The USAF 1951 Target

The USAF 1951 target is one of the most common test targets used and is comprised of sets of horizontal and vertical lines, called elements, of varying sizes.

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 L in the object plane. Each element has a unique set of widths and spacings and is identified with a number from 1 to 6. Six sequentially numbered elements are considered a group and each group has an identifying number that can be positive, negative, or zero. This group 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 one line pair (Lp) which is equivalent to one black bar and one white space.

Vertical bars are used to calculate horizontal resolution, and horizontal bars are used to calculate vertical resolution.
Qualitatively, the resolution of an imaging system is defined as the group and element combination that is located directly before the black and white bars begin to blur together.

USAF 1951 targets are designed so that higher-resolution elements are closer to the center of the target while lower-resolution elements are closer to the target edges. This arrangement is beneficial for 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 magnification decreases the FOV.

Ronchi Rulings

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. 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 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.

The Star Target

The multi-element start target is possibly the most powerful tool for testing the resolution and contrast of a system and combines many of the strengths of both the USAF and Ronchi targets. Each element of the star target consists of a circle formed of alternating positive and negative pie-shaped wedges that are tapered towards the center at a known angle. The element’s tapered wedges provide a continuous change in resolution that can be evaluated in both vertical and horizontal directions, along with a variety of other orientations, without repositioning the target.

Having many stars across the FOV makes comparing different imaging solutions easier by providing the ability to determine the best focus across the FOV while simultaneously analyzing horizontal and vertical information at a variety of resolutions.