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Birefringence patterned abstracts created via cross polarised transilluminated woven plastic media Flamenco Fusion Flight Tiptoe Heckler Cellist ... this was the very first attempted digital abstract image using the technique – as distinct from using Velvia film. A work in progress. BW, dunk

Simply put do UV cameras, or cameras that operate with only UV light, pick up dyes that are florescent even with out a black light to expose them? The dyes I'm talking about are common with 'invisible inks'( google image search for reference), but are also used for tattoos (reference), and even some hair dyes (reference). The best example of what I hope to achieve is this Wikipedia photo that shows two photographs of a man with sunscreen one normal the other UV. As anyone tried something similar but with those kinds of invisible inks mentioned above?

This is a stretch of the forum's general theme as its link to UV is only the recent topic about hummingbirds. This is only partly about visual iridescence. I hope I am forgiven. Last week I was out searching for suitable sites for photos when I stumbled over something I think are very interesting. I had heard about a small nature preserve with bank swallows. When I got there, without any camera, I found a group of nice ornithologists too looking for a more rare visitor, the Bee-eater. They had cameras with fast loong fast tele lenses. The sand bank is fenced off to protect the bank-swallow colony. You have to stay maybe 40m back from the brink. That beautiful bird is rarely seen at my latitudes. It has happened a handfull of times this millennia. It was a privilege just to see the bird hovering in the sky. There is/was a pair feeding small ones inside a deep nest hole in the sand bank. I decided to try to get some images myself. The following day and the day after I got there better equipped and had some luck. The first day the birds only took dragon flies, the second I saw bumble bees, butterflies, and bees in their beaks too. Here are a few of my better images. The last five are from a bird-landing sequence taken at 10 Frames/s Telyt 560/6.8 + novoflex 1.5x extender all images except for the sky image, then Canon EF 400/4.0 IS + EF 1.4x extender

Here's a recent article describing the colorful iridescence of hummingbirds that explores how it works and what it is for. Visible + UV iridescence is described. Lots of great hummer pics of course. https://www.allaboutbirds.org/news/what-is-the-essence-of-iridescence-ask-a-hummingbird Edit: I see there is a Fauna forum. Please move this post there.

I have found a software-tool for setting up and planning for photo stacking (not macro) and using Tilt-shift lenses. The Lumariver Depth of Field Calculator : https://www.lumariver.com/lrdof-manual/ It is also usable to set up an optimal hyperfocal-distance setting or optimal DOF for one shot. The tool is very powerful and maybe a bit difficult to grasp at first, but after a while it becomes increasingly more helpful and easy to use. I am still learning more each time I open the app. There are several tutorials on the site.

A long time ago I read something about the birefringence of ice crystals. Birefringence is a when a material has different refractive index in different directions, causing a ray of light to split into two rays, one called the "ordinary ray" which keeps going in the direction of the original light, and another called the "extraordinary ray," which can be at a different angle, depending on how the light hits the crystal. The two rays vibrate in perpendicular directions, or polarizations, and they move at different speeds. As a result, one ray can get delayed with respect to the other and interfere with it. When the crystal is placed between crossed polarizers, the interference makes lovely colors. In the last few weeks I noticed some giant icicles forming on the building next door to me, and bearing in mind what I knew about the birefringence, I decided to snap some icicles off and photograph them on a light table between some plastic polarizing sheets and a circular polarizer on my camera lens. I harvested the icicles during the daytime and stored them in my freezer. Then after work, I photographed them. For the camera, I used my brand new (to me - it's used) A7iii, which is unconverted at the moment. I put a Tiffen circular polarizer on the lens of my Nikon Micro-NIKKOR 55mm 1:2.8, which was mounted on my copy stand. I used an A4-sized polarizing sheet on my light table, hooked the camera to the computer with the focus approximately set, and got the icicle out of the freezer. I then fine-tuned the focus as fast as I could, and shot a lot of images at different exposures to bracket with. Then I tossed the icicle out the window, one second before I realized that I'd forgotten to take any NORMAL photos of it for comparison. Oopsie. Because birefringence depends on the angle, you get different colors when the object is at different angles. Here's a crop of a pretty part: Kind of opal-like! The colors have been pushed a bit in the above photos, because obviously seeing them is the whole point here. ---- Having photographed an entire icicle, the next day I started to think, "but wait, have I seen everything there is to see here? What about cross-sections?" Because icicles form layer by layer, so in principle you would expect them to have rings, like trees do. So out I went again to harvest another icicle. I got a huge one and stuck it in my freezer. Later that evening, I started to think about how to cut it into sections. My first thought was to try to cut it with a heated wire. I rigged one up, and indeed the wire sank nicely into my test ice cube, but the ice cube REFROZE behind the wire, as it went through! So that idea was discarded. My next try was cutting an ice cube with a sharp kitchen knife. The knife hardly sank in at all. Then I tried sawing it with a butter knife, and that worked a little. The serrated edge was the key ingredient, pulling ice out of the groove. Once I had that realization, I got a bandsaw and that turned out to be the way forward. The other thing I discovered is that the best way to hold an icicle is with a potholder, because towels and paper towels stick to the ice and leave behind ugly loose threads or fibers. I started cutting from the small end (bottom tip) of the icicle (opposite from the end shown above, which is the base). As expected, rings are clearly visible. I had been hoping the rings would be bright and colorful, but that's not what happened. I tried another cut a little higher up the icicle. This was more colorful, and the ice was getting clearer and less cloudy. Finally I tried a cut from the base of the icicle, which was very clear an glassy. This one produced extremely bright colors with no "pushing" at all. Aside from cleaning up dust and hairs that got on the ice, and adjusting the black level to make the background dark, this is mostly unmodified. The details here are fantastic. 1:1 crop:

Another Background-oriented Schlieren photo (using the usual random dot-background consisting of four A4 sheets taped to a foamboard). I will probably add a few more images later as I process them. Credit to Bernard for the idea of doing an iron. There was no water in the iron, so it's not steam.

By rights, this topic doesn't really belong on UVP, which is why I'm sticking it in the chat room, but I have a feeling many people here will be interested in it, because it's about another kind of invisible imaging. I've been working for several years with a Ph.D. student in Florida who is building a large scale wind tunnel for imitating thunderstorm downbursts over models of medium-sized buildings. The problem he has is how to visualize the flow over these models in a wind tunnel the size of small warehouse. Currently he is doing a smoke based system, but it occurred to me that another way would be to put pieces of black tape at various places on the model and heat them with spotlights or small lasers, then visualize the refractive index changes in the air as it flows. The imaging of refractive index gradients has a long history in fluid mechanics and is known as Schlieren imaging. It is usually done with large parabolic mirrors, but a new kind of computational Schlieren called Background-Oriented Schlieren has recently come of age, and it is extremely simple to do, as I will demonstrate below. The basic idea is that you take your warm object that is making the hot air currents and you place it in front of a large screen of randomly placed dots. Small changes in refractive index caused by warming the air make the dots "dance" on the background. Prior to placing the warm object in the scene, you take a "tare" image of just the screen with the dots, and then you can use special programs that use image correlations to determine how far each background dot has moved. The final output is then shown as a grayscale image where the amount of left-right movement of the dots is coded as a shade of gray (or sometimes a false color). The setup is shown below, both as a schematic from a paper by Gary Settles, "Smartphone schlieren and shadowgraph imaging," and also in my kitchen. My setup: Note that the size of the screen I used is actually much too small, which I knew ahead of time, but this was intended only as a proof of concept. I wanted to know how easy it was, and how practical it would be to build a set up in a large wind tunnel. I also wanted to demonstrate the concepts for my student, and show him how to do the processing. My tare image (actually an average of 30 aligned images to reduce noise) looked like this: A second image with the candle lit looked like this. No visible movement of the background is obvious and it couldn't be seen with the naked eye either. At this point I was very nervous that it wouldn't work! After processing the images, though, the airflow popped right out! Images were processed in MATLAB using a freeware program called PIVLab. Places where I had no dot screen or that had very little texture for the PIVLab program to detect came out noisy. I have also Photoshopped the candle and bowl back into the photo, which is standard procedure in BOS imaging. Here are some more examples: I do believe this is the first time THE AIR ITSELF has been imaged on UVP! I also took thermal photos, but what you see here is not the air but the soot from the candle flame (since gases don't emit blackbody radiation). And before Stefano points it out, yes, I COULD have visualized the CO2 absorption in MWIR with the other camera, but at the moment I don't have a way to support that 7kg camera in my kitchen. https://vimeo.com/535737212

After reading the Petapixel article dabateman linked, I had to run off and try it immediately, so here you go. Beauty is in the eye of the beholder, but the eye you behold is mine. Equipment Linear polaroid film taped over a ring light (Nikon macro speedlight SB-29), Sony A7S modified, BG38 2mm + Tiffen 2E for visible light photos + Tiffen circular polarizer, Micro-Nikkor 55mm/2.8 (old manual version). Flash at ~5 degrees to camera line of sight Another shot Flash at 45 degrees to camera line of sight UPDATE: I tried again using a Tiffen 12 + polarizer, followed by the usual Aerochrome-style channel subtraction and swapping. Got this result. I think you can't see the effect as well in IR, although that may be because the polarizer doesn't work quite as well there? This is mostly red and 700nm IR I think, so not very deep in NIR. UPDATE2: I took another visible light (BG38 2mm + Tiffen polarizer on the camera) image from a profile view.

I was reading this interesting cross polarization article on petapixel, and was surprised that brown eyes would have more melanin. https://petapixel.com/2021/01/20/photographing-rainbow-eyes-by-using-birefringence/ It seems to actually be true and blue eyes are more prone to UV damage. Yeah me, I didn't know my eyes were the worst for UV damage. Maybe make sense why I can see deep into UV. Thought I would post this to remind other lucky blue eyes people, to: ALWAYS WEAR YOUR EYE PROTECTION!!

Ok, firstly I apologise for the click-bait title. I was reading some articles for a book chapter I'm writing at the moment, and came across the following piece from New Scientist in 1998 - "Vikings were surprisingly well focused". They were apparently making aspheric, quartz lenses, about 1000 years ago. Link to the article here - https://www.newscien...y-well-focused/ Actually, there is a bit more on it here - http://news.bbc.co.u...tech/702478.stm - and it looks like they may have had an Eastern European origin.

I keep trying to get my head around what, if anything the false colours we see in a white balanced (against Spectralon) UV shot may correspond to. I decided I didn't have a suitable prism to generate a rainbow but the noticed the colours produced on a typical CD or DVD. These shots are meant to stimulate discussions. Camera was full spectrum modified Pentax K-5, lens was Nikon Rayfact PF1054MF-UV 105 mm f/4.5 Visible light shot of CD in sunlight, 1/50s @ f/11 ISO 200 B&W UV/IR cut filter UV light of CD in sunlight, 1.0 s @ f/11 ISO 200, BaaderU UV-pass filter, white balance in Photoninja on Spectralon (through central hole in CD) Visible light shot of DVD in sunlight, 1/50 s @ f/11 ISO 200, B&W UV/IR cut filter UV light of DVD in sunlight, 0.5 s @ f/11 ISO 200, Baader U UV-pass filter, white balance on Spectralon The CD pair clearly show that the visible colours are removed by the Baader U filter in the UV shot, but there are some "false colours" which do seem to resemble some of those found in UV images of wildflowers. The DVD appears to have a blue layer on the disc which biases the results compared to the CD. I suppose those of our members who have been shooting UV for a long time will have done this for themselves. Dave