LED illumination

There are many different types of light sources available including the following:

• Incandescent lamps
• Fluorescent lamps
• LED lights

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.

A general guideline for computing the peak wavelength of a LED is the following:

With:

• λ the peak wavelength
• E_gap the energy gap of the semiconductor material

For example, for obtaining a green LED (λ = 555 nm), a semiconductor with an energy gap of about 2 eV has to be used (for example: GaAlP semiconductor).

White light is produced as in the fluorescent lamps, where the blue light is absorbed and re-emitted in a broad spectrum slightly peaked in the green region.

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