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SPIE Professional October 2011

What color is that LED?

Describing color and other characteristics of an LED depends on your choice of technology and instrumentation.

By Randy Klimek

LED metrology image from SPIE Professional

The new, high-growth field of LED lighting presents exciting challenges and opportunities for measurement.

Industry professionals and researchers engaged in the measurement and characterization of LED sources work under the same fundamental measurement principles used for traditional lighting.

Their goal is to accurately quantify, characterize, and communicate LED light characteristics and color specifications, and they confront the same, age-old questions about choosing the correct instrument and technology: Should you choose an instrument that employs spectroradiometric technology? Or should an instrument based on tristimulus colorimetry technology be used to acquire data?

While much of the choice is application- dependent, there are key differences in the two broad categories of measuring technologies and instrumentation.

Tristimulus system

Tristimulus colorimetry is based on the three-component theory of color vision, which states that the human eye possesses receptors for three colors (red, green, and blue) and that all colors are seen as mixtures of these three primary colors.

The most important system is one developed in 1931 by the International Commission on Illumination (CIE). The CIE 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.


 CIE 1931 Yxy chromaticity diagram. In the Yxy color space, Y is the lightness and x and y are the chromaticity coordinates calculated from the tristimulus value XYZ. x = X/(X+Y+Z) and y = Y/(X+Y+Z).

XYZ tristimulus values and the associated Yxy color space form the foundation of the present CIE color space. (See image above).

Filters and humans

Tristimulus colorimeters are commonly referred to among industry professionals as filter-based instruments. They are desired for their low cost, speed, and portability. However, they contain inherent design limitations, which make them unsuitable 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. Matching 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.

Developments in recent years by instrumentation manufacturers have limited this deviation significantly by employing proprietary technology. Spectral fitting, or matching, of filter-based systems to the CIE standard observer curves is now available in the Konica-Minolta Chroma Meter CS-200, for instance, with significantly higher accuracy than in traditional systems.

Spectroradiometric method

Many different spectral-power distribution curves can yield the same visual effect, which we call color. In other words, the color of a light source does not tell us the nature of its spectral power distribution; two different light sources that have the same color in x, y, or color temperature might not exhibit the same spectral-power distribution, as below.


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

 

The reverse, however, is true. Knowledge of spectral-power distribution of light will enable us to describe the color.

Hence, the spectroradiometric method for LED measurement is the most accurate and complete for 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 color-matching function. The tristimulus values are then used to compute CIE chromaticity coordinates and luminosity, which provide complete description of the color.


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

Instrumentation that employs the spectroradiometric method is ideal for measuring spectral energy distribution of the LED light source. These instruments determine not only the radiometric and photometric quantities, but also the colorimetric quantities of light.

The instruments also 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 spectroradiometry by means of prisms or diffraction gratings.

The exact CIE Vλ curve and CIE color-matching curves are stored in the instrument 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 with spectroradiometers. Adequate sensitivity, high linearity, low stray light, low polarization error, and a spectral bandpass resolution of 5 nm or less are essential for obtaining good accuracy.

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

Traditionally, when compared to three-filter colorimeters, spectroradiometers have limitations in terms of speed of measurement, price, and portability. Recent developments in industry have changed that, however. For instance, Konica Minolta Sensing, Inc., and B&W Tek collaborated to introduce a miniature spectral irradiance meter, SpectraRad™, earlier this year as a lower-cost option for fast and precise LED characterization.


 SpectraRad™, TE Cooled Miniature Spectral Irradiance Meter.
How to choose the right instrument

If precise measurement of LEDs is required, the spectroradiometric method is the ideal and comprehensive method as it records the spectral characteristics of light and processes them mathematically to obtain radiometric, spectroradiometric, photometric, and colorimetric data.

When portability, speed of measurement, and cost of investment is a priority, tristimulus colorimeters are still preferred. However, some of the newer spectroradiometric instruments are just as inexpensive.


 Colorimeters mimic the response of the three cone receptors in the human eye.

 The spectroradiometric method is considered the most comprehensive.

It is important to ascertain whether a colorimeter is appropriate to measure the light source under test, considering its spectral energy distribution.

Finally, choose an instrument which makes direct measurements of light characteristics, such as luminance, illuminance, luminous intensity, and luminous flux. Conversions across measurement geometries should not be attempted in any form.

A good understanding of the measurable characteristics of light, and of exactly which characteristics need to be quantified for a particular situation, will ensure that the radiometric and/or photometric characteristics of an application are described correctly.

Randy Klimek

Randy Klimek is a project manager who specializes in light and color measurement instrumentation for Konica Minolta Sensing Americas (USA). He has a BS in imaging from the Rochester Institute of Technology.

 

 


Consumers want a pleasing LED light

The Spectrally Tunable Lighting Facility (STLF) at the National Institute of Standards and Technology (NIST) is one of several labs working to develop a new color quality scale for LED lighting.

Wendy Davis, Yoshi Ohno, and a team of other NIST scientists use the lab to study the relationship between physical measurements of LED light and human perception of light and color in order to help manufacturers develop LEDs for general lighting.

"Because the light emitted by LEDs is different from the light we get from other lighting technologies, the way that we measure color quality doesn't always work for them," Davis says.

While LEDs offer many advantages over incandescent and fluorescent lighting, they don't always emit light that looks 'right' to consumers, she says.

"Bad color means unhappy consumers," she adds.

More information on LED metrology:


Related articles in SPIE Professional

 

Have a question or comment about this article? Write to us at spieprofessional@spie.org.


DOI: 10.1117/2.4201107.10

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