In this article we discuss the theory and practicalities of color sensors. We will dwell on what color actually is and what a color measurement consists of. After that we will present a few industrial applications that involve color measurement and end with the different solutions that we provide within this.
What is color?
Color is a trait of wavelengths of which light consists of. The human eye is capable of perceiving color of light between wavelengths of 750 and 400 nm. All wavelengths together are called the electromagnetic spectrum of which the human can only see a small portion.
Color is generated when objects are illuminated by electromagnetic radiation in a wavelength area between 380 nm and 780 nm. This area is perceivable for the human eye. The radiation with a wavelength of less than 350 nm is called ultraviolet radiation. Radiation with a wavelength of more than 750 nm is called infrared radiation. These ends of the spectrum are invisible to the human eye. Color can also be generated by primary light sources that radiate light in the (for the human) visible range between the 380 and 780 nm.
How does a color measurement work?
Light is necessary to observe colors. We, as humans, can not distinguish colors if there is no light source present. Thus, everything would have been black. Color sensors that are used for the detection of color are equipped with an integrated light source for that reason. This light source provides steady lighting on the objects so the measured color values will always be consistent.
A step-by-step color measurement with a color sensor (also known as a spectrometer) is as follows:
1. Illumination of the measurement target
2. Receiving a reflection
The object that has to be measured, also called the target, on forehand has to be illuminated consistently. The sensor does this with the help of an integrated light source that emits a light beam to the object. This emitted light makes a color as measurable as possible for the sensor, because a deviation in the lighting can cause color values that are off.
The emitted light beam makes the object (and its color) provide a reflecting light that serves as an input for the color sensor (also known as a spectrophotometer). Thus, the light that is sent out is reflected by the object and received by the sensor again. Receiving the reflected light is through the optic of the sensor. This is a special lens that varies per area of application and makes the light shine on an internal color sensor.
3. Processing the reflection
4. Displaying the color value
The received light reflection from the object arrives at the internal color sensor that can process it. The internal color sensor measures the color values of the received light. Next, the measured data of the values are evaluated and with the help of (spectral) sensitivity functions it is processed into color values.
When the measured values of the reflected light are converted into color values, these can be displayed to the user and the system in which the sensor operates. The display of measured color values is possible in a variety of supported formats such as L*a*b*, XYZ and RGB.
Industrial applications for color sensors
Different types of optics make it possible to perform color measurements in a variety of applications. Below there are a number of application in which the color sensor excels.
Colored plastic or metal parts
Narrow plastic tubes
Detect color difference in production
The detection or measuring of glossy objects or for example coloured plastic and metal can be a challenge. With the help of a inclination angle of 45° and a diffuse light beam with a bigger range this is still possible. This enables colors of objects with different shapes and glossy surfaces to measure or detect.
The challenge here involves that you can see through the foil. By suppressing the gloss and placing a white background this can be realised. This way, the quality of foil can be measured and some errors in the imprint can be detected in a production environment.
A color measurement works with a measuring spot. This spot has to be big enough for a reliable measurement. Objecten with a diameter of only 1-2 mm can be meaured using different optics. These give a focussed light beam and with that a very small measuring spot (⌀0.3mm). This way, a color measurement is possible on a relatively small object.
When you anodize aluminum, the chemical composition determines the final color. By measuring the color value, the consistency of the final product can be monitored. But you can also think of other applications like printed surfaces, carpet, cosmetics, food, paint, etc. Everything where color is composed.
Glazed (floor) tiles and roof tiles
A reliable measurement can be realized by optimally making use of the glossy surface and using a large measuring spot against the irregularities of the surface.
The color of the packaging material has to be measured right after the coloring of the (paper) roll. A color sensor can also measure the color of the different types of paper.
The color of a glass coating can also be measured with the help of multiple sensors that are mounted with an inclination and provide diffuse lighting. Multiple mounting angles are possible.
The color values of laminate flooring can also be measured by using a greater measuring spot and thus to guarantee the color quality within the furniture industry. The obstacle of a glossy surface can be overcome with this.