RGB v Tristimulus Color Scales – HELP!

At my new quality assurance job, one of our product quality checks is to measure the color of our product and to ensure that we are producing our product that is within a preset acceptable color range. We use an instrument, a spectrophotometer, that reports Hunter L, a, b; XYZ, and L*a*b scales. I’ve always thought that color was measured in terms of RGB, like the way computer monitors and TV screens describe color. I’ve even created custom colors for fonts on my computer by manually adjusting the RGB quantities. Can someone explain why we would measure color using Hunter L, a, b; XYZ, and L*a*b instead of RGB?

It depends on what you want. Do you want White, Yellowish White, or Bluish White?

In simple terms, the primary difference between CIE Tristimulus Scales (Hunter L, a, b; XYZ, and L*a*b scales) and RGB is their purpose within the color world. RGB is a device dependent method of producing color and is not exact enough to be used to describe a color for quality control purposes. CIE XYZ color scales represents the true color of an object, while RGB describes a flat solid color representation of the average color of an object is displayed on a screen. One is used to provide color directionality (RGB), the other is used to precisely quantify a color (Tristimulus values). let’s illustrate…

Let’s take a drive to the Lincoln Memorial

Let’s pretend the Lincoln Memorial is not a physical object but rather a specific color, let’s say white, since in fact it is made of a very specific white concrete. To get there, should I use RGB or Tristimulus values? This will depend on how close to the Lincoln Memorial, or its specific color of ‘white’ you want to get. Using RGB to measure the color white and expecting analytical precision would be like trying to get to the Lincoln Memorial without the exact address and a GPS/map to guide you. While you may know that the Lincoln Memorial is located in Washington D.C., getting to the specific address would be a challenge.

RGB is very much like this in that you might know the general area of red, blue, green, or in this case ‘white,’ but getting to a precise color takes more than a general direction. Much like GPS, which uses three-dimensional physical coordinates that can guide you to within three feet of the desired address, CIE Tristimulus scales provide three-dimensional color coordinates to give you the exact address of a specific color with extreme precision. While RGB might drop you off on the Mall without any further direction, tristimulus coordinates will direct you precisely to a specific color with decimal precision, much like GPS will guide you to the Lincoln Memorial within three feet.

So, how is RGB used then?

RGB is an additive color scale that was created to represent colors on digital devices. RGB uses 0 to 255 different colors of light to create different colors. RGB color scales were created to display colors through pixel digitization, where a Red, Green and Blue phosphor are grouped together. This is what you are seeing when you are looking at your computer, TV, or phone screen.

Have you ever walked into Best Buy or Target and seen the rows of televisions all playing the same football game? As your eyes pan over the TV screens you might notice that some TVs look more washed out than others. One TV has bright neon grass, and the one next to it looks almost gray in comparison. Now the TV pans to the referee. You are now seeing a bunch of different skin tones on the same referee. He appears to be pink on one screen, orange on the other, and pale on the next one.

Although all the TVs are using RGB to display their colors, the machine’s output of the color determines what we perceive. Each TV is simultaneously getting a signal to display the RGB intensities R=100, G=200, B=150 but they are all slightly different colors. This is because different television manufacturers all use different display phosphors.

Now it is starting to make sense! Please, tell me more about these Tristimulus values

Long before television was invented, the International Commission on Illumination referred to as CIE (Commission Internationale de l’Eclairage) endorsed the XYZ color model as a standard transformation of visible light to how a Human retina would perceive it. CIE XYZ represents color through a mathematical table that weighs our retinal response to wavelengths in a range. The X value is the Red response, the Y value is the Green/Yellow response, the Z value is the Blue response.

CIE Tristimulus color scales such as Hunter L, a, b; XYZ, and L*a*b scales were developed to quantify color as perceived by humans. CIE Tristimulus scales are highly precise and can represent color to a second decimal place. This means that not only can many more colors can be represented with this color scale, but they are also more exact. Again, using our GPS analogy, they provide an exact color address vs. a general color area.

How does this relate to how humans ‘perceive’ color?

Color is not a primary physical quality, it is a psychophysical quantity. This is why there needs to be a common color language to give a representation of color in an exact and precise way. Breaking down how color is perceived helps to understand how color is calculated into a tristimulus color scale such as Hunter L, a, b; XYZ, and L*a*b.

In order for humans to perceive color, three things must be present. First, you need an object, then you need a light source, and lastly someone to observe the object. Humans perceive color by gathering information from the wavelengths emitted by the light reflected off an object and then make a subjective statement about the color. This is demonstrated by the physical perception of different TVs displaying different variations of the color green. We can say that all the TVs are displaying the color green, despite the slight differences between the colors. Color Spectrophotometers work similarly to the human eye in that the object is placed in front of a light source, the light is reflected off of (or transmitted through) the object, and the reflected (or transmitted) light is collected by precision optics that literally count the number of photons at specific wavelengths to calculate the three dimensional coordinates of the color of the object.

Are we there yet?

Analytical color measurement is a must for manufacturing due to the subjective nature of human color perception. Making a repeatable, objective, and precise definition of color without the proper color measurement technology is difficult, but with the right technology, we can easily and reliably arrive at our desired color destination, in this illustration of the Lincoln Memorial, a very specific ‘white’.

HunterLab creates instruments that measure precisely, while providing the best technology for its customers. Find out more about our products and the importance of quality control by checking out our previous article on how our New Digital Qualification Notebook is transforming quality control.

Do you want to learn more about Color? Stay tuned for our next article within this Color Education Series where we will be discussing the differences between Color Scales and Color Indices.

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