Digital BW, The Print

--- In DigitalBlackandWhiteThePrint@y..., Todd Flashner <tflash@e...> 
wrote:
> on 8/22/01 9:57 AM, chameleon@i... wrote:
> > Torchon--100% wood, flourescent, L=95.11, Brightness=97.54
> > Concord Rag--mostly cotton, non-flourescent, L=93.83, B=80.25
> > Somerset E.--all cotton, slightly flourescent, L=95.93, B=93.06
> > and finally...
> > Museo--all cotton, non-flourescent, L=95.35, B=89.5 (SFA is made 
> > to these same L and B specs)
 
> Robert,
> Could you define fluorescence, and elaborate more on how it relates 
> to brightness, and lightness, and if relevant, stability.
> Appreciate your insight,
> Todd

Hopefully this answers Todd Flashner's and Martin Wesley's specific 
questions about OBAs.  The sentences in quotations are from Wilhelm's 
book The Permanence and Care of Color Photographs.  Sentences without 
quotes are further explanations.

"Flourescent brighteners absorb UV radiation, causing the OBAs to 
flouresce (emit light) in the visible region, especially in the blue 
and green portions of the spectrum.  If the illumination source 
contains no UV radiation, flourescent brighteners are not activated 
and the paper appears 'dull' or subtly lacking in brightness."--page 
469

" A drawback of fluorescent brighteners in...papers is that when 
these products are exposed to light and UV radiation over time, they 
gradually lose their ability to fluoresce--in effect, the fluorescent 
brightener "fades".  Thus, the paper gradually becomse faintly yellow 
and less bright in appearance."--page 469

"The UV component of a "standard" museum display illumination must be 
precisely defined--both for conservation reasons and because of the 
different visual effects various levels of UV radiation have on 
fluorescent brighteners.  Incandescent tungsten lamps emit a 
relatively small amount of of UV radiation and have less effect on 
OBAs in papers than daylight or fluorescent illumination.
...Glass-filtered quartz halogen lamps typically emit approximately 
twice as much UV radiation as incandescent lamps. Thus, the whites & 
lighter tones of prints made on papers with OBAs look noticeably 
brighter..."--page 604

This is not metamarism, but a similar kind of effect, where a print 
on a paper like Eclipse, German Etching, etc will look different 
under different light sources.

Regarding the "fading" aspect of OBAs, Wilhelm's book is now about 10 
years old.  We are aware that there are supposedly some lightfast 
OBAs available, but they are VERY EXPENSIVE.  Consequently, in my 
most humble opinion, i doubt if any art paper would use these.  THis 
is my opinion and if I am wrong, I would hope my competitors will 
contradict this, and elaborate on the specific OBA they use.

hope this helps somewhat,
robert rex