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Greenlight Optics
How LEDs Appear Brighter to Our Eyes
Wednesday, May 21, 2014

Here’s a surprising truth about LEDs: they actually have a lower lumen count than conventional light bulbs.  How can this be? They’re clearly brighter than incandescents, as our senses confirm by comparing the two next to each other.  The answer lies in our eyes.

Lumens are the measurement of light given off by a radiating body, and are the standard unit of measurement for light bulbs.  Our eyes see light and perceive color with photosensitive cells on the retina called rods and cones.  Rods are more sensitive to the intensity of light, while cones are more sensitive to red, green, and blue light, the combination of which gives us color vision.  

In different lighting conditions, the rods and cones are activated in different combinations to adjust.  In white daylight, cones are primarily responsive, and at night, it’s rods; but in twilight our eyes use a combination of rods and cones.

Rods also control the size of the pupil, which determines how much light is actually received in our eyes.  Smaller pupils, improve your depth of field, sharpen acuity, and improve the quality of your vision, and the way most light bulbs attempt to shrink your pupil is to increase the amount of light - the lumens - that a bulb gives off. 

Research has shown, however, that the best way to decrease pupil size is to increase the color temperature of light, not the lumens the light produces.  LEDs have a much higher color temperature (higher color temperature is closer to white), even though they are technically lower in lumens.

So, while lumens may be empirically brighter, LEDs appear brighter because of the way the rods and cones in our eyes work.


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