Industry professionals and researchers engaged in the measurement and characterization of light sources are confronted by the age old question — should an instrument be utilized which employs spectroradiometric technology, or should tristimulus colorimetry technology based instruments be used to acquire data? While much of the question's answer is application-dependent, there are key fundamental differences in the technology employed to arrive at the measurements for each respective type of device.

CIE color matching functions for 2º and 10º observers.

In the past, various researchers and organizations have devised methods to quantify color and light so that the communication is more accurate. These methods attempt to provide a way of expressing color quantitatively. Instrumentation used for light source color specification and measurement can be broadly categorized into two categories — tristimulus colorimetry and spectroradiometry. This article summarizes the main difference between instrument types in an attempt to characterize and express light and color accurately.

Tristimulus Colorimetry

CIE 1931 Yxy chromaticity diagram.

Tristimulus colorimetry is based on the three component theory of color vision, which states that the eye possesses receptors for three primary colors (red, green, and blue) and that all colors are seen as mixtures of these three primary colors. The most important system is the 1931 Commission Internationale I’Eclairage (CIE) system, which defined the standard observer to have color-matching functions x(λ), y(λ), and z(λ). The XYZ tristimulus values are calculated using these three standard observer color-matching functions. XYZ tristimulus values and the associated Yxy color space form the foundation of the present CIE color space.

Tristimulus method.

Tristimulus colorimeters are commonly referred to among industry professionals as “filter-based instruments.” They are desired for certain properties such as cost, speed, and portability; however, they contain inherent design limitations which yield them an unsuitable choice for all measurement applications. Colorimeters receive information from three filtered light sensitive sensors which mimic the response of the three cone receptors in the human eye. Fitting the filters' response to that of the normal human eye as described by the CIE 1931 standard observer color-matching functions is subject to limitations or conditions present during the manufacturing of the filters. Therefore, filter-based instrumentation is susceptible to finite errors because of the deviation of the filter response from the ideal human eye response.

Spectroradiometry

Left: CIE 1931 color-matching functions and spectral response of a conventional tristimulus colorimeter. Right: CIE 1931 color-matching functions and spectral response of the CS-200.

Many different spectral power distribution curves can yield the same visual effect which we call color. It means that the color of a light source does not tell us the nature of its spectral power distribution. In other words, two different light sources which have the same color in x,y or color temperature might not exhibit the same spectral power distribution. The reverse, however, is true — knowledge of spectral power distribution of light will enable us to describe the color.

Sunlight and white LED spectral power distributions. Both can yield the same visual effect on color but might not necessarily have the same spectral energy.

Hence, the spectroradiometric method is the most accurate and complete method of specifying color. The spectral data can be analyzed visually and/or compared to data from another light source. However, the best use of spectral data with respect to color measurement is to calculate the CIE tristimulus values by mathematically integrating the data with the CIE colormatching function. The tristimulus values are then used to compute CIE chromaticity coordinates and luminosity, which provide complete description of the color.

Example of calculating colorimetric values.

Instrumentation that employs the spectroradiometric method are most ideal for measuring spectral energy distribution of the light source, which determine not only the radiometric and photometric quantities, but also the colorimetric quantities of light. These instruments record the radiation spectrum of the light source and calculate the desired parameters, such as chromaticity and luminance. Dispersion of light is usually accomplished in the spectroradiometer by means of prisms or diffraction gratings. The exact CIE Vλ curve and CIE color-matching curves are stored in the software and are used to process the data from the measured spectral energy distribution of the light source under test. Hence, the measurement error associated with photometers and filter colorimeters is avoided in spectroradiometers. However, adequate sensitivity, high linearity, low stray light, low polarization error, and a spectral bandpass resolution of 5nm or less are essential for obtaining good accuracy.

Comparison between tristimulus and spectroradiometric methods.

Non-thermal radiators, such as discharge lamps (which can be characterized by their noncontinuous spectral energy distribution), and narrow-band emitters such as light-emitting diodes (LEDs) can only be measured with precision by means of the spectral procedure.

Traditionally, when compared to three-filter colorimeters, spectroradiometers do have their limitations, in terms of speed of measurement, price, and portability. Recent developments in industry such as Konica Minolta Sensing, Inc. and B&W Tek‘s collaboration are offering spectrally-based instrumentation to the industry at high-speed, portability, and comparable price points to mid-level colorimeters. One such instrument, the SpectraRad™, is offering researchers and industry alike the ability to characterize today’s light sources with precision and speed at an affordable price point.

Conclusion

SpectraRad™, TE-cooled miniature spectral irradiance meter.

If precise measurement of light is required, the spectroradiometric method is the most ideal and comprehensive method as it records the spectral characteristics of light and further processes them mathematically to obtain radiometric, spectroradiometric, photometric, and colorimetric data. When portability, speed of measurement, and cost of investment is of priority, tristimulus colorimeters are still preferred. However, advancements in industry are offering spectroradiometric instruments at lower price points. It is important to ascertain whether a colorimeter is appropriate to measure the light source under test, considering its spectral energy distribution. Finally, one should choose an instrument which makes direct measurements of light characteristics, such as luminance, illuminance, luminous intensity, and luminous flux, and should not attempt any form of conversion across measurement geometries. A good understanding of the measurable characteristics of light, and exactly which of those characteristics of light need to be quantified for a particular situation, will ensure that the radiometric and/or photometric characteristics of an application are described correctly.


Lighting Technology Magazine

This article first appeared in the May, 2011 issue of Lighting Technology Magazine.

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