Lighting Requirements
The proper lighting is very important for
generating images that are detectable by an image sensor. Optimum lighting provides
a clear image, which is not too bright or too dark, and enables
a vision system to distinguish the features and characteristics
it needs to in order to accomplish the required task. The lighting
must be adequate enough to obtain a good response out of the sensor, but not
too excessive to cause blooming or saturation of the sensor.
The lighting needs to be uniform and consistent - therefore
controlled in such way that enhances features (contrast) that are
looked for and minimizes features that should be ignored. The lighting
also needs to eliminate or minimize effects of ambient lighting
and to simplify image processing.
Light
Sources
Spectral distribution is one of several
ways in which light sources differ. The spectral distribution of a
light source must be within the spectral response of the image sensor.
The following graphs show the spectral response of several different light sources and
a typical spectral sensitivity of the CCD sensor.
The light sources may also be classified according to their radiation
pattern (point, linear, hemispherical), distribution of light (spot,
diffused, collimated), geometrical shape, physical size, and efficiency.
For an object or a feature to appear in an image, light coming from
a light source must reflect off the object into the lens. Therefore,
the object is seen differently when illuminated by different types
of light.
A point source illuminates an object from a single direction (point)
and hence causes reflections and shadows – which are sometimes desirable
and other times unacceptable. Good point source illumination can
be generated by incandescent spotlights or by using fiber-optic
point sources.
Diffused light illuminates an object from all directions thus
creating little or no reflections, and no shadows. Good sources
of diffused lighting include fluorescent lights, fiber-optic diffuse
sources, and array of LEDs.
Collimated light is unidirectional and originates from a single
source optically located at an infinite distance. It creates reflection
and very sharp shadows.
Incandescent Light Bulbs – The light bulb can be used as
a point source. With proper optics it can also be used as a collimated
or diffused source. Their disadvantage is that most of their energy
is converted to heat, light intensity declines with time, and light
has a high infrared content.
Quartz Halogen Bulbs – They are
a much more efficient light
source with more white-light emission. Dichronic reflector eliminates
the infrared spectrum.
Discharge Tubes – Light is generated by the electrical
discharge in neon, xenon, krypton, mercury, or sodium gas vapor.
The light spectrum of emitted light depends on the gas and its pressure
– e.g. mercury discharge tube generates light in the UV band.
Fluorescent Tubes – They are mercury discharge tubes where
the UV light excites the visible fluorescence of a special phosphor
coating on the inside of the tube. Typically the light is white
but different light colors can be obtained with different phosphors.
Tubes are manufactured with different geometries – e.g. long and
straight, circled, U-shaped, etc. The fluorescent lighting is very
efficient, easy to diffuse, and matches spectral response of camera
sensors very closely. These lights typically pulse at rate
that is twice the power line frequency (120 Hz in US). However,
they can operate at much higher frequencies and thus produce a light
without any visible flicker.
Strobe Tubes – The strobe tube is a discharge tube driven
by the very short current pulse of a storage charge capacitor. As
it generates a short (10 – 20
msec)
pulse it is possible to capture an image of a moving part as if
it were stationary. Their disadvantage is a need for very precise
timing control of the light source and camera. The alternative to using
strobes is using a camera with a built-in shutter. However, shutter
is not a direct replacement for strobe. Compared to a high intensity
light of the strobe, the shuttered light source integrates much less light during the exposure time allowed to image moving
part, without getting a blurred image.
Arc Lamps
– They provide a very intense light in a narrow
spectral band. Their disadvantage is high cost, need for a high
voltage power supply, and a short life.
LEDs – The light-emitting diodes emit light in narrow spectral
bands - infrared, red, yellow, green, and white. As their output
energy is relatively low, they are combined into arrays of different
configurations in order to increase the light output and direct
light where is needed. They can be pulsed at very high frequencies
making them an alternative for strobe lights. LEDs have a very long
life, are highly efficient, and maintenance free.
Laser
– Lasers are monochromatic and coherent sources – meaning they have
the same frequency and phase and the wavefront is perpendicular to the
direction of propagation. Hence, the laser beam can be focused to a
very small spot with extremely high energy and be
perfectly collimated.
There are different types of lasers that have been developed –
gas, solid-state, injection, and liquid. The following are some
of the lasers used in the machine vision applications: He-Ne laser,
Argon gas laser, diode laser, He-Cd vapor laser, and gas injection
laser.
Lasers are used when a selective
high-intensity illumination is required and when changing reflection
of a part makes conventional
light sources difficult to use.
For more information, please contact
High-Tech Digital Technical Support.
310-265-8203
support@high-techdigital.com.
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