Color Measurement

This reflectance setup includes a customized shield for the probe.

Color is a fickle thing, as anyone who has ordered an item online only to have it arrive in a completely different shade will tell you. Similarly, two articles of clothing matched in the closet can suddenly appear to clash in daylight.

Color brings beauty to our world, but can be infuriating in its apparent subjectivity. Why is it impossible to remember the color of an object when our eyes are capable of detecting the tiniest differences?

Quantifying color can be equally frustrating until you understand the factors that affect how it is perceived. That chameleon-like sweater? It’s not in your imagination. Read on and we’ll try to put it all in black & white for you.

Advantages

Quantitative: Far more accurate than human eye, with detailed spectral information.
Flexible: Data acquired can be used to calculate multiple color parameters, varying factors like the observer and illuminant, even at a later date.
Comprehensive & multi-modal: Other metrics can be simultaneously measured, like brightness and output power for efficiency calculations, or other spectroscopic techniques.
Objective: Does not depend on a particular set of filters or an algorithm applied at time of measurement.
Remote access: Fiber coupling allows measurement in hostile environments without impact to the spectrometer.

Application & Technical Notes

Applications

Other Common Applications

Electronics manufacturing: binning of LEDs, quality control of LCDs and plasma screens
Lighting: monitoring lamps used in horticulture, hydroponics, street lighting, advertising signage and hospitality lighting; characterizing light sources for CCT, CRI
Medical diagnostics: assessment of tissue for skin cancer
Food & agriculture: standardizing orange juice color, assessment of raw cotton quality
Biology: measurement of leaf disease states, organism coloring as a function of environment, impact of plumage on bird behavior, correlations between frog skin color and pollution levels
Industry: standardizing anodization, quality control in polymer manufacture, curing of ink and paint
Art and textiles: mapping the “pixels” in a painting, quality control of fabrics, paint matching systems at hardware stores

What Is Color, Really?

Color is the sensation produced when light at visible wavelengths falls on the human eye, specifically from 380 – 780 nm. Do dogs see in color? Yes, but not the kind of colors we see. That’s because they are physiologically different from us.

Cone cells don’t actually detect color directly. They respond only to the energy they absorb. A single cone cell can tell only whether two objects reflect the same amount of light, not whether they are the same color. The magic comes in their variety. The human eye has three different types of cones, each with its own unique spectral response function. It is the combined and contrasting responses of these three types of photoreceptors that create the impression of a single color in the human brain.

The spectral responses of the three cone types correspond roughly to red, green, and blue – what we consider to be the primary colors in an additive color model. Inspired by this trichromatic response of the eye, many color spaces have developed which use three values to describe color: RGB, XYZ, L*a*b*, and uvw, to name just a few.

Though the individual values in these color spaces don’t correlate directly to the response of a particular type of cone cell in the eye, each color space has a place in defining the endless array of colors in a particular field or industry. Some color spaces weight one parameter to describe the luminance or “lightness” of a color, while the other parameters capture the hue.

Other color spaces use a single parameter to describe one key quality of an emissive light source, like how it compares to an old fashioned tungsten wire bulb, or how well it “brings out” color in other objects.

Why Use a Spectrometer to Measure Color?

More information. More flexibility. In fact, it seems a little odd not to use a spectrometer to measure color, but it is actually common practice.

Simple color meters use red, green and blue filters in combination with diodes or sensor pixels for measurement. More advanced systems use tristimulus filters that mimic the CIE color matching functions. These work well for incandescent light sources, but are less accurate for LEDs. Handheld color meters may measure up to 20 wavelength bands, but this is not enough for research or high-accuracy measurements.

To detect small color changes, very high color resolution is necessary. By capturing the complete spectrum, the color measurement made by a spectrometer allows careful and detailed analysis of data. Color meters and analyzers based on filters or detection over specific bands simply leave a lot of information on the table – information that can be used to better inform the color measurement.

Some color analyzers are also strongly dependent on lighting conditions, since objects tend to appear different colors under different illumination. With the right lighting, two objects can appear to be identical in color even if the reflected spectral power distributions differ, an effect called metamerism. If the lighting changes, however, the colors can look significantly different. This makes controlled lighting conditions essential to consistent results.

When color measurements are made with a spectrometer, a full reflected or emissive spectrum is the starting point for all calculations. That allows the data to be analyzed in different ways, and even recalculated at a later date to change the observer, the illuminant, or the color space. It offers maximum flexibility with the same high accuracy as if the calculation had been performed that way initially.

Example Setups

Featured Products for Emissive Color Measurement:

TORUS-25-OSF	Torus spectrometer (360-825 nm) has low stray light and high thermal stability. This version includes a 25 µm slit and order-sorting filter, but your choice of optical bench accessories very much depends on the LED wavelengths you are measuring.
FOIS-1	Integrating sphere that collects light from emission sources such as LEDs
LED-PS	Power supply powers the LED, displays the LED drive current and holds the LED in place
QP400-2-VIS-NIR	Premium-grade patch cord directs light collected at integrating sphere to the spectrometer
HL-3-INT-CAL	Radiometrically calibrated light source designed for use with an integrating sphere
OceanView	Spectroscopy software

Featured Products for Color Measurement:


TORUS-50	Holographic concave diffraction grating spectrometer (360-825 nm) with 50 µm entrance slit
HL-2000	Tungsten halogen light source (360-2400 nm)
QR400-7-VIS-NIR	Reflection probe (6-around-1 fiber design), optimized for VIS-NIR response, 2 m length
RPH-1	Fixture for holding 1/4″ (6.35 mm) diameter reflection probes
WS-1-SL	Diffuse reflectance standard has 99% reflectivity from 400-1500 nm
OceanView	Spectroscopy software

Making Measurements

What is a reflected color measurement?

The color that we see from objects almost always comes from scattered light. That means that a reflected color measurement is really just a diffuse reflectivity measurement, optimized for the 380 – 780 nm wavelength range and analyzed mathematically to yield colorimetric quantities like xyz, RGB or L*a*b*.

Details regarding measurement configurations and techniques for reflection are covered elsewhere. We recommend reviewing the reflection material before learning how to optimize your system for color measurements and understanding how the color data is analyzed.

How do I optimize a reflection measurement for color?

LIGHT SOURCE:

A light source for reflected color primarily needs to have good output from 380 to 780 nm, the range over which the human eye detects light. The broad, smoothly varying output of a tungsten halogen light source works well, and is economical and versatile for other applications. Its output at short wavelengths, however, is not as good as the bluLoop light source.

The bluLoop is an LED-based light source with four different bulbs to yield balanced spectral output over the visible range. It is ideal for color measurements due to its unique spectral shape. A xenon light source is a poor choice for color by comparison due to its jagged, pulsed spectrum and the need for high averaging to get good quality measurements.

SAMPLING OPTICS:

Either VIS/NIR or UV/VIS fibers can be used for color measurements, since both transmit well between 380 – 780 nm. UV/VIS fibers have a slight advantage in transmission below ~400 nm, which may be useful if S:N for the system needs to be improved at shorter wavelengths, but not otherwise.

If using a reflection probe, remember that it will illuminate and detect from the same direction, only seeing part of the reflected light or color of the object. This is fine for most samples, but be careful when measuring iridescent samples and highly reflective ones where the color changes with the angle of illumination or viewing. This can happen with rough surfaces like brushed metal, fish scales, seeds, and reflective signs or material.

An integrating sphere might be a better choice for these samples, since it both illuminates and collects light at all angles. An integrating sphere is also good for looking at convex curved surfaces, or to measure the color of objects that are small enough to fit into the sphere. Color measurements made using an integrating sphere with a lower port-to-diameter ratio yield the most accurate results, particularly on the L*a*b* scale. (Measurements made using the ISP-REF tend to show errors of ~5% or more, so this is not the preferred integrating sphere for color.) Also, think about whether gloss should be included in your color measurements.

Though it might be tempting to use a variable-angle reflection sampling system to look at color at different angles, it is designed for specular reflectance only. As a result, both the angle of reflection and the angle of incidence will change as it is adjusted.

Collimating lenses can be used at the ends of individual fibers to truly customize the angle of incidence and angle of collection when making reflected color measurements, though the collimating lenses need to be adjusted carefully to avoid beam divergence and get good signal. We usually find that color measurements taken using collimating lenses and fibers are not as accurate as those made using an integrating sphere, so be sure that the extra fixturing and alignment is justified before using this method.

SPECTROMETER:

If making color measurements with a tungsten halogen light source as the source, consider using a grating #2 in the USB2000+ or USB4000 spectrometer. It peaks at a shorter wavelength than the grating #3 used in preconfigured VIS-NIR units. When combined with the silicon CCD and a tungsten light source, the resulting spectral response varies less with wavelength, resulting in slightly better S:N over the important 380 – 780 nm range. Color is a slowly varying spectral feature, so high resolution is not really needed to make good measurements. Resolution of ≤ 2 nm should suffice.

Our Maya LSL spectrometer is also very well-suited to color measurements due to its improved response at blue wavelengths and low stray light.

What reflectance standard is best for reflected color?

The best reference is one that is similar to the sample to be measured. Color almost always comes from scattered light, so a diffusely reflecting reference is almost always the right choice. The WS-1 diffuse reflectance standard is matte white in color and is > 98% reflective over the range of color measurements. The WS-1-SL can be a good choice when working in the field or in dirty environments, since it can be smoothed, flattened and cleaned if it gets pitted or dirty.

What is emissive color measurement?

In an emissive color measurement, the sample IS the light source. Good examples are lamps, LEDs, LCDs, plasma screens, and displays. A sampling optic with the right field of view is used to collect a portion (or all) of the emitted light from the sample, routing it to a spectrometer for measurement.

Just like in a relative or absolute irradiance measurement, the spectrometer needs to be calibrated first. In fact, an emissive color measurement begins with a relative or absolute irradiance measurement. Establishing an accurate scale for the spectrometer’s response at each wavelength is absolutely essential to calculating color.

We have already discussed in detail the measurement configuration options and methods for relative and absolute irradiance in a separate section, so take a look here first, and then come back to learn more about how to optimize your system for color measurements and analyze the data.

How do I optimize an irradiance measurement for color?

SAMPLING OPTICS:

The same wide variety of sampling optics can be used for emissive color measurements as for irradiance, so it is only the field of view that really needs to be considered. Either VIS/NIR or UV/VIS fibers can be used for routing light to the spectrometer, as both transmit well between 380 – 780 nm. UV/VIS fibers have a slight advantage in transmission below ~400 nm, which may be useful if S:N for the system needs to be improved at shorter wavelengths, but not otherwise.

A bare fiber or flame probe will give a 25° full angle field of view, sampling a spot size with a diameter equal to ½ the distance to the object or plane of measurement. These are good for getting a general view of the color of an emissive object. A Gershun tube kit can be used to vary the field of view from 1° to 28° using various apertures and configurations, making it easier to look at a specific object at a distance.

A cosine corrector increases the field of view to 180°, sampling the light directly incident on it and gathering the most light from the smaller angles (Lambertian response). It works well to measure the color of light incident on a plane.

An integrating sphere uses a measuring port cut into the sphere to collect light from a sample surface. This sampling configuration gives no bias or weighting of collection by angle. It is ideal for measuring the color of any source or surface and is particularly useful if a light source can be placed inside the sphere, like an LED. For the most part, ambient light is excluded from the measurement in this configuration or is easily subtracted. Color measurements made using an integrating sphere with a lower port fraction ratio yield the most accurate results, particularly on the L*a*b* scale.

A fiber terminated with a collimating lens is the most directional option, as it collects only light incident parallel to the lens housing axis. It works well for looking at how emitted color varies with angle for light sources or displays.

SPECTROMETER:

If using a USB2000+ or USB4000 spectrometer, a grating #2 will provide the best overall dynamic range. It peaks at a shorter wavelength than the grating #3 used in preconfigured VIS-NIR units, resulting in better S:N at the shorter wavelengths. If using an integrating sphere, remember to use a detector collection lens and/or a larger slit to increase sensitivity. Color is a slowly varying spectral feature, so high resolution is not really needed to make good measurements. Resolution of ≤ 2 nm should suffice.

Our Maya LSL spectrometer is also very well-suited to color measurements due to its improved response at blue wavelengths and low stray light.

COLLIMATING LENS:

If properly aligned for collimation, it samples a spot size equal to its diameter. Collimation can be checked by routing a light source in reverse through the fiber/lens pair. It works well for sampling light from a specific location at a distance.

Advanced Measurements & Samples

Calculating color

The CIE Colormetric System and YYZ Color Space

Color-matching functions:

When convoluted or folded with an emission spectrum of a light source, I(λ) and integrated over the response wavelengths of the human eye, these color-matching functions yield a set of three tristimulus values, XYZ, which together define numerically the color of an emissive sample. These values are the basis for the majority of other color spaces, each of which seeks to improve on earlier work or adapt itself to the needs of a particular industry.

How do I take the best dark measurement?

When taking a dark measurement with a reflection probe or integrating sphere, it is best to block the light at the light source if possible. Turning the light source off and then on again will throw the light source out of thermal equilibrium and require a new reference measurement. Another option is to point the lit probe or opening of the integrating sphere into a dark space to take the dark measurement so that no light scatters back in. A background measurement taken this way is more accurate because it includes any scattered reference light that will be present in the sample measurements, allowing it to be subtracted properly.

Resist the urge to point the sampling optic at something black (like a piece of paper or a cover cloth). Objects that appear to absorb all wavelengths usually reflect some colors better than others.

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