Industrial color sensors

Industrial color sensors (cromaview, color sensor) are devices dedicated to the automatic inspection of the color of products moving along production lines. They use RGB or white LED light sources and photodetectors with three or more channels to measure the color coordinate of a point on the product surface quickly (in milliseconds). They are used for the inspection of paints, colored plastics, labels, food, textile products, paper, packaging, pharmaceuticals and cosmetics, for shade control and for the identification of color codes. They are an effective replacement for visual inspection by an operator, eliminating subjectivity and ensuring 100% inspection.

Operating principle

Industrial color sensors use a stabilized light source (white or RGB LEDs) to illuminate the surface of the product; the reflected light is picked up by photodetectors fitted with colored optical filters (red, green, blue – tristimulus filters according to CIE) that measure the light intensity in each of the three spectral bands. The internal algorithms convert the three values into standard color coordinates (CIE L*a*b* or L*c*h*) or into RGB intensity values for each channel. For advanced applications, multispectral sensors with more than 3 channels are used to improve discrimination between similar colors (metamerism).

Operating modes

Industrial color sensors work in several modes: “teach-in” mode (the operator shows the sensor a reference sample; the sensor stores the values of the sample as “good” and flags as “reject” any product that deviates beyond a configurable tolerance); “match” mode (the instrument compares the measured color with a library of stored colors and identifies the closest one); “ΔE” mode (calculation of Delta E – the color difference perceived by the human eye according to the CIE76, CIE94 and CIE2000 formulas – above a ΔE of about 2-3 the color is perceived as different). Configurable GO/NOGO thresholds trigger alarms and automatic rejection.

Types

The main types are: spot sensors (with illumination and reception concentrated on a small spot of 1-20 mm, for precise measurements at a specific point); fiber-optic sensors (with a miniaturized measuring probe connected to the electronics by an optical fiber – for applications in confined or hard-to-reach spaces); line sensors (with a field of view extended along a line across the product, for the inspection of continuous webs); and full color vision systems (with RGB cameras and complete software analysis – for complex inspections of products with graphics and colored patterns).

Industrial applications

The applications are wide-ranging: the automotive industry (color control of bodywork and interior parts); the packaging industry (color control of labels, printed board and sleeves); the food industry (quality control of products by color – for example meat, fish, baked goods, fruit and wine); the textile industry (control of colored yarn, fabrics and dyed threads); the pharmaceutical industry (identification of tablets and capsules by color); plastics (color control of profiles and molded parts); the wood industry (checking the color of the finish on furniture); and the automatic identification of components on an assembly line.

Integration

Industrial color sensors are integrated into line control systems through: digital outputs (PNP/NPN) to actuate devices (ejectors, markers); analog outputs for display on an HMI; and digital interfaces (RS-485, Profinet, EtherCAT, IO-Link, OPC UA) for full integration with PLC and MES. Parameter setting (teach-in, thresholds, tolerances) is carried out locally with dedicated keys or from a PC using the manufacturer’s software. The sensors are generally rugged, with an IP65-IP67 protection rating, and need no integrated batteries (industrial 24 V DC supply).

Selection criteria

The choice of color sensor takes into account: the range of colors to be discriminated (some sensors are optimized for certain colors); the color resolution required (discriminating similar colors calls for higher sensitivity); the working distance and the spot size; the speed of the product (for synchronization with the line trigger); the interfaces for integration with the PLC; the operating conditions (dust, vibration, temperature, humidity); and the ease of parameter setting and product changeover (for production runs with frequent code changes).