The pattern standard and the engineering use
NBS 1963A is an ISO 3334-style five-bar pattern family. Its original reference use is finite-conjugate microcopying, while engineering teams also place the target at a collimator focal plane for infinity-conjugate testing. These are different geometries and their reported results must not be mixed.
Read the five-bar group
Each field contains five horizontal and five vertical bars. A line and an equal clear gap form one cycle. Count all five bars and the four gaps, then use the frequency table belonging to the approved artwork. The labels are not USAF group and element numbers.
| Frequency (cycles/mm) | Period (µm) | Single line (µm) |
|---|---|---|
| 1 | 1000 | 500 |
| 5 | 200 | 100 |
| 18 | 55.56 | 27.78 |
| 128 | 7.81 | 3.91 |
| 228 | 4.39 | 2.19 |
Set up the collimator
Place the target at the collimator objective focal plane and illuminate it uniformly. The collimator converts each target point into a direction in the outgoing parallel beam. The tested lens is focused at infinity and forms the target image on its sensor.
- Record the collimator focal length f_c and verify the target is at its focal plane.
- Confirm the tested lens infinity-conjugate side faces the collimator.
- Check the empty-field illumination before interpreting a five-bar group.
Convert cycles/mm to angular resolution
For target frequency ν_target in cycles/mm and collimator focal length f_c in mm, the angular frequency is ν_ang [lp/mrad] = ν_target × f_c / 1000. The period angle is θ = p / f_c in radians, where p = 1/ν_target mm.
Include the tested lens and sensor geometry
For a lens focal length f_lens, the approximate simulation magnification is M = f_lens / f_c. The image-plane period is p_image = p × M. Compare the resulting image frequency with ν_Nyq = 1/(2 × pixel pitch) before treating the reading as optical resolution.
Position the target across the field
Use a centre target and targets near the long-edge midpoint and a corner. At off-axis positions orient the pattern toward the field centre. An asymmetric result can arise from lens aberration, target tilt, collimator field quality, illumination or vibration.
Reject false resolution
A valid group has five distinct bars with the expected order. Four apparent bars, contrast reversal or a finer group reappearing after an unresolved group are reasons to check focus, phase, aliasing and processing. A visual bar reading is a resolution criterion, not automatically an MTF curve.
Report the test so it can be repeated
Report the chart family and version, artwork frequency, substrate and polarity, collimator focal length, tested-lens focal length, aperture, field position, illumination, processing and the last valid five-bar group. If a certificate or measured coordinate record is supplied, keep it with the target identity.