Scratch Dig measurement finally enters the 21st Century. Long regarded as a subjective measurement, scratch/dig remains one of the main industry standards defining the cosmetic finish of optical surfaces. In practice however, the practicalities of performing this assessment leads to a result that is highly subjective and prone to discussion. Although the use of "comparator plates" enables the inspector to roughly qualify the level of any gross imperfection, the intricacies of MIL13830 calls for the operator to make a pass/fail against the combined length of all scratches relative to the size of the optic; a challenging feat for even the most experienced technician.
Over recent years Crystran has seen an increase in the number of customers specifying against MIL13830 so we’ve decided to do something about it. Working with an advanced metrology company, we commissioned the design and development of a dedicated scratch/dig instrument to automatically assess components up to a clear aperture of 300mm. This instrument is calibrated against a set of industry recognised Davidson Optronics standards, and is able to resolve surface imperfections as small as 5µm using a dark-field measurement technique.
The instrument analyses the complete optic and provides an objective measure of scratch/dig that accounts for all the accumulation rules specified in the MIL standard. Additionally, any individual imperfection (or group of imperfections) can also be accurately assessed via an integration of the total amount of light being scattered.
Crystran would be pleased to discuss the use of this new instrument with you and, if you like, measure any samples you’d care to send our way.