Color Perception

May 31, 2016

Color is not simply a physical property of an object. It is the relationship between light, an object, and an observer. Our brains manage and process this information at amazing speeds, leading us to barely even notice the conclusions and the adaptations that they have made. But, these same wonderfully adaptive qualities of our eyes can trip us up when we are capturing color or even simply trying to remember a color precisely. Understanding color perception can help you to get accurate capture-to-print matching, understand the pitfalls of color correction and even to pick out the exact perfect shade of paint for your walls.

The Light

We all know that light is necessary to see color, but did you know that not all light sources are created equal?

You probably have noticed differences in the color of your light. For example, daylight at noon can seem quite bright and harsh compared to the warmer sunlight of the “golden hour” in the late afternoon. This difference in color is described as the color temperature of your light, and it is measured in Kelvin units - a measurement of heat. For example, sunlight at noon will usually measure around 5000 to 6500 degrees Kelvin, representing a neutral, even light. However, the warmer light of the evening may measure much lower at around 4000 degrees Kelvin.

Not only is the color of your light important, but understanding the spectral power distribution (SPD) of your light is critical to understanding how your colors will look under various lighting conditions. Spectral power distribution is sometimes visible to the naked eye. For example, you can usually see the difference between the warm yellow light of a tungsten light bulb (CIE Illuminant A) compared to the cooler light that emits from a computer monitor. However, in the case of fluorescent lighting, the SPD is not linear, which means that the near ultraviolet range, which is just outside the normal visible range for humans, and the blue-green area of the spectrum is much more heavily represented than the orange-red colors. This means that a red color viewed under fluorescent light will look significantly different than it appears under daylight.

The Object

Now that we understand the role that light plays, let’s take a look at the next aspect of color - the object that we are observing.

The colors we see when we look at an object are the colors that have been reflected, or to put it another way, the areas of the spectral wavelength that have not been absorbed by the object. Next, consider the surface of the object. Most organic objects are not perfectly smooth, so when light bounces off of them, it does not all bounce in the same direction. This scattered reflection affects the way we interpret the spectral reflectance of the object we are observing, making a color appear darker or lighter than the actual spectral measurement.

When we observe color, we observe not only the actual color, but we observe the color in the context of all of the surrounding colors. The accompanying optical illusion (image below) demonstrates that difference. Although the color in the foreground seems to be bright orange while the color in the background is brown, they are actually the same color. You can check this for yourself by opening the file in an image editing application and comparing the RGB values of each circle.

Finally, keep in mind that fluorescence can play complex tricks on your imaging systems. Optical brighteners are common in everyday objects like paper and fabrics. Because they are just outside of our visual range, they have the effect of making objects appear more bright and white to a human observer. However, because cameras are slightly more responsive to ultraviolet light, the bright white subject matter is sometimes interpreted by the camera with a subtle blue tint, especially in situations where UV light is common, such as daylight.

The Observer

Possibly the most important aspect of color perception is the observer. After all, our entire goal in reproducing color is to make sure that the images we create match our vision.

The human eye is a wonderfully complex and sensitive instrument. Since it is always with us, we may not notice how much it is affected by sleep, stress, medications, alcohol, and numerous other factors. To get the most out of your eyes, try to avoid eye strain, get plenty of rest and give yourself frequent breaks when you are making color critical judgments. If you are curious how your color vision measures up, try the Farnsworth Munsell 100 Hue Test.

However, the brain also plays a critical role in interpreting the colors we see, and this interpretation can be subject to situational factors, cultural influences, and even the emotional connections we have with certain colors. If you’ve ever heard a color described as “warmer,” “happier,” or with “more pop,” you’ve experienced the emotional connection that we make to certain colors.

One of the most interesting color perception effects is color constancy. When we know what colors to expect, our brain automatically interprets the color information from our eyes to that color. For example, if we see a white horse standing under a green tree, our brain automatically ignores the green that is reflected onto the horse. We know that horses are not green, so our brain makes a quick shortcut to interpret the color as white. However, our imaging devices do not have the same adaptations, and the result is that the colors we capture may look drastically different from our memories.

Most people see colors in opponent pairs of light-dark, red-green, and yellow-blue. If you’ve looked at a stereographic optical illusion (most commonly known as Magic Eye®) you have experienced the opponent pairs at work. For example, if the primary color in that image is yellow, your yellow vision will gradually fatigue and the blue secondary image will become visible, creating a 3D effect. Similarly, when you are viewing colors in your image editing workflow, allow yourself breaks to keep your color perception accurate.

People are well adapted to notice when color is not as they expect. By having an understanding of the many complex factors that go into human color perception, you can set yourself up for successful color reproduction throughout your imaging workflow.

The CDLC contributors are compensated spokespersons and actual users of the Canon products that they promote.

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