Letters
Letter on night vision
In your Newsletter 2/95 (issue no. 11, vol. 4) you featured the article by Mark S Rea "In the dark about the lumen ". The research mentioned by Rea-trying to find the most efficient light for the night-driving situation-is very important. In his article Rea discusses the change in luminous efficacy that occurs for different light sources when adaptation changes from photopic to scotopic.This is of great importance, and one has to congratulate Dr. Rea for the very clear presentation of how colour temperature and adaptation level affect the relative luminous efficiency and lumen-per-watt efficacy of different light sources.
We at CIE also believe that it is important that these concepts be well understood by the general public. Publication CIE 41-1978* summarises the changes in relative luminous efficiency with colour temperature, field size and adaptation level, and in Publication 81-1989** the equivalent luminance predictions for different calculation methods studied by the CIE are summarised for the mesopic range together with photopic and scotopic luminance data.
There are CIE committees dealing with both the fundamental and applied sides of this problem, and all parties interested in the subject are more than welcome to provide input to this research. There is still much important research needed concerning vision in connection with nighttime driving, such as brightness evaluation and the perception of details. The spectral responsiveness of the human eye to these two phenomena is, however, different.
I appreciate that in the editorial introduction you had to find an eye-catching phrase but, to avoid confusion, may I point out that there is no such thing as "night-time lumen". Rea, of course, was careful to refer only to "night-time spectral sensitivity" or "luminous efficacy" as the lumen is the unit of luminous flux (light), and no adjective should be used with the unit.
In calculations, the luminous flux can be weighted photopically or scotopically. When the unit of intensity, the candela, was re-defined, the CIE and the Consultative Committee for Photometry and Radiometry (CCPR) went to great lengths to ensure that the photopically and scotopically weighted luminous intensities of the 555 nm monochromatic radiation were equal.
May I thank you, on behalf of the CIE, for airing this important subject in your newsletter.
J. Schanda
Technical Manager, CIE
*CIE 41-1978: Light as a true visual quantity: principles of measurement
**CIE 81-1989: Mesopic photometry: History, spectral problems and practical solutions.