TRANSDUCERS, TRANSMITTERS & SENSORS SHINING A LIGHT ON COLOUR SENSING


It is difficult to define colour, but now colour measurement sensors can do just that, offering solutions for industrial applications, says Glenn Wedgebrow, business development manager, Micro-Epsilon UK


measurement tasks. Colour is a great challenge for industrial measurement technology since it is an individual, visual sensation elicited by light. When measuring colours, everything is about comparing the measured colour with the actual colour perceived by the human eye. The crux of the matter in most applications is to detect very slight deviations in colour. Painted car parts, printing matter or facade panels are just some examples of the many products whose colour must be inspected during production. Colour inspection in industrial applications


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requires suitable sensor technology that corresponds to the colour perception of the human eye. The latter is invoked by light in the wavelength range of 380nm to 780nm. The human eye has different sensor cells: 120 million rods for light/dark differentiation and 6 million cones for colour perception. There are three different types of cones,


each of which is receptive to a different wavelength range of the visible spectrum. Rods are much more sensitive than cones, which is why colour vision depends on illumination. Based on human colour impression, colours can be described differently. As there are three different cone types, the colour space consists of three dimensions. Since 1931, the CIE 1931 standard colour space stipulated by an international commission has ensured comparability in colour descriptions. This colour space is based on a study that examines the colour impression of the test persons. At the same time, parameters such as observation conditions and illumination were stipulated in order to ensure comparability of the colour measurements. In technical applications, the CIELAB colour space is more common. It is generated from the standard colour space by transformation. The coordinates of this colour space are ‘L’ as a measure of the brightness, ‘a’ (green/red saturation) and ‘b’ (blue/yellow


etermination, measurement and detection of colours in industrial applications are complex


saturation). The advantage of this colour space is that each hue perceived as a separate colour by the human eye has the same volume. The HSV/HSI colour spaces are rarely used. The RGB and CMYK colour spaces used with monitors and printing technology are smaller than the CIE standard colour space. This means they cannot image all colours that the human eye recognises and so are not suitable for precise, industrial colour measurement. In addition to the colour spaces, other


definitions are important. Therefore, regulations regarding lighting and observations distance were stipulated by the CIE commission. Another important parameter for industrial applications is the so-called colour distance, Delta-E, which is the distance between two colours in the colour space. Depending on the colour, the human perception is limited at 0.5 to 1. The automotive industry requires Delta-E < 0.1. When the observation geometry is


determined, colour is (in physical terms) 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. A colour sensor detects the reflected spectrum and imitates the principle of the human eye. For measurements, the reflected light is refracted into its spectral parts. The easiest method is to use filters where only one part of the spectrum can pass through. Most conventional CCD cameras whose sensors are divided into green, red and blue pixels are also based on this technique. A more accurate principle is used by colour


sensors for industrial applications such as the new colorSENSOR CFO series from Micro- Epsilon. The specimen is illuminated by a light source, which in most cases is a white light LED. The light reflected by the probe hits the sensor where the light passes through different filters and eventually reaches a light-sensitive


In order to ensure high quality standards, measurement of colour is an invaluable technique in numerous industrial applications. For example, in tablet production, the slightest colour deviations must be detected. In transparent blister packs, different colour shades would give a poor impression


sensor element. The absorption spectra of the filters used should overlap. The filters divide the light into long-wave (X), medium-wave (Y) and short-wave (Z) parts. The individual signals are then transformed into L*a*b* colour values. This is how measurement values are


generated which enables the assignment of colours according to the colour perception of the human eye. Therefore, we refer to these as perceptive colour sensors or true colour sensors. They are ideal for recognition of colour deviations. A teach-in function enables the user to program the desired colour on the sensor and to determine the maximum permissible colour deviation. Next, the sensor compares the colour of the products during operation and can for example send a signal via a digital output if the colour of the specimen is within the tolerance limits. Another functional principle is applied by


colour measurement systems such as the colorCONTROL ACS7000 from Micro-Epsilon. This divides the spectrum of the incident light via refraction on a grid into 256 parts, which are imaged behind the grid onto a CCD sensor line. This is how the complete visible spectrum can be accurately measured with a spectral resolution of 5nm. For more complex measurement tasks, for


example, with structured, highly reflective or shiny metallic surfaces, the ACS2 circular sensor with 24 circular arranged lighting optics at a measuring angle of 45°c:0° is used. This ensures continuous, homogeneous lighting for measurements, regardless of the angular position of the target object. For colour measurements of transparent objects such as glass, the ACS3 transmission sensor is required where illumination and receiver are arranged at an angle of 180°:0° to each other. This receiver sensor also measures colours of self- luminous objects. No lighting unit is required.


Micro-Epsilon E: info@micro-epsilon.co.uk


NOVEMBER 2021 | PROCESS & CONTROL 47


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