Colour, one of the most influential attributes of light, has a distinctive importance in various industries and scientific applications. The colour of a material can be used to evaluate the properties of that material. Colour sensors are employed to recognise/detect the colour of a material in RGB (red, green, blue) scale, while rejecting the unwanted infrared or ultraviolet light. The ultimate challenge with colour sensing has been to detect subtle differences among similar or highly reflective surfaces. Fortunately, the advances in electronics, optics and software technology have led to the development of colour recognition techniques that involve outputting the reading intensity and colour value. These colour recognition systems are deemed highly suitable for quality control applications in various industries, such as food, automotive, glass, manufacturing, for ensuring high productivity and cost reduction.
A typical colour sensor comprises a high-intensity white LED (light-emitting diode) that projects a modulated light onto the target. The white light holds a mixture of three basic colours having different wavelengths as mentioned above. These colours combine with one another to form different other shades of colours. When the white light falls on any surface, based on the properties of the surface material, some of the wavelengths of light are absorbed and some are reflected. A human eye detects the colour of the material when these reflected wavelengths fall on it.
Colour recognition systems based on white LED allow for a greater spectrum evaluation than those based on RGB LED. Apart from LEDs, there are various integral light sources such as fibre optics, lasers, and halogen lamps that can be used in the design of colour sensors.
Colour sensors are developed based on diffuse technology that can detect a wide range of colours. The combination of colour sensitive filters and sensors array perform colour sensing, which is further used to analyse the colour present in an image or in a specified object. The colour measurement process involves a light source to illuminate the surface, the target surface, and a receiver that measures the reflected wavelengths. A white light emitter is used to illuminate the surface. The sensor then activates three filters with three wavelength sensitivities to measure the wavelengths of RGB colours respectively. Based on these three colours, the colour of the material is determined.
Modern colour sensing has seen the involvement of fibre optics in the colour detection process. In this technology, the light transmission to the object and back depends on the optical glass fibres, which operate on the principle of total internal reflection. This phenomenon causes fibre to act as a waveguide and enables the complete reflection of light. In a fibre optic colour sensor, the white light spot is projected via the fibre optic onto the target surface; the part of light that is reflected back from the target is directed onto the detector via the same optical fibre. The reflected light is then separated into long, medium and short-wave light components. and transformed into L*a*b* colour coordinates.
In order to measure the difference between two colors, the distance within the color space is measured. The color distance can be determined using the following equation:
ΔE = √ (L*₁-L*₂)² + (a*₁-a*₂)² + (b*₁-b*₂)²
Here, L*₁-L*₂, a*₁-a*₂ and b*₁-b*₂ denote the distance between the color places in a color space and ΔE is the required color distance.
The value of ΔE is further categorized on the scale of color difference. For example, ΔE < 0.1 implies that the colour difference are negligible. This is generally the standard used in the automotive industry. ΔE > 5 is considered a noticeable difference and is often not tolerated in almost all type of applications.
Colour sensors are majorly used to grade coloured products, distinguish coded markings, detect the presence of adhesives or data codes on a package. The technology has a wide range of applications in various industries such as textile, automation, automotive, food, printing, pharmaceutical, and many more. These sensors can also be programmed to identify any one colour or multiple colours for sorting applications, based on the level of sophistication required in the colour measurement process.
In most industrial applications, colour sensors are mainly used in the quality control stage as visual inspection tools. For example, in the food industry, colour sensors are used for monitoring colour changes in the plastic in which the meat product is wrapped to detect whether the meat quality has deteriorated. It can also be used for process control such as in controlling the temperature to achieve ideal roasting time for coffee bean.
In the textile industry, colour sensors can determine whether the colour of the manufactured fabrics match the reference. This is a useful tool as some colour may not be easily distinguished by visual observation due to lighting. In the manufacturing sector, the growth in demand for smartphones and tablets also require colour sensors to be used to check whether the colour of the finished products match the advertised colour.
Spectral sensing has recently been emerging as a new trend in the colour measurement market. These sensors are used in certain specific applications such as analysing colour of coffee beans during roasting, measuring highly reflective surfaces and lustrous metals, measuring self-luminous and transparent surfaces such as foil, glass or plexiglass, and other inline colour measurement applications. Other applications include the recognition of anti-reflection coating on lenses, comparison of colour of parking sensor and car body, colour detection of kitchen fronts, colour and intensity tests of vehicle lights and many industrial applications.
To sum it up, colour sensors have a long way to go in production and quality assurance processes, mainly in low-volume manufacturing applications. The powerful sensing device can be integrated in automation systems reducing the amount of human intervention required for operations. With the advanced technology and the availability of memory loaded with colour intensity data, the controller enables evaluation and storage of unlimited number of colours virtually.
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