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.