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Reflected-UV photography

reflected-UV Forensic Science

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#1 Sian Smith

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Posted 20 April 2016 - 08:49 AM

I was wondering if anyone had explored using reflected UV photography with any of the photogrammetric methods as a way of enhancing surface detail? I know that reflected UV is usually done using film but there are some papers that have used digital cameras as well. If this is possible then I could capture images in the same way as usual and create the model using these. 

 

My project is looking at identification and analysis of skeletal trauma - UV light doesn't penetrate the bone as deeply as visible light so you can increase the clarity, but my thinking is that it might also reveal some surface scratches or even trace elements from the weapon. 

 

I would really appreciate anyone's expertise here



#2 GeorgeBevan

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Posted 21 April 2016 - 08:51 PM

Hi Sian, 

 

We've done lots of reflected UV photography and it is a very powerful technique for observing certain types of surface features. I assume you're talking about the UVA range, not UVB and UVC. There have been some experiments with film in the deeper UV range but it is very tricky and dangerous light to work with, particularly UVC. Recently I worked on a project using reflected UV to identify the degradation mechanisms of the patterns on the wings of moths in natural history collections. It remains, as you indicated, a much less explored technique than IR, but there are some practical reasons for that.

 

1) you need lots and lots of UVA light across the 300-400nm range. Be prepared for long exposure times. I would recommend using strobes converted to output UV. Remember that most commercial strobes have coatings intended to reduce UV output so you'll have to do some research. 

2) normal photograph glass absorbs UVA to a high degree. If you're serious about reflected UV consider purchasing the Coastal Optics 60mm UV/IR lens: http://www.jenoptik-...-apo-macro.html

It is VERY expensive but worth every penny, particularly because it is focus-corrected across the UV-VIS-IR range. That means you can focus through the viewfinder in visible light and then drop the filter in place and get the same tack-sharp focus. Non-focus corrected lenses require an adjustment when you move out of the visible range to keep focus. 

3) you need a really good filter. The Baader "Venus" filter is currently the best UVA cut filter on the market but is quite expensive in itself: http://www.company7....ilter_bpu2.html

you'll also need a pile of adapters to put this relatively small astronomical filter on a DSLR lens.

4) you'll need to get your DSLR converted to remove the Internal Cut-off Filter that is intended to remove extraneous UV and IR light so as to improve exposure and metering in the visible range. A monochrome sensor would be ideal with no Bayer filter to further cut-down on UV transmission. I've been meaning to experiment with Foveon sensors in this application to see if they can get us better sensitivity. 

 

Once you've mastered UVR for single images then you could consider using it for RTI and photogrammetry. I hope that helps get you started!


  • GOKConservator and Ayman Yaghi like this

#3 GOKConservator

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Posted 26 August 2016 - 04:41 PM

Great advice, George and great question Sian. Would UV-induced autofluorescence photogrammetry shooting in the visible also be useful? I've done this with RTI because there is no specular and the assembled normal map is pretty flat because the surface behaves as if it is totally lambertian. This also works in photometric stereo using Matlab.



#4 Taylor Bennett

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Posted 28 August 2016 - 03:13 AM

I've been doing reflected UV using a modified mirrorless, micro four-thirds camera, which allows you to focus in live view without having to refocus when you capture the images.  I use a small, 1-watt UV-LED flashlight as both a focusing aid and as the UV-A radiation source for reflected UV imaging.  I use a relatively inexpensive Noflexar Novoflex 35mm f/3.5 macro, which has good UV transmission properties because of its lack of UV coatings.  Here are links to information about this lens as well as other specialized lens options on Dr. Klaus Schmidt's UV photography and macro lens websites (for which I'm relying on his prior permission to cite): 

http://photographyof...parison_30.html

http://www.macrolenses.de/ 

 

Some versions of this lens reportedly perform better than others in the UV and it's worth checking before buying. [Disclosure:  I purchased my Noflexar macro from Dr. Schmidt.]  But they're much less expensive than the apochromatic lenses and allow UV imaging with reasonable exposure times (10-30 sec on my camera, usually).  As George points out, you'll still get some focus shift using this lens when you compare IR and visible images to the UV image.  For stacking and image registration, there are several options (Photoshop, CHI's Imalign tool, and others).

 

I agree with George that the Baader Venus filter is the best for UV reflectance.  I find I get better contrast with shorter exposures using the Baader filter.  The Baader filter is expensive and delicate; you can also use a variety of combinations of UV-pass + IR-blocking filters, which have varying performance, such as those discussed here: 

http://photographyof...ltraviolet.html

and here:

http://www.ultraviol...mission-charts/

 

It's important to block any IR component in the UV source to get good UV reflectance images, because silicon CCD and CMOS sensors are more sensitive in the IR than in the UV, so even a small amount of IR leakage will ruin the UV image.  Since the Baader filter is so effective at blocking IR, you can even use daylight or any source of UV-A with the Baader filter if you give adequate consideration to the sensitivity of the object to UV, but this might not be a concern with skeletal remains.  Wear UV-protective plastic glasses to protect your eyes from UV-A (they're inexpensive and easily available).  There are several sources for how to process the images; probably the best source is the AIC Guide.

 

For UV-induced visible fluorescence that Dale suggested, you'll need a dark room with little to no ambient visible light; a good UV + IR cutoff filter (visible bandpass filter) such as a Hoya IR/UV cutoff filter; and a clean source of UV radiation with no visible blue component.  This usually requires filtering the UV source, even if it says "365 nm LED" or such.  These LEDs nearly always have a blue tail that extends to wavelengths around 400-410 nm, which interferes with the induced visible fluorescence and degrades the images.  I put a Hoya U-340 filter on the UV-LED flashlight to reduce the blue tail, which gives me a peak wavelength of 371 nm and near-zero blue tail.

 

For UV reflectance and UV-induced fluorescence using the flashlight, I use a long exposure (usually between 10 and 30 sec at around f/5.6 to f/11) and sweep the flashlight beam across the surface to get an even exposure.  I might do this 3-4 times and choose the best image.  Because you're capturing lots of images and good, even exposure is critical for photogrammetry, this technique won't work for photogrammetry.  So you'll probably want to get a stronger UV source or a pair of sources that you can leave stationary relative to the object.  Strong sources of UV-A with good filtration to remove the blue tail tend to either cost a lot or they're bulky and awkward to handle, which is why you may want to follow George's suggestion of modifying strobes.  I've attempted this but found they didn't produce enough UV-A for my needs, a topic for another discussion. 

 

The trick to getting good UV-induced visible fluorescence images is good filtration on the UV source to eliminate the visible blue tail; good filtration on the lens to cutoff the UV source and any stray IR (only if you're using a modified full-spectrum camera); plus a dark room to reduce stray ambient light.  However, at least it doesn't require a modified camera or a special lens, and it has other benefits as Dale suggested.

 

Because UV-C has shorter wavelengths, it has special optical properties that are useful, but you can't capture UV-C reflectance images using any consumer camera with a silicon-based sensor.  You need both special optics and a camera with a specialized sensor to capture UV-C reflectance, and as George mentioned, UV-C is dangerous to work with.  You absolutely need to wear protective eyewear and cover skin, and do due diligence.  However, you can capture UV-C induced visible fluorescence with an unmodified consumer camera if you put the right filtration on both the source of UV-C and the lens (to eliminate any visible components of the UV source or stray ambient light).  These topics are also covered in the AIC Guide and elsewhere.



#5 Ayman Yaghi

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Posted 12 July 2017 - 08:16 PM

Dear all 

I've tried to do RTI captures by using a special camera for UV reflection and UV flash , but after processing the photos in RTI builder , i'm getting unsharp view of the object in RTI viewer , have u tired before ( UV reflection with UV flash ( ? any suggestions ? 

best regards 

 Ayman 



#6 Carla Schroer

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Posted 21 July 2017 - 12:05 AM

Ayman - this really belongs as a new topic in RTI processing - this is a photogrammetry forum.  I can move topics, but not individual posts.

 

At any rate, I suspect it may be that you have some movement somewhere in your image set.  Either the subject moved, or the camera moved.  Even a few pixels can cause the RTI to appear blurry.  In RTiBuilder, you can sometimes spot this, by choosing the first image then the last image in the sequence, after the sphere is detected.  If the circle isn't in the same place on the sphere, then that means either the camera or the sphere moved.  If it was the subject that moved, then this won't tell you.  You can try loading the first and last image in layers in photoshop and zooming way in like 400%) then turning a layer on and off.  That can tell you if there is a misalignment.

 

We have a tool to fix misalignment in image sets, though we haven't had the fund to finish it and release it.  I do have a beta version for Mac only, if that helps.  Otherwise, you can try aligning the images in photoshop, or some other tool. 

 

Hope this helps.

 

Carla






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