Iluminantes

Insight on Color

May, 2005, Vol. 17, No. 5

Equivalent White Light Sources and CIE Illuminants
What is the Difference between a Light Source and an Illuminant?
A light source is a physical emitter of radiation (i.e., candle, lamp, or sunlight) that can be
characterized numerically by a spectral power distribution curve of power versus wavelength.
For each light source, the wavelengthtells us where light energy is present. The relative energy
values (power) indicate how much energy is present at a particular wavelength.
The CIE (Commission Internationale de L’Eclairage, http://www.cie.co.at) has codified the
spectral power distributions of different types of white light sources and called them
“illuminants.”
An illuminant is a set of numbers (relative energy versuswavelength) that represents the
spectral quality of a type of white light source and is used in the calculation of color
measurements in colorimetric software. The choice of illuminant is independent of the
instrument lamp.

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Applications Note

May 2005

Vol. 17, No. 5

Common CIE Illuminants
While there is a wide range ofilluminants available, most industrial users will be using one of
four CIE illuminants representing the general types of white illumination found in the home,
office and exterior conditions. These illuminants are:
A, which represents an incandescent or tungsten light source (2856° Kelvin) found in the home.
F2 (also called F, F02, Fcw, CWF, CWF2), which represents the spectral quality of the mostcommon fluorescent lamp—cool white fluorescent—found in office environments.
D65, which is the most commonly used daylight illuminant representing noon daylight (6504°
Kelvin).
C, which is an historical representation of average or north sky daylight (6774° Kelvin), still in
active use.
If you have no prior conditions or requirements (test methods, SOPs) that tell you to use another,
use the CIED65 illuminant. It is the most commonly used illuminant and represents a full, even
spectrum.

CIE Incandescent or Tungsten Illuminant
There is only one incandescent illuminant, and that is CIE Illuminant A (2856° Kelvin).

CIE Daylight Series of Illuminants
There are several CIE standard illuminants that represent various daylight lighting conditions
throughout the day.
D50, sometimesreferred to as horizon light, represents warm daylight at sunrise or sunset (5000°
Kelvin).
D55 represents mid-morning or mid-afternoon daylight (5500° Kelvin).
D65 is the most commonly used daylight illuminant, representing noon daylight (6504° Kelvin).
C is an historical representation of average or north sky daylight (6774° Kelvin), which is still in
active use.
D75 represents overcastdaylight (7500° Kelvin).

CIE Fluorescent Illuminants
Historically, a series of some 20 different fluorescent illuminants were represented as codified
tables of numbers by the CIE, of which only 12 are in active use today. Illuminants F2, F7 and
F11 should take precedence over others when a few typical fluorescent illuminants need to be
selected.
F2 (non-standard names include F, F02, Fcw, CWF,CWF2), the fluorescent illuminant in most
common use, represents cool white fluorescent (4100° Kelvin, CRI 60).
F7 represents a broadband fluorescent lamp, which approximates CIE illuminant D65 (6500°
Kelvin, CRI 90).
F11 represents a narrow tri-band fluorescent of 4000° Kelvin color temperature, CRI 83.

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Applications NoteMay 2005

Vol. 17, No. 5

Fluorescent Illuminant Specifications
White light sources and their corresponding illuminants usually have correlated color
temperature (K) and color-rendering index (CRI) values associated with them.
White light sources vary from warm white to cool (blue) white. Expressed in color temperature
values in degrees Kelvin (K), the higher the color temperature, the...