When defining surface irregularities more often than not it is useful to represent these as slope errors rather than surface
errors. However, surface error specifications are more commonly used even though they do not accurately represent
performance but simply physical characteristics. An example of slope measurement technique and specification is a
method developed by the University of Arizona called SCOTS (Software Configurable Optical Test System) which has
showed to be accurate, while very fast and easy to setup. It directly measures the transverse ray aberration/slope of an
optical system using transmission deflectometry.