Color Temperature & Color Rendering Index DeMystified
|"What is color? No object of itself alone has color.
We know that even the most brightly colored object, if taken into total darkness,
loses its color. Therefore, if an object is dependent upon light for color, color must be a property of light.
And so it is."
Paul Outerbridge, Photographer 1896 - 1958
Color Temperature Defined
Color temperature has been described most simply as a method of
describing the color characteristics of light, usually either warm (yellowish)
or cool (bluish), and measuring it in degrees of Kelvin (°K).
That's a little too simple to be of more than introductory value.
A more technical definition assigns a numerical value to the color emitted
by a light source, measured in degrees of Kelvin. The Kelvin Color
Temperature scale imagines a black body object--- (such as a lamp filament)
being heated. At some point the object will get hot enough to
begin to glow. As it gets hotter its glowing color will shift, moving from
deep reds, such as a low burning fire would give, to oranges & yellows,
all the way up to white hot. Light sources that glow this way are called
"incandescent radiators", and the advantage to them is that they have a
continuous spectrum. This means that they radiate light energy at all
wavelengths of their spectrum, therefore rendering all the colors of a
scene being lit by them, equally. Only light from sources functioning this
way can meet the truest definition of color temperature.
Note - the light spectrum is wider than our ability to see it. Light values
falling beneath the visible part of the spectrum are referred to as
infrared, and above the spectrum as ultraviolet. Each can adversely
affect an image, and you may need to add some filtration to remove
Light sources that are not incandescent radiators have what is referred
to as a "Correlated Color Temperature" (CCT). It's connotations to any
part of the color temperature chart are strictly visually based. Lights with
a correlated color temperature do not have an equal radiation at all
wavelengths in their spectrum. As a result, they can have disproportionate
levels (both high & low) when rendering certain colors. These
light sources are measured in their ability to accurately render all colors
of their spectrum, in a scale is called the Color Rendering Index (CRI).
Incandescant radiators have a CRI of 100 (the max.) More on this
Color Temperature in Imaging
The above is not a true Color Temperature
chart. Instead it is a hybrid, showing the color
temperatures of light sources most commonly
encountered in professional imaging. In our
scale, tungsten-halogen has a color temperature
of 3200°K. Household fluorescents are
accepted to be around 4500°K, depending on
the lamp. (They are shown for reference, but
would not be part of a true Color Temperature
chart, for reasons described below).
Sunlight is 5600°K, with shade & skylight hitting higher
temperatures. These are basically averages
which became standards when they were
selected, back at the beginning of color film
manufacturing, as the choices for various
emulsions to be made sensitive to (daylight
film, tungsten film, etc).
There are variations on these standards, but
this is a good start to understanding the relationship
between different colored light
sources. It's important to keep in mind that
even tho' reddish light has a technically lower
color temperature, its frequently described as
warm. Bluish light, which has a higher color
temperature, is described as cool. In this
instance, warmer & cooler describe color, not
The greenish color of 4500°K fluorescent would not appear in a true
Color Temperature chart because a fluorescent lamp does not get its
color by heating a black body object to the point of glowing, it uses
gases & phosphors instead.
Household quality fluorescent lamps can have either too much green
or magenta rendered in their color. The degree to which this occurs
will affect the lamps CRI rating.
See Color Rendering Index (CRI) below for more information.
Tungsten incandescent, most common in household lamps, has a
slightly lower color temperature at 2900°K than tungsten-halogen
(aka quartz) at 3200°K, so its output will be slightly warmer.
Incandescant lamps also shift their color, growing warmer as they
age, something tungsten-halogen lamps don't suffer from.
Cinematographer Tom Robotham has a theory that we prefer the
warm color of tungsten in our living environments because of our
long pre-historic practice of sitting around campfires, and our pre-electric
history of lighting early dwellings with fireplaces & candles.
How many people have a wall dimmer in the dining room to set an
intimate mood? Notice that when you dim tungsten lights, they get
warmer in color, closer to flames & candles. Their color temperature
is shifting lower when this happens.
Daylight is not the same as sunlight. Sunlight is the light of the sun
only, where daylight is a combination of both sunlight & skylight.
Outdoors, shadows are lit by skylight, since sunlight is being blocked
to create them. This is why shadows in exterior day-lit shots are
bluish in color.
Sunlight changes its
color as it crosses the
sky (or more accurately,
as the Earth rotates
in relation to it).
At dawn & sunset the
sun appears more reddish,
due to the filtering
nature of the
layer it's rays are passing
thru at that angle. It
has a correlated color
temperature of approximately
2000°K at sunrise
/ sunset, and
5600°K when directly
Sunset Photo by Marrike Van Irsel
Color Rendering Index
Color Rendering Index (CRI) Defined
A simple definition of Color Rendering Index (CRI) would measure
the ability of a light source to accurately render all frequencies of its
color spectrum when compared to a perfect reference light of a similar
type (color temperature). It is rated on a scale from 1-100. The
lower the CRI rating, the less accurately colors will be reproduced.
Light sources that are incandescent radiators have a CRI of 100
since all colors in their spectrum are rendered equally. As stated earlier,
light sources that are not incandescent radiators will have
Correlated Color Temperatures.
Examples of light sources with Correlated Color Temperatures, having
CRI levels that are less than 100 would include: HMIs, and also most
photo quality fluorescent lamps, as well as LEDs. With lower CRI ratings
these sources may also have too much green or magenta in
their spectrums. An acceptable Color Rendering Index level for professional
imaging is considered to be 90 or above.
The lamp formerly known as
Quartz has a more stable color
temperature throughout the life
of the lamp, than tungsten-incandescant. These lamps get hot
and have shorter lifespans than
some others. CRI is 100.
Fluorescent (Photo Quality)
3000-3200K, or 5000-5600K
Fluorescent lamps made for photo
use are available in tungsten-halogen
or daylight colors, with CRI's over 90.
High frequency ballasts are flicker
free even when shooting slo-motion.
HMI stands for Hydrargyrum
Medium-arc Iodide lamp. This
discharge lamp has a very high
output of daylight color (usually
6000K), normally with a CRI of
LED (for imaging) 3000-6000K
LED stands for Light Emitting
Diode. A semi-conductor based
light source, that is energy efficient,
with a long life. CRI is normally
70 - 90+, but subject to
Comparison of High & Low CRI Fluorescent Lamps
These 2 images are worth really studying. Both were shot with daylight colored fluorescent lamps and a camera with daylight preset
white balance. The image on the left was shot with Lowel 27w day-flo lamps (CRI 92+), and the image on the right side was shot with a
household day-flo lamps (CRI not listed, but assumed to be aprox. 80). Compare the details of each image, noting where the colors
are pretty similar, such as the red & orange peppers, and the radishes. Then look at the items with colors rendered differently, such as
the floor, cutting board, carrots, cabbage, and lettuce leaf, for example. A low Color Rendering Index does not mean all colors will shift,
and no 2 lamps with the same low CRI rating will necessarily have the same errors in rendering.
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