LED illumination

There are many different types of light sources available (Fig. 3) including the following:

  • Incandescent lamps
  • Fluorescent lamps
  • LED lights

LED lights are by far the most commonly used in machine vision because they offer a number of advantages, including:

  • Fast response
  • Suitable for pulse and strobe operations
  • Mechanical resistance
  • Longer lifetime, higher output stability
  • Ease of creating various lighting geometry
Different emission spectra
Emission spectra of different light sources.

Incandescent lamps are the well-known glass bulbs filled with low pressure, inert gas (usually argon) in which a thin metal wire (tungsten) is heated to high temperatures by passing an electric current through it. The glowing metal emits light on a broad spectrum that goes from 400 nm up to the IR. The result is a white, warm light (corresponding to a temperature of 2870 K) with a significant amount of heat being generated.

Fluorescent lamps are vacuum tubes in which UV light is first produced (by interaction between mercury vapor and highly energetic electrons produced by a cathode) and then is adsorbed by the tube walls, coated with fluorescent and phosphorescent material. The walls then re-emit light over a spectrum that again covers the whole visible range, providing a “colder” white light source.

LEDs (Light Emitting Diodes) produce light via the annihilation of an electron-hole pair in a positive/negative junction of a semiconductor chip. The light produced by an LED depends on the materials used in the chip and is characterized by a narrow spectrum, i.e. it is quasi-monochromatic. White light is produced as in the fluorescent lamps, but the blue light is absorbed and re-emitted in a broad spectrum slightly peaked in the blue region.

Next →