Advanced Imaging

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Advanced Imaging Magazine

Updated: January 12th, 2011 09:49 AM CDT

Choosing the Proper Illumination

First of all, test different forms of lighting on your target object
NET
With multiple lighting solutions available, including these LED products from NET, end-users have a variety of choices to use in their applications.
This table can serve as a general guideline for lighting choices; however illumination always should be tested on the object first to be absolutely certain. Key to table: ++ = very suitable; + = suitable; 0 = suitable under certain conditions; - = unsuitable; Dif = lighting with IR diffuser = lighting equipped with infra-red LEDs; UV = lighting equipped with UV LEDs, if applicable with devitrified CCD sensor (depending on the UV-wavelength); (*) Daylight cut filter required in front of the camera; yellow = direct lighting; violet = indirect diffuse lighting; blue = coaxial lighting.
Dome lighting with additional coaxial light prevents the camera from reproducing its own image in the case of reflecting surfaces.
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By Dr. Hans Ludwig, NET

As anyone involved in the field of industrial image processing knows, the choice of lighting is extremely important for producing good images, because the better and richer in contrast the image is, the less effort is required from the software. However, lighting is not just lighting! There is a big difference in results if a coin is illuminated using diffuse lighting, side-lighting (dark field) or coaxial incident lighting. The basic rule is that the lighting should be tested on the object. In the following, different lighting techniques are briefly described and the general application possibilities are listed.

POSSIBILITIES

Incident lighting, transmitted light or backlighting, and self-luminous. Incident lighting means that the object is directly illuminated, transmitted lighting means that the object is trans-illuminated, backlighting means that the back of the object is illuminated and self-luminous objects, such as melting glass or steel, do not normally require any extra lighting.

Direct and indirect lighting. Direct lighting means that the light falls directly on the object (this may result in annoying specular highlights if shiny objects are being photographed). Indirect lighting prevents the light source from reflecting so that there are almost no specular highlights.

The angle of incidence. Here, with direct lighting, all angles between 0 (vertical to the optical axis) and 90 (parallel to the optical axis) are possible. In the case of diffuse lighting, the light should fall as homogeneously as possible from all directions onto the object.

Coaxial lighting is a special case. Here, the light is parallel to the optical axis. Coaxial lighting causes the areas of the object that are vertical to the optical axis to appear bright, and areas that are diagonal to the optical axis such as the edges of a scratch, for example, to appear dark.

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