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Illumination

Illuminate features & make defects visible

LED illumination

Light Emitting Diodes (LEDs) have been used in industrial image processing since the late 1990s. The continuous improvement of semiconductor materials has led to an extreme increase in intensity. Today, LED lighting covers the majority of industrial applications.

Fluorescent lamps were only used for a long time for extremely large-area lighting, such as in robotics or in the field of line scan camera technology, where a lot of light and/or the illumination of large areas is required. However, LED lighting has also solved such applications in recent years.

Reasons for the widespread use of LEDs in machine vision

  • Low power consumption, therefore low heat generation
  • Extremely long life, from 30,000 to 100,000 hours
  • Low premature failure; a cluster of many LEDs makes the failure of a single LED uncritical
  • Smallest dimensions, directional light due to the lens optics of the housing
  • High resistance to shock and vibration in industrial applications
  • Can be easily switched or flashed to increase lifetime or output
  • Available in a range of colours, not just white
  • The small size allows many different lamp designs and shapes to be used in a variety of lighting applications.

Features of industrial LED lighting

Working principle of the LED

Electroluminescent diodes work like semiconductor diodes that generate light in a forward direction. This is how light is emitted. The abbreviation LED stands for "Light Emitting Diode". The energy and colour of the light produced is determined by the semiconductor material used and its doping. Mixed crystals such as gallium arsenide (GaAs), gallium arsenide phosphide (GaAsP), indium gallium nitride (InGaN), etc. are used.


The LED consists of an n-doped semiconductor material with a very thin p-doped layer on top, which is deficient in charge carriers. When a voltage is applied, an excess of charges is created in the junction, which recombine with holes in the p-doped material and release their energy there. Because the top layer is very thin, the light flash can escape from the material. The light produced in this way is monochromatic due to the precisely defined energy levels of the semiconductor material. Unlike an incandescent lamp, the light always has a specific colour.

White LEDs also produce only monochromatic light. A trick is needed to produce white light: The light from a 100% blue LED stimulates yellow fluorescent dyes arranged around the light-emitting crystal. White light is mixed from the direct emission of blue light and the fluorescence of yellow-green and red light.

Typical LED spectra

Finally, optical elements focus the emerging light beam into a defined cone of light.
The intensity of the light increases in proportion to the intensity of the current, which is used in industrial image processing to generate large amounts of light with high-power LEDs. A consumption of 5-25 watts for LED lighting is typical.

The energy yield can be further increased by increasing the current above 100% of the allowable value. Continuous operation would of course destroy the LED immediately. However, with the aid of a flash controller, short current pulses (1x per inspection) can be overflashed at 3 to 6 times the value to produce a multiple of the light output.

Important for Machine Vision

  • LEDs age much faster when heated than when adequately cooled. The heat generated should be dissipated by ventilation, thermoconductive mounting or appropriate triggering.
  • Warming reduces the light output. Brightness depends on current flow and temperature. Ideally, a flash controller/current limiter should be used to keep the LED cool.
  • Frequent switching (per inspection) will not damage the light but will greatly extend its life.
  • LED life is defined as the age at which the LED emits 50% of its initial brightness. Total failure is very rare and the ageing process of LEDs is almost linear.
  • By using a flash controller, the intensity can be increased by several hundred percent.
  • Important for colour applications: even white LEDs change their colour temperature over their lifetime. They age very quickly due to the aggressive exposure of the fluorescent dyes to large amounts of light energy. The LED becomes more bluish. Recalibration may be required from time to time.

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