Master Thesis - Fachbereich Informatik

2.2. ILLUMINATION 17
tubes alternate at about 25kHz, what is far beyond what can be captured by a video
camera.
Fluorescent lights exist at different sizes, shapes, and setups. Beside the common light
tube, there are also fluorescent ring lights or rectangle area lights. Low costs and a long
life-time make fluorescent lights even more attractive.
Light Emitting Diodes A LED is a semiconductor device that emits incoherent, monochromatic
light with the wavelength depending on the chemical composition of the semiconductor.
Today, different wavelengths of the visible spectrum for humans ranging from
about 400 to 780nm, as well as ultraviolet or infrared wavelengths, can be covered by
LEDs. The emitted visible light appears for example red, green, blue or yellow. Furthermore,
it is possible to produce LEDs that appear “white” by combining a blue LED with
a yellowish phosphor coating.
LEDs have many advantages compared to other light sources. Due to the small size, they
can be used for a variety of lighting geometries [18]. This includes ring lights, dome lights,
area or line lights, spot lights, dark-field lights and backlights. Theoretically, each single
LED in a cluster can be controlled independently. Thus, it is possible to generate different
illumination conditions (for example different lighting angles or intensities) with a single
setup by enabling and disabling certain LEDs, e.g. automated or software controlled. It
is also possible to use LEDs in strobe light mode.
Another advantage of LEDs is their energy efficiency and long lifetime with only a little
loss in intensity over time. Thus, LEDs have low maintenance costs. Operated at DC
power, LEDs do not produce any flickering visible as intensity changes in the video image.
Halogen lights Halogen lamps are an extension of light bulbs and filled with a halogen
gas (e.g. bromine or iodine). With respect to machine vision applications, halogen lamps
are often used in combination with fiber optic light guides [18]. The emitted light of a
light source is transferred through this fiber optic light guides, allowing for very flexible
illumination setups and geometries. This includes ring lights, dome lights, area or line
lights, spot lights, dark-field lights and backlights as for LEDs. Furthermore, there are a
range of fiber optic bundles at different sizes to route and position the light for user-defined
lighting.
One disadvantage of halogen lamps is a large heat development. Thus, usually active
cooling is required. Nevertheless, due to the bright “white” light emitted by halogen
lights (color temperature of about 6000K), they are also called cold light sources in the
literature. If heat development of the light source can be harmful to heat sensitive objects,
fiber optics can be useful to keep the light source away from the point of inspection. Like
LEDs, halogen lamps do not produce flickering effects, if the light source is DC-regulated.
Thus, halogen lamps qualify for high accuracy inspection tasks.
Xenon lights, often used for strobe light mode, are quite similar to halogen lamps. These
lights allow for very short and bright light pulses, which are used to reduce the effect of
motion blur.
Besidethedifferentwaysoflightgeneration,therearemultiplepossiblesetupsofhow
light sources are arranged. Especially LED lights and fiber optics are very flexible as
introduced before. They can be adapted to a wide range of machine vision tasks at almost
any size and geometry.