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Verify a UV filter?

Discussion in 'Accessories' started by BigOwl, Dec 1, 2013.

  1. BigOwl

    BigOwl Mu-43 Regular

    61
    May 3, 2013
    San Antonio, Texas USA
    Al
    Is there any easy way to verify that a UV filter is really blocking out UV? Other than just trusting the brand, how can we know the filter is not just a thin piece of clear glass? For mechanical protection of the lens it doesn't matter, but why pay good money for a filter that is not delivering what you pay for? You can't see the UV and some people say digital sensors do not respond to UV like film does, so how can we know?
     
  2. Ned

    Ned Mu-43 Legend

    Jul 18, 2010
    Alberta, Canada
    You don't need any extra UV filter anymore. People just use them as a protective element these days. Whether or not your UV filter is actually filtering out UV is a moot point in this age.
     
  3. wjiang

    wjiang Mu-43 Hall of Famer

    A UV filter can still reduce purple flare caused by UV on modern sensors, but even the coated ones tend to make general flare worse...
     
  4. mister_roboto

    mister_roboto Mu-43 Top Veteran

    637
    Jun 14, 2011
    Seattle, WA, USA
    Dennis
    If people want to use a filter for protection, I won't dissuade them- however I will say you should use a "not cheap" filter. Get a good multi coated filter, to reduce any bounce back glare you may get.
     
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  5. nickthetasmaniac

    nickthetasmaniac Mu-43 All-Pro

    Jan 11, 2011
    You can't know, but that's the whole point - it doesn't matter.
     
  6. janneman

    janneman Mu-43 Veteran

    414
    Dec 6, 2012
    Netherlands
    Jan (John) Kusters
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  7. jamespetts

    jamespetts Mu-43 Top Veteran

    802
    May 21, 2011
    London, England
    See here for a video testing whether an ultra-violet filter makes any difference (it doesn't).
     
  8. mf100

    mf100 Mu-43 Regular

    95
    Aug 26, 2012
    Sawbridgeworth, England
    Matthew
    I agree. I've found a few situations where I've needed to take the UV filter off (which I use to protect the lens) due to bounce back glare.
     
  9. wjiang

    wjiang Mu-43 Hall of Famer

    With the PL25 I've removed the UV filter permanently (unless I go somewhere really bad) as I got annoyed by flare at night. Since then I've noticed that purple fringing at high contrast edges has gotten a bit worse...
     
  10. yakky

    yakky Mu-43 Top Veteran

    662
    Jul 1, 2013
    I seem to get flare no matter what brand or price I try. The only difference is in coated filters, they seem to lessen it. That said my $10 Agfa does as well as a $50 filter I have.
     
  11. 0dBm

    0dBm Mu-43 Top Veteran

    859
    Jun 30, 2011
    Western United States
    To answer your question technically, you will need the use of a spectrophotometric measuring device: a spectrophotometer. This instrument provides a spectrum of light as the source. The filter is placed between that light source and the photo detector, then the lambda (UV wavelength) is programmed, and the detector reads the magnitudinal value of the light that passes through the filter. The lower the value, the more efficient (better) the filter. The spectrophotometer is first calibrated using a deuterium artifact standard traceable to the US National Institute of Science and Technology (NIST). Spectrophotometers are highly specialized electronic instruments costing up to 100K.
     
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  12. BigOwl

    BigOwl Mu-43 Regular

    61
    May 3, 2013
    San Antonio, Texas USA
    Al
    Thanks for that answer. It is technically very satisfying, but since I do not have access to a spectrophotometer I will have to go with Plan B, which is to use a filter when I want to protect the lens and remove it if I get concerned about flare or purple fringing. Thanks to all who responded to the OP. Time for me to stop worrying and start clicking.
     
  13. Petrochemist

    Petrochemist Mu-43 Top Veteran

    651
    Mar 21, 2013
    N Essex, UK
    Mike
    If you have contacts with access to UV spectroscopy equipment with a large enough sample area it's relative easy to check the UV transmission spectrum. I suspect most people don't have the right contacts, I have the equipment available at work but I'm in the UK so not practical for Texas.

    Another possibility is to jury rig a cruder check with a full spectrum camera (there may be members on the forum near you with suitable cameras)
    Paying a lab to test the filter would be excessive IMO.

    I see no reason to doubt them, UV absorbing filters are cheap to make, and as others have pointed out normal digital cameras already have a UV block built in. Surface reflections is a much greater issue on the specification of filters, and more difficult to measure with routine lab equipment.
     
  14. BigOwl

    BigOwl Mu-43 Regular

    61
    May 3, 2013
    San Antonio, Texas USA
    Al
    Thanks, Mike. I am retired from a research institute and have contacts there who could probably run a test for me, but it's not worth the effort. But since you opened the door for this by admitting to being a "photographic dabbler", I'll tell you about an experiment I ran on my own. I have a UV source (black light lamp) that is used for defect inspection using fluorescent particles or penetrants. Since white cotton shines brightly under UV, I thought to put a UV lens filter between the lamp and a white handkerchief. The filter in no way reduced the fluorescence of the cotton, suggesting to me that the filter had not reduced the UV reaching the cloth. Granted, I do not know the spectral range of the lamp, nor the UV wavelength of the filter, so the result is not conclusive, but it did prompt the question in my original posting. Thanks to all. Now back to clicking.
    Cheers.
    :Al
     
  15. wjiang

    wjiang Mu-43 Hall of Famer

    Surely the filter just attenuates specific wavelengths, rather than stopping UV per se. If you use a strong UV lamp I'm sure a lot of UV energy will still go through the filter even after attenuation.
     
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  16. 0dBm

    0dBm Mu-43 Top Veteran

    859
    Jun 30, 2011
    Western United States
    Mike,
    Reflectance can be measured (quantified) with modern spectrophotometers such as the Perkin-Elmer Lambda 950; however, the technique is more involved with the use of an integrating sphere to account for back scatter when the photons hit and bounce off the detector then hit and bounce off the filter surface. That is when an anti-reflective (AR) coating is of benefit.

    wjiang,
    The stopping factor of UV filters is invariably measured by the amount of transmission attenuation. Yes, stronger light levels can and will overpower the intrinsic attenuation of typical, commercially-available UV filters; however, minimizing the transmission plane (by stopping-down the lens) will help attenuate the transmission magnitude by limiting the sheer amount of light (photons) that hit the camera sensor.

    Did I obfuscate that?:tongue:
     
  17. wjiang

    wjiang Mu-43 Hall of Famer

    That made perfect sense (engineering degree helps), somewhat expected coming from a handle called 0dBm :p

    I'm not sure what you were trying to get at though - that UV purple globs are reduced if you stop down?
     
  18. 0dBm

    0dBm Mu-43 Top Veteran

    859
    Jun 30, 2011
    Western United States
    Yes, UV purple blobs can be minimized when the lens aperture is stopped-down. Not always, though. Depends on other parasitic, peripheral light sources.
     
  19. nickthetasmaniac

    nickthetasmaniac Mu-43 All-Pro

    Jan 11, 2011
    Sounds like the basis for next year's LensRentals 'photo geek' competition entry :wink:
     
  20. Petrochemist

    Petrochemist Mu-43 Top Veteran

    651
    Mar 21, 2013
    N Essex, UK
    Mike
    I've used our Lambda 35 for checking many of my filters, having just checked my limited data on the only UV filter I've run I got an absorbance of 0.7A at 300nm rising fairly steeply after that whilst the visible region gave between 0.015A & 0.1A.
    In transmission terms 20% transmitted at 300nm, & 80% transmitted at 400nm (the boundary between UV & visible) with ~95% transmission through the bulk of the visible region.
    This was probably a fair cheap filter, the poor transmission in the visible region suggests it's no more than singly coated (uncoated glass gives ~4% reflectance loss at each air glass boundary).

    With BigOwl's crude black light test that 20% transmission at 300 might easily be enough to excite any available chromophores, so no change would be seen. Black light lamps generally emit into the visible spectrum too, and if 380nm UV is enough to produce flourscence there's even more of that transmitted.

    We should also remember that the transission between UV & vissible isn't really quite as well defined as people tend to think, some people see further into the UV than others, rather than the convient 400nm switch over. Apparently younger people can usually see a little more, and at one point just after eye surgery I had a distinct diference between my eyes. I don't remember where I saw wavelength data for visible /UV transitions but I don't think many people see below 390nm.

    My point was that the integrating sphere isn't standard lab equipment. I've only seen them in manufacturers demo labs, though I'm sure there are industries where they are in common use.

    Any of the 6 full blown UV/visible spectrometers I've used would be capable of measuring the transmission/absorbance spectra, on filters up to ~ 80mm diameter.
    The cheaper spectrometer I use for water testing (CODs etc) has a more specialised sample compartment in which a photography filter wouldn't fit, and have a user interface designed only for specific tests, So wouldn't be any use.The very basic ones I used at college could probably have been made to work if I'd been willing to cut the filter into a 1cm wide strip.
     
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