Traceability to NIST:
We state our traceability to our in house +/- 0.5% NIST standard as +/- 3%. This is to allow for the transfer from the NIST standard to our secondary standards and then the transfer to the probe being calibrated. What actually happens in our 365nm standard (for example) is that the transfer to the secondary standard from the +/- 0.5% NIST standard is more like +/- 1% or better. We then set the customer probe being calibrated to as close to 0% as possible so that we can hold any customer to a better than +/- 1% between probes.
In the semiconductor industry, this is imperative so we hold that tightness so our customer can, through testing, establish the exposure time necessary for the process to provide the best yield. We lock the customer to better than 1% ongoing so we do not cause a problem in the processing of the wafer. In many areas of exposure the allowable tolerance is +/- 5%. We still try to hold to as close to 0% as we can. We can hold to customer requested offsets from the standard value. Locking to a fixed calibration value provides repeatable calibrations year after year.