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JMC Scientific Consulting Ltd

UV Polarizers – stability under studio flash exposure
This summary includes the results of exposure of two different UV polarizers to studio flash conditions and the effects it has on their transmission spectra

The two polarizers tested were:

  • Knight Optical – yellow UV polarizer – described as “Ultra-High durability UV Linear Polarizer 25.4mm OD w/PSA SAMPLE”.
  • Knight Optical – grey UV polarizer – described as “UV Linear Polarizer 25.4mm OD No PSA SAMPLE”.

The yellow polarizer is a dye based polarizer and 50mm disk of this was recently bought by JMC for testing (the two samples outlined above were sent along with the order for testing purposes – Sales Order 26878).

Figure  1

JMC believes the grey polarizer is the HNPB replacement material sold by Knight Optical, but the product code has not been given on documentation sent by Knight Optical with the order, so this is TBC.

The disks used are shown below in Figure 1, along with the packaging in Figure 2.

Figure 1. Disks used for the assessment. Note the protective film is still on the disks here but was removed for testing.

Figure 2. Packaging for the disks (yellow packaging on the left and grey on the right).

Method

Transmission spectra were measured using an Ocean Insight FX spectrometer between 250nm and 800nm. A 600 µm fiber was used the transfer to the light from an Ocean Insight DH-2000-BAL light source to a UV-Vis collimator. It then passed through the sample before being collected by another UV-Vis collimator and going through a 600 µm fiber to the spectrometer.

On the light source either the deuterium and halogen lights were used (for the 250 nm to 800 nm scans) or just the deuterium light (for the 250 nm to 400 nm scans, except where noted in the text). The lights were left to warm up for at least 20 mins before use. ‘Transmission’ mode was used on the spectrometer and the device re-zeroed before each set of scans.

The light source was a Bowens GM500 studio flash with a bare quartz flash tube and run on full power. An 18cm reflector was attached to the front of the flash and the polarizer disk held just in front of the reflector, equating to a distance of about 8 cm from the flash tube.

The disks were exposed to 10 flashes then their transmission measured. They were then exposed to 40 flashes (for a total of 50 flashes) before being measured again. Exposure then continued until they had been exposed to 100 and 150 flashes, which was the maximum exposure tested.

There was no significant difference to the yellow dye-stuff based polarizer between the ‘no exposure’ baseline at the beginning of the experiment and after 150 flashes. Some minor variation in transmission was observed for the filter as it was moved around in the beam, but JMC has assumed that was due to the presence of a PSA layer which was visible on the surface.

The results of the flash exposure on the grey polarizer are given in Figure 5 and 6.

Figure 6. Transmission of the grey polarizer before any flash exposure and after 100 and 150 flashes.

The optical transmission of the grey polarizer was stable up to 50 flash exposures but then started to drop in the UV at 100 flashes, dropping further again after 150 flashes. There were 2 measurements taken after 100 flashes for the 250 to 800 nm region. Note in Figure 5, the 150 flashes scan was run using both the deuterium and halogen light switched on. This has resulted in some stray light within the spectrometer a small transmission signal between 250 and 280 nm which is not present for the other scans.

With the grey polarizer, the drop in transmission in the UV after exposure to the flash seems to be matched by a slight increase in transmission in the red and IR part of the spectrum.

Although not shown here, even after 150 scans the grey polarizer still continued to block the light effectively when used in a cross polarized manner, so it would seem that it is only transmission that has been impacted by this experiment.

Conclusions

Overall the yellow and grey UV polarizers behaved differently as a result of exposure to a studio flash light. The yellow dye-stuff based polarizer was much more stable and did not change significantly as a result of exposure to the flash during the experiment, while the UV transmission of the grey (likely, iodine-based) polarizer dropped as a result of exposure to the flash light.

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