To produce good quality image data, the choice and design of the illumination is as important as the choice of the camera and the optics. Very seldom the ambient
light provides controlled and adequate illumination for the measurements.
Illumination setup
Direct spot-light illumination is rarely utilized in measurement applications. Instead, one can choose from a wide variety of setups: in-line, diffuse, ring-light,
light-sheet or back-light illumination. The appropriate choice or their combination depends on the application.
Pulsed vs. continuos
Imaging moving objects requires pulsed illumination or short exposure time. Pulsed illumination can freeze the motion and concentrate all the power at correct moment.
An every-day example of pulsed illumination is a flash-light included in most consumer cameras. Pulsed illumination can also be produced by lasers and LED-devices. Lasers can provide pulse durations down to
5 ns, which is enough to freeze any fast moving object in the image. Light pulses can be generated at several thousand Hz frequencies and synchronized to high-speed cameras.
Monochromatic vs. white light
Most light sources, such as halogen, xenon and many LEDs provide a broad band (i.e. white) illumination. White light source is needed for a measurement utilizing color
information. However, in many applications greyscale cameras combined with monochromatic light sources are preferred. Lasers providing monochromatic light have several
properties, which make them ideal for measurement applications. Lasers provide a very high power and the light is easily collimated and focused. For example the diode-lasers
coupled with fiber optics allow versatile illumination arrangements.
|
|
|
