I'm looking for a cheap spectrophotometer (at least I think I am)

First let me describe the experiment (you can find more details in my ever-evolving paper on Color Vision). In order to perform this experiment, we commence by painting a board with a wide variety of colors in various interlocking geometric shapes. Next, three light sources – which generate pure red, green, and blue light – are all set to the same intensity and used to light up the board. The combination of these pure sources effectively illuminates the board with white light.

Setting up the experiment.

Next, a spectrum analyzer is pointed at the board. This analyzer is able to separate and distinguish the various bands of the optical spectrum that it's receiving.

The analyzer also has a telescopic lens, such that it can be focused on individual colored shapes on the board. Consider a shape that's painted primary red. In this case the paint will reflect most of the light from the red light source and it will absorb the majority of the light from the green and blue sources. Thus, if we were to point the spectrum analyzer at this red area, the light received by the analyzer will show a large red component, along with relatively small green and blue components as shown at point (a) in the following illustration:

Pointing the analyzer at areas filled with primary colors.

Similarly, if we point the analyzer at areas that are painted primary green or primary blue, it will see large green or blue components as shown in points (b) and (c), respectively.

Now, suppose we consider two shapes on the board that aren't painted in primary colors. Let's say that one of these shapes is painted a light-ish brown, while the other has a pink-ish sort of hue; also that both colors reflect some amount of red, green, and blue light, but in different proportions. Thus, if we pointed our spectrum analyzer at these two shapes in turn, we'd be able to detect the differences in their color components as illustrated below:

Pointing the analyzer at areas filled with non-primary colors.

Our obvious reaction is that the differences between the amount of red, green, and blue light being reflected from these shapes gives each of them their own distinctive color, and that's certainly true to an extent, but there's more to this than meets the eye (if you'll forgive the pun).

Suppose we point our spectrum analyzer at the pink-ish shape, and then vary the intensities of our three light sources to create an artificial environment in which the light that's reflected from this shape has exactly the same characteristics we previously recorded from the brown-ish shape. The question is: "What color is the pink-ish shape now?"

Common sense (which actually isn't as common as it used to be) would dictate that if we're now receiving exactly the same color components from the pink-ish shape that we originally received from the brown-ish shape, then the pink-ish shape should look exactly the same color as the brown-ish shape used to (and also that all of the other shapes on the board will have changed color to one degree or another).

Well, hold onto your seat because here comes the amazing part ... it does and it doesn't and they do and they don't. (If you think this is difficult to follow, just wait until you try to explain it to someone else!)

What do I mean by this? Well, let's suppose we take a large piece of white card the same size as the board; that we cut out a piece exactly the same size, shape, and position as our original pink-ish area; and that we then place the card in front of the board. In this case – assuming that that we leave the light sources generating our artificial lighting environment – our shape that was originally pink-ish will now appear to be our brown-ish color. Similarly, if we did the same thing for any of the other shapes, they would all appear to have changed color to some extent.

However, if we were to remove the white card so that we could again see the entire board, the pink-ish shape would once again appear to be, well ... pink-ish, and all of the other colors would appear to be pretty much as we'd expect!

As I mentioned earlier, I actually saw this experiment performed on television years ago and it was amazing. What's happening is that if you can see only the one shape, then your brain has no other recourse than to assume that its color is determined by the different proportions of red, blue, and green light that are being reflected from that shape. By comparison, if you can see that shape's color in the context of all of the other shapes' colors, then your brain does some incredibly nifty signal processing, determines what colors the various shapes should be, and corrects all of the colors before handing the information over to the conscious portion of your mind.

To put this another way, your brain maintains a three-dimensional (3D) color-map, in which every color is weighted in relation to every other color. Thus, when you can see the whole board, your brain automatically calculates all of the color relationships and adjusts what you're actually seeing to match what it thinks you should be seeing.

A call to action: As I said at the beginning of this blog, I want to perform this experiment for myself. In order to do this, I need variable intensity red, green, and blue light sources. I also need some form of spectrum analyzer. I think the type of analyzer I want is called a "spectrophotometer," but there may be others that are better suited to what I'm trying to achieve. Ideally, I'd like the spectrophotometer (or whatever) to have some sort of output that could be fed to a computer to display the results on its screen (from there it could be fed to some sort of projector so the audience can see the red-green-blue values that are being detected).

And, just to make things really tricky, I'm on a very limited budget (you know how it goes, some years are better than others). So, do you have any ideas as to (a) what type of equipment I need and (b) where I can get this equipment for not-so-much money?

Questions? Comments? Feel free to email me – Clive "Max" Maxfield – at max@techbites.com). And, of course, if you haven't already done so, don't forget to Sign Up for our weekly Programmable Logic DesignLine Newsletter.