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Section 12: Testing and Targets
Section 12.1: Resolution and MTF Testing
Six-hundred and forty (640) pixels was the standard for camera
sensors only a few decades ago. It is common to see more than six
million pixels on sensors today. When sensors only off ered several
hundred pixels, it was customary for the imaging lens to outperform
the sensor. Today however, pixels on sensors are getting smaller and
more numerous. To keep up with sensors, lens manufacturers have
increased their lens selections, while pushing the boundaries of lens
design technology and manufacturing. To take advantage of pixels
decreasing in size and increasing in number, lens performance must
increase. Selecting the right lens requires understanding and proper
characterization of a lens’s performance.
To measure how well a lens performs requires the use of correct metrology.
To characterize lens performance, a set of objectives based
on lens specifi cations must be designated before selecting test methods.
It is important to remember that no one test method can fully
characterize every lens specifi cation. Moreover, many tests useful for
characterizing performance exclude real-world and often applicationspecifi
c, external eff ects. Therefore, a test plan that uses multiple
methods may be necessary.
Resolution is often the most important specifi cation for an imaging
lens; it is a continuous function that tells the user how small a detail
can be before it’s indistinguishable from its surroundings at a specifi c
contrast level. The performance of a lens can vary across diff erent
points within the image and can also vary with WD, f/#, and other
parameters. When measuring resolution and contrast, it is important
to manage expectations and set reasonable system boundaries.
Some resolution test methods can also reveal additional information
about other parameters such as distortion and relative illumination.
Common tests for lens resolution include reverse projection testing,
modulation transfer function (MTF) testing, slanted edge MTF testing,
and camera testing. Each of these methods provides a unique set
of benefi ts and drawbacks.
Reverse Projection Testing
In reverse projection testing, a lens test projector is used. The pattern
from a high-accuracy test target is placed at the image plane and is
projected through an imaging lens to a specifi c WD (essentially, imaging
in reverse) and observed in a dark room. Because the modulation
of light through a lens is a reversible process, this method is a
simple and eff ective test of resolution. This method is additionally
useful because resolution specifi cations are already given as image
space values. A common test target used in reverse projection is the
USAF 1951 target, which consists of a several orthogonally oriented
bars of increasing frequency that spiral into the center. The bars are
dispersed across the entire fi eld allowing operators to focus the lens
to optimize resolution at specifi c fi eld regions. As such, this method
can be used to test multiple fi eld points at once.
Reverse projection is low-cost and a fast method to test lens resolution
and astigmatism. Operator training is relatively easy, and equipment
cost is inexpensive compared to other methods. One important
drawback of this test is the inability to measure contrast levels since
this test method depends on operator eyesight. Human eyes can typically
detect the lowest resolvable contrast, which is approximately
20%, but not specifi c contrast values.
Modulation Transfer Function (MTF) Testing
The most comprehensive representation of the resolution performance
of an imaging lens is a modulation transfer function (MTF)
curve. MTF curves are used to directly compare the resolution of one
lens to another. Commercial test benches are available that allow for
the characterization of lenses within a three-dimensional coordinate
system (Figure 12.2).
Figure 12.2
An operator conducts an MTF test by passing an impulse signal
through a lens, typically in the form of light from a point source
against a dark background. Careful attention is paid to the position
of the source and the location of the image. The impulse response
is then used to determine a response at any spatial frequency up to
the Nyquist (the highest, resolvable, sampled frequency). Because
the testing environment is so tightly controlled, the measurements
obtained are purely descriptive of the performance of the lens. External
phenomena, such as stray light and falloff , are not factored
into system-level performance specifi cations and the actual achiev-
Figure 12.1: An operator performing a reverse projection test. The
circles labeled 11, 9, and 6 correspond to image circles of ⁄", ⁄,",
and ⁄" sensors, respectively.
Figure 12.2: A commercially available Trioptics branded MTF test
station testing an imaging lens.
Figure 12.1