Journal of the Society of Motion Picture Engineers (1930-1949)

mittance through a single coated surface may be plotted in terms of transmittance density versus wavelength for varying film thicknesses. From these curves we can tabulate our density difference Z)4oo — D-iQQ for each of several film thicknesses. If we do this for glass indices from 1.50 to 1.90, we find that for a given film thickness the color contribution jD40o — AGO plotted against glass index gives very nearly a linear relation. This provides us with an important simplification, for, if we have a lens consisting of glasses of differing indices, rather than compute the effect for each index we can use a "weighted index" for the lens as a whole. This "weighted index" is determined by using for each lens element its refractive index times zero, one or two, depending upon the number of its coated surfaces. These weighted values are added together for the entire lens and divided by the sum of the weightings, that is, the total number of coated surfaces. We now use a table having as entries the film thickness and the weighted index. The body of the table consists of the color contributions from the coated glass surface. We have found it convenient to have a table for four, six and eight coated surfaces. The dispersion of glass has been taken into account in setting up the tables, although this is a secondary effect. The color contribution arising from glass absorption is always a positive quantity, i.e., the Z>4oo — Aoo is always greater than zero since the glass transmittance density in the blue region is greater than in the red. The contribution from the coating, however, takes on negative values as well as positive values and this is what enables us to control the transmission color of the lens. A coating having its minimum reflection at about 500 mfj. has a zero value for the term Z>40o — D7QQ regardless of glass index. Thinner coatings have negative values, thicker ones positive values. There is of course a limited amount of control offered by the coating; the higher the glass indices and the more coated surfaces, the greater the control. For example: a lens having a weighted index of 1.70 and eight coated surfaces can have a negative color contribution of as much as 0.11 while the maximum negative contribution from a lens having four coated surfaces and a weighted index of 1.55 is 0.03. It is now a simple matter to determine the combined effect of glass and coating on the transmission color of a lens. But we are still without a limiting value for this color. It was felt for the most critical lenses, interchangeable cine lenses, the departure from neutrality should not exceed the amount introduced by the lightest filter that may be used. In the Kodak Wratten series of Light Balancing Filters the lightest is the No. 81 . It has a value for Z)4oo — -Dyoo of 0.08. It was found that for the simplest cine lenses 0.02 represented the minimum color contribution. This range 0.02 to 0.08 was therefore taken as a reasonable range of color contribution values for cine lenses. Other camera lenses being less critical for color could have the range 0.0 to 0.10. To avoid any possibility of the spectral curves becoming too highly inflected between 400 and 700 rm«, it is suggested that the expression Z)70o — Aninimum have a maximum value of 0.04, that is, the density at 700 m/i must be within 0.04 of the minimum density wherever it is. There will of course be camera lenses which cannot be made to meet these color specifications. Large aerial lenses are examples, but in such cases we do not ordinarily have rigid requirements for neutral transmission. 194 September 1952 Journal of the SMPTE Vol. 59