Zygo has built Smart Accessories for its Qualifire Laser Interferometers, transmission spheres and flats that carry their own calibration data and hand it to the mainframe when mounted. Zygo says the approach is not a new calibration algorithm. It is a new way to deliver calibration data to the instrument.
In Fizeau interferometry, the reference surface has its own figure error, and that error shows up in every optical path difference map. Shops typically deal with it by buying better reference optics, applying polynomial corrections, or running absolute calibration routines, each of which costs labor, equipment, or operator skill. Zygo's alternative puts a persistent electronic storage device inside the transmission element housing. Electrical contacts on the accessory mate with connectors built into the mainframe's bayonet receptacle. Zygo chose a wired link over wireless because it gave the most stable connection.
On mounting, the software fills in measurement fields with the part number, description, serial number, calibration date, and F# for spheres, so each measurement carries accessory metadata. For a quality engineer, that means traceability records no longer depend on an operator typing in the right serial number. The software also enables only the controls relevant to the installed accessory and syncs the stored error map to the instrument.
The map itself comes from Zygo's labs, where metrologists use absolute calibration methods with uncertainties of a few nanometers PVr. Accessories can be returned for recalibration, which refreshes the onboard file for quality-system compliance.
Aligning that map to a live measurement relies on permanent fiducials etched into the reference surface. A transmission flat has three. The system finds them in the image with sub-pixel precision, checks them against the locations stored with the calibration file, then centers, rotates, resizes, and interpolates the map to fit. Because the full map is applied, the system can correct mid and high spatial frequency errors that a low-order polynomial fit would leave in. The same registration gives an automated lateral calibration. With the true pixel pitch known, the software can size the clear aperture, evaluate slopes, and set filter cutoffs, including the radius-of-curvature adjustments needed for spheres.
Zygo tested the method with a transmission flat carrying roughly 15 nm PVr of figure error, measuring a part with about 6.3 nm PVr of actual form error. Without correction, 17 nm PVr of reference error read as part error. With automated subtraction on, the cavity measured 6.5 nm PVr, against 6.1 nm PVr from a full absolute calibration. The 0.4 nm PVr gap separated a result that took a few minutes from one that took half a day.
Zygo also points to future software updates that could use the same accessory data to compensate for self-weight deflection when a flat is mounted vertically or to correct retrace errors.
