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Monday, October 29, 2012

Update: Study on UVB for Optical Testing

Many optical tests are performed using the D65 (outdoor daylight) source condition.  Within instrumentation this is done by using a pulsed xenon lamp. 

The long-held thought was that only the relative quantity of UV energy was important, not where the UV energy was coming from (UVA: 315-400 nm, UVB: 280-315 nm). Therefore, in the calibration of optical instruments there is a UV-trim filter which is adjusted in and out to achieve the desired UV level to simulate D65.

One issue is that a xenon lamp has energy in the UVB below 300 nm which the theoretical D65 does not have (see above). The ISO TC6 (pulp, paper and board) WG3 (optical properties) has undertaken two studies so far to better emulate the theoretical D65.  

The first study in 2011 involved 5 instruments all of the same manufacture but different age.  Ten (10) different papers were used of different UV excitation.  There were three different UVB cutoff filters used to see which one best approximated D65 and provided reasonable repeatability and reproducibility.  These results showed that instruments that had a filter cutting UVB excitation would not underestimate whiteness and brightness values for papers containing fluorescent whitening agents that have low excitation in the UVB.

The second study in 2012 involved 6 instruments (2 each from 3 different manufacturers) and different age.  Twelve (12) different papers were used of different UV excitation.  Only one specific UVB cutoff filter was used.  The intention was to see if the results of the 2011 study could be replicated over a much wider sampling of instruments.  The results showed instrument-dependent and sample-dependent trends for the measurement of ISO Brightness and D65 Tint.  However, the second study showed no clear evidence of improved inter-instrument agreement.

Even though the concept of cutting off UVB energy to better approximate D65 UV Level has been shown to affect the results of ISO Brightness, D65 Whiteness and D65 Tint, there still needs to be more work to eliminate variables in the process in order to provide consistent results and improved inter-instrument agreement of different make and model.  ISO TC6 WG3 is investigating options for another study to improve the reproduction of D65 and at the same time to produce the same or better inter-instrument agreement.