Illumination

Illuminate features & make defects visible

Lateral illumination

Selecting a suitable angle

Light from the side can be relatively steep or arbitrarily low. The effect on the camera image can be very significant; in extreme cases, the image information can be virtually inverted.

This is very easy to test: simply take a light that does not shine over too large an area and move it from a vertical position to a lateral position and observe the camera image of the illuminated object! Similar effects can be achieved by tilting the camera axis towards the surface of the component while keeping the direction of illumination constant.

Lateral light as bright-field illumination

With more vertical lighting, this is referred to as ‘bright field illumination’, as the object can be brightly illuminated. Scratches, steps and bevelled edges are shown darker or brighter depending on the angle.

Diffuse illumination of flat surfaces

With less shiny, slightly rough surfaces, there is usually enough stray light reaching the camera to illuminate the flat object 'diffusely'. Diffuse lighting can therefore be dispensed with if there is sufficient ambient light.

Perpendicular from top

front light reflection on matt metal finish, perpendicular illuminated

Light tilted by 10 degrees

Direct incident light on matt metal surface, but tilted by 10 degrees

Lateral lighting from several directions

To avoid shadows on the side of the object, it is a good idea to illuminate from at least two sides. This also avoids a drop in brightness from one side of the image to the other, which can cause problems when analysing the image.

Instead of ring-shaped illumination, rectangular light fields or bar-shaped light sources are now commonly used.

Laterally diffuse light can be a viable alternative on flat surfaces where diffuse illumination is required. However, they are less easy to handle than coaxial lighting with a beam splitter.

Functional principle lateral lighting

Oblique light (beginning dark field)

The illustration shown here already shows the smooth transition from bright field to dark field illumination. The surface is still brightened by scattered light effects on the slightly structured surface to such an extent that the component is easily recognisable. However, bevelled edges and notches are already brighter than the other image areas, as is typical for dark-field illumination.

Side orientated light

Surface slightly lightened by scattered light, deep edges of the notch lettering and slanted contours lightened.

Working principle

Edges facing away from the light appear dark, edges facing the light appear light.

Laser marking also involves a very strong roughening of the material. The resulting microscopic, uneven surface appears smooth and metallically bright at first glance, but generates a lot of scattered light and reflections when illuminated from the side:

Direct incident lighting

All metallic conductors and the laser-marked Datamatrix code are highly reflective.

Lateral illumination

Smooth metal conductors reflect light away from the camera, while the rough surface of the Datamatrix code scatters light towards the camera.

Lateral light used as dark field illumination

An illumination angle of 45 degrees or more is called dark field illumination. Light is reflected off the surface and the field of view appears dark. This effect is most apparent when the light is incident at an extremely shallow angle, as no stray light is reflected towards the camera. See the "Dark field illumination" section for more details.

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