Question: The User's Manual shows the didymium tolerance to one decimal point and the EasyMatch QC Diagnostics program shows to two decimal points. For example, @430nm the manual says by no more than 1.2 %T and the QC Diagnostics shows +/- 1.25 %T . Which of these values are correct?
Significant digits are important. The UltraScan Pro has fixed reporting using double precision floating point. This means that every measurement is calculated to 64 decimal points. EasyMatch QC then rounds those readings to report from 0 to 4 decimals places based on user preference.
The manual states the acceptable tolerance with the words "by no more than". This is equivalent to saying "less than or equal to". Taking the 430nm tolerance, we specify 1.2 %T as the tolerance. Mathematically this means all of the following possibly results from EasyMatch QC would produce a Pass +/- 1.2, +/- 1.24, +/- 1.249, +/- 1.2499.
The author of the report felt that a tolerance of 1.24 was not as pleasing as 1.25, so three thousands of a nanometer acceptance to the tolerance at each test point was added when the Didymium Filter report was modified. For an average filter curve, remember the tolerance width does vary slightly by the actual filter being used. There is a nominal thickness variation from filter to filter. A 1.249 %T shift equates to a 0.898 nm shift and a 1.254 %T shift equates to a 0.901 nm shift. I would humbly suggest that this difference is quite a bit less than the measurement to measurement variance. This amount isn't causing any significant change in determining the wavelength performance of the instrument.
We may need to either make a change to EasyMatch QC or to the User's Manual if the explanation above does not satisfy the customer.