FUV-0.5 Unique Combined UV/VIS Light Photometric Accuracy & Stray Light Calibration Standard

How To Use The FUV-0.5 Stray Light Filter for Stray Light Analysis Without H-F3-0.5

FUV-0.5 stray light tests do not necessarily rely on using the H-F3-0.5 filter. Just prior to testing for stray light performance, all spectrophotometers must be calibrated for photometric accuracy. A customer can use their own photometric accuracy kits in both the UV and VIS range, or if a user does not have their own kits they can purchase and use H-F3-0.5 for VIS photometric accuracy calibration prior to using FUV-0.5. In this case (when using H-F3-0.5 prior to FUV-0.5), the FUV-0.5 can check photometric accuracy in both VIS and UV but in the UV range the FUV-0.5 will not be able to determine the affect of stray light, if any is present in a spectrophotometer.

What Is Stray Light?

Stray light is unwanted light inside a spectrophotometer which usually has different optical path from the one originated by the light source of the spectrophotometer.

Stray light reaches the spectrophotometer’s detector and deteriorates the quality of spectrophotometric analyses of samples. Depending on why and where exactly inside a spectrophotometer the stray light propagates, it can reach the detector either through a sample positioned in a cell-holder inside a spectrophotometer or directly (from a direction or directions outside the sample area).

While a spectrophotometers’ base-line correction allows compensating, to a large degree and sometimes - entirely, for presence of stray light reaching the spectrophotometer’s detector directly, stray light passing through a sample may not be compensated by the base-line correction feature because of often unpredictable interaction of stray light and the sample.

Unlike a typical neutral density filter, FUV-0.5 substantially suppresses stay light (SL) passing through it while properly attenuating the original optical beam (OOB) of a spectrophotometer. Therefore, a typical neutral density filter causes the detector of a spectrophotometer to measure M1 = (OOB + SL) while FUV-0.5 causes the detector of a spectrophotometer to measure only the M2 = OOB. Therefore, SL= M1 – M2.

Example Of Stray Light Test

The above equations mean that, in terms of stray light in VIS, a user has to first take a ND measurement with a traditional ND 0.5 filter (i.e. H-F3-0.5), make a record of the obtained results, and  compare the records to the trace-table supplied with H-F3-0.5. [IMPORTANT: tolerances of the trace-table for H-F3-0.5 are set in accordance with NIST 930e standards].

For example, a user measured ND=0.4923 at 500nm. The trace-table of the H-F3-0.5 gives ND=0.4511 at 500 nm. The measured difference is (0.4923-0.4511) = 0.0412 A which translates to about 10% T, which exceeds both –NIST tolerances and the spectrophotometer’s tolerances, which is definitely a bad sign of either photometric and/or a stray light problem.

Now we will conduct measurements with FUV-0.5 at 500nm

Now if the obtained ND measurements are within the tolerances of the user’s spectrophotometer (taking in consideration the NIST tolerances), the user can proceed with the measurements in VIS using FUV-0.5. Let's say, the user measured ND=0.5022 at 500nm and the trace-table of FUV-0.5 gives ND=0.5001. The measured difference is (0.5022-0.5001) = 0.0011 A, which translates to only 0.25%T, which is well inside NIST 930e tolerances.

Conclusion: The error in measurements when using H-F3-0.5 was caused by stray light.

Notes:

IMPORTANT NOTE: THE SAME APPROACH AS DESCRIBED ABOVE IS APPLIED TO THE UV RANGE OF A UV-VIS SPECTROPHOTOMETER. 

Users are also advised that FUV-0.5 could be used as true-ND filter across both – UV and VIS ranges without using any additional filters. In this case stray light performance of a spectrophotometer will remain unknown.