Science of Light

What does “Full Spectrum” Mean?
Natural White is a “Full Spectrum” lamp. The IESNA Lighting Handbook (3-26) defines full spectrum lighting as having spectral emission in all parts of the visible spectrum and in the near UV, with a correlated color temperature of 5000K or more, and a CRI of 90 or more. Natural White lamps have spectral distribution with output in all parts of the visual spectrum and feature a CCT of 5000K and a CRI of 90+. In fact, Natural White lamps mimic the color of natural daylight at 5000K.

What is Correlated Color Temperature?
Color temperature is defined as the absolute temperature (expressed in Kelvin units) of a theoretical black body whose chromaticity most nearly resembles that of the light source. The CCT rating is an indication of how “warm” or “cool” the light source appears. The higher the CCT value, the cooler the lamp color will appear. The lower the number, the warmer the lamp color will appear. However, CCT is only one aspect of color. The true “color” of a light source is derived from a complicated relationship of CCT, CRI, and spectral distribution.

Why do Natural White lamps offer superior color rendering?
The “color” of light sources is derived from a complicated relationship drawn from a number of different measurements, including correlated color temperature (CCT), color rendering index (CRI) and spectral distribution. In general, color is most accurately described by a combination of CCT and CRI. Since Natural White lamps feature a CCT of 5000K, a CRI of 90+, and a spectral distribution featuring light output in all parts of the visual spectrum, color is greatly improved. Blues are true blue, whites are whiter and reds are more vibrant.

What is Color Rendering Index (CRI)?
In general, CRI is a numeric indication of a lamp’s ability to render individual colors accurately relative to a standard. The CRI value is derived from a comparison of the lamp’s spectral distribution to the standard (e.g. a black body or the daytime sky) at the same color temperature. However, CRI is only one aspect of color. The true “color” of a light source is derived from a complicated relationship of CCT, CRI, and spectral distribution. When we look at a light source, the eye “perceives” a single color. In reality, we are seeing literally thousands of colors and hues made up of a combination of different wavelengths of light. These different combinations and the relative intensity of various wavelengths of light are used to determine the CRI of a light source.

What are Photopic Lumens?
Photopic lighting is light measurement based upon the response of the human eye under daytime conditions. Published lumen ratings reflect photopic lumens.

What are Scotopic Lumens?
Scotopic lighting is the light measurement based on the response of the human eye under nighttime conditions. Scotopic lumens are higher for high CCT light sources.

What do Rods and Cones Have to do with Lighting?
There are two types of light detectors in the eye, the cones, and the rods. Cones dominate during daylight conditions and rods dominate at night and at low light levels. Photopic lumens are based on cone response and scotopic lumens are based on rod responses.

What is an S/P Ratio?
An S/P ratio is the ratio of light measured by the scotopic and photopic methods. Some common S/P ratios are listed below. Lamps with high S/P ratios (high scotopic content) also improve one’s ability to see both outdoors and indoors.
– The S/P ratio of 5000K daylight light sources is 2.1.
– The S/P ratio of 4000K light sources is 1.65.
– The S/P ratio of 3000K light sources is 1.4.
– The S/P ratio of yellow HPS light sources is 0.64.

What does the S/P Ratio Tell Us?
Lamps with high S/P ratios (high scotopic content) also improve one’s ability to see. Human studies have shown that the S/P values, though previously applicable to only nighttime conditions, can be used to describe the “effective lumens” of indoor lighting as well.

The following examples describe the effect of S/P in relation to outdoor and indoor lighting. Outdoor Lighting: The result is that a 4000K light source is at least twice as effective for viewing at night as HPS sources. Similarly, a 5000K light source is more than 3 times as effective as HPS Sources. Indoor Lighting: If two rooms are lit separately with 4000K and 5000K lamps to equal photopic luminance, the 5000K room will appear 13% brighter [(S/P)0.5 = 1.13].

How does the S/P Ratio relate to Natural White?
With its high S/P ratio (2.1), Natural White lamps provide customers with an:
– Improved ability to see indoors
– Improved nighttime vision

This makes Natural White an excellent product for either indoor or outdoor lighting applications. Combined with high CRI and high CCT, the Natural White lamp/ballast system features superior color rendering, as well as all the benefits of the Uni-Form arc tube and Opti-Wave ballast technology.

What are “Effective Lumens”?
As defined by Dr. Ian Lewin, the term “effective lumens” describes the modified lumen output of a lamp, taking into account the shifting color sensitivity of the eye at low light levels.

The effective lumen rating of Natural White lamps reflects lumen output relative to 4000K lamps. The formula used to determine this rating indoors is (S/P)0.5. This provides the end user, lighting designer or contractor with an accurate comparison of traditional 4000K lamps to 5000K lamps.