Test & measurement
MEASURING COLOUR IN INDUSTRIAL PROCESSES
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n many industries, particularly consumer goods, colour implies quality. For any goods that arrive at the final customer, the exact colour shade affects the value and identity of the product and the brand. Colour not only leaves behind an impression of quality but can also be used as an indirect quantity to control the process. For example, colour sensors are used to monitor the presence of adhesive beading, to sort parts or to determine active ingredients. In most cases, colour sensors are the more cost- effective solution as they provide more precision than conventional switching sensors.
VARIOUS SENSOR TYPES
In physical terms, colour is a reflected intensity spectrum in the visible wavelength range. This reflection spectrum depends on the object colour and the illumination. Illumination is defined by different light sources, e.g. light bulbs, daylight, fluorescent lamps or cold white LEDs. A colour sensor detects the reflected spectrum and imitates the principle of the human eye. The ability of a colour sensor to determine or measure the colour presented to it is dependent on the detection technology used in the sensor.
RGB sensors Narrow-band colour filters divide the received light spectrum into the three colour coordinates: Red (R), Green (G) and Blue (B). The sensor evaluates the intensity of the respective colour
In many industries, colour implies quality. It is therefore crucial to match the correct colour shade in the production process and to produce it homogeneously throughout numerous batches, says Glenn Wedgbrow, business development manager at Micro-Epsilon UK.
coordinates and outputs them as an analogue signal. The three colour values are often combined into one displayed digital value and therefore have only little information value about the colour detected. Furthermore, an RGB sensor cannot separate colour information and brightness, i.e., changing the brightness also changes the displayed colour. Also, the RGB filters only cover a small portion of the spectrum and thus not all colours can be seen.
True Colour (XYZ) sensors for precise colour recognition
Three high-resolution colour filters divide the received light spectrum into the following three colour coordinates: X = long-wave, Y = medium- wave and Z = short-wave. Similar to the cones in the human eye, these colour filters image the entire spectral sensitivity of the eye. Due to these broad-band colour filters, the wavelengths of the light spectrum are imaged in more detail and contain more information about the colour composition. Based on stored standard formulas, the values are converted into a standardised colour space (e.g. L*a*b*). Due to this
standardisation to L*a*b*, changing the brightness (e.g. fluctuating distance between sensor head and target) has only little influence on the colour value. True Colour sensors, such as the Micro- Epsilon colorSENSOR CFO series, are designed for relative colour inspection and are ideal for recognising deviations from a reference colour.
Colour spectrometer for high precision Another functional principle is spectral technology and is applied by colour measuring systems such as the colorCONTROL ACS7000 from Micro-Epsilon. With a grid, the received light is refracted into individual wavelengths and projected onto a CCD line. Each of the 256 measurement pixels of this line is assigned to a certain wavelength. Based on interpolation, an intensity value (0-100 per cent) of the received colour wavelength is then output for each pixel. This allows for the computing unit to image and output the full spectral curve of the visible light. Based on its spectral curve, the colour obtained can be clearly identified. With the freely selectable standard observers and illuminants stored in the controller, the values and spectral
May 2024 Instrumentation Monthly
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