Now’s the part of this series where we get technical–it’s the only way for us to explain why LED lights look brighter than traditional lights despite having lower lumen counts.
Let’s start with how our eyes work.
Simply (yet extremely complexly) light passes through the cornea and pupil on to the lens, which focuses it on the back of our eye called the retina. The retina is covered with two types of photoreceptors called rods and cones.
These photoreceptors are responsible for our ability to see light and perceive color.
Rod cells line the outer surface of the retina and allow us too see motion, peripherally, and assist most of our vision at night or in dimly lit situations. They are much more sensitive to light intensity rather than color itself which is why it is hard to distinguish colors in the dark. Our eyes typically have more rods than cones.
Cone cells on the other hand, line the inner surface of our retina (towards the center) and allow us to see color but are less sensitive to light. They are sensitive to red, blue, and green in bright conditions and rods are more sensitive to the color blue and primarily control the opening and closing of the pupil.
Cone light receptors control photopic vision, the scientific term for color detection in high light, while rods control scotopic vision, the scientific term for visual perception in dim light.
Cones are commonly described to handle “daytime vision,” while rods are described as handling “nighttime vision.”
Rods, specifically, are widely thought to only be important for night vision, when in actuality they contribute largely to vision behavior at various light levels. There are a range of light levels called the mesopic region, where both rods and cones affect the human visual system. This is where most outdoor lighting falls within.
Surprisingly, lighting companies and consumers only rely on photopic lumen measurements to describe a lamp’s brightness. Photopic lumens are the only lumens measured in LM79 tests and other current light simulation tests, which lamps are put through in order to determine their brightness (luminous flux), light distribution angles, and efficacy.
Photopic lumens are usually the only lumens that are mentioned in marketing materials, and therefore the only lumens considered when purchasing a bulb.
So what about scotopic vision? Two scientists, Dr. Sam Berman and Dr. Don Jewett, give a better understanding of the performance of the eye for lighting practice and lighting energy efficiency.
The two scientists show that rods primarily control the opening and closing of the pupil of the eye and that the perception of room brightness is substantially influenced by rod’s functionality. Their tests show that lighting would be improved if it accounted for the responsiveness of the rods during workplace environments.
The rods’ control of pupil size significantly influences vision; smaller pupils improve your depth of view, provide better acuity, and allow a higher quality in vision at certain light levels and color temperatures (Kelvin) illuminating the eyes. Unfortunately, mostlighting practitioners crank up the light levels to reduce pupil size, which only wastes energy, adds glare and creates headaches for the onlooker.
Instead, the best way to reduce pupil size is to increase the color temperature, not increase the light level. The higher the color temperature, the more the lighting is scotopically elevated, or contributing to more rod activation.
This is proven by studies that show subjects choose the scotopically enhanced light as brighter than a photopically enhanced light, even though the light level is measured 30% lower by the light meter. Today’s lighting consumers are too focused on what the light meter claims and not enough on what they actually see.
That is why the brightness of LEDs are more than what meet the eye, making them the superior choice when retrofitting, or from the beginning. In the photo above, colors are clearly distinguishable in the cool white light from the LED luminaire compared with the high-pressure sodium light, which washes out the area in a golden light (and not the pretty kind either).