Lighting

Illumination vs. industrial image processing

A suitable illumination is decisive for the quality of the image acquisition and simplifies the subsequent image evaluation considerably.

In practice, the correct selection of the light source is one of the more complicated and often neglected problems in image processing.

Image processing systems usually consist of a camera or a sensor and a corresponding lens and a processing unit, which is equivalent to a PC and corresponding software. In addition to these components, the illumination, which plays a decisive role, must not be missing. Since "image processing" does not check the object itself, but only a visualized image, stable and reproducible conditions must exist in order to obtain a constant image of an identical object. (Fig. 1) This also includes a correspondingly stable illumination, because, for example, fluctuating lighting situations do not stand up to strict quality criteria.

Only if it is possible to display the desired inspection characteristics with sufficient contrast, can the processing software subsequently provide a corresponding evaluation.

Application

There are a number of test objects that require a special form of illumination. Take a transparent liquid with a transparent cell, this is symbolic the white mouse on the white paper. The test object and characteristics are thus made of the same, transparent material. But even scratches on a metallic surface, for example, usually do not cause any changes other than a recess, but this also needs to be detected.

A key role is played by light and its interaction in a triad of lighting, object and camera unit.

In most cases it is the perfect utilization of the properties of light source, test object and camera unit that enables the solution of difficult applications.

Decisive characteristics are among others:

- Light: wavelength (color), direct or indirect illumination, polarized or non-polarized, angle of incidence

- Object: material, surface, geometry, color

- Camera unit: resolution, sensitivity, monochrome or color, CCD or CMOS

In exceptional cases the objects are also dyed or UV-active dyes are added to them.

Light sources

The light to be used can be generated in different ways. Depending on the problem, lighting requirements, object size and installation, it is usually used:

- LED lighting

- Metal halide light source (cold light source with glass fibre transmission)

- Laser lighting

- Fluorescent light (high frequency)

- Halogen lamps

Color

Usually white light is used, but for special applications colour is also used. Frequently, red is used as well.

Lighting techniques

The angle of incidence of the light on the object also influences the result. We speak of various techniques such as reflected or transmitted light, direct or diffuse light, bright or dot-field illumination.

• Direct incident light - a ring light shines directly on the object (parallel to the optical axis) The image appears homogenous. (Bild 1)
• Diffuse brightfield illumination - the image appears more homogeneous. There is a high contrast between the object and the background. The disadvantage is that a glossy surface is over-illuminated.. (Bild 2)
• Diffuse Dunkelfeld-Beleuchtung – das Licht strahlt über ein Ringlicht in einem schrägen Winkel auf das Objekt und dadurch sind mehr Detail sichtbar, zudem treten so gut wie keine Schatten auf. (Bild 3)
• Diffuse darkfield lighting - the light radiates via a ring light at an oblique angle onto the object and therefore more detail is visible, moreover there are virtually no shadows. (Bild 4)
• Transmitted light method - Light is directed from the back of the object towards the camera. Only where there is no obstacle, the light shines through or transparent objects can be displayed from a different angle. (Bild 5)

Each of these five different lighting techniques leads to a different result.