Controls: Form of actual surface relative to perfect form
Related Terms: Form Error, Rt, Rq
Units: Linear units
Specification Must Contain: Value (RMS, Peak-to-Valley, +/- Sag Error or combination) over an aperture
Optimax Preferred Specification: +/- Sag Error in units of microns
Reasons: Change is relative to nominal form
Manufacturing Standards: ISO 10110-12, OEOSC OPI.006
![Irregularity: Profilometer](https://optimaxsi-com.dev.webhost.io/wp-content/uploads/Irregularity-Profilometric-Measurement.jpg)