
pixel spacing at the focal plane is known. Making brightness measurements in
the image is also useful, and it requires knowledge of the relationship between
specimen brightness and gray levels in the digital image.
2.8.1 Spatial Calibration
The pixel spacing can be either calculated or measured. Calculation requires
knowledge of the pixel spacing at the image sensor and the overall magnification
of the optics (recall Eq. 2.3). Often it can be calculated from
dx ¼
Dx
M
o
M
a
(2:23)
where dx and Dx are the pixel spacing values at the specimen and at the image
sensor, respectively, and M
o
is the magnification of the objective. M
a
is the
magnification imposed by other ...