40 Spyder2 User’s Manual
03-032-10091-06 Teledyne DALSA
• LED light sources are relatively inexpensive, provide a uniform field, and longer life
span compar ed to other l i ght sources. H ow ev er, they also r equire a camera w i th
excel lent sensi ti v i ty, such as DA LSA ’ s Sp y d er 2 camera.
• Halogen light sources generally provide very little blue relative to IR.
• Fi ber-optic light distribution systems generally transmit very little blue relative to IR.
• Some light sources age; over their life span they produce less light. This aging may not
be uniform—a light source may pr oduce progressively l ess li ght i n some areas of the
spectrum but not others.
Filters
CCD cameras are extremel y responsiv e to infrared (IR) w avel engths of light. To prevent
infrared from distorting the i m ages you scan, use a “ hot mi r r or” or IR cutoff filter that
transmits visible wavelengths but does not transmit wavelengths over 750nm. Examples
are the Schneider Optics™ B+W 489, which includes a mounting ring, the CORION™ LS-
750, w hich does not include a mounting ring, and the CORION™ HR-750 series hot
mirror.
Lens Modeling
Any lens surrounded by air can be modeled for camera purposes using three primary
points: the first and second principal points and the second focal point. The primary
points for a lens should be available from the lens data sheet or from the lens
manufacturer. Primed quantities denote characteristics of the image side of the lens. That
is, h is the object height and h
′
is the i mage hei ght.
The focal poi nt is the point at which the image of an infinitely distant object is brought to
focus. The ef f ect i ve focal l en gt h (f
′
) is the distance from the second principal point to the
second focal point. The back focal l engt h (BFL) is the distance from the i mage si de of the
lens surface to the second focal point. The obj ect di st ance (O D ) is the distance from the first
principal point to the object.
Figure 10. Primary Points in a Lens System
Magnification and Resolution
The magnification of a lens is the ratio of the image size to the object size:
where m is the magnification, h’ is the image height (pixel size)
and h is the object height (desired object resolution si ze).
By similar triangles, the magnification is alternatively given by:
These equations can be combined to give their most useful form:
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