Digital BW, The Print

> In a post by "yohnnyboy" chhopk@... the writer ...
> asserts that the Wilhelm data is misleading. ...
> since Wilhelm's tests are based on fluorescent lighting, 
>they in no way reflect accurately what we can expect for print 
> life for our prints which are exposed to daylight ...

Sunlight contains a lot of energetic, short-wavelength UV that is filtered
out by standard window glass.  So, for outside display UV coatings are much
more important, and prints can be expected to last much less time. 

I think Wilhelm is fairly clear that he tests for indoor display.  For that
I think his work is fine, although any "years of display" estimate must be
taken as a very rough estimate that is subject to lots errors due to
differing display conditions.

Livick prefers sunlight, but that is only better if you'll be displaying
your prints in sunlight.  Aside from the UV content of direct sun, it is
probably the amount of light -- the intensity -- that is most relevant.  So,
the relative results of testing with either should be close.

How one defines the "normal" display lighting conditions is an issue Wilhelm
has pointed to in his criticism of what Kodak is doing.  Kodak seems to
assume very dim normal display lighting.  I'm not sure what the "correct"
amount of light is, but when it comes to stating results in terms of "years
of display" the assumption about display light intensity obviously has a
direct bearing.  

Other factors like humidity may also be important.  The brighter and hotter
the light, the more the print will be artificially dried and the results
exaggerated.  On the other hand, excess heat might be more like accelerated
aging than fade testing.  Many companies use the Xenon faders, which are
very fast and hot.  I think MIS used one of these for its UT testing.  With
the commercial Xenon light faders there are usually filters that one inserts
to change from outdoor to indoor lighting conditions.  Sometimes what filter
was used is not clear when the results are displayed.  However, this might
make a difference.

I don't think any procedure is perfect, so I look at all the results and try
to learn what I can from them.

For my own uses, I find fluorescent light easier to control in terms of
consistency and repeatability.  It's cool and efficient, but it also takes a
long time.  

I just do comparisons of test strips that are in the fader at the same time,
and I only look at the relative results.  Assigning a "years of display"
seems like a stretch.  So, I'll leave that to others (and let them defend
the results).

Paul
www.PaulRoark.com