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www.edmundoptics.co.uk/imaging 171
resource guide fixed focal length telecentric liquid lens / specialty filters/accessories
cameras illumination targets
microscopy /
objectives
Limitations of USAF 1951 Targets
USAF 1951 targets do have drawbacks from the higher-resolution elements
being centered. Lenses produce different levels of resolution
at the center and corners of the FOV. Moving away from the center
of the field usually causes resolution to decrease, making it important
to check resolution and contrast at many field positions. This requires
repositioning the target around the FOV and taking mulitple images
to evaluate a system’s full performance, increasing testing time. This
can also cause issues depending on whether the system is focused
at the center of the FOV or across the entire field; because resolution
varies across the field, it can be difficult to determine when the
full FOV is in best focus. Some lenses obtain very high resolution at
the center of the FOV, but very low resolution at the corners when
the lens and camera system is focused on the center of the image.
Defocus the lens slightly to balance the resolution across the full field.
However, doing this is at the detriment of the center resolution. The
loss of center resolution is not necessarily bad though, because the
lens may still meet the demands of the application while still achieving
a balanced focus (Figure 12.6).
The potential for resolution variability across the FOV reinforces the
need to analyze all field positions before drawing conclusions on a
system’s performance. The lens that performs optimally with the
target at the center may not perform the best overall. However, it
is critical to perform all the analysis at a single focus setting. It may
seem intuitive to determine the system’s best performance through
the middle of the lens and then refocus to see the performance in
the corner, but this will not show how the system will perform once
deployed because refocusing during operation is often not possible.
There are variations of this target that allow for analysis across the
entire FOV by repeating the patterns in numerous locations on the
target (Figure 12.7).
Some issues associated with the USAF 1951 target are overcome
using the target known as a Ronchi ruling. This target has repeating
lines at one spatial frequency in one orientation that covers the
target’s full surface (Figure 12.8). Because there is detail across the
full target, the system’s best focus across the full field can be evaluated.
For applications needing only one frequency analyzed, this is an
easy-to-use, straightforward tool. However, there are two drawbacks
to using the Ronchi ruling. First, since a given target provides only
one frequency, a new target is required for each frequency. Second,
non-symmetrical resolution reductions across the field that are the
result of factors such as astigmatism cannot be analyzed because the
lines only propagate in one direction. To overcome this, the target
Figure 12.7a
Figure 12.7b
must be rotated by 90° and a second image must be used to analyze
the resolution. Additionally, while a lens’ focus can be balanced for
best focus, even for cases of astigmatism, it can be difficult to find this
balance when alternating back and forth.
Section 12.4: Ronchi Rulings
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0 Group or Element #
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Element and Group Number Corresponding to Frequency for USAF 1951 Test Targets
Group Number For High Res Only
Element -2 -1 0 1 2 3 4 5 6 7 8 9
1 0.250 0.500 1.00 2.00 4.00 8.00 16.00 32.0 64.0 128.0 256.0 512.0
2 0.280 0.561 1.12 2.24 4.49 8.98 17.95 36.0 71.8 144.0 287.0 575.0
3 0.315 0.630 1.26 2.52 5.04 10.10 20.16 40.3 80.6 161.0 323.0 645.0
4 0.353 0.707 1.41 2.83 5.66 11.30 22.62 45.3 90.5 181.0 362.0 –
5 0.397 0.793 1.59 3.17 6.35 12.70 25.39 50.8 102.0 203.0 406.0 –
6 0.445 0.891 1.78 3.56 7.13 14.30 28.50 57.0 114.0 228.0 456.0 –
Figure 12.6a: USAF 1951 Example: The center and corner of an image
that has been repositioned so that the best focus is only in the middle
of the target.
Figure 12.6b: The center and corner of an image that features balanced
focus across the entire field.
Figure 12.7a: A USAF 1951 pattern wheel target.
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Figure 12.7b: A USAF 1951 variable contrast and field target.
Figure 12.8: A Ronchi ruling.
/imaging