Camera measures color precisely

OCTOBER 31--Several vendors, such as DVT Corp. (Norcross, GA) and Cognex Corp. (Natick, MA), have announced additions to their smart-camera lines that include models specifically developed for capturing and processing color images.

Oct 31st, 2001

OCTOBER 31--The use of color-based camera systems can be more effective than monochrome cameras in industries such as printing and textiles, where specific colors must be precisely measured and matched. To address these requirements, several vendors, such as DVT Corp. (Norcross, GA) and Cognex Corp. (Natick, MA), have announced additions to their smart-camera lines that include models specifically developed for capturing and processing color images. These smart cameras use CCD sensors to capture RGB images that are then processed using color-image-analysis software.

"While RGB cameras are not good absolute-measurement devices such as the spectrophotometer," says Ali Zadeh, a senior research-and-development engineer at DVT, "they are good relative measurement devices. You can use these cameras to find a good sample and monitor it for any changes, so that when a light blue changes to a dark blue, for example, you will know."

To allow such quantitative measurements to be made precisely, the Commission Internationale de l'Eclairage (CIE; Wien, Austria) developed a color model based on how human beings perceive color. Known as the XYZ Standard Observer, the model uses three stimulus values that provide the basis for an objective description of colors. With known tristimulus values, both the color and the luminance (or brightness) of the object can be described unambiguously.

To obtain these color values, Delta Danish Electronics, Light & Acoustics (Lyngby, Denmark; www.delta.dk), has developed an Imaging Color Analyzer Module (ICAM) that is both a digital camera and a color meter. Using an off-the-shelf CCD chip, Delta uses a number of filters in a filter-wheel configuration to obtain the tristimulus color values.

Delta initially measures the monochrome CCD spectral sensitivity. Then, custom filters are developed that map the spectral sensitivity maps to the three tristimulus x-y-z values of the CIE standard observer. Once complete, each filter wheel can produce the exact x, y, or z value that is then digitized by the monochrome camera. A patent has been filed on the optical-system configuration in the camera and for its use in a wider scope.

With a resolution of 1024 x 768 x 8 bits, the camera is capable of capturing one color image in about 2 s. To interface the camera to a PC-based system, the ICAM features a FireWire interface and has been qualified with the IEEE 1394-based Meteor II/1394 PCI-based frame grabber from Matrox Imaging (Dorval, Quebec, Canada). According to Jens Jorgen Jensen, Delta product specialist, the company is already seeing use in print and textile applications, where real-time requirements are not a necessity.

For more information, see the November 2001 Vision Systems Design.

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