Small, inexpensive, hand-held optoelectronic color-measuring devices based on metal-film/dielectric-film interference filters are undergoing development. These color meters could be suitable for use in a variety of applications in which there are requirements to quantify or match colors for aesthetic purposes but there is no need for the high spectral resolution of scientific-grade spectrometers. Such applications typically occur in the paint, printing, and cosmetic industries, for example.

The figure schematically depicts a color meter of this type being used to measure the color of a sample in terms of the spectrum of light reflected from the sample. Light from a white source (for example, a white light-emitting diode) passes through a collimating lens to the sample. Another lens collects some of the light reflected from the sample and focuses the light onto the input end of optical fiber. Light emerging from the output end of the optical fiber illuminates an array of photodetectors covered with metal/dielectric-film interference filters like those described in "Metal/Dielectric-film Interference Color Filters" (NPO-20217), NASA Tech Briefs, Vol. 23, No. 2 (February 1999), page 70. Typically, these are wide-bandpass filters, as shown at the bottom of the figure.

Several Photodetectors Covered by Filters measure incident light in several wavelength bands. In this example, there are three broad wavelength bands corresponding approximately to the primary additive colors.

The photodetector array need not be of any particular design: it could be something as simple as an assembly containing several photodiodes or something as elaborate as an active-pixel sensor or other imaging device. What is essential is that each of the photodetectors or each of several groups of photodetectors is covered with a metal/dielectric-film filter of a different color. In most applications, it would be desirable to have at least three different filters, each for a spectral band that contains one of the three primary additive red, green, and blue colors. In some applications, it may be necessary to have more than three different color filters in order to characterize subtle differences in color (or in the sensation of color) that cannot be characterized with sufficient precision by use of the primary colors alone.

This work was done by Yu Wang of Caltech for NASA's Jet Propulsion Laboratory. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Innovative Technology Assets Management JPL Mail Stop 202-233 4800 Oak Grove Drive Pasadena
CA 91109-8099 (818) 354-2240 E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Refer to NPO-30858.

Topics:
Assembling Coatings, colorants and finishes Coatings, colorants, and finishes Fiber optics Imaging and visualization Light emitting diodes (LEDs) Manufacturing processes Optics Photonics Research and development Sensors and actuators Spectroscopy
This Brief includes a Technical Support Package (TSP).
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Hand-Held Color Meters Based on Interference Filters

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    Photonics Tech Briefs Magazine

    This article first appeared in the May, 2004 issue of Photonics Tech Briefs Magazine (Vol. 28 No. 5).

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    Overview

    The document is a Technical Support Package from NASA's Jet Propulsion Laboratory (JPL) detailing the development of hand-held color meters based on interference filters. It introduces a novel, low-cost, and compact color-measuring device that leverages thin metal film interference filters and digital tunable filters to enhance color matching capabilities across various applications.

    The primary motivation for this innovation stems from the limitations of existing color measuring devices, which typically use three dye color filters. These traditional filters often fail to provide accurate color matching due to their differing interactions with human vision and the insufficient number of colors for high-quality results. In contrast, the new device employs thin metal film color filters that can be designed to transmit specific wavelengths, allowing for more precise color matching. The flexibility of using multiple filters—three, four, or even more—further enhances the device's performance.

    The document outlines the technical aspects of the device, explaining how it operates. Light from a white LED is collimated and directed onto a sample, with the reflected light collected and transmitted through an optical fiber. This fiber acts as an optical integrator, ensuring uniform light distribution before it reaches an active pixel sensor equipped with thin metal color filters. The sensor captures the intensity of the reflected light, providing detailed color information that can be analyzed for accurate color matching.

    Applications for this technology are broad, including the paint and printing industries, cosmetic dentistry, and the cosmetic industry itself, where matching shades of foundations and other products is crucial. The document emphasizes the importance of accurately quantifying human color vision, referencing the CIE (Commission Internationale de L'Éclairage) standards for primary colors, which are essential for achieving a full spectrum of human color perception.

    In summary, this Technical Support Package presents a significant advancement in color measurement technology, offering a practical solution for industries that require precise color matching. By replacing traditional dye filters with advanced thin metal film filters, the hand-held color meter promises improved accuracy and versatility, making it a valuable tool for various commercial applications.