Resolved detail or an MTF curve?
USAF 1951 and NBS 1963A bar targets help identify the finest visibly separated lines under defined conditions. Slanted-edge targets support a different measurement: software estimates an edge-spread function and derives spatial frequency response. Select the target around the output your test procedure requires.
Directional focus and periodic patterns
A Siemens star presents radial sectors and helps compare sharpness with direction. Ronchi rulings provide one or more fixed spatial frequencies. A Sayce logarithmic target varies frequency with position. Binary patterns have harmonics, so their measured contrast is not automatically a sinusoidal MTF result.
Camera geometry and lens distortion
Use dot arrays, checkerboards or line grids when you need feature coordinates across the field. Match the layout to your detection software, including row offsets or internal corner counts. Flatness and feature-position error belong in the accuracy budget alongside camera and lens effects.
Pixels, divisions and physical length
A stage micrometer is placed at the specimen plane to relate pixels or eyepiece divisions to a known physical interval. An eyepiece micrometer sits inside the microscope viewing system. Recalibrate after changing objectives, adapters, camera sampling or image scaling.
Frequency, pitch and line width
One line pair is one dark line plus one clear space. A 100 lp/mm pattern has a 10 µm period; with a 50% duty cycle each line and space is 5 µm wide. These are geometric examples, not a manufacturing tolerance. Always state whether frequency refers to the object plane or the image plane.
Inspection records for your measurement
Specify nominal geometry, permitted error, measured features and report requirements separately. If your workflow requires traceable calibration, define the traceability chain and measurement uncertainty required. Product geometry alone does not establish an accredited calibration service.
Match spatial frequency to magnification and sampling
For magnification M, an object-plane frequency f maps to image-plane frequency f/|M|. If sensor pitch p is expressed in millimetres, the image contains |M|/(f × p) pixels per line-pair period. This separates target geometry from camera sampling when choosing a frequency range.
Confirm the software geometry before ordering
For a checkerboard, distinguish square count from internal-corner count. For a dot array, confirm symmetric or asymmetric rows, centre spacing and the software coordinate origin. Match the printed or coated field to the actual field of view, including the border needed for reliable feature detection.
| Task | Pattern to consider | Specify before manufacture |
|---|---|---|
| Visible detail check | USAF 1951 or NBS bar targets | Frequency range, polarity, magnification and reading rule |
| Camera geometry or distortion | Checkerboard, dot array or coordinate grid | Detection geometry, spacing, active area and flatness |
| Pixel-to-length conversion | Stage micrometer or calibrated scale | Interval, object plane, field position and calibration record |
| Quantitative SFR/MTF | A target compatible with the specified analysis method | Edge or pattern requirements, illumination and software method |
Control the comparison, not only the chart
Record illumination, exposure, aperture, focus, working distance and image processing. Automatic sharpening or denoising can change the apparent bar contrast and invalidate a before/after comparison. Compare the same field positions and orientations, and keep centre-field performance separate from edge-field performance.
Provide a DWG for a custom target layout
For a custom chart, supply a DWG CAD drawing with the active field, true feature dimensions, fiducials, polarity and inspection references. A named standard pattern still needs its actual range and plate format specified. A resized marketing image cannot establish traceable physical spacing.