Journal of the Society of Motion Picture and Television Engineers (1950-1954)

layers is more nearly complete. Highly reflecting films can be applied to both surfaces of a glass plate advantageously in many cases; however, this scheme cannot be used in any case where double images are undesirable. Transmission characteristics for an interference mirror which reflects efficiently through the visible spectrum and efficiently transmits infrared radiation are shown in Fig. 10.* In this case, several sets of layers controlled for maximum reflection at different positions in the visible range, are deposited on one surface of a plate. In this case, no attempt is made to prevent interference between the sets of layers. Interference between the sets of layers improves the efficiency of the film. Interference mirrors can be made which reflect all wavelengths of the visible spectrum with approximately equal intensity. Such mirrors can be made to reflect as much as 45% of the incident light. A relatively thin layer of low-index material is first deposited on the glass surface, followed by a thicker layer of a material with a high index of refraction. The interference effects of such a film design cause all wavelengths of the visible spectrum to be reflected with approximately equal intensity because of the relative thickness of the component layers for each wavelength. The reflectivity of such a film is plotted with respect to the film thickness in Fig. 11. The low-index film is slightly less than J-wavelength thick for blue light. The high-index material is deposited until a reflection maximum is passed and the reflection drops to about 85% of the maximum. Since the low-index layer is not J-wavelength thick at the point of maximum reflectance, the interfering rays are not exactly in phase. This maximum is, therefore, not as high as it * G. L. Dimmick and M. E. Widdop, "Heat transmitting mirror," Jour. SMPTE, 58: Jan. 1952. *A ix Jx FILM THICKNESS IN WAVELENGTHS Fig. 11. Design of an achromatic film. would have been if the low-index film had been J-wavelength thick. For longer-wavelength green light, the effective thickness of the low-index film is less than for the blue light. The maximum intensity obtainable for this wavelength is, therefore, less than it was for blue. For these film thicknesses then, the reflected intensity for green light has just passed its maximum point, and is approximately equal to the intensity for the blue. And for the red light, having the longest wavelength, the interferring rays are at a point of maximum reinforcement, but the resultant intensity is equal to that of the green and blue. The transmission characteristics of a mirror of this type are shown in Fig. 12. Discussion A. V. Loughren, Chairman of the Session (Hazeltine Corp.} : The importance of these dichroic mirrors in the development work, which has been going on recently both at RCA and many other places in color television, is something to which I can certainly testify personally. Our own developments at Hazeltine came along far faster than they would have otherwise because it was possible to make arrangements to get some of the dichroic mirrors designed by Miss Widdop and use them in our own work. I have no doubt myself as to their importance. L. L. Ryder (Paramount Pictures Corp.) : Is any work being done in conjunction with these mirrors for high-intensity light levels, for instance where they're used in conjunction with arcs and similar light sources? Mary Ellen Widdop: Dichroic Mirrors 3*5