USAF resolution targets consist of bars organized in groups and elements. Each group consists of six elements (i.e. elements 1-6) and each element is composed of 3 horizontal and 3 vertical equally spaced bars. Each element within a group corresponds to an associated resolution, based on the bar width/space. The resolution of an imaging system is defined by the group and element just before the black and white bars start blending together.
Note: Although the test yields precise resolution values, this is a qualitative test which depends on the user's definition of acceptable blur. The vertical bars are used to calculate horizontal resolution and the horizontal bars are used to calculate vertical resolution. One line pair equals one black and one white bar.
We offer 2” x 2” and 3” x 3” 1951 USAF Glass Resolution Targets in both standard and high resolution versions. Standard resolution targets range from Group -2 to Group 7 and Element 1 to Element 6; they can resolve up to 228 lp/mm. High resolution targets range from Group -2 to Group 9 and Element 1 to Element 6; they can resolve up to 645 lp/mm. High resolution targets can resolve much smaller features than their standard resolution counterparts.
The pattern on a positive target is most often made from a reflective material, such as chrome, deposited on a clear background. Positive targets can also be referred to as reflective targets since incident light hitting the chrome pattern will reflect. The pattern on a negative target, on the other hand, is clear and the background is chrome. Negative targets, or “transmission targets”, are most often used with a backlight in order to increase contrast between the target and the background. In terms of a technical difference, both types of targets will have the same maximum resolution regardless of their reflective or transmissive construction.
Ronchi Rulings are made up of many pairs of opaque and transparent lines. 1 Line Pair (LP) = 1 cycle for a 100 LP/mm ronchi ruling. 1mm / 100LP = 10μm, or a cycle of 10μm. The width of the line pair is 10μm, so each line thickness will be ½ the line pair thickness, or 5μm.
To calculate how many LP/mm you need based on the cycle length, divide 1mm by the cycle length: 1 mm/ 20μm = 50 LP (½) x 20μm = 10μm line thickness. In other words, you need 50 LP/mm to get a 20μm cycle length with a line thickness of 10μm.