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The light receptors in our eyes called cones and rods convert light into elecrified signals that are sent to the vision centers of the brain. The number of rods outnumber the number of cones 10 to 1. Lighting manufacturers have utilized light meters to determine lumen output of a lamp (bulb). They calibrated the meters to examine the eye's sensitivity to only cone activated vision (photopic). They assumed and practiced in error that the more light sensitive rods only functioned at very dim light levels. These meters ignored the effect of rod activated vision (scotopic) completely.

S/P Ratios

Studies now indicate without question that rod photoreceptors are active not only in dim light but at typical interior light levels as well. In fact, they seem to be more sensitive than cones to blueish-white light sources or (warmer) color temperature light. This explains why some warm white (3000K) or cool white (4100K) fluorescent lights appear less bright than a higher temperature light (5000K) or more. Also, that explains why many lumens from HPS and metal halide sources are not visually "seen".

Combining the scotopic and photopic lumen (S/P Ratio) output of a light source to be more favorable to how a human perceives light is most important to providing better visual acuity. Lamps with high S/P ratios can result in faster reading time, reduced visual fatigue, reduced glare and improved human performance. They also become more efficient using their watts to produce ("seeable" lumens) light.

Brightness of lighting can be better understood now that both the scotopic and photopic responses by both the cones and rods are included.

This S/P Ratio is actually a fixed constant for any light source that can be measured with the proper instruments. We know that by using a light source that enhances the eye's scotopic response by making the pupil of the eye smaller, the eye works less and there is less eye fatigue.

You can do that by using a light source with a high S/P Ratio.

Induction lighting carries a high 2.0 S/P ratio.


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