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What is Color Rendering Index (CRI)?
Color rendering describes how a light source makes
the color of an object appear to human eyes and how
well subtle variations in color shades are revealed.
The Color Rendering Index (CRI) is a scale from 0 to
100 percent indicating how accurate a "given"
light source is at rendering color when compared to
a "reference" light source.
The higher the CRI, the better the color rendering
ability. Light sources with a CRI of 85 to 90 are considered
good at color rendering. Light sources with a CRI of
90 or higher are excellent at color rendering and should
be used for tasks requiring the most accurate color
discrimination.
It is important to note that CRI is independent of
color temperature (see discussion of color
temperature). Examples: A 2700K ("warm")
color temperature incandescent light source has a CRI
of 100. One 5000K ("daylight") color temperature
fluorescent light source has a CRI of 75 and another
with the same color temperature has a CRI of 90.
To further understand the physics of color rendering,
we need to look at spectral power distribution.
What is spectral power distribution?
The visible part of the electromagnetic spectrum is
composed of radiation with wavelengths from approximately
400 to 750 nanometers. The blue part of the visible
spectrum is the shorter wavelength and the red part
is the longer wavelength with all color gradations in
between.
Spectral power distribution graphs show the relative
power of wavelengths across the visible spectrum for
a given light source. These graphs also reveal the ability
of a light source to render all, or, selected colors.
Below see how a typical spectral power distribution
graph for daylight.
Notice the strong presence (high relative power) of
ALL wavelengths (or the "full color spectrum").
Daylight provides the highest level of color rendering
across the spectrum.
Compare the daylight spectral power distribution with
that for a particular fluorescent lamp.
The most obvious difference is the generally lower
level of relative power compared to daylight - - except
for a few spikes. All wavelengths (the "full spectrum)
are again present but only certain wavelengths (the
spikes) are strongly present. These spikes indicate
which parts of the color spectrum will be emphasized
in the rendering of color for objects illuminated by
the light source. This lamp has a 3000K color temperature
and a CRI of 82. It produces a light that is perceived
as "warmer" than daylight (3000K vs. 5000K).
It's ability to render color across the spectrum is
not bad, but certainly much worse than daylight. Notice
the deep troughs where the curve almost reaches zero
relative power at certain wavelengths.
Here is another fluorescent lamp.
This spectral power distribution looks generally similar
to the one above except it shows more power at the blue
end of the spectrum and less at the red end. Also, there
are no low points in the curve that come close to zero
power. This lamp has a 5000K color temperature and a
CRI of 98. It produces light that is perceived as bluish
white (similar to daylight) and it does an excellent
job of rendering colors across the spectrum.
Above are links to linear fluorescent light
bulbs from Topbulb that have a CRI of 90 or higher.
If you want a high color rendering bulb to produce light
perceived as warm white, choose a bulb with a color
temperature of 3000K or 3500K. If you want a high color
rendering bulb to produce light perceived as white,
choose a bulb with a color temperature of 4000K. For
a bulb that simulates daylight, choose a color temperature
of 5000K or higher.
Note: all incandescent and halogen light bulbs, by
definition, have a CRI close to 100. They are excellent
at rendering color. However, except for some halogen
bulbs, most incandescents produce a warm 2800K color
temperature. The only way to achieve the bluish white
appearance of daylight with incandescent bulbs is to
use bulbs coated with neodymium. However, these bulbs
have a CRI much lower than 90. They are not good for
accurate color rendering across the spectrum. See neodymium
light bulbs available from Topbulb.
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