Measuring Color

The best way to measure opaque liquids, solids, pastes, or powders is to use a 45/0 degree geometry instrument with a horizontal sample port. A liquid sample can be poured into the sample cup and measured. Blocks of cheese or slices of meat can be placed directly to the sample port aperture. With a circumferential illumination and a large measurement port, flakes, chips, and/or chocolate disks can be measured. A QC may have a standard target color that must be repeatedly manufactured by the production team. Colorimeters are ideal when the standard and measured batch are non-metameric, e.g. production batches. Natural colors such as chlorophyll, carotenoids, and anthocyanins can be measured in a colorimeter and quantify the pigments present in a food.

An inline color monitoring system mounted over a production line can give real-time data. Translucent samples will pose a concern and a “ring and disk” assembly is used to measure this type of sample. Brewed tea, for example, can be poured into the transmission compartment and a reading can be obtained.

The amounts of red, green, and blue needed to form any given color are called the “tristimulus” values, X, Y, and Z, respectively. The measurement is expressed in terms of X-Y-Z and the user can pinpoint the differences in lightness, chromaticity, and hue between the target and the sample. The color measurement taken in one location can be compared with another location or a different time in an internationally accepted terminology. This eliminates color perceptions and judgmental differences between technicians.

The Commission Internationale de l’Eclairage (CIE) defined the color of an object on three primary stimuli: red (700 nm), green (546.1 nm), and blue (435.8 nm). Sometimes, tristimulus systems of representation of colors are not easily understood by the users in terms of object color. Other color scales, therefore, were developed to relate better to how we perceive color, simplifying the overall understanding.

A three-dimensional rectangular L, a, b, color space uses L (lightness) axis – zero is black and 100 is white; a (red to green) axis – positive values are red, negative values are green, and zero is neutral; and b (blue to yellow) – positive values are yellow, negative values are blue, and zero is neutral.

There are two popular L, a, b color scales in use today: Hunter L, a, b and CIE L*, a*, b*. They are similar in organization, but will have different numerical values. Hunter L, a, b and CIE L*, a*, b* scales are both mathematically derived from X, Y, and Z values. Hunter scale is over expanded in the blue region of color space, while CIE scale is over expanded in the yellow region. The current recommendation of CIE is to use L*, a*, b*.

A spectrocolorimeter is a hybrid instrument that gives data such as X, Y, and Z or CIE L* a* b* values. These are priced similar to the spectrophotometer. They are basically a spectrophotometer except that it does not output spectral data (%R) at various wavelengths. They are mostly QC lab type instruments.

Spectrophotometers measure light reflected, transmitted, or absorbed from a food product to a known standard. They have more sensors and measure spectral reflectance of an object at each wavelength on a visible spectrum continuum. They work best for liquid samples. A specimen is exposed to light and the reflected light waves are displayed as a curve on a graph. The size and shape of the curve is called a reflectance curve and is unique to each color.

Reflectance measurement (reflectance factor) is basically a reflectance of a food sample at a given wavelength compared to reflectance of the perfect diffuse white measured under the same exact conditions. The reflectance color measurements are more rapid. These are expressed as %R. If transparency of a dye solution is measured, it is denoted as %T. This quantity is equal to the percent of light at a given wavelength, transmitted through a thickness of 10 millimeters.