Fixed-period and progressive patterns answer different questions
A Ronchi ruling repeats one period over its active field, making the same detail available for comparisons of focus or configuration. A Sayce-type logarithmic target progressively changes stripe spacing, allowing the transition in reproduced detail to be located along the pattern. The exact position-to-frequency map belongs to the artwork. A Ronchi wavefront test uses its own grating placement and optical geometry, distinct from photographing the stripes.
Convert pitch, frequency and duty cycle
If one period has pitch P millimetres, its frequency is f = 1/P lp/mm. One line pair contains a dark line and its neighbouring clear space. At dark-line duty cycle D, dark width is D × P and clear width is (1 − D) × P. Clarify whether a quoted lines/mm value counts complete pairs or individual lines.
Check sampling before accepting visible stripes
With magnification |M| and pixel pitch p in millimetres, pixels per line-pair period equal |M|/(f × p). Two pixels per period is a sampling boundary, not proof of reliable separation. Stripe phase, focus, pixel aperture and processing affect the image. Aliasing can make detail reappear with a wrong period or direction; compare with the known geometry and repeat under controlled conditions.
Stripe contrast and sinusoidal MTF are different
Binary clear/opaque stripes contain the fundamental frequency and higher harmonics. Measuring image contrast from these stripes gives a response to that waveform, not directly the response to a single sinusoidal frequency. Recovering sinusoidal MTF requires an appropriate method and its assumptions. A visual coarse-to-fine reading is most useful as a repeatable screening result; a slanted-edge analysis gives a different, software-based measurement.
Report the plane and frequency map
For a fixed-period target, record pitch, duty cycle, orientation and polarity. For a progressive target, retain the artwork revision and frequency corresponding to the observed position. State whether frequency is reported at the target or image plane, together with magnification, field position, exposure, focus and processing. Use the same conditions when comparing a retained baseline.