16
LIGHTING
Light Source Type
The camera is designed to be used with Tungsten Halogen
illumination as this most closely matched the camera
spectral sensitivity. Other sources that emit radiation in
within the camera’s wavelength sensitivity can be used but
will deliver a higher proportion of light outside of the
camera sensitivity and hence more heat.
Mains or DC
Mains operated lighting will fluctuate at twice the mains
frequency; I.e. if your mains frequency is 50Hz then the
light from you lamp will have a small variation with a period
of 10 milliseconds. This fluctuation will appear on the
image as vertical stripes if it is not corrected. OSIRIS can
remove this effect by having the time the detector is
integrating the light the same as light fluctuation period.
DC operated lights will allow shorter integration times and
hence shorter image capture times.
For all sources it is important that the light has had
sufficient time to warm up and stabilise before acquiring an
image. This will be typically a few minutes.
How to Illuminate
As with all imaging the captures image will only be as
uniform as the lighting of the object. A single light source
to one side of the camera will provide more illumination to
one side of the object. Two light sources placed either side
of the camera will provide more uniform illumination across
the object but will still have a minor variation vertically. An
array of sources either side of the camera will provide even
better uniformity.
13
THE USER INTERFACE
The combination of low sensitivity and short exposure times
would require greater light levels without any corresponding
advantage.
CALIBRATING THE CAMERA
The sensor in OSIRIS has been calibrated at the factory to
allow for variations in the both the black level signal (Dark
current correction) and the white level gain (Shading
correction).
Both these calibrations should be done for each combination of
exposure time and gain that is set. If you try to run the camera
at an exposure time and gain that has not been calibrated then
you will receive the message
“A shading calibration for an exposure of
… msec can not be found. Would you like
to use an exposure calibration of …?”
The software will offer a calibration closest to the one you have
requested. If it is close then the image will have acceptable
quality otherwise the image may not be of optimum quality. You
may then receive the message
“A Dark current correction for an exposure of
… msec can not be found. Would you like
to use a dark current calibration of …?”
The software will offer a calibration closest to the one you have
requested. If it is close then the image will have acceptable
quality otherwise the image may not be of optimum quality.