Machine Vision Light Sources
Light sources for machine vision systems include:-
We mention incandescent lighting here for completeness rather than as a suitable lighting source for a machine vision application. Normal incandescent lamps are very rarely used because of the heat they produce and their variation in brightness due to fluctuations in the power supply frequency.
In recent years LED, or Light Emitting Diode, lighting has become a popular light source in vision applications. They are very durable (service life of up to 100,000 hours) when compared to other light sources, rather than fail completely they tend to lose part of their luminosity as they get older. LEDs are compact, can be arranged in a variety of configurations and in their simplest form only require a basic DC power supply to energise them. Although constantly illuminated LEDs are only suitable for low speed applications where long exposure times are permissible strobed LED sources can provide light intensity close to that of Halogen lamps. The ability to synchronise the strobe with the image acquisition make them ideal for capturing objects moving through the field of view.
LEDS are available in a selection of colours making them suitable for monochrome, colour and infra red applications. A disadvantage of LEDs when trying to achieve high levels of illumination is the complex circuitry required to control the strobing.
Fluorescent tubes come in a variety of geometries ranging from the common strip light to ring lights. They provide bright homogeneous light , however standard varieties are susceptible to changes in power supply frequency and offer light concentrated in the blue area of the spectrum. More suitable fluorescent tubes operate at high frequencies (>30KHz) and provide better colour balance. Fluorescent tubes are often a good choice for providing area illumination either as diffused front lighting or as a means of back lighting in gauging applications.
Halogen lamps provide very intense light. Typically they are used in conjunction with fibre optic light guides whereby a halogen bulb is housed in a light source box with the control and feedback circuitry for adjusting the bulb intensity. The light from the bulb is focused onto the end of a fibre optic cable attached to the box by means of a ferrule. As a rule the fibre optic cable should be kept as short as possible, the longer the cable the greater the loss and the more expensive they become. A practical limit is about 5m. Halogen lamps are available in numerous configurations such as ring lights with built in polarising filters to reduce specular reflections or line lights for use with line scan cameras.
Lasers are typically used to provide structured lighting in 3D applications and in situations where there is no grey scale contrast. Often the laser is used in conjunction with a band pass filter on the lens to ensure only the reflected laser light reaches the camera.