Management & MTF Testing
6 +44 (0)1904 788600 • EDMUND OPTICS® imaging lenses filters microscopy cameras illumination targets imaging resource guide
A few decades ago, six-hundred and forty pixels were the standard
for a camera sensor. Now, it is common to see more than six million
pixels. When sensors only o ered several hundred pixels, it was customary
for the imaging lens in the system to outperform the sensor.
Today, pixels on sensors are getting smaller while their numbers are
increasing., Due to this, lens manufacturers have increased their selections
while simultaneously pushing lens design and manufacturing
boundaries. It is imperative to use the appropriate metrology to measure
how much performance increases with these advancements. In
order to characterize the performance of a lens, a set of speci cation
objectives should be selected before choosing the appropriate test
method, as there is no one test option able to fully characterize every
factor of the performance of a lens. Many tests that are useful for
characterizing lens performance exclude external e ects. While such
tests are useful for evaluating the maximum performance limitations
of a lens, they disregard real world application factors. Thus, there are
alternative test methods to characterize lens system performance in
the intended application environments.
Resolution is often the most important speci cation parameter for an
imaging lens, as it illustrates how small a feature can be before the
lens is unable to distinguish it from its surroundings. Resolution is
a function that relates the amount of contrast in an image at every
two-dimensional point on the image plane, as well as at every unique
conjugate or working distance, to the spatial frequency of the corresponding
point in object space. As pixels continue to decrease in size
and increase in number, lens resolution must increase as well.
Resolution is a continuous function and so, it is important to manage expectations
and set reasonable system boundaries in order for contrast
to be measured by sampling the appropriate region across as many
pixels as necessary. Typically, resolution test methods allow the user to
garner additional information about other important parameters such
as distortion and relative illumination. Common tests for lens resolution
include reverse projection, modulation transfer function (MTF) testing,
slanted edge MTF testing, and camera testing. Each of these methods
provide a unique set of bene ts and drawbacks.
During reverse projection, the pattern from a high accuracy test target
gets placed at the image plane, projected through an imaging lens to a
speci c working distance, and then oriented in the reverse direction.
This method e ectively tests resolution because the lens modulation
of light rays is a reversible process. Going from image to object space
is virtually the same as going from object to image space, if you disregard
magni cation. Additionally, this method is useful because resolution
performance speci 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 way the bars are
dispersed across the entire eld allows an operator to focus the lens in
order to optimize the resolution at speci c eld regions and can test
multiple eld points at once.
Figure 1: Showing an operator performing a reverse projection test.
The circles labeled 11, 9, and 6 correspond to image circles of a /",
/,", and /" sensor, respectively.
Reverse projection is a quick and low cost method to test the resolution
and also the astigmatism of a lens. Operator training is relatively
easy and the cost of equipment is inexpensive when compared to
other methods. One important drawback of this test is the inability to
detect di ering levels of contrast since this test method is dependent
on the operator’s eyesight. Our eyes are typically able to detect the
lowest resolvable contrast, which is approximately 20%, but not speci
c contrast values.
Modulation Transfer Function (MTF)
The most comprehensive representation of the resolution performance
of an imaging lens is a modulation transfer function (MTF) curve. This
curve displays the amount of contrast that a lens can distinguish for a
range of spatial frequencies. As such, MTF curves can be used to directly
compare the imaging capabilities of multiple lenses. Test benches
are commercially available and allow for the systematic testing of
lenses within a three dimensional coordinate system (Figure 2).
Figure 2: A commercially available Trioptics branded MTF test station
testing an imaging lens.
An operator conducts an MTF test by passing an impulse signal
through a lens. This signal is typically in the form of light from a point