According to the Mayo Clinic, Seasonal affective disorder (SAD) is a type of depression that's related to changes in seasons — SAD begins and ends at about the same times every year. If you're like most people with SAD, your symptoms start in the fall and continue into the winter months, sapping your energy and making you feel moody. These symptoms often resolve during the spring and summer months.

Often touted as a treatment are “SAD lights”, or full spectrum artificial lighting to trick your body into thinking that more daylight is available to offset the symptons of SAD. Maybe. Maybe not.

Lamp Color Temperature. This standard was created using a reference piece of metal, heated up, and linking the color it glows at what temperature; measured in Kelvins.

The old fashioned incandescent filament lamps burned between 2800-3200 Kelvins, a warm yellowish light.

The once-widely used fluorescent “cool white” was 4100 Kelvin, a dull white with a tinge of subtle green.

Outdoor daylight can range from 5000-7500 Kelvin, depending on season, time of day, and direction of sunlight. 5500 Kelvin is the usual “standard”, although the deep Northern sky can be 7500 Kelvin or better.

Color Rendering Index (CRI). This rating determines how accurate colors appear under any given light source. The outdated low pressure sodium street lights emitted a single frequency of pure yellow. Objects under a low pressure lamps appeared black and white, thus, a color rendering of 0 (zero).

Various fluorescent lamps generally rated from mid 50’s to mid 70’s, with a few higher rated exceptions, “Deluxe Cool White” and the pinkish “Natural”. When Duro-Test introduced the “full spectrum” Vita Lite, the CRI was closer to 90. In 2004, Winbeam Light Labs improved upon the Vita Lite by making better use of scotopic energy and deep reds found in natral sunlight. Discussion of scotopic versus photopic light are important in understanding how the eye responds to light- but is beyond the scope of this discussion. Searches for theses terms will bring you to plenty of insightful reading.

Incandescent lamps are rated at 100 CRI @ 3000 Kelvin. Outdoor sunlight also rates at 100 CRI but @ 5500 Kelvin.
This seems odd that two very different light sources and colors can both be 100 percent accurate in color rendering. However, there are two standards of CRI, one at 3000K and the other for outdoor daylight.

Lumens. How much light a lamp produces, or, how bright a given space is. Can be calculated into LUX.

An example of the same display as seen under three different light sources. “The Skylighter” was considered to be a “full spectrum”, or more accurately, and “Equalized Spectrum” lamp.

The eye is least sensitive to reds. When outdoors, there is so much red light available, it makes little difference.
However, undeer the typical lower levels of indoor light, extra red needs to be manufactured into the lamp for reds to be rendered properly. This is the basis for an “Equalized Spectrum”.

Mass produced fluorescents lamps used a cheap orange phosphor to get into the reds.
This is a prime reason fluorescent lighting was not flattering.

The Skylighter used an expensive rare earth phosphor to reproduce deep reds.

Scotopic light at 507 nanometers is what the rod cells, your black and white (and low light) receptors see. It ALSO triggers a reflex which closes the pupil. Just like a camera, the smaller the lens, the greater the depth of field. This can notably improve reading.

The color cone cells in your eyes respond to photopic light, which peaks at 550 nanometers. The more energy at 550, the brighter a lamp appears to be. However, a lamp can only produce a given amount of light per watt, concentrating too much at 550 will sacrifice the other portions of the spectrum, resulting in poor overall light quality.

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