Section 13: Testing and Targets
170 +44 (0)1904 788600 • EDMUND OPTICS® imaging lenses imaging resource guide filters microscopy cameras illumination targets
Target Applications Pros Cons
Simultaneously test the vertical and horizontal resolutions
at discrete spatial frequencies
Qualitatively, the resolution of an imaging system is de ned as the
group and element combination that is located directly before the
black and white bars begin to blur together. Quantitatively, resolution
(in terms of lp/mm) can be calculated by Equation 1.
A chart showing how groups and elements match to their associated
lp/mm frequencies can be found on page 168.
USAF 1951 targets are designed so that the elements increase in
frequency along a spiral towards the target’s center; higher-resolution
elements are placed in the middle of the target. This arrangement is
bene cial when testing zoom lenses because it avoids the need to reposition
the target by allowing the higher resolution elements to remain in
the FOV as the lens magni cation causes the FOV to decrease.
Test targets are designed to help evaluate or calibrate the performance
(imaging quality) of an imaging system. This could include troubleshooting
a system; benchmarking, certifying, or evaluating measurements; or
establishing a foundation to ensure multiple systems work well with one
another. Because image quality can be de ned by di erent components,
particularly resolution, contrast, Modulation Transfer Function (MTF),
Depth of Field (DOF), or distortion, di erent systems may require different
targets, and some systems may require more than one.
It is important to keep in mind that the results of using a test target
are subjective if only viewed visually; using visual observation is
dependent on who is looking at the target. For instance, someone
with 20/20 vision is typically capable of discerning higher resolution
or more detail than someone with 20/25 or 20/30 vision. Additionally,
individuals that regularly look at these targets can have
their brains trained to see detail that may not actually exist due to
viewing the target’s repetitive frequencies or patterns. While visual
inspection can help compare two di erent systems, it does not always
validate results. If possible, it is important to use software to
truly validate measurements.
Targets for Resolution Measurements
Test resolution in vision systems, optical test equipment,
microscopes, high magnifi cation video lenses, fl uorescence and
confocal microscopy, photolithography, and nanotechnology
The USAF 1951 Target
One of the most commonly used test targets is the USAF 1951 target,
which is comprised of sets of horizontal and vertical lines, called elements,
of varying sizes (Figure 1). The horizontal and vertical elements
are used by a system to simultaneously test the vertical and horizontal
resolutions at discrete spatial frequencies (line pairs per millimeter,
or lp/mm) in the object plane. Each element has a unique set of
widths and spacings and is identi ed with a number from 1 to 6. Together,
six sequentially numbered elements are considered a group,
and each group has an identifying number that can be positive, negative,
or zero. Typically, this number ranges from -2 to 7. The group
number and element number are then used together to determine
spatial frequency. The resolution is based on bar width and space,
where the length of the bars is equal to ve times the width of a bar
(Figure 2). One line pair (lp) is equivalent to one black bar and one
white bar. Vertical bars are used to calculate horizontal resolution,
and horizontal bars are used to calculate vertical resolution.
Must reposition target to fully evaluate a system’s performance
Can be diffi cult to determine when the entire FOV is in best focus
Testing resolution and contrast
Can simultaneously determine system’s best focus across
Different target required for each frequency that needs to be
Diffraction testing Non-symmetrical resolution reductions cannot be analyzed
Comparing highly resolved or magnifi ed imaging systems Potentially most powerful for testing resolution and contrast
Diffi cult to determine exact resolution that the test system is
achieving at each element
System alignment Can evaluate continuous change in resolution across multiple
orientations without repositioning target
Requires advanced image analysis software
Assembly assistance Eases the comparison of different imaging systems
Table 1: Applications, Pros, and Cons of Resolution Targets.
Figure 2: USAF 1951 Target Speci cations.
Resolution = 2 (GroupNumber + ElementNumber-1) 6 1
Figure 1: Example of a USAF 1951 Target.