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Seeing Eye to Eye: Overview of Spectrophotometers

(April 2003) posted on Tue Apr 01, 2003

Here's a look at how they work, what they do, and how you can use them to avoid color conundrums in your screen-printing operation.

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By Ben P. Rosenfield

Is a color hot pink or flamingo? Forest green or hunter green? Where an ink mixer sees sky blue, does the customer see powder blue? The labels humans use to identify the world of color reflect the subjective nature of color perception and hint at the challenges a screen printer faces in matching colors correctly. The printer's problems only escalate when we consider the factors that influence perception, from viewing environment to metamerism--the phenomenon by which two color samples may appear identical under some lighting conditions, but not others. These obstacles can make accurate color matching as futile as searching for the pot of gold at the end of a rainbow.

But the gold is well within reach, thanks to the spectrophotometer. Spectrophotometers, or "spectros" as they're referred to by their manufacturers, are designed to evaluate colors in ways similar to the human eye. But unlike subjective human perception, spectrophotometers measure colors by their spectral values, which remain constant regardless of lighting condition.

Spectrophotometers measure color reflectance quantitatively within the visible spectrum (wavelengths of approximately 360-750 nanometers). The devices report color values as numbers in a three-dimensional color space (e.g., L*a*b), thereby transcending vague color names such as midnight blue and candy-apple red. And when used with modern color-management software, these tools allow you to store and recall color information on a computer, making it possible to deliver accurate colors time and again on repeat orders.

How spectrophotometers work

Reflective spectrophotometers are the most common form (tranmissive models will be discussed later) and are typically solid-state devices that rely on a small number of complex parts to do their jobs. Inside these spectrophotometers, you'll find a light source--generally a xenon-flash bulb or bank of super-bright light-emitting diodes (LEDs), a holographic defraction grating that separates the light reflected from the color sample, a diode-based detector array that measures the amount of light coming from the defraction grating, and the circuitry that handles the mathematical tasks involved with color measurement.

During spectrophotometer operation, the unit uses its light source to illuminate a color sample, scans and records the spectrum of reflected color, and provides readings that relate to how humans view color. The device can also provide readings that predict how the color sample would appear under different light sources, such as fluorescent lamps, tungsten-filament bulbs, or daylight.

"Any color we can perceive can be quantitated," explains Hal Good, director of marketing services for Hunter Associates Laboratory. "The eye can perceive nearly 10,000,000 shades of color--very sensitive, but not repeatable or reprodu-cible--and that's why these instruments are used."

Three distinct optical geometries are most commonly employed in the design of spectrophotometers: integrating sphere, 45/0 (also known as bidirectional), and multiangle. In general, the sphere type is the most useful for screen-printing applications.

Spectrophotometers designed with sphere geometry illuminate samples diffusely--from all angles equally--and pick up measurements about 8


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