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Lighting and the Human Eye

Part 2 of 3
Human eye vision has been discovered to be a variable combination of scotopic* and photopic** vision. The eye is commonly in a transition state between different levels of lighting.
* Scotopic Vision (Night Vision) from the Greek word, skotos, meaning "darkness", and -opia, meaning "a condition of sight". Scotopic vision is produced exclusively through rod cells.

** Photopic Vision (Daylight/Well-Lighted Vision) from the Greek word, photo, meaning “light”, and -opia, meaning “a condition of sight”. Photopic vision is the vision of the eye using cone cells, under daylight or well-lighted conditions, which allows color perception.
It is not a case of either/or with just cones operating during the day, and just rods being active in the eye at night. The human eye uses both rods and cones to see.
To accurately measure how the eye sees light, it must be measured on a different scale, where both rods and cones are in use, and such a Scale has been created.

The Scotopic/Photopic Ratio Scale, or S/P ratio , is a multiplier that compensates for different types of light, and evaluates lumens of light very differently. The S/P Ratio determines how much light is “useful” to the human eye on a scale and provides a means of calculating Visually Effective Lumens (VEL’s) for a variety of light sources.
CLICK to OPEN PDF & Download this interactive
" Visual Effective Lumens " Calculator.
The VEL Calculator uses the S/P Ratio to converts light meter lumens into Visually Effective Lumens so that a comparison can be made between different types of lights. In the screen capture entries below, the VEL Calculator was used to create Visually Effective Lumens from two different lights.
Bill Nagengast, Lighting Engineer
Solas Ray Lighting
Holds over 20 patents in the lighting industry.
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TJ-51 11.15.18