Tuesday, June 06, 2006

Optical Brighteners

Uwe at Outback Photo has some interesting comments about optical brighteners. I found I had a particular problem with papers as illustrated below:

In selling at the local farmers market lit with a combination of incandescent and mercury vapour - I am unable to sell prints without OBA - they look strongly yellow, not cream like at home. Those with lots of OBA like Moab Entrada Bright White and Enhanced Matte look the best, Hahnemuhle Photo Rag is fair, Crane Museo was unusable (and I have a box of 32X40 paper!). Personally I like the bright white of the Moab paper though confess that finding a matte board that is even remotely as white is nigh on impossible, making white matting with the signature showing on the print problematic. I did find one board labeled 'digital white' but can't remember the manufacturer - not one I'd heard of before but app. fairly big in the art world. It wasn't quite as bright as the paper but had the same colour - all the others looked sickly next to the Moab paper.

I have not been overly concerned about optical brighteners since they have a 50+ year history of use in photographic papers that no one seems to have complained about. I personally don't like uv protective glass - I'm convinced it has a definite green tinge to it. Ordinary glass has some uv protection - try getting a tan through a window - can't do it. With ordinary glass, I don't notice a radical change in the paper brightness or colour behind the glass suggesting that the paper whiteness isn't just about brighteners. Photographic paper has a baryta coating which makes it white. OBA's are added to that. I'm sure that bright white papers also have a white coating which affects the colour and doesn't in fact fade. The OBA's are just the icing on the cake.

If you look at the back of enhanced matte, you will notice that the paper stock is very warm coloured - clearly the paper has a white coating that is independent of brighteners. Try putting the backside of enhanced matte under uv protective glass next to the front side and compare - I suspect they will still be radically different - ie. it isn't the oba's that make paper white - they just make it bright.

A check on the net shows that baryta consists of barium sulfate - the same thing patients get to drink for x-rays.

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