Tuesday, November 5, 2013

What is CRI?

I had a customer ask me yesterday what CRI is and why it is important.
CRI is Color Rendering or the Color Rendering Index. Just today I received an email about CRI from one of my vendors, with the subject fresh on my mind from a conversation yesterday I found this information very useful. 
The following information was sent in an email from Nicor written by, Trevor Shaw. with added illustrations from Google Images.
The human eye was created to perceive colors in direct sunlight, thus all color perception under artificial light must be judged against this standard.
Various lighting technologies have differing abilities to show or render color accurately. This ability is measured by a scale called the Color Rendering Index (CRI). Created by the International Commission on Illumination (CIE), the Test-Color Method, and mainly the Color Rendering Index, has been accepted by the industry as the standard method for measuring color rendition.
A test light is used to illuminate a panel of 8 "standard" colors. For each sample, the light source is evaluated for its ability to render (Ra) the colors compared to a reference source (usually an incandescent source) and rated on a scale of 0 to 100. A score of 100 would be comparable to sunlight. A score of less than 100 indicates some colors may appear unnatural.
Photo from Google Images
The CRI of various light sources available on the market is as follows:
Low Pressure Sodium
Standard Warm/Cool White Fluorescent
Premium High Pressure Sodium Conventional Metal Halide
Thin Coat Tri-Phosphor Fluorescent

Standard CRI LED
White High Pressure Sodium, Warm Metal Halide
Thick Coat Tri-Phosphor Fluorescent

High CRI Fluorescent
Incandescent / Tungsten-Halogen/Sunlight
Which Source to Choose?
High CRI sources are the clear choice for proper color rendering and a more pleasing space, and are becoming a requirement in some areas , so how does one go about choosing the proper lighting technology?
For a standard working space that requires 90+CRI, LED and Fluorescent are the main technologies used. Halogen lighting also has excellent CRI but is inefficient and operates at very high temperatures. While these technologies will list >90 CRI on data sheets, the lighting they provide is not identical. Spectral Power Distribution (SPD) shows the energy of the light source at differing wavelengths of energy. In other words, it shows which colors will show best under the light source.
The SPD curves of the technologies (See Figures 2 and 3) show that the LED source provides a more even spread across the full color range versus the peaks of the Fluorescent, therefore providing more accurate color rendering across a full range of color. High CRI LED = more color, more accurately.
Photographs often do not demonstrate color rendering well due to differences in printing or video display. However, given the opportunity to compare colorful objects under fluorescent and High CRI LED first hand, the difference is striking. LED is the clear winner.
With its many other advantages (efficiency, robustness, lifetime) added to a CRI advantage, LEDs are the clear choice for creating an ideally lit space with excellent color rendering.