjective brightness (SR) .
10. Subjective brightness (B,,) .
J. (The relation between £sr and Bs<, determines the extent to which the desired subjective impression of tone is accomplished. B*o = f (B*») .)