In this roundtable discussion, Ocean Optics experts describe the basics of color, differences in emissive and reflective color measurements, and color applications that transcend aesthetic considerations.
About Our Experts
Originally captured as part of the Applied Spectral Knowledge Podcast series, this conversation on color was led by Matt Kremens, Ocean Applied’s Director of Engineering for Applied Systems, and features Ocean Optics colleagues James Gass, Manager of Test and Calibration, and Ethan Montag, Senior Scientist. Collectively, the group has several decades of experience in color measurements.
Matt Kremens: Let’s start with the broadest question of all: What is color?
James Gass: It’s actually a very complicated question. There’s a whole color science, but really, color is one of the ways in which humans perceive light. So, the easy dimension to understand of human perception of light is intensity. If I increase the power of a light bulb, we see that it has more output. It looks brighter, it looks more intense. Color is another dimension of perception that is separate from intensity.
MK: What makes color complicated?
JG: I’ll discuss the origin of color and we’ll understand a little bit why. Within our eyes, we have four distinct receptors. Some of those receptors are called rods and those are responsible for low light perception, like night vision. The rods, however, do not really perceive color. And there’s only one type of rod.
We have another receptor within the eye called cones. There are three types of cones and each cone responds differently to different wavelengths of light. They have overlapping regions of response, though. And that overlapping region is critical to color perception, but it also makes the models very complicated. Here’s the other thing we should think about with color vision: You have these eyes that are like wavelength sensors, but you only have three of these sensors. And that’s important because when you think of a spectrum of light, there are many different wavelengths that are covered within human vision. Human vision goes from about 380-780 nm and you can have many wavelengths in that region.
But even though there’s in theory, infinite, different spectra, they collapse down to a three-dimensional space. That’s what you see. And so, as a result, two spectra can look quite different and yet be perceived as the same color. So, in color science, that’s called metamerism. That’s a very interesting phenomenon.
MK: There are also segments of color measurement, like emissive and reflective color measurements. What is the difference between those types of measurements?
JG: Emissive color means measuring the color of something that is emitting light. So, you’re talking about a light source — an LED, a light bulb, or maybe a computer monitor or the sun. Even bioluminescent things like fireflies emit color. So, you’re measuring the thing that produces the light directly. Reflective color is looking at color reflecting from the surface.
Ethan Montag: Reflective color is probably what most people think of when they think about the color of a surface. And so, what happens is you have an illuminant — a light bulb or the sun or whatever — shining on an object. And the object has reflective properties so that it reflects light at different wavelengths by different amounts. And then that light reaches your eye. And as James described before, we have the cones that detect the light. So, the product of the illumination and the reflectance and the sensitivity of your cones gives the color signal.
MK: What else is important about reflected color?
EM: When we look at an object the cones are compared, and we get a color from it. But it’s a little more complicated than that because it takes into consideration the color of the illumination – what looks like white when you’re looking at something – and the texture and geometry of an object, which affects its appearance.
If you have something that’s matte, it has light that reflects in all different directions uniformly. And if you have something that’s glossy, then the reflection of the illumination will come off more directionally. This gives things their characteristic appearance, like something that’s metallic or matte – like a piece of paper or fabric — that you can perceive because of its spectral characteristics. And because the appearance of objects is affected by their geometry, we should be careful about the way that we measure them. And so, there are international standards that have been developed over the years so that when you measure surface reflectance, you use certain geometries so you can define how an object is being measured. Because part of color measurement is not only measuring the color, but it’s a way to communicate the color. So, you define ways of making measurements.
MK: What are some of the standard methods that have been developed to measure color?
JG: Off-axis color measurements are usually how the standards are set up. For example, a diffuse illumination and an 8° measurement is one of the very common ones. I think there’s another standard where you illuminate at 0° and then your probe is set off at a 45° angle. So, you’re not catching the specular reflection. The idea is that setup correlates closer to how humans perceive such things.
EM: Well, you’re right. A typical method is to illuminate with a directional light source. For example, our spectrophotometer [model FD-D8R] has a light source at 8° from vertical and the reflected light is then collected in an integrating sphere. And then your option is you could have a hole in that integrating sphere to let the specular reflection from that 8° light go out of the sphere and not be part of the measurement. Or you could have it included.
MK: How would you decide whether to include specular?
EM: If you have an object that’s glass, you might want to have the specular light included because it gives a better correlation to what people see than when it’s excluded. We make measurement devices that can do specular and diffuse at the same time, so it correlates with the color that the person making the measurements is more interested in. Also, there are measurements where the light is directed at the object at 45° and then it’s measured at normal perpendicular to the object. And so, any specular reflection from that is totally omitted from the measurement.
JG: You really do have to specify the sampling geometry or else it’s almost meaningless.
EM: Another thing that the standards bodies have come up with is two different standard observers that are part of the specification of the color. There’s the 2° observer and the 10° observer. The 2° observer is used to measure small spots, and the 10° observer, as the name implies, is used to measure larger spots. Throughout the literature, and through experiments with people making color matches and judging colors, the standards bodies found that there’s a difference in the 2° and 10° observer behavior. [Editor’s note: For a deeper discussion on color standards and observers, review the color podcast. The discussion begins at 11:24.]
MK: We’ve discussed color theory, but what are some examples of color applications?
JG: So, why would somebody want to do an emissive color measurement? A real-world example that I have worked on is LED sorting. When LEDs are produced, the manufacturing process control is not tight enough to keep the colors consistent. And so, the industry standard is to measure and sort the LEDs. You’ll have many different whites that end up getting separated into one bin because they’re slightly bluish. And then others that are slightly reddish getting separated into another bin. And that’s how they guarantee that the LEDs look similar.
MK: When would consistent color in LEDs be important?
JG: For example, if you’re in a very high-end jewelry store, and the store is using white light LEDs to illuminate the countertops, they really don’t want a lot of blotchy, subtle color variations across the illumination of their very expensive merchandise. Color matching in a scenario like that is considered very valuable for those vendors.
MK: Let’s move on to reflective color. What are some example applications?
EM: You can imagine a scenario where you’re producing your product — your widget. Your widget might be assembled from different parts. And sometimes the parts are made in different places. And when you assemble them, you want the different parts to match. So, if they don’t match people will not like the way they look and they won’t buy the product. So, you want to measure the different parts and make sure that they’re within a certain tolerance of each other so that when you assemble your product or put them next to each other in your store, they’ll all be the same or be perceived as the same color.
Now, in the old days, you couldn’t measure color very precisely. You might have an expert with very good color discrimination and training who would look at a sample from a batch of your widgets and compare it to a standard and decide whether that batch matched or didn’t match. But if that person became fatigued or was out for the day, you’d be out of luck.
Now, we can use a color measurement device like a spectrophotometer, which measures the complete spectrum and then computes what the color of the widget is for a standard observer and use that to determine your tolerances for your measurements. Not only do you gain precision because you can be much more precise than a human observer, but you can also gain speed. And you can start doing it for more than just one sample per batch. You can check many of your product very quickly in a factory. And that way, batch variation doesn’t become a problem anymore. You can you can check everything much more quickly and more accurately than a human can do it.
MK: Let’s address different illuminants, how we handle things being presented under different lighting conditions, and the advantages of using a spectrophotometer to make those measurements.
JG: As we discussed before, part of the perception of color is the color of the illuminant. So, when you look at an object, the color is influenced by the illuminant. There are a couple of things that can happen. One is illuminant metamerism. You can have two colors that match under one illuminant and then you go under another illuminant and they don’t match anymore. And people might have experienced that, like when they put on a shirt and pants and the colors match when they’re in their bedroom under some sort of incandescent light. And when they get to work under fluorescent lights, the clothes don’t match anymore.
When we make a measurement with a spectrophotometer, the instrument measures the spectral reflectance. And because you have that information, you can apply any illuminant that you want. If you’re interested in having colors match in your store and your store is illuminated by fluorescent lights, you can make that calculation and make sure all the objects or the parts match; you’ve accounted for the illuminant metamerism in your store. Part of the specification of a color is the illuminant. And so, there are different types of illuminants that have been standardized.
MK: We’ve talked a lot about trying to match product colors or colors of screens and LEDs, things that are aesthetic or somewhat superficial. What are applications where color is used as an indicator of some other criteria?
JG: That’s an interesting question. There are these applications where color is a proxy for something else. And one I can think of right off the top my head is for oil and gas analysis. There’s an index that effectively measures the “yellowness” of the fuel. Nobody really cares if fuel is yellow, aesthetically, but it can be an indicator of fuel quality or other criteria.
Also, there are standards like that in food safety, where the color of ground beef, for example, is really a stand-in for what’s happening chemically to that sample.
EM: There are many other examples. For example, in this time of COVID-19, there are biological assays in which dyes don’t have any color until they’re bound with certain proteins. So, there are tests out there for viruses, let’s say, that change color to signify that the virus is present in the sample, just like pregnancy tests also have color changing indicators.
JG: There are bunch of examples like that. And even though those tests are really designed for a human to look at, they often can benefit from using a spectrophotometer to measure the color, especially if you’re trying to find very fine levels of discrimination.
MK: You have another example that’s close to home.
JG: Yes, a color test goes to the origin of Ocean Insight. The founder of Ocean Insight was doing pH measurements of the ocean and he wanted to get a much finer discrimination of his color-changing pH indicator than the human eye. And, he wanted to do it automatically because he was trying to put these pH sensors on buoys out in the ocean. And so that’s kind of why he built a miniature spectrometer, because he was trying to do color measurements with a machine.
MK: That’s awesome. That take us full circle here. Thanks to James and Ethan for talking about color today.
Note: This discussion has been edited for length and clarity. To hear the complete discussion, tune in to our Understanding Color Science podcast.