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3. Theory of Focal Spot Reproduction
7
Theory
1.0B
RTI Slit Camera User’s Manual
3.
Theory of Focal Spot Reproduction
3.1.
Theory
The focal spot is being reproduced (depicted) according to the IEC 60336 [Reference
1], standard through a 10 µm wide slit, and then registered with an image receptor (not
included in the RTI Slit Camera). This is described in figure 1 below. The reproduction
is a one-dimensional “picture” of the focal spot in one direction and a perpendicular
measurement has to be done to get the characteristics of the focal spot in both length
and width directions.
Figure 3–1 Focal spot reproduction. The grey part to the right shows an image taken
with a film in the picture plane. Note the lighter part in the middle indicating the dip
between the two peaks.
As shown in Figure 3–1, you get a measured width of the focal spot, f
m
. This measured
focal spot is an enlargement of the true focal spot on the anode, f
w
, magnified by the
factor E, which is n divided by m.
The distance from the true focal spot to the slit is m, and n is the distance from the slit
to the image receptor. The slit width is denoted by
s
. The true focal spot width can thus
be calculated as:
f
w
=
m
n
f
m
(Eq. 3.1)
where
represents the penumbra from the slit.