rfcurry Posted November 23, 2014 Share Posted November 23, 2014 Someone inquired recently about his UV-bandpass filter fluorescing. I thought that I would provide my reply to the forum as not everyone may be aware of the fluorescence of optical glass. Schott says of their filter glasses -The more or less pronounced luminescence of the optical filter glasses is only interestingfor practical purposes if these filters are to be used to measure the luminescenceof materials. Here, the application of optical filter glasses as excitationfilters, i.e. for spectral isolation of the exciting radiation, presents no problem inmost cases. from http://www.schott.co...pt_2013_eng.pdf Since the glass is absorbing some UV photons, and emitting a longer wavelength, there is some loss of transmission. That, however, is the nature of the beast. Schott has an excellent article on the fluorescence of their clear optical glass here - http://fp.optics.ari...al_glass_us.pdfSee also http://www.laserfocu...n-expected.html Link to comment
Andrea B. Posted November 26, 2014 Share Posted November 26, 2014 Reed, thank you for these links. It is a good idea to shine a UV torch into a lens under consideration for use as UV-capable to see if there is any fluorescence from the glass, the lens interior, the glues and so forth.Also we should check our sensor stack to see what might be happening there as was discussed in a recent thread. The (above linked, relinked here) Schott booklet entitled Optical Glass is worth reading for many reasons. I encourage everyone to look through it. You can skip the math! (But you shouldn't, said the ex-math-teacher.) One good point to be made is that our filters do not last forever. They are subject to various kinds of chemical deterioration. For example: The S8612 and BG39, which many of us use as IR blockers, are Schott Group 3 filters possiblly subject to a change in the glass surface after a few months of normal storage unless coated or laminated. Then there are the effects on a filter simply by coming into contact with ordinary handling and cleaning. Link to comment
colinbm Posted November 27, 2014 Share Posted November 27, 2014 Thanks for posting this interesting material, Reed.Col Link to comment
rfcurry Posted November 27, 2014 Author Share Posted November 27, 2014 Another thing to consider when buying old lenses for UV work, is the glass composition. Some of the old "radioactive" Takumars develop such an amber hue that they pass only miniscule amounts of UV. This is due to the addition of thorium oxide to the glass, up to 30% by weight. Most of the old German glass, both crown and flint, is pretty clear, but Zeiss did use thorium oxide in some lenses, so you might want to avoid those. (see a list at http://camerapedia.wikia.com/wiki/Radioactive_lenses ) Link to comment
OlDoinyo Posted November 28, 2014 Share Posted November 28, 2014 Supposedly, the yellowing developed by thoriated lenses can be reversed by bleaching with UV light. Link to comment
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