Spectrophotometers are a commonly used tool in nearly every industry, yet this valuable technology is often under-utilized. Companies purchase color measurement equipment out of necessity and then find themselves using the bare minimum applications for the job. Understanding tristimulus values and the different geometric degrees and angles can be overwhelming, making instrument selection challenging. Well, not anymore! In this post, we will give a simple and straightforward explanation of tristimulus values and color measurement, plus provide resources that will help you choose the right instrumentation for your application needs. Finding the support you need to use your color measurement tools correctly and efficiently will ensure that you are getting the best value for your investment.
Image Source: Flickr user mckinney75402
Human eye technology and color measurement
Image Source: Wikimedia Commons user Sakurambo
Spectrophotometers utilize human eye technology to create accurate yet objective and quantifiable color measurement values. These values can then be repeated for color quality and consistency in product development. Based on human visual perception, color measurement relies on a system of three color values which are referred to as tristimulus values.
This system mirrors the color receptors in the human eye, referred to as cone cells. Tristimulus values measure light intensity based on the three primary color values (RGB), typically represented by X, Y, and Z coordinates. The tristimulus values system is the foundation of color language, also referred to as the CIE color system, and is used to communicate precise color values around the world.
Other factors that affect color measurements
Tristimulus values are the backbone of color measurement, but many other factors can influence color perception. Variations in light sources, viewing angles, and field of view can alter visual perception; therefore color measurement instrumentation must be designed to account for these variables.
The standard field of view is the CIE 10 degree standard observer, with has been proven to be the most accurate degree of observation for matching human visual perception. Alltristimulus values are calculated from this standard degree of observation, so that manufacturers can be confident in meeting exact color specifications. Calculations are based on the XYZ tristimulus values in accordance with the type of illumination and reflectance of the sample.
Color-matching and quality regulations
The tristimulus values provide a reliable system that provides manufacturers and customers a way to communicate color preferences in order to ensure precise color matching and product compatibility. The ISO (International Organization of Standardization) utilizes these specific methods and the tristimulus value system to regulate and ensure compatibility standards between various industries. Understanding the basis of color measurement is the first step toward utilizing your color measurement instrumentation at its full potential.
Choices in instrumentation
Spectrophotometers are available in a variety of styles and types, which vary greatly depending on industry needs and regulations. HunterLab is a leader in spectrophotometric technology and has over 60 years of experience working with various industry leaders to develop to the most efficient and reliable instrumentation available. HunterLab is a trusted name in color measurement and we pride ourselves in customer support and relationships. We are here to help you understand your color measurement needs and how to utilize your instrumentation to get the most out of your investment. Contact HunterLab today to learn why more companies trust us to handle all their color measurement needs.
Mr. Philips has spent the last 30 years in product development and management, technical sales, marketing, and business development in several industries. Today, he is the global market development manager for HunterLab, focused on understanding customer needs, providing appropriate solutions and education, and helping to solve customer color challenges across these industries and cultures.
Simon Morice
I have yet to come across a good explanation of this diagram. Nobody ever describes the units of the X and Y axes. It is a very clever diagram but a more comprehensive, but simple explanation is required.
Grayson Lang
Simon,
The X and Y axes are the CIE 1931 X and Y primaries. The graph is of a three-dimensional colorspace where you’re looking down the Z axis (the CIE 1931 Z primary), though the color solid has been projected down onto the X+Y+Z=1 plane. By projecting it onto this plane, the colorspace flattens along the luminance axis. Thus colors with the same hue and saturation but different luminance values collapse onto each other. Chromaticity refers to this fact, it’s an objective specification of the quality of a color regardless of its luminance. Chromaticity consists of two independent parameters hue and saturation.
Hope this helps,
Grayson
Mayank Shukla
Thanks, @Grayson Lang. I have been trying to understand this from months and I thought I understood things….but reading this opened new Pandora box in my mind (The dots connecting feeling).
Thank you again, Grayson.
Moon
Hi,
I have 2 different constructions of the optical measurement system, a) testprobe with CIE ILED-B measurement geometry and b) integrating sphere providing the 4 pi measurement. I’d like to understand will these consider 2 view angles to lead to significant impact on CIE xy measurement data? Thanks!
Trejkaz
Chromaticity is one thing but if you’re trying to convert XYZ to a spectrum, or a spectrum to XYZ, units do become important, and nobody ever talks about what units XYZ is in, which makes it difficult.
The only thing that’s obvious is that a spectrum with twice the height of another (but the same shape) would convert to an XYZ with double the values. But the scaling to use when converting one to the other is completely non-obvious and nobody ever talks about it.
The SI unit for luminance is candela per square metre.
A spectrum of light tends to be in watt per steradian per square metre per hertz.
Obviously _some_ of this scaling is done by using the colour matching functions. Maybe those have units but I have never seen that mentioned either.
Trejkaz
I think I found the answer but it required some amount of searching and discussion with other.
The units of spectral radiance (what you have for a ray sample) are power (watts) per unit solid angle (steradian) per unit of viewing area (square metre) per unit of wavelength (metre).
When you do the summation/integration by wavelength, the per metre drops off the end, leaving watts per steradian per square metre.
But there is a normalising constant you are supposed to multiply through the result with, which is 683 lumens per watt.
So the resulting units are lumens per steradian per square metre, which is the same as candela per square metre.