Of colorimeters and spectrophotometers

Today’s secret word? Spectrophotometer.

Pronounce it with me: SPECK-troe-fuh-TAH-muh-terr. The three parts of the word are 1) spectro-, which comes from the fact that they take slices from the rainbow, 2) -photo-, meaning light, and 3) -meter, which comes from the fact that they measure the light in each of those slices of the rainbow.

The part in the middle, -photo-, may sound redundant since we are already talking about rainbows. But this middle part helps distinguish a spectrophotometer from a spectrometer. (Note that the latter omits that middle part. The photo in spectrophotometer designates that this sort of device comes with its own flashlight. It shines that flashlight on your sample and measures the reflected light.

A spectrometer (note the omission of the -photo- part) does not have such a flashlight, so it is used to measure emitted light from things like computer monitors, LEDs, and members of the family of insects lampyridea. These cute little buggers put flashlights on their tushies to make themselves sexier. It didn’t do much for me, but it seems to help them at pickup joints on a Saturday night.

Ray the Firefly

Here is some news on the spectrophotometer front. DataColor, manufacturer of handheld and benchtop spectrophotometers, has come out with a small, inexpensive color measurement device, the ColorReaderPro. It talks to your smart phone via Bluetooth.

The ColorReaderPro in action

This is already a crowded field, with a lot of recent hoopla coming from the folks at Color Muse and Nix. My good friends at Color Technology Consultancy recently blogged on a variety of inexpensive color measurement devices.

Handheld colorimeters on the market

Here is a little tidbit just for my loyal newsletter readers: While the Color Muse, Nix, and ColorReaderPro measure color, and SPECK-troe-fuh-TAH-muh-terrs also measure color, these small and inexpensive devices are properly called colorimeters (KUH-luh-rim-uh-terrs).

Colorimeters measure color by mimicking the human eye in how they respond to light. This mimicry is done with colored filters that approximate the color response of the cones in the eye. Spectrophotometers apply those filters with math, after capturing all those slices of the rainbow.

Colorimeters are in general much less expensive than spectrophotometers. I don’t have a price on the ColorReaderPro, but the Color Muse is $59, and the Nix goes for $349. Spectrophotometers are generally multiple thousands of dollars.

So, why would someone want to spend a whole bunch of money to buy a spectrophotometer? It comes down to accuracy. If you want to pick out a paint color to match your drapes, then a colorimeter is a good thing to put in your purse or murse. If you are contemplating using it as part of your color quality assurance project, I would recommend investing in a good set of dice. Your results will be just as good, and you will be able to play craps at lunchtime.

Which of the three different measurement devices is appropriate for your process control?

Are colorimeters appropriate for process control?

We gosh-darn sure would like to use this inexpensive and convenient new crop of colorimeters for all of our color measurement needs, including process control. We would like to reach into our purse or murse or pocket and pull out this cute little gadget to check to see if production is acceptable. We would like to deploy these little puppies all through our supply chain so that the designer and the janitor and the guy who drives the fork truck in the factory can check the tolerances of color throughout the process.

But

There is a rule of thumb for statistical process control. It's called the 30% rule, mainly because it includes the percentage 30. Here it is: If your measurement device eats up more than 30% of your tolerance window, then the measurement device will help you out by increasing your variation. You will be potentially chasing bad color that wasn't really bad, and neglecting bad color that you thought was good.

So, if your customer has allotted you a tolerance of 4 DE_ab, then your measurement device must be accurate to within 1.2 DE_ab.

I have done some extensive testing of the Nix and the Color Muse on a simple set of samples, all of them white. black, or gray. I have seen typical disagreement of a several DE_ab, with some up to 6 DE_ab. 

So, I'm gonna just go ahead and add that one to the list that your mom started, and Jim Croce added to later on. Don't run with scissors. Don't pull the tail of the neighbor's bulldog. Don't add water to acid. Don't spit into the wind. Don't pull the mask of the old Lone Ranger. And don't use a colorimeter for process control.

Unabashed promotion of some related blog posts

Just in case you missed a series of my blogs on spectrophotometers…

I started off my highly-acclaimed series with three blog posts that are prerequisites to talking about different devices to measure color: What color is water? and When light reflects from stuff. There is also a slightly more technical blog post called An illustrated compendium of the indicatrix throughout history. These posts all talk about how light interacts with matter.

The four colors of water

I followed up with no less than six blog posts about the different devices used to measure color.

If you are into color and are in the graphic arts, then I recommend reading What to use in the graphic arts. There are some notable special cases of inks that require special attention: Measuring metallic inks, and Measuring wet ink. These lead to Variants on the graphic arts spectrophotometers

If you are Measuring cloth or paint, or maybe soda cans, then you might want a different type of device.

Even when you are using the same sort of device to measure color, there are still choices for how the color is computed: The wonderful thing about standards is that there are so many to choose from.