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Optical design reflectors: part 3; author's reply to comments W. B. Elmer 2 Chestnut Street, Andover, Massachusetts 01810. Received 15 August 1978. 0003-6935/78/1201-3706$0.50/0. © 1978 Optical Society of America. A statement of the desired visibility objective of a new light is occasionally all the instruction that a reflector designer is given when asked to make the design. In the absence of the usual concise beam specification, he himself must then es- tablish the characteristics of the required beam before he can make the design. He must therefore possess at least a basic knowledge of physiological optics, a discipline not his own though closely parallel thereto. In my paper 1 I therefore included a section comprising a series of short statements relative to vision, each intended to be helpful to the practicing reflector designer. These state- ments reflected for the most part the conventional wisdom among illuminating engineers, which, although not always rigorously exact, nevertheless does serve a sufficient pur- pose. It is especially appreciated therefore when an optical physiologist of the stature of Dr. Wohlbarsht gives some so- phisticated insights in this area. 2 For example, he gives us the academic explanation that it is not aging per se that causes the eye to lose clarity, but the environmental influences at work on the eye over a lifetime of use. The comment re atomic fireball damage to the eye is of in- terest, although no distance is specified upon which the dis- tinctions are made. It has been clearly (and even emphatically) stated by in- vestigators in the Illuminating Engineering Research Institute that visibility improves continuously without limit as illu- mination is increased. There must be some limit residing in collateral effects such as glare, dazzle, or adaptation delay. It is declared that man can see best under direct sunlight. Be- cause of the eye's miraculous ability to adapt over an ex- traordinary range of illumination, vision is still fully adequate under one-fortieth or one-fiftieth of full sunlight, the values mentioned by Wohlbarsht. My remarks re the Purkinje effect stand. In writing the American Standard Practice for Street and Highway Lighting, we specified lumens rather than watts for the various specific applications. Since the efficacy of yellow sodium lamps is much higher than that of blue mercury lamps, less wattage of sodium is needed to provide any given level of illumination. 3706 APPLIED OPTICS / Vol. 17, No. 23 / 1 December 1978 I compared lights of equal intensity, not equal wattage, in my paper. This is why mercury (or blue) lights are superior to sodium in the remote fringes of rural areas. References 1. W. B. Elmer, Appl. Opt. 17, 1308 (1978). 2. M. L. Wolbarsht, Appl. Opt. 17, 3705 (1978).
