Design and Applications: Compact, low-cost instrument measures color

Alexander E. Martens and Cornelius J. McCarthy

The human eye discerns small color differences, but few people can remember a specific shade of color. Consequently when someone buys matching paint for a wall, he or she has to bring along a color chip and try to match it with a color sample in a store. Paint manufacturers use specific codes to identify product colors, thereby enabling employees in stores to add colorants to a base paint and achieve a desired color. Instruments are also available that measure and match colors accurately and report the results in internationally accepted units or in manufacturers' color codes.

Many of these instruments use color filters or diffraction gratings, tungsten light sources, and photomultipliers, making them large, heavy, expensive, and suitable for use only by skilled personnel. The availability of a wide variety of light-emitting devices (LEDs) that generate light covering the entire visible spectrum, however, has made it possible to develop a hand-held, battery-operated colorimeter for color measurement and color matching that is smaller, lighter, more rugged, less expensive, and easier to use than most traditional instruments of comparable performance.

RIGHT. AccuProbe Colorimeter uses LED-based technology to simplify color measurement tasks for a variety of applications, such as choosing a paint color to match wallpaper.

In most cases, the newer technology allows a user to simply turn on the measuring colorimeter, place it on a color surface to be measured and press one button (see photo). The colorimeter can then identify the color by manufacturer's code or provide color identification in one of the internationally accepted color designations, such as X, Y, Z color coordinates of the CIE chromaticity diagram, R, G, B tristimulus values, or paint formulations. An RS232 interface kit allows the colorimeter to be connected to a personal computer to take advantage of software packages providing features that include color quality control, on-screen color rendition, and statistical process control as related to color.

Once the colorimeter has measured and recorded the color characteristics of the sample, this information can be used to identify the close and neighboring colors in a vendor-specific color set, such as a paint fan deck, to advise if the color is within tolerance of a pre-established standard, or to provide a formula for mixing the measured color or correcting a mixture. These operations can be performed with the instrument alone or in conjunction with a personal computer. For subsequent processing, the instrument remembers measurements taken during portable operation, and it can be controlled by an external computer using standard RS232 communications.

The heart of the colorimeter is an integrated color-sensing module that contains a complement of LEDs, photodetectors, a temperature sensor, and optics. The color of the LEDs and the spectral response of the photodetectors are selected to provide a broad spectral range covering the visible spectrum of 400 to 700 nm and high sensitivity. The range can be expanded to include the near-IR. Depending on the output energy of a particular LED, more than one of these devices of a given color can be included, as is the case, for example, when multiple middle-green LEDs are used in current designs, as these emitters have lower efficiency. A thermistor in the module is used to sense the temperature of the substrate on which the LEDs are mounted and provide an input to a control circuit that compensates for the effects of temperature fluctuations, thus ensuring high stability of the operation.

Spectral characteristics of LEDs and detectors are used to achieve good color discrimination. The spectral response of each combination of emitter type, sample, and detector is the product of their respective spectral properties. Thus, based on the spectral characteristics of each photodetector and LED, the color of the object can be accurately determined using proprietary algorithms.

In operation, the LEDs are sequentially and very rapidly turned on and off, and the emitted light is directed to the object's surface. The reflected light is detected, correlated to a particular LED, and subsequently quantified. The geometry of the optics in the color-sensing module conforms to the internationally accepted standards for color-measuring instruments. The size of the entrance aperture can be selected depending on the application. The smallest aperture is only 3.5 mm in diameter; hence, very small samples can be measured.

The best-developed current application area for the colorimeter is in color identification and matching at retail paint counters (see box for other uses). A set of preprinted color chips and formulas is established for each paint line by the paint companies. Using the formula that defines how much of each basic colorant has to be added to the base, the paint is mixed.

Often, however, the customer may want a color to match a competitor's chip, a wallpaper sample, or other material. Sometimes also the existing formula may result in a color unacceptable to the customer. In such cases the color can be quickly matched with a colorimeter interfaced with a personal computer. Colorimeters based on LED technology do not require frequent calibration and are light and portable.

ALEXANDER E. MARTENS is a technology consultant; e-mail: [email protected]. CORNELIUS J. MCCARTHY is president of Accuracy Microsensors, 1800 Monroe Ave., Pittsford, NY 14534; e-mail: [email protected].

Colorimeters useful here

Point-of-sale: for color identification in paint, fabric, wallpaper, cosmetic, and other retail stores where color is a major factor

Design: for use by professionals such as architects, interior designers and decorators, and artists, also in industrial laboratories involved with color

Industrial: (using the color sensor module) in process control where color is an indicator of the status of the process or product quality, for example, chemical and plastic industries, textile and carpet mills, food-processing companies, paper mills, and others

Medical: especially in dermatology and dentistry, where color often is an indicator of a disease or where color needs to be matched to make naturally appearing prostheses, such as dentures

Reprographics: (using the color sensor module) in color printers, scanners, facsimile machines, and copiers to achieve reliable and true reproduction of color, irrespective of ambient temperature, humidity, paper, and toner, by providing real-time feedback to the control systems and allowing for correction of color reproduction with respect to the input color