Understanding non-contact colour measurement

Colour measurement sensors have been used for several years in R&D and production environments. However, in recent years significant improvements in measurement speed and accuracy have created many more uses for these sensors. Therefore, to successfully utilise colour sensors, it is important to understand how sensors measure colour, says Stephen Smith, Product Sales Engineer at Micro-Epsilon UK.

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, warehousing, packaging, wrapping and other production processes. Measured objects can take the form of solids, powders or liquids and it is also possible to measure the colours of transparent or translucent surfaces such as coatings, labels and plastics, as well as LEDs and glass objects.

How is colour measured?

The human eye has three colour receptors, which is why 3 Dimensional colour models are used to clearly identify colours and to compare these with other colours. When defining a colour of an object, the term ‘colour space’ is often used. One of the most frequently used colour space models is the L*a*b* colour space.

The L*a*b* colour space comprises all colours perceptible to the human eye. In this 3D colour model, the L-axis is a correlation of lightness, the a-axis relates to redness and greenness, while the b-axis describes the colours from blue to yellow. The L*a*b* colour space serves as a device or sensor-independent model. The colours are defined regardless of their method of production and their output technique.

Colour distance and Delta E

A colour sensor compares colours, or more accurately, checks if colour values match. The measurement object is illuminated using a light source (normally LED) and the reflected colour components are then evaluated. The colours of the inspected object can be taught to the sensor and stored in a colour memory. These taught colours can be assigned acceptable deviation tolerances. The stored colour values are compared with the determined values by calculating the colour difference, Delta E (ΔE) between object colour and the taught reference value. The colour difference ΔE is the result of the three coordinates in the L*a*b* colour space: position on the red-green axis (a), position on the yellow-blue axis (b) and brightness (L). If these values match (within the tolerances), a useable output signal is generated. One of the benefits is that the sensor evaluates the colours in the same way as the human eye, which is why the sensor is called a perceptive or true colour sensor.

The accuracy of most inline colour sensors in the marketplace are typically defined with a ΔE of 1-1.5, which represents similar performance to the human eye. However, modern colour sensors offer a ΔE of 0.8 at relatively competitive price levels. This is enabling more applications to be solved in-process, measuring 100% of produced parts that previously would have to be measured offline.

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