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How To Choose The Right Lighting For Camera Applications

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How To Choose The Right Lighting For Camera Applications
How To Choose The Right Lighting For Camera Applications

Video: How To Choose The Right Lighting For Camera Applications

Video: How To Choose The Right Lighting For Camera Applications
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The test results in an application with a camera sensor are largely determined by the lighting. However, given the multitude of different lighting technologies available for camera sensors, it is often difficult to choose a solution for a specific application: Both for incident lighting (here the light source is above an object level) and transmitted light lighting (the light source is below one) Object level) there are numerous solutions for which several lighting technologies are also available.

Incident light illuminations and their possible uses

The following lighting techniques are generally available for incident light:

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  • directional coaxial lighting (e.g. ring lighting, dome lighting)
  • diffuse coaxial lighting (e.g. ring light with diffuser, flat dome)
  • telecentric lighting
  • Dark field lighting (flat ring lights, line lights or spotlights)

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Directional coaxial lighting

With directional coaxial illumination, the light is projected onto an object parallel to the axis of the camera optics. The term "coaxial" describes an equiaxial radiation of the light to the camera optics. The light source is therefore to a certain extent arranged around the optics of the camera, e.g. B. ring lighting.

Since the light sources now consist almost entirely of LEDs or LED arrays, the lighting technology used for incident light sometimes integrates a diffuser in the form of a diffusing screen or matt cover in order to obtain the most homogeneous light distribution possible on a test area. The emerging light is emitted directly onto the object level (Fig. 1).

Potential areas of application: In particular test objects with a flat, smooth, reflective or glossy surface, such as washers (e.g. testing the object diameter) or electronic components on which e.g. B. the completeness of contacts should be checked (Fig. 2).

Dome lighting with coaxial light source

Under the directional coaxial lighting techniques, the dome lighting enables extremely uniform illumination of a test object. Since the camera optics positioned above the dome require an opening to record the object plane, a dark area from which the light is not reflected would normally be seen on the camera image in the center of the object plane. To avoid this effect, high-quality dome lighting usually integrates a light source installed to the side of the camera optics, which directs the light towards the object plane via a beam splitter (Fig. 3).

Potential areas of application: More complex reflective objects such as B. foils, bottle caps or data carriers such as blue rays, CDs, DVDs.

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Diffuse coaxial lighting

Diffuse coaxial lighting (Fig. 4) can e.g. B. realize by a diffuse cover on a ring light. The emerging light radiates in an undirected or strongly scattered manner onto the object level. With this lighting technology, a diffuse and therefore very homogeneous, i.e. evenly distributed light is generated.

Potential areas of application: Ideal for reflective objects with uneven surfaces, e.g. B. for testing pipes, whereby not only their apex, but also peripheral areas can be recognized more clearly by the better light distribution and thus also stand out more clearly or with higher contrast from a background.

Diffuse coaxial lighting: flat dome

The so-called flat dome (Fig. 5) represents a special form of diffuse coaxial lighting and provides an undirected light that is distributed very evenly on an object surface. However, the lighting must be brought very close to the object surface in order to obtain evaluable image results.

Potential areas of application: Glossy, uneven object surfaces, which can also vary in position during the test.


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Telecentric lighting

In contrast to directional or diffuse coaxial lighting, the light beams do not have an opening angle with telecentric or collimated lighting (Fig. 6). Instead, the light beams hit a test object parallel to the camera optics, for example with the aid of special directional optics.

Potential areas of application: Examination of scratches, edges or surface structures using incident light.

Bright field / partial bright field lighting

The lighting techniques described so far can be referred to as brightfield or partial brightfield illuminations with a view to the directions of illumination. They are used to evaluate the direct reflection of the light rays from a test object or from an object surface. Strictly speaking, a “real” bright field can only be generated by using telecentric lighting technology in reflected light, since here the light rays are mainly reflected by the object surface in the direction of the camera optics. The amount of light reflected is very strongly dependent on the surface structure, ideally resulting in high-contrast images of the surface structure.

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Dark field lights

Illumination techniques for generating a dark field are usually positioned at a very short distance from a test object. Dark field illuminations can consist, for example, of flat ring lights, line lights or so-called spotlights (Fig. 8)

The light reflected by a test object is largely directed to areas outside the camera optics in the case of dark field illumination with incident light. The camera optics only detects the reflections of the light beams from the subunits of the object, so that errors, defects or specific object features in the camera image can be recognized as bright areas.

Potential areas of application: Ideal for detecting defects on object surfaces (e.g. scratches or marks) or for checking engravings (Fig. 8).

Transmitted light illuminations and their possible uses

With transmitted light or background lighting, a distinction is made between the following lighting technologies:

  • telecentric lighting
  • Bright field transmitted light illumination (diffuse or directional transmitted light illumination)
  • Dark field transmitted light illumination
  • transmissive lighting

Telecentric lighting

In transmitted light, telecentric lighting technologies provide an exact image of the test objects, largely free of diffraction effects (Fig. 9). If, on the other hand, an object is illuminated from behind with standard transmitted light illumination, its silhouette or the silhouette of the test area changes with the distance between the illumination and the object (Fig. 9). The silhouette is no longer clearly delimited due to the deflection or diffraction of the light rays at the object edge. The more telecentric the light (collimated beam), the weaker this effect.

Potential areas of application: For precise measurement tasks on test specimens in transmitted light. B. for deep holes (Fig. 10).

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Bright field transmitted light illumination

With this lighting technology, a test object is illuminated from below or from behind so that the rays of the light source are directed towards the camera optics (Fig. 11). This creates a kind of silhouette of the test object, in which the object background can be recognized as a light area in the image of the camera sensor.

Potential areas of application: Check whether, for example, specific features are present on objects or not (such as punched holes or holes), but without placing high dimensional requirements on the test result. Depending on the requirements of the test task, light sources with diffuse or directional lighting should be used. A classic application is the testing of injection molded plastic parts, whereby the shadow cast by the test objects in the camera image is used to check whether certain product areas are missing or differ from the desired shape.

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Dark field transmitted light illumination

Certain specific surface features of transparent objects can be highlighted very well with this lighting technology. B. scratches on a plexiglass pane. With dark-field transmitted light illumination, the scratches on the object surface are clearly recognizable as bright areas in the camera image, which are high-contrast and thus clearly stand out from the rest of the plexiglass pane (shown as a dark area in the image output) (Fig. 12).

Transmissive lighting

The transmissive lighting is a special form of dark field transmitted light lighting, because with this technology the light is virtually injected into a transparent object (Fig. 13). The device under test thus has the role of a light guide through which the light can pass.

Christian Fiebach, Managing Director, ipf electronic GmbH
Christian Fiebach, Managing Director, ipf electronic GmbH

Christian Fiebach, Managing Director, ipf electronic GmbH

Potential areas of application: The inspection of transparent objects, for example for cracks, scoring, scratches, but also deformations. Such defects can be seen very well in the camera image, since the light breaks at them, the light rays being reflected towards the camera optics.

* Christian Fiebach is managing director of ipf electronic GmbH

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