 Most cameras have
gain control, which tries to even out the signal over the full image.
Hot spots will drive the camera to reduce the signal level, possibly
erasing desired data. Conversely, a large amount of dark area will
drive the camera to boost the signal and over drive certain areas. The
ability to illuminate an object so as to clearly see what is desired,
and only that, is still more of an art than a science. Video imaging is
further complicated by the camera's ability to see lower light levels
and longer wavelengths than the human eye.
Transmitted Illumination
Transmitted Illumination is generally used with transparent or
translucent objects. Sometimes used to cast a shadow simulating
comparators. Entering angle and stray light can be significant.
Incident (Coaxial) Illumination
Incident (Coaxial) Illumination is most practical when used with highly
reflective or specular objects, such as wafers of other polished
specimens. Coaxial is very efficient mode of illumination. It provides
light on opaque surfaces at high magnification. However, it is
difficult to fill a large field-of-view with coaxial illuminators.
There are two methods of producing incident (coaxial) illumination in
your Optical Systems.
- By injecting a beam above the taking lens
with a beam splitter. In this method, the lens becomes the condenser
and the light is highly controllable. The draw back is that some light
will always reflect back off the lens surfaces and reduce contrast. The
coaxial illuminators shown on the systems diagrams are of this design
and provide excellent illumination when used within the confines
indicated by the accompanying chart. Due to the angles of incidence,
this type of illuminator cannot provide even illumination on large
fields. It is restricted to higher power auxiliary lenses and higher
recommended limits on the zoom setting dependent upon your camera
format and TV Tube.
- The second method is
to inject light below the lens. Qioptiq Imaging Solutions's new LF Coaxial
Illuminator accessory is designed to attach to any existing Thales
Optem Zoom system.
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