Guide to UV Measurement
Advanced - Filter and Detector Specifications
Making of the optical filter, detector and other components is an art and it is normal to expect small variations in the optical performance between two filters.
How close to specified center wavelength (CWL) is the actual filter? What is the repeatability between filters on either side of the CWL? The slight differences between two filters can lead to slight variations between two instruments. How well does the instrument reject UV that is out of specified bandwidth? The way that your instrument deals with energy outside the band you are looking at (out of band rejection rate) can also influence the readings. How well does the unit deal with infrared or visible energy?
Most instrument manufacturers test individual optical components and accept only those that meet their criteria.
Requiring even tighter specifications or tolerances on optical components could easily price the instruments out of the range of all but a few users.