Basic Knowledge on LEDs
Comparison of Light Sources for Image
There are many different types of light sources available 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
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)
LEDs (Light Emitting Diodes) produce light via the annihilation of an electronhole 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.
Although the emitting principles are basically the same, they are available in the following types of shapes.
In machine vision, light is mostly characterized by its wavelength, which is generally expressed in nm (nanometers).
Basically light is electromagnetic radiation within a certain portion of the electromagnetic spectrumas following figure: it can be quasi-monochromatic (which means that it is characterized by a narrow wavelength band, i.e. with a single color) or white (distributed across the visible spectrum, i.e. it contains all colors).
Light visible to the human eye has wavelengths in the range of 400-700 nm, between the infrared (with longer wavelengths) and the ultraviolet (with shorter wavelengths): some special applications will require IR or UV light instead of visible light.