The importance of measuring colour in industrial processes

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

In 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 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 co-ordinates: 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%) 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 curves received can be converted and output in the desired colour space. “White referencing” makes it possible to largely eliminate any ambient influences onto the measurement result.

Sensors operating according to the spectral technology are colour measuring sensors.

Application examples

Colour measurement is required in many industry sectors, enabling companies to select, monitor, differentiate, grade and sort various types of coloured objects involved in manufacturing, automated handling and other production processes. Here are some examples:

Colour recognition for seam stitching in automotive interiors

In the assembly line of a car manufacturer, car interior parts are distinguished on the basis of different seam stitching colours. Micro-Epsilon colour sensors automate this process. Due to their high measurement resolution (colour distance ΔE = 0.5), these true colour sensors easily distinguish colour shades that look virtually the same to the human eye. 

Inline detection of protective film on PVC window frames

After their extrusion, a protective film is applied onto the PVC frame profiles. This film protects the frames from scratches and dirt. Applying the transparent protective film changes the colour of the window frame slightly (colour change of ∆E = 0.1). The colorCONTROL ACS7000 colour measuring system checks if the protective film has been applied, and whether it is applied correctly. 

Colour measurement of tablets in pharmaceutical production

In vitamin tablet production, different ingredients are used. The tablet colour depends on the concentration of ingredients and might vary from pure white through to beige and yellow. The colorCONTROL ACS7000 inline colour measuring system with a 30° sensor head accurately measures the slightest colour differences, particularly those finely-graded colour shades between white and beige. This colour information provides a statement about the quality of the ingredient composition.

Colour detection of furniture and kitchen fronts

Kitchens are available in many different colours. In order to ensure homogeneous colour application throughout several production batches, the colorSENSOR inspects the colour of kitchen fronts in the painting plant to recognise the slightest of colour deviations which are imperceptible to the human eye. The inspection also covers colour fluctuations that might occur over the course of time.

Inline colour measurement of injection-moulded plastic parts

In plastic injection moulding, the exact colour shade of the products is important. As the colour changes during cooling, it was only possible to determine the colour via random checks of cooled parts. The colorCONTROL ACS colour measuring system inspects the products as they are extracted from the mould. The system uses an empirically determined correlation of the colour between warm and cold pieces. This enables any colour deviations to be recognised at an early stage, thus avoiding waste. 

Colour and intensity tests of vehicle lights

After the assembly process, vehicle lights are tested in terms of colour and intensity. Homogenous distribution of light should also be ensured with fluctuating LED batches. The LED colorCONTROL MFA Analyzer performs this colour inspection. Fibre optics enable the lights to be measured at different points simultaneously, which ensures that every single LED is tested. With the colorCONTROL MFA-7, between 7 and 28 measuring points can be tested at the same time.

Inline colour measurement of transparent films

As well as colour fluctuations, streaks can occur during production. The colour homogeneity of transparent film strips is monitored inline without making contact. As transparent films are translucent, the colour is measured in transmission, i.e. a transmitter illuminates the receiver and the film in between. This is possible using the high speed, high precision colorCONTROL ACS7000 inline colour measuring system, which is connected to a transmission sensor head (ACS3) comprising a transmitter unit (tt) and a receiver unit (TR). This system enables the early detection of any slight changes in colour and streaks, allowing production parameters to be modified accordingly. 

Colour measurement of liquid paint

Measuring the colour of liquid paint is extremely challenging. To date, paints have been typically applied on a test area first and then tested only after the drying process. If the colours deviate, the paint containers must be mixed again or even disposed of, which partly involves long waiting times. In order to accelerate this process, Micro-Epsilon has developed a system that enables the measurement of liquid paint. In colour measurement technology, the spectral distribution of the reflected light depends on the distance. Therefore, even changing distances larger than 0.05mm will influence the measurement results. However, the height tolerance with which paints can be filled into the sample container is limited to ±2mm, which means distance measurements and controls are required to achieve high precision and reproducible measurement results. This application therefore includes the colorCONTROL ACS7000 colour spectrometer for colour measurements and the optoNCDT 1420-50 laser distance sensor. A linear unit offers automatic readjustments. This ensures the correct distance between the colour sensor and target.

Marking detection on cosmetics bottles

Colour sensors from Micro-Epsilon are not just used for colour measurements but also in detection, testing or positioning tasks. When automatically printing on semi-transparent glass ceramic bottles, it is necessary to determine the exact position for the printing. Before the printing process, a position mark is embossed onto the bottles. The colour of the embossment deviates slightly from the rest of the bottle surface. The colorSENSOR CFO100 detects this colour difference, which enables the exact determination of the printing position. If the marking is missing, the bottle is considered ‘faulty’ and will be rejected immediately. Thanks to its high speed measurement frequency, the True Color sensor outputs a complete OK / NOK signal within the specified cycle time of less than two seconds. Its small measurement spot size with a diameter of just 0.6mm ensures reliable and precise embossment detection.

Glenn Wedgbrow is Business Development Manager at Micro-Epsilon UK.