Color should be a pretty simple concept. Most of us had learned the basics of Color by the time we were six years old, and were virtual experts in color matching and mixing by the end of our fingerpainting phase.
Often, one of the primary goals of color measurement is to precisely model the human eye. Ideally, the hardware instrument used for this task would have a perfect correlation with what your brain tells you that your eye sees. The first thing that we need to remember is that Color does not exist at all outside of our eye/brain interpretation of it. What we perceive as Color is nothing more than energy contained in wavelengths in the visible spectrum (approx. 380 nm to 780 nm).
There is no Red, Green or Blue. There is no Yellow, Magenta or White outside of our eye/brain interpretation of it. There is no Color "out there" at all. There are only electromagnetic wavelengths. The only reason this fact is reviewed is that it becomes important in instrumented color measurement, as the instrumentation will "see" light and color quite differently than we humans do.
Perfect. We measure the energy in the visible spectrum with a hardware instrument capable of such measurements, and we have an instrument that models the eye! Right?
Unfortunately not. While such a measurement could certainly provide the spectral power distribution (SPD) in the range that our eye can see, the eye is a very complex organ with a lot of surprises in store as we attempt to model it. I will leave out the rods, cones and optic nerve discussions to keep this topic on track, but the short reason is that the eye is very complex.
Two of the major difficulties in precisely matching instrumentation to the human eye are the following:
The former problem means that most (affordable) hardware color measurement instrumentation can not accurately 'see' light in environments as dark as the eye can see, nor can it 'see' light as bright as the eye can see without saturating. However, it is really the second problem of color matching that will be the focus of the remainder of this article.
The eye is not only non-linear in it's response, but is also very complex in it's processing of the visible spectrum. Again, without going into the rods and cones discussion, suffice it to say that humans have three different unique responses that create our interpretation called "Color". You will recall from when you were six years old that the responses can roughly be characterized as Red, Green and Blue for additive color systems. Consequently, most display devices will use those three 'primary' colors to produce their image. Let's take a closer look at how and why Red, Green and Blue come to be the three primary colors, which will lead us into the color measurement techniques that attempt to model the eye.
Since this discussion will be devoid of mathematics,
graphical examples will help to demonstrate the textual points
being made. The graphics will be taken from the
Milori ColorFacts application.
Here is the first response, the one that is sensitive to the shortest wavelengths. You will see that the wavelengths are all in the range of the spectrum that we would perceive as "Blue". The eye begins to perceive wavelengths at just above the Ultraviolet (UV) range, at approximately 380 nanometers.
Note that the graph has been scaled to show detail in the response. On the next page, you will see the three responses scaled correctly in proportion to each other.
Here is a graph of the wider second human eye response, the one that we would call "Green". It has a peak at around 550 nm, in the greenish-yellow part of the spectrum, which is the single wavelength that the human eye is most sensitive to.
So far, so good. It is becoming clear that the human eye has different responses to different parts of the visible spectrum. Two of the responses correlate with wavelengths that we perceive as the colors "Blue" and "Green". Although the responses have different distributions (take a look at the shapes of the two graphs), it should be possible to model this with hardware.
However, we haven't seen the third response, the one we call "Red" when referring to the three primary colors. Here it is:
What we call the human Red response actually has two separate peaks! In fact, even this is simplified (as is most everything in this discussion to keep it on track). The smaller Red peak is actually negative, but normally all three responses are scaled and presented as all-positive responses for easier calculations. Clearly this third response will be a trickier response to model than the first two, and will be a significant factor in any color measurement hardware device designed to model the eye.
Let's take a closer look at what these three responses mean in our quest to have a color measurement instrument that closely models the eye (next page).