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UltravioletPhotography

Barrier Filters: what are they and who needs them?


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Okay, so a "barrier filter" apparently is only used for Blue/Violet Induced Visible Fluorescence, another topic that I have had questions about but haven't found being covered in detail on this forum.

 

Assuming we are photographing within the visible spectrum by using either an unmodded camera with a UV/IR cut filter in front of the sensor, or a full-spectrum camera with a UV/IR cut filter in front of the lens, a barrier filters purpose is to be placed in front of the camera lens to block Blue/Violet light from entering the camera, so that the only light your images show is the visible spectrum with Blue and Violet removed or at least reduced, so you only see the fluorescence, and not the original light source which is causing the fluorescence, just like with UVIVF. Doing this is usually done when photographing underwater marine life.

 

A barrier filter is not needed for Ultraviolet Induced Visible Fluorescence (UVIVF) because humans and camera sensors with UV/IR cut filters in front of them cannot detect UV light, so UV light is already blocked - it is invisible by default. Some might say a barrier filter is needed for UVIVF because 365nm UV LED torches still emit out some visible light. Well, if that is true, couldn't that be eliminated at the light source (ie. putting a UV-only-pass filter in front of the LED)? I suppose you could eliminate the small amount of visible light from a UV LED by getting rid of it at the source or by placing a UV-Blocking filter in front of the lens, either way would seem to work I would think.

 

Does everyone agree with these statements or is my terminology and understanding about this off?

 

Does anyone have any example images to show regarding this topic (maybe images that show what something looks like with and without a barrier filter)

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Some might say a barrier filter is needed for UVIVF because 365nm UV LED torches still emit out some visible light. Well, if that is true, couldn't that be eliminated at the light source (ie. putting a UV-only-pass filter in front of the LED)? I suppose you could eliminate the small amount of visible light from a UV LED by getting rid of it at the source...

 

That is what I do. There are a lot of objects that emit blue light when subjected to UV, so using "yellow" filter on the camera would eliminate large part of that fluorescence.

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We have had lengthy discussions about proper filtration for fluorescence photography. (Links in next post.)

 

The basic "rules" for proper fluorescence photography are:

  • Filter the illumination source
  • Filter the camera lens
  • Shoot in the dark

Mostly here we do UV-induced Visible Fluorescence. The UV illumination is blocked typically with a filter like the Baader UV-Pass filter in an attempt to block any IR or Visible leakage from the light source. 365nm UV-LEDs typically shed some blue light, for example.

The lens is blocked typically with a filter like the Baader UV/IR-Cut filter so that only Visible light is recorded by the sensor.

 

You cannot usually depend on an internal sensor filter to block all UV or IR light. Sensor filters are stronger than they used to be, but it is surprising how much still gets through.

 

I'll go do a search and post some links to previous discussions.

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Info in the following links about fluorescence photography, filters, lights, etc.

 

The very important Shiny Metal Test.

This will contain some images made with and without filters on the light source.

http://www.ultraviol...visible-output/

 

Fluorescence discussion. Also some images with various combos of filtration.

http://www.ultraviol...light-standing/

http://www.ultraviol...ndex.php/topic/

 

Examples of fluorescence photography.

http://www.ultraviol...d-fluorescence/

http://www.ultraviol...resce-under-uv/

 

Blak-ray lamp for fluorescence.

http://www.ultraviol...vf-photography/

 

Infrared fluorescence.

http://www.ultraviol...d-fluorescence/

 

UV contamination in fluorescence photography.

http://www.ultraviol...fl-photography/

 

For some stunning UVIVF photographs go to the Advanced Search: http://www.ultraviol...earch_in=forums

Then enter the following:

  • Find tags: fluorescence
  • Search in section: forums
  • Find in forum: Ultraviolet Botanicals
  • Display results: as topic list

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Evan's post has prompted me to start writing a Fluorescence Sticky so that we will have the basic info all in one place rather than scattered across several topics. :rolleyes:

 

The Fluorescence Sticky

http://www.ultravioletphotography.com/content/index.php/topic/1456-fluorescence-sticky-in-progress/

 

Input Thread - suggestions, comments?

http://www.ultravioletphotography.com/content/index.php/topic/1459-white-balance-fluorescence-stickies-what-to-include-input-please/

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It's weird how you need to filter both the lens and the light source - when both filters are just seperating UV from VIS.

Why do you need to do that? Is 1 filter on the light source filter not strong enough? If it was made thicker, would you still need to place a UV/IR block on the lens?

Couldn't you just use a UV/IR blocker on the lens and forget about filtering the light source?

If the filter does what it says it does, I don't get why you need double filtration when once seem like it would do it. The only reason I can think is that the filter filters out some light but not all - some still gets passed it.

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Couldn't you just use a UV/IR blocker on the lens and forget about filtering the light source?

 

As mentioned somewhere, most UV light sources leak some visible light in addition to the UV. That visible light would be recorded as light reflected from the subject in addition to the fluorescent light emitted by the subject. Thus you would not have a pure fluorescent photograph. The fluorescence would be contaminated by the reflected light.

 

IF you had a UV light source which produced ONLY UV

and

IF you shot in absolute darkness,

then

you could get away with filtering only the lens to block UV and IR

in order to record Visible fluorescence.

 

The difficulty lies in finding a UV light source which produces only UV.

 

Many people don't block the lens if the camera is unmodified. That is making a presumption (unproven) that your camera's internal blocking filter is strong enough to block the UV illumination and any possible IR-fluorescence. This is probably mostly true, but someone sooner or later will challenge you on it anyway.

 

*************

 

Also, is there a reason why you do tests on shiny metal, and not any other material?

 

Actually, I'm not sure I understand your question ??

You want something that reflects the light rather than absorbs it ??

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