Re: Phosphors
Re: Phosphors
- Subject: Re: Phosphors
- From: Robin Myers <email@hidden>
- Date: Sun, 10 Aug 2014 11:24:29 -0700
>> You are correct in how the optical brighteners (OB) or fluorescent whitening agents (FWA) in paper operate.
>
> Also, to clear up something else I think might be confusing Steve: narrowband and especially monochromatic effects are the exception, not the rule -- and even in sources we generally think of as being very "spiky."
>
> A great example is mercury, the main source of emissions in fluorescent bulbs. Start here:
>
> http://physics.nist.gov/PhysRefData/ASD/lines_form.html
>
> and put in Hg I for the spectrum, 350 and 800 for the lower and upper wavelengths, and tick the button for the line identification plot. At the very bottom of the form, click the "Retrieve Data" button. On the next page, you'll see every emissions line NIST knows about for Mercury in the visible spectrum -- and there're at least a couple hundred such lines. At the very bottom is a link to a PDF with the location of each line. There're a lot of noticeable gaps, but there're also lots of overlapping sections. However, all but of a few of those lines are vanishingly faint in relation to the few prominent lines you might be familiar with. Same thing even with Hydrogen, the least cluttered of spectra; it's very messy in the far violet / near UV range.
Thanks for the link to the NIST site. That saves some work looking up emission lines in my old copy of the CRC Handbook of Chemistry and Physics.
> If you want to see this in effect for yourself...get or make yourself a spectrometer for visual observation and some fluorescent objects. For light sources (such as bulbs) too bright for safe or comfortable direct observation, a diffuser of PTFE thread seal tape is perfect -- poor man's Spectralon. Either bounce the light source off a few layers of the tape, or cover the entrance slit of the spectrometer with the tape and point it at the source. You'll see the sharply-defined lines (with the sharpness of the lines depending on the design and quality of the spectroscope), but you'll also faintly see most or even all of the rest of the spectrum. (Of course, this also depends on how well you block out stray light.)
If someone is not very mechanically inclined, I recommend a cheap spectroscope such as the Hand-Held Spectroscope ($10.95) or Pocket Spectroscope ($52.95) on the Educational Innovations (http://educationalinnovations.com). From Edmund Scientifics there is the Spectroscope ($1.95) (http://www.scientificsonline.com/spectroscope.html) or the Desktop Spectronomy Kit ($49.95) (http://www.scientificsonline.com/desktop-spectronomy-kit.html). I have found such devices useful for some color seminars.
> So, while quantum mechanics tells us that these effects are quantized and come in neat discreet packages, quantum indeterminacy also tells us that you've got a bit of wiggle room...and the sheer complexity of systems of so many different elements and compounds (just how pure _is_ the mercury vapor in that glass tube?) makes it such that it's very, very hard to ever actually encounter truly monochromatic effects -- and, most of the time, what you get is broadband.
>
> For optical brighteners, this is a good thing. The ideal optical brightener would perfectly balance out the natural gradual drop in reflectivity at the blue end of the spectrum found in most substrates -- which means the most fluorescence at violet and a slow taper to no fluorescence somewhere around yellow, in inverse proportion to the spectrum of the paper. Or, you can go with a coating (with barium sulphate being perhaps ideal) that itself has a flat spectrum without any need for fluorescence, which is what the very best papers do.
>
> ...thinking about it, baryta-coated Tyvek might be the ultimate in neutral white papers intended for framing. Any paper manufacturers reading this who want somebody to play with such a paper, give an holler....
Tyvek is also available with an inkjet printer compatible coating, but the company that makes this (I believe it is Dupont) used an FWA in the coating. It has a nice smooth matte surface, but is useless as a white reference.
Also, do not over-estimate the usefulness of FWA/OB agents for papers. Due to light exposure and chemical processes these agents can fade rapidly (days to weeks), resulting in a yellowish substrate and rendering their purpose moot.
There are some very white (CIECAM02 bc value < 1) papers available on the market. I have a list of papers without FWA/OB agents on my website at http://rmimaging.com/information/fine_art_paper.html. The CIECAM02 J, ac, and bc values for the papers are listed in the table.
Robin Myers
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