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Factors to Consider when Purchasing a Colour Measurement Sensor

13-01-2020

Colour subconsciously affect the choice of the food we eat. Instinctively, humankind prefer the bright colours such as red, orange and yellow over darker hue such as blue or green. Research has also shown that colouring influence our perception of taste, flavours, odours and we often associate certain colours with its unique tastes. For example, pink represents sweet stuff, green represents bitter and red with spicy good. The food industry takes colours into serious consideration for developing, processing and producing their products.

In food manufacturing, ready-to-use ingredients are commonly subjected to processes such as baking, roasting, colouring, fermentation to create food that are ready-to-consume. Colours can be an indicator of the quality of the final products. Examples of the applications may include roasting of coffee beans, baked breads and processing of colour-dyed foods. Integrating colour sensors in the processing line allows manufacturers to capture the colour information that assist in determining good or botched batches, inform operators of process anomaly which allows them to make necessary adjustment as well as streamline the quality control process.

Determining the exact colour of an object in is a complex measurement task. Colour recognition becomes quite challenging especially when it comes to measuring subtle differences among similar or highly reflective surfaces or identifying different colour spot on a uniformly-coloured surface. The optical properties of the light source, the target material, and the angle of observation altogether affect the colour properties such as brightness, saturation, and hue of the target. Nevertheless, colour sensors are still worthy to be considered in the production line as part of the quality control or monitoring unit to achieve uniform product quality and consistency.

Find out more about the operating principle of colour sensors here.

When it comes to purchase a suitable colour sensor, there are several factors that have to be taken into account to ensure that the new colour sensor is suitable for the intended applications. Listed below are several key parameters to be considered.

Required Accuracy

The first thing manufacturers need to figure out is the type of colour measurement they require for their application. Based on whether they require just the colour detection or they need to recognise deviations from a reference colour, the appropriate sensor can be selected. All this depends on the ∆ E factor that is retrieved from the measured values. Colour difference or ∆E is used to sort the closeness of the measured colour with the reference colour. It is mathematically achieved by calculating the visible difference or error between two colours. The value of ΔE is also categorised on the scale of colour difference. For example, ΔE < 0.1 implies that the colour differences are negligible. This is generally the standard used in the automotive assembly and manufacturing. On the other note, ΔE > 5 is considered a noticeable difference and is often not tolerated in almost all types of applications. For applications requiring absolute colour measurement, the value of ΔE must lie in the range of 0 to 1.0.

Type of Surface of Target Material

For accurate colour measurement, ideally the measured surface should be smooth and flat. This may be difficult to achieve as not all objects exhibit this feature and the requirement to prepare the sample prior to measurement may be complex and challenging. A new sensor head or a light source can be changed to accommodate for colour measurement in other type of surfaces, such as textured or structured surfaces as well as surfaces of transparent and translucent objects. Also, depending on the difficulty of measurement on surface, multiple sensors can be installed to measure at different measuring points on the sample prior to averaging these values.

For measuring curved surfaces, a system different from the one used for flat surface is required. This is because on a curved surface, the light from its source is reflected to the receiver differently as compared to a flat surface. In this situation, the sensor controller is set up to keep the measurement opening fully covered. Before beginning with the colour measurement process, it is also ensured that the curved radius of the sample is fewer than 10 times the diameter of the measurement field. Considering all these factors is essential for achieving highly accurate measurement.

Environmental Conditions

The accuracy of a colour sensor is strongly affected by its surrounding temperature. The value of ΔE changes as the temperature increases. Sensors that are designed based on optical glass fibre are suited for measurement in harsh ambient conditions as they are able to withstand extreme mechanical stress and high temperatures. It is also strongly advised to check with the sensor supplier to clarify the effect of temperature on the measurement accuracy of the sensor.

Inline colour sensors are suitable for measurement in high-speed production line as they can be integrated with other industrial control/quality control systems. The system also able to interfaces with industrial protocols via Ethernet, EtherCAT, and RS422 to remotely monitor the process. The advanced colour measurement systems also offer colour recognition feature. It is done by comparing the measured data to reference values and using the reflection spectrum to ensure unique identification.

Lenses and Filters Selection

Lenses have an important role to play when it comes to colour measurement. There is a wide range of combination of colour sensors available based on the type of lens and filters used. For measuring large distances and measurement on matt surfaces, clear glass lenses are preferred. Sensors with diffusive lenses are more suited for measurement on inhomogeneous surfaces as well as on textured and shiny surfaces. A polarising filter can also be considered as an add-on to enhance colour saturation and eliminate reflections. This is suitable when colour sensor is used for measurement in highly reflective surfaces. Last but not least, sensors with UV and LED light sources emits little visible lights and may be suitable for measurement on fluorescent objects.

As a leading supplier of state-of-the-art sensors and measuring instrument in Oceania, Bestech Australia has vast experience in sensors integration for various types of applications and measurement requirements in industries. The sensors can be integrated in automation systems to reduce the amount of human intervention required for operations. If you have questions, contact us and one of our application engineers will attend to your technical queries and recommend the suitable solutions for your applications.

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