How We See Color

It is estimated that humans receive 70% to 80% of their sensory information through sight. This means they also make about the same percentage of their decisions based on what they see. A big component of this visual perception is color, and the color humans see is based on the color characteristics of the light being used. Although people perceive various light sources as “white light,” in fact, depending on the source, the light can be biased quite heavily toward the red/yellow, blue or green portions of the visual spectrum. These various sources will substantially shift color perception.

Although the human visual system has a broad range of color shades that can be perceived (the average observer can see over two million colors, while a trained observer can see over four million), the visual system also adapts to the lighting environment. It scans the scene and picks the lightest color, perceiving it as “white,” and all the other colors fall into place as necessary. Through the course of human existence, this has allowed people to identify a ripe red apple in the warm yellow-red glow of early morning and still see that same apple as red and ripe in the middle of the day, when the light is bluest. For survival, the human system adapts.

We see color because an object either absorbs or reflects light. If that object absorbs all the colors, we “see” black. If it reflects all colors, that object will appear white. If the object absorbs all colors but red, that red light is reflected and we see the object as red. As more colors are reflected, different colors are perceived. If the light source has a high amount of energy in one area of the spectrum--for example, blue--there will be more energy to reflect back to the eyes. In comparison to the other colors, a blue object will appear bluer under such a light when compared to a more equal energy light source with equal amounts of red, green and blue light energy.

This allows us to use the spectral output of the light to our advantage. For instance, when lighting the display case in a supermarket's meat department, using a source with larger proportions of red energy will make the meat appear “redder” than it would under most commercial fluorescent store lighting. The produce in the vegetable department benefits from a light source with higher amounts of green energy, such as the common cool white fluorescent lamp, which makes green vegetables appear greener.

Unfortunately, some light sources are not well suited for certain foods. For instance, a typical tungsten-style light, which has a great deal of red and yellow energy, has very little blue energy. Because of this, not only will it subdue blue, it also will make dark blue shades very difficult to see. Blueberries appear almost black under such a light.

Understanding Light

This graph shows us the light source has a great deal of red energy (600nm to 700nm), some green energy and very little blue energy. A typical commercial fluorescent lamp is pictured in the chart “Cool White Fluorescent SPD Curve” and shows it has very little red and blue energy but large amounts of green energy. The “spikes” in the graph are called “mercury spikes” and are a result of the small amounts of mercury used in a fluorescent lamp that allow the lamps to work. They are in all fluorescent lamps, and studies indicate they have little effect on overall color perception because they are so narrow. The graph shows there is some blue energy, very little red energy and large amounts of green energy. A typical spectral curve for a light source that simulates natural daylight is provided in the chart “Re-creating Light.”

Such graphs only tell part of the story. How well a light source renders colors is rated using the Color Rendering Index (CRI). For color evaluation and matching, a CRI of 90 or better is required.

A better indicator for daylight simulators is the International Commission on Illumination's (CIE, Vienna, Austria) Publication 51 rating. This is a very stringent test to determine how well a light source will render colors that are very close together. For critical color matching and evaluation applications, a rating of “BC” or better is required.

The condition in which colors that match under one light source but then do not match under another spectrally different light source is called a metamerism. It is not uncommon for objects using different types of dyes to achieve the same color to experience this problem. Two samples, say one plastic, the other ink-on-paper, may match in color under a specific light source, such as daylight. But when another light source is used, such as incandescent lighting, for instance, the samples no longer match. The effect can be insignificant, or it can be dramatic. One thing is for certain, if the samples are of two different types, or the color is achieved using different dyes or pigments, there will be metamerism to some degree.

Shedding Light on Foods

When performing the evaluation, assess the sample under the primary source and then switch to the secondary source to observe what happens to the color. Evaluators should note extreme color shifts, objectionable colors or color incompatibilities (such as between different color cake icings, for instance).

After making this observation, they should then switch between the secondary and the third sources, and then between the primary and third sources. This helps formulators develop an understanding of just how the product will appear when a consumer sees it in their lighting conditions. It is a good idea to evaluate foods with dyes using this type of light testing. As different dyes can be used for different products, color shifts or metameric differences are likely. Even natural foods or food colorants will produce color shifts that may be objectionable.

To make certain the test is accurate, each light source must have the same intensity, and a common neutral color surround is used. The surround is usually a medium gray color, commonly known as Munsell N7. The neutral surround is necessary to keep colors from the surrounding area from influencing both the light quality itself and the observers' color perception. Only the color of the object is to be in the viewing environment. The lighting environment described is typically found in a light booth available from various manufacturers.

Color is an important sensory sensation for many foods. Through color, consumers judge freshness, quality, consistency and, in some instances, even perceived flavor. To understand this sensation, processors need to understand light, and how it relates to color. No one wants a product that seems washed out.