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UltravioletPhotography

Laser-Stimulated Fluorescence: first tests


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I first read about Laser-Stimulated Fluorescence (LSF, sometimes also called Laser-induced Fluorescence, LIF) in the paper, "Laser-Stimulated Fluorescence in Paleontology" (Kaye et al., 2015) which has spectacular photos of fossils produced by this method. The main advantages of LSF over UVIVF is simply that laser beams are (1) columnated, so it's possible to do fluorescence photos at a distance from the subject where a torch is impractical, and (2) extremely intense, depending on the laser, with the possibility to see faint fluorescence which would otherwise not be able to be imaged. My main motivation is actually item 1 — I would like to do fluorescence photos of large objects (especially the kind of faded signs that I used Independent Component Analysis on here).

 

I highly recommend reading that Kaye et al. paper, which has almost all the information you need to get set up, as well as beautiful fossil photos. Kaye and another student have also written another paper on LSF in caves, which has gorgeous photos. I highly recommend both of these papers as "required reading" if you are interested in pursuing LSF. If you prefer to get your information in audio format, both authors talk for 38 minutes

, which has some info that isn't mentioned in either of the papers as well as their personal experiences.

 

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Basic Setup and Methodology

 

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(from Kaye et al., 2015)

 

We are all familiar with the light painting technique, but in this case, because lasers usually produce only a single bright spot, it isn't practical to directly implement it. Instead, the optimal setup is a laser with a special lens called a Powell lens which spreads the beam into a uniform line. If you buy a laser online, they will sell you one with a "line lens" but it will be a cylindrical lens that does not produce uniform flux. Powell lenses are much better for this application. They can be purchased many places and I haven't bought one yet -- I am using a cylindrical lens for the experiment below, however I plan to get a Powell soon. The authors recommend Powell lenses that have very small angles to keep the irradiance high and keep the beam easy to direct.

 

ThorLabs link to Powell lenses

 

For the choice of laser, in the papers above they usually use powers between 150mW and 500mW and 405nm. Technically it is a violet-induced fluorescence rather than UVIVF. I chose to buy a 100mW laser with a 120 degree cylindrical lens (cheap Chinese laser, $30) for my first attempt, and then once I understand the method and limitations better, I plan to buy a more powerful laser and a Powell lens of perhaps 10 or 20 degrees.

 

WARNING: Do NOT use any laser beyond 500mW! Not only is it overkill for this application, laser power can be non-intuitive for people who haven't used lasers before: a 1W laser can burn matches, pop balloons, start wild fires. A 5W laser will cut plastic sheets nicely and put an eye out with ease.

 

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For filtering the source, no filter is needed on 405nm lasers, but if you try a green laser, you may need to filter out infrared with a BG38 or something because green lasers are really infrared lasers that go through a crystal that converts the beam to green. Many of them leak infrared, sometimes dangerously.

 

For filtering the camera, I am currently using a Tiffen Haze 2E filter + BG38 2mm just like I use for other fluorescence photos. I am not yet sure whether the 2E has enough blocking at 405nm to stop the laser. Some experiments need to be done to verify this.

 

The authors have built a motorized setup to do the light painting automatically. This is not necessary, but in my first test I found that it's very easy to linger too long in one location with the laser-line and make an accidental bright spot. I think building a motorized setup is a good idea to get uniform results.

 

Safety

Lasers are a completely different beast than the sources we are used to on here and need different safety precautions.

 

1) You NEED safety glasses. They are not optional, not even for the lower end of the power scale mentioned above. These are Class IIIB lasers, they can cause eye damage fairly quickly at close range, even when spread into a line.

- When purchasing safety glasses, you must select glasses that filter the wavelength of laser you are using (405nm in this case) and with sufficient OD for the power of laser (mine are rated to OD6).

- Look for glasses that are wrap-around and protect the sides and bottom from reflections. With lasers, the reflections can be almost as bad as the original beam. Even DIFFUSE laser light over sufficiently long periods can cause eye problems.

 

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2) My friend who has worked with lasers cautioned me that the safety glasses must NOT be scratched because that significantly weakens their protection, so always keep them in their case when not in use.

 

3) All reflecting items (mirrors, shiny surfaces) should be covered beforehand.

 

4) Outdoors, lasers should not be used around other people. Obviously never shine them where they might hit a driver, a pilot, or shine in someone's eyes. As well as immoral, it's very illegal in the US (and probably in most countries). My plans with regard to imaging signage is to pick locations that are abandoned.

 

5) It's probably a good idea to know beforehand whether (or at what distance) the beam produces significant heating. You don't want to set your subject (or worse, dry vegetation) on fire. Goes double if you live in a dry place like the US southwest. Know before you go.

 

In addition to the above, I found a forum for laser hobbyists that has a nice sticky on getting started with lasers, including an (even more) elaborate section on safety than I included here.

 

First Test

 

Here is the same gourd that I did UVIVF on previously. The exact same color profile was used here to allow for comparison. The laser was light painted from very close to keep the intensity very high. The beam spread was 120 degrees, which is inconveniently large, and no significant heating was observed at any distance.

 

LSF, Micro-Nikkor 55mm/2.8, BG38 2mm + Tiffen Haze 2E

Laser: 405nm, 100mW (nominal), cylindrical lens, distance roughly 1/3 meter.

F/8, 30", ISO100

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For comparison, here was the UVIVF result from before. Note that several months have passed and additional mould has grown.

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Final Notes

My experience with the Chinese seller, Laserlands, was atrocious. They did not respond to repeated emails, nor to Skype, nor to a Facebook contact, for weeks. I will not buy any more lasers from them and would recommend keeping far away.

 

Also, my intent, if Andrea agrees and if there's sufficient interest, would be to expand this post into a sticky.

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Interesting. Are you scanning the subject with the laser line? If so, would making the laser beam a uniform "plane" make the system more practical, even if you lose the intensity advantage?

 

I am glad you talked about safety. Staring into a laser is not like staring into an LED. I got an afterimage last three days in my right eye due to a laser like your, 405 nm (dot, not line).

 

Definitely do this indoors, or in your garden at most. Apart from shining lasers at people, in some countries (like Italy) it is actually illegal to carry a laser more powerful than 5 mW outside your house without a valid reason. It is considered the same as carring a knife or a baseball bat.

 

Looking forward for more.

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Colin, the line appears to be about 3-5 mm or so, at a distance of a meter, and obviously it gets somewhat wider as you get further away.

 

Stefano, I think keeping it a line is much better. You lose the entire advantage of the collimation if you spread out the beam. The original reason I decided to try this, aside from being impressed with the above paper, was that I tried to image some graffiti using the Nemo torch at a distance of 3-5 meters or so, and the beam was much too dim to get a useful result, not even with image subtraction of the ambient light. This will not change if you use a laser, light is still light. If you spread it out to much, you won't get any signal.

 

Regarding legal stuff: obviously know the rules of your home country. In the US I didn't see any laws aside from the one that says you can't shine it in anyone's eyes or especially in the air (pilots) without getting in huge trouble. In the US, most regulations are for the sellers.

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Yes exactly colin. You want to keep it narrow for two reasons: 1) higher intensity, 2) less chance of causing unintended reflections or hitting something you don't want to illuminate (whether that is a person or a window or...). 120 degrees is much too wide, I plan to ditch this laser soon and find something a little more controllable.
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Colin, I added a photo of the setup in the Kaye paper to the original post above to help make things clearer.
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Colin, I added a photo of the setup in the Kaye paper to the original post above to help make things clearer.

 

Excellent thanks Andy.

You could put this laser on a remote motorised gimbal to scan the scene.

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Yes, that's what they did! I am thinking of rigging up something with an Arduino and a stepper motor, perhaps triggered by the camera's flash shoe?
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Very fun times. You are getting closer to my PhD using 2-photon light scanning to image fluorescent aggregates on the surface of cells. Only on a much more macro level.

Oh lasers are dangerous, and should not be under estimated. They may also have a spread light leak point as a reflection of any front lens. So watch out for the sides as well, and make sure to test around it with paper, as that should light up well with your 405nm beam.

 

Many labs I remember visiting would mark a spot in the room where you should never stand due to reflectance from the optical bench. One lab in NIH focused the leak point near the supervisor's door. So when ever he left his office he needed to have his eyes closed. Claimed not intentionally, but we all know better.

 

You should consider this as if not more dangerous than a 254nm light.

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dabateman, keep in mind it's spread into a line, so the flux is much much lower than it would be if focussed to a point. I could not see any local heating at all when I shined the laser in my bathtub to check for burning. Not even warmth. I suspect that the actual flux is probably similar to a 5mW laser pointer in the 120 degree case. I do intend to be careful but given it wasn't even warm at close range, I'm not sure how worried I am. That would change if I get a different laser or focussed the beam more.
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Fascinating, Andy. I won't be touching lasers after all the warnings in the posts above - still looking for a lead suit to use with the hand-held 254nm light I've bought.

 

Why did you choose 405 nm? I haven't looked, but I would have thought that you can get lasers at a variety of wavelengths across UV, Vis, and IR. With the light being almost perfectly monochromatic there must be the opportunity to try different frequencies to get UVIUVF, UVIVF, UVIIRF, VIVF, VIIRF, IRIIRF (OK - enough with the abbreviations already)

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Fascinating, Andy. I won't be touching lasers after all the warnings in the posts above - still looking for a lead suit to use with the hand-held 254nm light I've bought.

 

Why did you choose 405 nm? I haven't looked, but I would have thought that you can get lasers at a variety of wavelengths across UV, Vis, and IR. With the light being almost perfectly monochromatic there must be the opportunity to try different frequencies to get UVIUVF, UVIVF, UVIIRF, VIVF, VIIRF, IRIIRF (OK - enough with the abbreviations already)

If you have a reasonably limited budget and want something portable semiconductor lasers are the best alternative.

Then the 405nm variant is the shortest wavelength available.

There are not that many alternative wavelengths to choose from.

https://en.wikipedia.org/wiki/Laser_diode

 

There are laser with shorter wavelengths but then they are playing in an entirely different division.

https://en.wikipedia.org/wiki/Excimer_laser

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Yes YouTube, Netflix and Amazon have killed the disc race, at least for me. I don't buy DVD (650nm laser) or DVD-R (635nm laser), let a lone Blu-ray (405nm laser). So the push for mass produced UV lasers fell apart. We online video viewers are to blame.

4K Discs still just use 405nm laser diodes and the 365/370nm diodes just didn't happen. But at least we got cheap LED flashlights for their work to that point.

 

Bernard the other option is the same as a Mercury bulbs, just lines off a Mercury or Argon light. Thus why 488nm and 365nm lines are popular on microscopes. It the same source lamp for your line.

 

Taken from wikipedia:

"Argon-ion lasers emit at 13 wavelengths through the visible and ultraviolet spectra, including: 351.1 nm, 363.8 nm, 454.6 nm, 457.9 nm, 465.8 nm, 476.5 nm, 488.0 nm, 496.5 nm, 501.7 nm, 514.5 nm, 528.7 nm, and 1092.3 nm."

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Colin NO NO NO, that is MUCH TOO POWERFUL. That laser could easily punch holes through things. You want a laser in the 100-500mW range (note the crucial "m" in "mW").

 

I would start with something towards the 100mW end, like I have, and increase power very gradually and only as necessary.

 

You do also need a Powell lens (or at minimum a line lens).

 

I am going to add some bold face warnings up above about suitable laser power.

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Yep that one looks much better for this.

 

Check out what a 20W laser can do! Laser cutting starts at 1:29.

 

Don't forget your safety goggles! Look for ones that have side protection (wraparound).

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Andy Is this like what is needed.....it will need the 'line' lens.

The ad is 5.5W but it has a 20W sticker ?

https://www.ebay.com...er/223817937695

Andy is absolutely right about the power need, stated above. TOO POWERFUL!

 

Col, If you read the text below the picture in the add you linked you will see that 20W is the input power and 5.5W is the optical output power.

Just as with LEDs there are a lot of thermal losses that needs some cooling.

 

This is a laser for basic engraving and cutting with a tiny focussed spot, used in some DIY maker systems.

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