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A comparison between Filter Colorimeters and Spectroradiometers
Insuring that self-emissive technologies, such as information display monitors, LED lighting, or digital
projectors, meet or exceed performance specifications requires precise characterization through
accurate measurements. While there are several measurement technologies available for device
characterization, two of the most commonly-used instrument types are filter colorimeters and
spectroradiometers.
Filter Colorimeters
A basic filter colorimeter consists of:
• Collection Optics: This could be a lens, contact probe, translucent disc or integrating sphere,
depending on the application.
• Detector: A light sensitive device that converts photons into electrons. A colorimeter may contain
three or more detectors, which are typically photomultiplier tubes (PMT’s) or silicon photodiodes.
• Tristimulus filters: Red (two lobes X/red and X/blue), Green (Y) and Blue (Z) absorptive filters
work with the x-detector and the International Commission on Light (CIE) Color Matching
Functions to create a match to the color vison capabilities of the cone cells of the human eye.
This functionality is also known as the CIE Standard Observer.
Figure 1 is a detailed illustration of an advanced filter colorimeter with rotating filter wheel. Each
position in the wheel contains a Red, Green or Blue CIE filter. Sequential measurements are taken
through each filter to obtain the CIE tristimulus values X, Y, and Z. Colorimeters of this caliber are
supplied with hand matched CCIE filters.
FIGURE 1 - FILTER COLORIMETER
The CIE recommends the following equations for determining the Red (X), Green (Y) and Blue (Z)
values for any give color stimulus:
X EQ 1 FIGURE 2 - CIE XYZ COLOR MATCHING FUNCTIONS
Y EQ 2
EQ 3
Z
Where R(
) is the spectral power, X( ), Y(
), Z(
) are the CIE Color Matching Functions and ( ) is
the data increment in nanometers.
The ultimate goal of the filters utilized by filter colorimeters is to modify the detector’s spectral
responsivity to the desired function. Filters are typically constructed as laminates (multi-layered) or
thin film deposits.
Figure 3 is a typical silicon photodiode response curve illustrating how much of a change is required
to modify the detectors’ native spectral responsivity to create the Tristimulus functions necessary to
conduct CIE chromaticity measurements.
FIGURE 3 - SILICON PHOTODIODE RESPONSIVITY