28
Appendix B – Interpreting the A-1519/1520 Type II
Calibration Reports
The A-1519-2.4 Target has a 0.5 micron resolu-
tion and 3.5 micron accuracy, versus the A-1520-
2.4 Target, which has a 0.25 micron resolution
and 1.5 micron accuracy. When the elevation of
either target is set near zero (within ± 1 mm from
zero), the most accurate part of the sensor is in
use. The calibration graph on Page 29 shows that
the error in the central part of the sensor for the
A-1519-2.4 is less than 1 micron, which is better
than the total error of the A-1520-2.4 (± 1.5 mi-
crons). Furthermore, when a target is zeroed on a
given spot on the sensor for high-accuracy meas-
urements (for example, measurement deviations
of less than 25 microns), the error in the meas-
urement from one point to the next is extremely
small.
When a target is calibrated, measurements are
taken every 1 mm and the error in between is
interpolated. This makes is very likely that the
error in measurement at the point where the
target is zeroed is nearly identical to the error in
each subsequent measurement because the
difference in sensor area between the two points
is less than 1 mm. So in effect the error really does not matter – it’s like having the same error “offset” at
each point. The errors start becoming important only when large deviations from zero are being
measured, for example 1 mm or more.
The accuracy of the A-1519/A-1520 Type II targets is specified in the report below as an error of 3.2
microns (µm). This means that the maximum error of a given measurement could be ± 3.2 µm over the
central 80 percent of the measuring area of the target.
For example, if one measurement point was at -12.5 mm (-.492 in.) and the next measurement point was
+12.5 mm (+.492), then the maximum error in the 25 mm deviation would be no more than 6.4 µm
(.00024 in.).
When measuring small deviations in flatness/straightness (less than .1 mm), the maximum error is much
lower (usually about 1 micron or better). See the explanation of the graph below for more details.
