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Night Vision Theory

Implications

The LRC's system of mesopic photometry enables light sources to be better evaluated over the entire range of visual adaptation important to electric lighting applications. This can lead to more efficient and effective lighting practice.

Problem

The current practice of measuring light and lamp efficacies is inadequate for many low light conditions such as roadway, parking lot, and security lighting, because a photopic, pure cone, response of the eye is assumed. Under mesopic, or low-light, conditions, both types of photoreceptors in the eye, rods and cones, contribute to visual response.

Since reported lamp efficacies are inappropriate for low-level conditions, specifiers cannot easily determine the most efficacious source for the task. As shown above, a large part of the limited range of visual adaptation covered by electric lighting is in the mesopic region. This reveals a large potential for improving lighting practice.

Approach

Mesopic photometry requires a methodology that reveals the combined response of rods and cones, but we feel that a practical system must avoid the nonadditive chromatic channels of vision. We chose reaction times as a way of isolating the luminance channel, with the expectation that reaction times would measure an additive visual response basic to human vision.

The LRC developed a novel technique, called the binocular simultaneity method (BSM), for measuring differences in reaction times for eyes adapted to two different light sources. Figure 1 and figure 2 show the apparatus. Changing the light level in one chamber changes the perceived simultaneity of the flashing off-axis targets; the higher the light level, the quicker the eye responds. By measuring the light levels when the targets appear to flash simultaneously for different monochromatic light sources, the LRC determined the mesopic luminous efficiency functions shown in the figure 2.

McGowan, T. and Rea, M.S. 1995. Visibility and spectral composition: Another look in the mesopic. 70 years of CIE Photometry. Vienna: Commission Internationale de I'Eclairage.



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