Filters for UV induced visible Fluorescence

Here now an overview and comparative test of filters for UV induced visible fluorescence, as an outcome of a recent test I made using a Phalaenopsis as a target. I was using for that test my newly developed High Power UV Flash with UV transmitting front filter to stimulate UV induced visible fluorescence.  As optics I used the X35 lens at f8, 1/160 exposure time and the various mentioned filters (but any good lens for visible photography would obviously do).

Just a reminder: The intention was to find a filter, which allows to record visible fluorescence in the 400-650nm range. Deep red or NIR flourescence >650nm is tricky to record, since it coincides with NIR (near infrared) transmitted from the flash, so it would be wiped out in the resulting images (IR leakage). To record that, either a NIR blocking but UV transmissive flash front filter has to be used (a bit tricky, but could be done by sandwiching a BG38 or BG40 filter in front of the UV transmissive flash filter), OR a excinting source like the Nichia 365nm UV LED has to be used, which emits no NIR/IR at all.

[click on image to see a larger version]

1. Schott BG38 filter, transmits UV, but filters out IR - NOT useful


2. Commercial noname IR Cut filter, obviously transmits UV, blocks IR - NOT useful


3. Omega UV cut, IR blocking filter (from 650nm onwards) - too much blue also cut off (missing blue)


4. Baader UV IR Cut filter - nice colors, but transmits to about 700nm, which creates an
issue with most UV transmissive flash filters, since NIR is transmitted (too bright reds)


5. Baader UV IR Cut filter + IR cut filter (from 650nm onwards) - nice and UV flash issue
also nicely covered; but needs two stacked filters


6. Xcut filter, UV + NIR cut, transmits ca 410-650nm - works nicely, the best, all in just one filter


7. Xcut2 filter, UV + NIR cut, transmits ca 400-650nm - works nicely, bit on the cold side with enhanced blue, all in just one filter


8. Tiffen Hotmirror - quite some UV bleed through - NOT useful
[identical to the Canon Hot Mirror Filter btw.]


9. B+W 489 IR blocking filter - quite some UV bleed through - NOT useful
[the B+W 486 digital UV IR cut filter cuts on at 370nm, so it will be about the same, NOT useful result]


10. Noname Hot Mirror Filter - quite some UV bleed through - NOT useful


...and just for the fun of it and for comparison....purely reflected UV...
Baader U-filter ("Venus") 310...390nm


Stay tuned, more will follow on that fascinating subject...

More info on this very interesting field may be found on my site http://www.pbase.com/kds315/uv_photos

High Power UV Flash for stimulated visible Fluorescence

Here a recent test I made to see if my newly developed High Power UV Flash would be able with just one pop to stimulate UV induced visible fluorescence. I used the X35 lens at f8, 1/160 exposure time and a Xcut blocking filter (prototype) ,which only allows 400-650nm vivible light to pass and thus also controls the risk of IR leakage coming from the flash, since Xenon emits plenty of IR. The flash itself is modified for high UV output and uses a quartz flashtube plus a UV transmissive, visible light blocking filter.

The following pic is (C) Michigan State University and shows clearly the high IR output of a normal Xenon flash (red line). The blue line however shows the effect of a flash circuit modification, allowing more amps per square centimeter to pass through the Xenon plasma, so as to enhance UV output and suppress the usually high IR content.

Here now the shot and the proof that the High Power UV Flash idea works to stimulate visible fluorescence:

[click on image to see a larger version]

And now how fluorescent minerals look like using that flash.

All fluorescent minerals from the famous Franklin Mine/USA:

Stay tuned, more will follow on that fascinating subject... More info on this very interesting field may be found on my site http://www.pbase.com/kds315/uv_photos

Principle Thoughts about Lenses + Filters for UV

Some priciple thoughts, why normal lenses and filters do not work for reflected ultraviolet (UV) photography.

I get quite a few questions, why a certain camera, lens, filter setup does not or not satisfyingly work for UV photography. I will try and show the effects of that optical system using some transmission graphs I recently made.

So here some results using one my X-lenses (red line) and a very modern, multicoated lens (purple line), a Baader 2" U-filter (dark green line) and a common UV transmissive filter (very light green line) such as a Schott UG-1, Hoya U-340, B+W 403, Kodak Wratten 18A, Nikon FF etc. sometimes also called "Woods Glass".

Let's look at the filters first. The older, common UV filter has a benefit, quite a high peak UV transmission of about 90% - but it also shows IR transmission from about 670nm onwards. In former times of film based cameras, that was no issue, since these film had no IR sensitivity. Today in the times of digital cameras, and especially the modified ones, their IR sensitivity is quite resp. very high. So that leads to quite some contamination of that supposedly UV image due to "IR leakage". One cure would be to add ("stack") another filter in front which passes UV but blocks or reduces IR. Such a filter would be the Schott blue-green BG-38 or BG-40 filter. But that does not completely cure that problem. The german astronomy company Baader developed because of that fact their Baader-U filter (also called "Venus" Filter, since astronomers use it to make details of the surface of planet Venus visible), with perfectly suppressed IR, as the dark green line nicely proves. This fact made it my "filter workhorse" for reflected UV photography, even if the peak transmission is a bit lower at about 80%.

Now about the lenses. Usually a photographic lens should reproduce visible images, which is the wavelength band of about 400-700nm. This is why lens makers apply a special multicoating (aside from achieving higher light transmission), which usually blocks light outside this visible waveband. The purple line of a recent multicoated lens proves, how succesful this is done today. For UV photography, however, the UV-A band (300 - 400nm) is the interesting one. So using a modern lens and that Baader U-filter leads to that mini UV transmission of about 5% only, which usually causes very long exposure times, grainy images and a lot of frustration for some who wanted to step into that field. Here the wrong lens is the reason for that, not the filter!

One solution would be to get one of the highly specialized $$$$ quartz / fluorite lenses I have reported about here, but this is just for a few who can afford that or have access to one of these lenses - surely a pleasure to use. But the ambitioned amateur or researcher on a tight budget needs a solution too.

This was the reason why I started some reasearch to find lenses which would allow similar image quality, but at affordable cost. So my series of X-lenses was born, the most prominent being the X135 (f=135mm), X50 (f=50mm) and X35 (f=35mm), one of them shown as bright red line which shows quite some more and deeper into the UV reaching transmission, one of them to about 310nm.

The system lens + filter using an X-lens now leads to a much higher UV output to the camera chip (assuming a modern digital SLR camera, which also has to have some UV sensitivity, such as a Nikon D70(s), D40 or modified D200) of about 7 times more, a gain of 3 stops or 3EV in photog speak (this is just about 1 stop less than using the UV Nikkor!). This allows much shorter exposure times, better quality in terms of graininess (noise) and UV reproduction, since also a broader UV waveband is covered.

Just a remark, the high UV transmission is a must for a suitable UV lens, but also good sharpness, high contrast, flare resistance, no hot-spots, etc. are requirements such an X-lens has to meet, and I have not jet mentioned an important point - focus shift. This latter point was solved by calibrating such a lens for UV use, to allow for a sharp visible and UV focus.

[click on image let's you see a larger image]

The explanation is not yet complete, two important parts of the system are not covered here in detail, the camera and the light source(s), which should be used for UV photography. I will elaborate on that at a later time here.

Stay tuned, more will follow on that fascinating subject...

More info on this very interesting field may be found on my site http://www.pbase.com/kds315/uv_photos

Teeth and UV I

Today about teeth and ultraviolet photography.

I found in some older books some contributions of Prof. Woods and esp. Ch. Lunnon about photograping teeth using ultraviolet light.

So here some results using my calibrated for UV X35 lens, a Baader 2" U-filter and my newly developed High Power UV flash.

The top image clearly shows, repairs, cracks, plaque etc. whereas the second one using a special filtering technique also shows bacteria build up (plaque) as reddish cast and repairs having being done. The small size of the images here do not really show the amount of detail which gets visible using these techniques.

[click on image let's you see a larger image]





Stay tuned, more will follow on that fascinating subject...

More info on this very interesting field may be found on my site http://www.pbase.com/kds315/uv_photos

Lens Comparison: UV Nikkor 105mm, COSI 105mm, B&L 9"

Today I got an old lens, which was made many years ago by famous lens maker Bausch & Lomb, Rochester NY. It is a golden color single coated, heavy and well made f4.5 9" lens which attracted me, since it was made to record images emitted by a P-16 phosphor screen. Nothing special actually, but P-16 radiates at about max. 380nm, so now you know what attracted me most...

I ran a quick test using my "standard test subject", a roof of the neighbours house against the Coastal Optics Micro Apo 105mm and the Nikon UV Nikkor 105mm. The B&L lens results are shown on the right side of the comparison images. The B&L images have been scaled down to match the shorter focal length of the other two lenses. These are 100% center crops. The Baader U-filter (310 - 390nm) was used for the reflected UV shots. [click on image gets you a larger image]

Here the results in visible light for the UV Nikkor (left):




and for the Coastal Optics 105mm (left):




Here the results in reflected UV light using the Baader U-Filter (310 - 390nm) for the UV Nikkor (left):




and for the Coastal Optics 105mm (left):




So now it is up to you to understand my surprise, how amazingly sharp this B&L lens is....

Stay tuned, more will follow on that fascinating subject...

More info on this very interesting field may be found on my site http://www.pbase.com/kds315/uv_photos

Allium (Onion) in UV

So this is about onions in UV light. These have very cute ball-shaped flowers and I wondered how these would look like in UV "light". The petals seem to have quite some strong UV reflectance to attract pollinators.

So have a look, left is the visual image, middle the reflected UV image and right the image with UV remapped into the visual as blue [click on image gets you a larger image]:




And this one in a bit higher resolution:




So I hope you like these as much as I do....

Stay tuned, more will follow on that fascinating subject...

More info on this very interesting field may be found on my site http://www.pbase.com/kds315/uv_photos

UV Images Exhibit in Weinheim

So this is about my upcoming next UV picture exhibit which will be opening September 20st, 2008 in Weinheim, Germany in cooperation with very creative and esthetic Christian Mayer Floristik. That same weekend "Weinheimer Hebst" ("Autumn in Weinheim") will take place, with all shops open during the whole weekend. (No big deal you think? Not in Germany I tell you...)

So have a look, these images will be on display there for a few weeks [click on image gets you a larger image]:



















and this is how it looks now there:

[exhibit ended Nov 7th, 2008]

So I hope you like these as much as I do....

Stay tuned, more will follow on that fascinating subject...

More info on this very interesting field may be found on my site http://www.pbase.com/kds315/uv_photos