Color sensor
Under color sensor (English Color Sensor ), a in the automation technology used signal forming optical device for the selective detection and measurement of the visible spectral range understood. The selective recording and evaluation of the visible spectral range provides suitable color measures for further processing . Typical names for color sensors are, for example, LAB color sensor , true color sensor or RGB sensor . Even print-mark sensor is a term for a particular color sensor.
Demarcation
Color measuring devices do not count among the color sensors in the sense of this definition . The main aim of color measurement devices is to record color measurements with high absolute accuracy. They mostly work on a spectrophotometric basis and are comparatively expensive. Furthermore, the pure primary color sensors (mostly photodiodes with upstream color filters ) are not counted among the color sensors, as they are not independent devices.
Devices similar to color sensors are called electronic color fans and are used to determine color numbers or color names according to a fixed color system ( e.g. RAL or Pantone ). These are hand-held devices for simply displaying the color numbers or names.
Areas of application
The areas of application of color sensors are diverse. Important areas of application can be found in quality control and process control . Quality control includes, for example, the testing of paints, varnishes, primers , greases or other coatings applied to the surface . In addition, presence checks (seals, lids, closures, locking varnishes , fastening elements ) are carried out in the area of quality control with color sensors . The review of light sources on color and brightness (eg. LEDs in Automotive - interior ) is one of the quality controls.
Color sensors can easily be used for process control. The high processing speed of the color sensors in the 10 kHz range is an advantage in automation technology. This means they can be used for faster production processes. In the printing area, for example, colored print marks are evaluated to control offset machines . Often parts are marked in color in order to be able to differentiate the quality of the parts according to “good / bad”. Color sensors recognize the colored markings and trigger a control process in the process.
functionality
Color sensors mostly work according to the three-range method . The spectral curve shape of the three-range filters used largely corresponds to the standard spectral value functions . Modern color sensors work perceptually . This means that they evaluate color differences according to the color perception of a human observer. The basis for this is the higher colorimetry.
For object lighting, broadband light sources (mostly high-performance white light LEDs) are used with color sensors. Photodiodes with integrated color filters are used as detectors. The color filters work on an interference basis . The photocurrents are converted into voltages by means of a transimpedance converter, filtered and amplified. Digitization then takes place. In a microcontroller, the digital values are converted into color dimensions and transformed into suitable perception-based color spaces . This is followed by a comparison of the current color index with tolerance values stored in a table or other comparison color indexes in the microcontroller. The result of the comparison provides a binary switching decision. This process is also known as color recognition . For the robust function of the color sensor, the extraneous light is eliminated from the measurement environment using suitable modulation methods and existing drift phenomena are compensated.
Earlier color sensors often used a broadband photo receiver and therefore worked with various single-color light - emitting diodes as the light source , which had to be switched on sequentially. Disadvantages here are a process-related lower processing speed and a limited color selectivity of these color sensors due to the limited spectral properties of the light-emitting diodes.
literature
- DIN 5033: color measurement . (Part 6 three-area procedure, August 1976).
- Manfred Richter: Introduction to colorimetry . 2nd Edition. Walter de Gruyter, Berlin, New York 1981, ISBN 3-11-008209-8 (Chapter 17, Higher Colorimetry).
- Ansgar Wego, Gundolf Geske: Correct recognition of colors and surfaces with color sensors . In: Photonics . No. 5 , 2010, ISSN 1432-9778 , p. 38-42 .