Tuesday, October 18, 2005

DTP70 Review Update 1: UV filter or no filter ?

While installing the DTP70 -the actual user's manual is on CD- I came across the instructions for moving the built-in UV-cut filter into or out of the spectro's optical path. Although I had noticed this filter in an earlier glance at a sample instrument , I'd forgotten all about it. A check showed me the filter of the unit I just got was in place.

Should I filter the UV, or not ?

Let's remember that the filter is not there to filter UV from the light reflected from the specimen. There's no point in doing this because the spectro can easily ignore any UV bands - after all it's a spectro and bandwidth discrimination is its job !

Indeed, the filter is there to remove UV from the illuminant . Because UV light hitting the specimen might cause fluorescence i.e.. light re-emission in a visible band. In such a case, the reflectivity of the specimen in that band would be remarkably enhanced, and might even rise above 1!

Now that we understand the reason for the filter, we have informed our decision as to its desirability. For proofing, we are simulating one substrate with another, and we really don't want some accidental fluorescence of the proofing paper to perturb our simulation of the printing paper. So we filter out the UV.

On the other hand, as a photographer, I expect people to view my prints in real-life conditions which may well contain some UV. So when making photo prints I would not want to filter out the UV.

The DTP70 is exceptional in that the UV filter can be flipped in and out of the illuminating path. But our discussion is also informative as regards the Pulse and the EyeOne: Printing-trade members might wish to purchase the more expensive UV-cut versions of these instruments, while the photo trade can safely employ the cheaper standard unfiltered models.

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