December 2021

F amous scholars such as Leonardo da Vinci and Isaac Newton have already tackled this question in the late Middle Ages. State-of-the-art colour measurement technology not only answers this question but also offers solutions for industrial applications. 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, as even these can be perceived by the human eye. Painted car parts, printing matter or facade panels are just some examples of the many products whose colour must be inspected during production. Human colour perception Colour inspection in industrial applications 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 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 significantly 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 Process, Controls & Plant Focus on: Sensors & Measurement 30 | Plant & Works Engineering www.pwemag.co.uk Annual Buyers’ Guide 2022 Determination, measurement and detection of colours in industrial applications are complex measurement tasks. Compared to other physical quantities, it is difficult to define colour. Glenn Wedgbrow* reports. A colourful challenge for measurement technologies When measuring colours, everything is about comparing the measured colour with the actual colour perceived by the human eye