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Spatial Resolution
Assuming the spatial resolution is CCD limited, the smallest spatial element in the image
is represented by a “pixel”, having the dimension ΔX by ΔY. Depending upon the image
magnification, a certain total number of image pixels of a CCD are applied to an object
area, limiting the spatial resolution to a certain number of pixels per object area, often
referred to as pixel density. To simplify, the pixel limitation of resolution is usually quoted
for only one dimension (as in pixels/mm), assuming that X and Y magnification are
identical and the pixels are square. The pixel limited spatial resolution of the IS4000R is
variable (zoom lens), ranging from a low resolution of about 10 pixels/mm to a high
resolution of about 10 pixels/mm.
Other measures or quotations are often associated with spatial resolution. The measure of
dots/inch or dots/mm is often used in the printing industry, and is a true measure of printed
resolution; it is clearly analogous to the pixels/mm quoted above. Another common
measure used in imaging is line-pairs/mm, a number which corresponds to one-half of the
above pixels/mm; two pixels are needed to present the same information as one line-pair.
As a general rule of thumb, a minimum of two image pixels or one line-pair must be
applied to an object to “resolve” the object in the image. The same applies to the distance
between two objects that are to be resolved. For example, objects 0.2 mm apart will
require at least 10 pixels/mm or 5 line-pairs/mm to discern the space between them
(spatially resolve the objects). While this general rule may be a mathematical necessity for
spatial resolution, it is not sufficient. Although similar objects within an image captured at
a high signal/noise may be well resolved using a spatial resolution of two or more image
pixels per object, objects having extreme differences in signal may not be resolved, or
objects captured with a low signal/noise may not be resolved.
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