Contrast sensors and how a contrast measurement works

In this article we discuss contrast measurements. Questions that will be answered are: What is contrast? How does a contrast sensor work in a contrast measurement? In which applications is contrast measurement used? After reading you will have obtained some basic knowledge of contrast and the measurement of it, giving you better directions in finding the solutions for your application!

What is contrast?

Contrast is the difference in opposites. Like dark and light. In optical sensors contrast means the difference in brightness between two objects. A rule of thumb is: the bigger the difference, the higher the contrast. Both objects have a different brightness value that has to be detected by the sensor. In a picture, for example, this is the darkness of the black and the brightness of the red. In the case of objects this is the same. By this difference in contrast it is not only possible to detect two different colors but also differents shades within the same color.

How does a contrast measurement work?

A contrast measurement uses the difference in contrast between objects or in multiple colors on one object. This measurement is performed by a contrast sensor. An example is the SPECTRO-1-POF, which has a fiber connection and is versatile in use. By making use of values for the background and target object, the sensor can detect a difference in contrast. The object is the detection target. The background is the color of another object or the background which the object is in front of.

Step-by-step a contrast measurement works as follows:

1. Teach-in contrast values

2. Emitting light

To perform a contrast measurement the sensor needs reference values with which the detected objects will be compared. Desired values for target (the object) and the background have to be taught. It is possible to choose for a strongly or barely contrasting background. Furthermore, it is possible to define tolerance values within this in order to group measurements in terms of deviation from the reference value. These parameters can be configured with the help of the SPECTRO1-Scope software for Windows systems.

To perceive the difference in contrast a consistent source of light is necessary so the sensor can perform all measurements under the same situational lighting. All measurements are comparable because of this. For this, the sensor is equipped with an integrated light source that illuminates the object and causes a reflection coming from the object.

3. Receiving and process reflection

4. Evaluate received reflection

The light that is reflected by the object that has to be measured makes is possible to determine a difference in contrast. This is because the reflected light is received in the optics of the sensor which refracts it to the internal light sensor. This sensor determines the contrast values for the received light. The contrast values are then divided into different shades (gray values).

As a rule of thumb, the gray values are determined over the complete visible range on the color spectrum. The evaluation of contrast values by our sensors is color neutral because of this. This means that the contrast sensor can perform a measurement on every color. The gray values of the received light are compared with the reference values from the teach-in. Based on this, the sensor tells whether the object meets the reference values and what the eventual deviation from it is.

Industrial applications with contrast measurement

The SPECTRO contrast sensor can be used in a great number of applications, because of the possibility to parametrise the measurements as desired with the SPECTRO1-Scope software. Some examples are:

Detection of text, numbers and barcodes

Detect markers

When numbers or other characters and symbols like letters and bar codes on packaging has to be checked in industries such as food, beverages and pharmacy the sensor is an excellent choice. It serves its purpose by detecting errors early on. Contrast sensors have a high switching frequency which makes detection almost realtime, so downtime is prevented or solved as soon as possible.

With markings that serve for further processing of packaging, for example on tubes of toothpaste, it has to be made sure that these are printed correctly. The contrast sensor can detect whether a tube is provided with a right marking and makes sure that tubes missing markings can be sorted out in the following processing steps.

Prints detection

Check adhesive edges

In order to prevent or decrease printing errors in the packaging industry a contrast sensor can be used. These identify de taught values on a packaging and make sure printing errors are corrected or filtered out of the production process. The sensor is insusceptible for gloss and thus can be used on glossy and matte packages.

Some packages like (cardboard) boxes or drinking packages are sealed with an adhesive edge. With products of which this edge is sealed incorrectly this can cause packages to be closed improperly or not at all. The contrast sensor can also detect these adhesive edges in order to maintain the sealing of packagings.

Monitoring sealings

On a conveyor belt, in the beverages industry, over which sealed bottles are transported and have to be inspected the contrast sensor can be used. The sensor can inspect the bottlecaps for the presence of a sealing. Unaffected by the transparency of the sealing on the bottles or the bottlecaps themselves.