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UltravioletPhotography

Full-colour/Tri-colour UV and IR


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A process for full-colour UV (actually, UVA) photographs using tri-colour separation images is covered in the thread at https://www.ultravio...__fromsearch__1 . An equivalent process for IR (actually, NIR) uses the same methods, but with different filters (and without the problems associated with the camera's low sensitivity in UV).

 

In this post are some images which show typical results from these techniques. Unlike simulated Aerochrome images, no visible light is involved – the images are pure UV or pure IR.

 

Just as a reminder, the tri-colour separation images used the following filters:

UV:

Red Channel: 380BP20

Green Channel: 345BP25

Blue Channel: 315BP25 (peak transmission at about 323nm)

 

IR:

Red Channel: CWL at about 1,000 nm

Green Channel: CWL at about 850 nm

Blue Channel: CWL at about 735 nm

 

Camera in all images is a full-spectrum Sony A6000.

 

Firstly a few shots showing groups of related items. Here are some printed materials (Visible, UV, then IR images; Lens = Focotar-2):

 

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Next, various containers with metal, plastic, and paint (Visible, UV, then IR images; Lens = Focotar-2). In UV, a lot of plastics come out as brown (i.e. increasing absorption as wavelength decreases) irrespective of their visible colour, and the same plastics tend to come out white in IR. (Oil-based paints similarly come out brown in UV.). There is also a glass of water here. In UV this is yellow, which is because of the absorption of shorter wavelengths by the glass; in IR it is blue, because of the increasing absorption at longer wavelengths by the water. The plastic bottle of isopropyl alcohol at bottom left shows a similar effect in IR.

 

post-245-0-31482500-1592939578.jpg

 

post-245-0-70221900-1592939577.jpg

 

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Now, some fruit and vegetables (Visible, UV, then IR images; Lens = Focotar-2). Relatively little colour, with the objects looking dark/rotting in UV and shades of white in IR.

 

post-245-0-46845600-1592939596.jpg

 

post-245-0-96146200-1592939595.jpg

 

post-245-0-19185500-1592939394.jpg

 

Glazed Pottery (Visible, UV, then IR images; Lens = Focotar-2):

 

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My long-suffering wife. The red hair in the IR shot is close to what it was like when she was younger. The skin has a slight cyan colouring, indicating higher reflectance at the shorter IR wavelengths (the yellow patches are probably down to facial movement between shots). The redness in IR of the visibly red bricks is noticeable. The UV shot shows up the freckles, and the sun-blocking effect of face cream (rather than specific sun-block cream) applied several hours earlier: the brown colour of the face cream area indicates decreasing absorption as wavelength decreases. The mauve of the T-shirt is interesting – indicates that reflection dips in the middle of the UVA range (Visible, UV, then IR images; Lens = Focotar-2):

 

post-245-0-78402500-1592939623.jpg

 

post-245-0-25423900-1592939623.jpg

 

post-245-0-84035700-1592939413.jpg

 

Non-glazed Pottery (Visible, UV, then IR images; Lens = Focotar-2):

 

post-245-0-57045600-1592939599.jpg

 

post-245-0-09562300-1592939599.jpg

 

post-245-0-15699300-1592939412.jpg

 

Finally in this sequence, a car windshield (Visible, UV, then IR images; Lens = Focotar-2). The UV image shows very strong absorption, especially at shorter wavelengths, which is not so surprising. But I was surprised to see that there was quite a lot of IR absorption, predominantly at longer wavelengths.

 

post-245-0-45701300-1592939597.jpg

 

post-245-0-95732600-1592939596.jpg

 

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The above UV image brings to mind what the great Richard Feynman said about the first atomic bomb test at Los Alamos: "They gave out dark glasses that you could watch it with. Dark glasses! Twenty miles away, you couldn't see a damn thing through dark glasses. So I figured the only thing that could really hurt your eyes - bright light can never hurt your eyes - is ultraviolet light. I got behind a truck windshield, because the ultraviolet can't go through glass, so that would be safe ... this tremendous flash out there is so bright that I duck ... So I look back up, and I see this white light changing into yellow and then into orange ... Everybody else had dark glasses, and the people at six miles couldn't see it because they were all told to lie on the floor. I'm probably the only guy who saw it with the human eye."

 

 

Looking at buildings now, it is interesting that brick and roof tiles that are red in the visible also come out reddish in IR. So an IR colour image could almost be mistaken for a standard visible colour image - until you have a true visible colour image for comparison (Visible, UV, then IR images; Lens = Focotar-2):

 

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A couple of noteworthy points about the next trio of images: the window frames and doors in the buildings to the left are brown in UV because they are plastic coated or painted with oil-based paints. In the building just right of centre, the wooden beams, which are weathered to grey in the visible image, come out brown in IR: this is a typical rendition of wood in colour IR (Visible, UV, then IR images; Lens = Focotar-2):

 

post-245-0-87162300-1592939545.jpg

 

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(Visible, UV, then IR images; Lens = Focotar-2):

 

post-245-0-69107200-1592939544.jpg

 

post-245-0-09613800-1592939544.jpg

 

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In this image, the columns are painted white using an oil-based paint, and so come out brown in the UV image (Visible, UV, then IR images; Lens = Focotar-2):

 

post-245-0-87266300-1592939520.jpg

 

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The next image is a vertical panorama. This church is on a hilltop and is a reporting point called "Golden Ball" for aircraft flying in to the local airfield (Visible, IR; Lens = Focotar-2):

 

post-245-0-50535400-1592939392.jpg

 

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The following image shows the limitations of this technique when using focal lengths shorter than 50mm (with an APS-C sensor) and dichroic filters. You can see the colour shift towards the edges in the IR shot, and the additional problems caused by the small diameter (25mm) of the UV filters (Visible, UV, then IR images; Lens = Soligor 35mm f/3.5 enlarging lens):

 

post-245-0-58143900-1592939519.jpg

 

post-245-0-88891100-1592939518.jpg

 

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Turning to landscapes, this is where IR is at its best. UV landscapes show very little colour, and of course haze in the distance is more pronounced (which might be an effect you want). Skies come out blue in IR (adding to the effect of sometimes appearing as almost normal colour images); this is less noticeable in UV, with skies often appearing white – perhaps as a result of the sky burning out, because if you deliberately under-expose you can get a blue sky) (Visible, UV, then IR images; Lens = El-Nikkor 105mm):

 

post-245-0-05991700-1592939517.jpg

 

post-245-0-48878600-1592939516.jpg

 

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I have added a funky fail in to the following trio: this was an IR shot where the fast-moving clouds caused their shadows to move while I was changing filters for the three tri-colour separation exposures (Visible, UV, IR, then IR Funky Fail images; Lens = El-Nikkor 105mm):

 

post-245-0-81326700-1592939495.jpg

 

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I like this shot in IR – it almost looks like a normal colour image, but then you have the surprising white background. Also, another funky fail (Visible, UV, IR, then IR Funky Fail images; Lens = El-Nikkor 105mm):

 

post-245-0-19312100-1592939518.jpg

 

post-245-0-58133700-1592939517.jpg

 

post-245-0-16700700-1592939370.jpg

 

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This IR image shows subtle variations in colour between different areas of vegetation. (Visible, IR; Lens = Focotar 2):

 

 

post-245-0-73077600-1592939370.jpg

 

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In this trio, we see clearly the effects of atmospheric scattering: the distance in the UV shot is very hazy, and the IR shot shows blue-green colouration in the distance. The building with chimney stacks towards the top-left of the image is about 19 Km away (it is all that remains of a power station which used to be a major landmark for local light aircraft) (Visible, UV, then IR images; Lens = El-Nikkor 105mm):

 

post-245-0-67896600-1592939620.jpg

 

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Note here the brown colour of the bridge in UV, which again is down to the use of oil-based paint. The IR image shows subtle variations of colour in foliage, and the white paint on the bridge is obviously not white in IR (Visible, UV, then IR images; Lens = Focotar-2):

 

post-245-0-99835500-1592939515.jpg

 

post-245-0-39249200-1592939515.jpg

 

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Finally, flowers. I have not included any IR shots here, because these always come out as white, although you can squeeze a bit of colour out of them by ramping up the saturation to extreme levels. But here we have only UV shots.

 

As a general (but not universal) rule, blue and white flowers come out red – presumably because the colourant reflecting blue light doesn't stop reflecting at 400 nm, but continues into the longer UV wavelengths. (Blue flowers also come out blue in straight shots taken through a Baader U, presumably because the longer UV wavelengths just happen to be triggering the blue channel.)

 

Wild Strawberry (Visible, UV; Lens = Focotar-2):

 

post-245-0-37923100-1592939548.jpg

 

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Cornflower (Visible, UV; Lens = Focotar-2):

 

post-245-0-72169400-1592939622.jpg

 

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Bindweed (Visible, UV; Lens = Focotar-2):

 

post-245-0-68990900-1592939436.jpg

 

post-245-0-16827500-1592939436.jpg

 

Mock Orange (Visible, UV; Lens = Focotar-2):

 

post-245-0-03036700-1592939463.jpg

 

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Campanula – note how the difference in visible colour does not come through in UV (Visible, UV; Lens = Focotar-2):

 

post-245-0-88965200-1592939460.jpg

 

post-245-0-37161400-1592939460.jpg

 

Margerite (Visible, UV; Lens = Focotar-2):

 

post-245-0-95768000-1592939461.jpg

 

post-245-0-43620600-1592939461.jpg

 

Sweet Pea (Visible, UV; Lens = Focotar-2):

 

post-245-0-35389000-1592939547.jpg

 

post-245-0-39003100-1592939546.jpg

 

Dead Nettle (Visible, UV; Lens = Focotar-2):

 

post-245-0-24567000-1592939491.jpg

 

post-245-0-72404100-1592939490.jpg

 

Dog Rose (Visible, UV; Lens = Focotar-2):

 

post-245-0-58515000-1592939493.jpg

 

post-245-0-04923900-1592939493.jpg

 

And yellow flowers tend to come out slightly cyan, indicating more reflectance at shorter wavelengths. (These come out slightly yellow with the Baader U).

 

St. John's Wort (Visible, UV; Lens = Focotar-2):

 

post-245-0-52424800-1592939573.jpg

 

post-245-0-91614300-1592939572.jpg

 

Buttercup (Visible, UV; Lens = Focotar-2):

 

post-245-0-83467800-1592939459.jpg

 

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Dandelion (Visible, UV; Lens = Focotar-2):

 

post-245-0-82845100-1592939458.jpg

 

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Hawksbeard (Visible, UV; Lens = Focotar-2):

 

post-245-0-84465600-1592939437.jpg

 

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The rest of these images do not follow those general rules:

 

Geranium (Visible, UV; Lens = Focotar-2):

 

post-245-0-51297600-1592939434.jpg

 

post-245-0-98869600-1592939433.jpg

 

Iris (Visible, UV; Lens = Focotar-2):

 

post-245-0-94917300-1592939432.jpg

 

post-245-0-41386800-1592939433.jpg

 

Pansy (Visible, UV; Lens = Focotar-2):

 

post-245-0-62890500-1592939435.jpg

 

post-245-0-02718700-1592939435.jpg

 

Mallow (Visible, UV; Lens = Focotar-2):

 

post-245-0-44512200-1592939492.jpg

 

post-245-0-85024200-1592939491.jpg

 

Clematis (Visible, UV; Lens = Focotar-2):

 

post-245-0-57411200-1592939549.jpg

 

post-245-0-00840200-1592939549.jpg

 

Loosestrife (Visible, UV; Lens = Focotar-2):

 

post-245-0-95421400-1592939575.jpg

 

post-245-0-33425700-1592939575.jpg

 

Aquilegia (Visible, UV; Lens = Focotar-2):

 

post-245-0-89493600-1592939415.jpg

 

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Snapdragon (Visible, UV; Lens = Focotar-2):

 

post-245-0-69278000-1592939621.jpg

 

post-245-0-19373700-1592939621.jpg

 

Tulips. Flowers of the same species but with different visible colours usually look the same in UV, but these tulips show that that is not always the case (Visible, UV; Lens = Cassar S):

 

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Link to comment
Andy Perrin
AMAZING work! I am still trying to find time/money to get an equivalent set of SWIR filters put together, but whenever that happens I look forward to imaging some of the same flower species at least.
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Thanks, Cadmium & Andy.

 

It would be great to see comparative images from those of you who are not limited to the 320-1100nm range that I have to work within.

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Thanks Bernard

Wow wow wow, what can I say, three filters is a good way to see what moves in the shoot out.

Excellent quality works.

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Excellent series and analysis.

Rainbow IR skies and sheep seem to work.

 

Be careful about drinking that yellow water. Make sure not a sample first.

Link to comment
Andy Perrin
Bernard, I noticed that if I take your photos and do a red/blue channel swap, the colors seem to partially match the usual colors that we get with the Bayer array (which fall on a blue-yellow spectrum with little to no red-green variation). So for example, we are used to yellow flowers like St. John's Wort having predominantly false yellows in UV, and if you channel swap your photos, then that will still be the case. I mention that because it makes it easier to see how the "full color" UV extends the usual gamut that we work with.
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Bernard, I noticed that if I take your photos and do a red/blue channel swap, the colors seem to partially match the usual colors that we get with the Bayer array (which fall on a blue-yellow spectrum with little to no red-green variation). So for example, we are used to yellow flowers like St. John's Wort having predominantly false yellows in UV, and if you channel swap your photos, then that will still be the case. I mention that because it makes it easier to see how the "full color" UV extends the usual gamut that we work with.

 

Andy,

 

I sort of tried this from the opposite direction - took a standard shot through a Baader U and swapped blue to red. This gives a result very similar to the red areas of the tri-colour images, but of course you lose the nuances of the other colour. Although the Baader U transmits well across pretty well all of the UVA area, the camera sensitivity and lens transmission plummets as wavelength decreases - for example, my exposure through the 315BP25 filter (peak transmission 70%) is 64 x exposure through 380BP20 (peak transmission 50%). So when you take an image through Baader U, you are effectively only recording the longer UVA wavelengths. The typical "natural" Bayer colours for UVA wavelengths is given at http://www.savazzi.net/photography/uv-b.html - but you rarely see the green or cyan in a Baader U image because of the low sensitivity at that end of the spectrum. So as you say, the tri-colour approach increases the gamut of colours, and it's interesting to see what this increase is.

 

As you probably realise, my colour assignment is chosen to mimic the visible (blue = shortest wavelength, etc.), which makes it easier for me to interpret the colours and preserves effects like blue skies and other atmospheric scattering consequences.

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Bernard,

Assigning specific wavelengths to each channel is an interesting technique. Lots of color in those UV images. Great to see the effect on so many different subjects and materials.

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  • 3 years later...

I can finally comment on this topic (it used to be in a gallery).

 

For me at least, this topic is one of the most beautiful ever posted on this forum. It really is about what I like the most: showing how the world appears outside the visible spectrum, but in color, where colors are analogous to those we see: red for the longest wavelengths, green for medium ones, and blue at the short end (there should also be violet for the very shortest wavelengths, but blue is enough).

 

Bernard's work is what inspired me to try the TriColour technique myself. I did succeed, but I still think he got better results than me. A monochrome camera would greatly help with the UVB channel (which I use for the blue channel).

 

Also, I hope Bernard is doing OK. He hasn't posted for a while.

Link to comment
lukaszgryglicki

I'm looking for a filter wheel (not a cinematography one) that would be purely mechanical (no motors etc) so a big wheel with 3 52x0.75mm filters inside.

One end would be mounted on a camera on a tripod and then on the other end, there will be 3 52mm filters: UV, UV/IR cut, and IR. Then I can just take a photo in UV, spin the wheel, take UV/IR cut, spin the wheel, and take the IR - that way there is the biggest chance to register an image that is not moved and the time difference between shots would be minimal.

I even asked Rafcamera to make such a filter wheel for me, but they are not sure if they can make one :(

 

Link to comment
1 hour ago, lukaszgryglicki said:

I'm looking for a filter wheel (not a cinematography one) that would be purely mechanical (no motors etc) so a big wheel with 3 52x0.75mm filters inside.

One end would be mounted on a camera on a tripod and then on the other end, there will be 3 52mm filters: UV, UV/IR cut, and IR. Then I can just take a photo in UV, spin the wheel, take UV/IR cut, spin the wheel, and take the IR - that way there is the biggest chance to register an image that is not moved and the time difference between shots would be minimal.

I even asked Rafcamera to make such a filter wheel for me, but they are not sure if they can make one :(

 

You would still need to refocus your lens, unless you use a UV-to-IR corrected one.

Link to comment
lukaszgryglicki

That is correct, but I can use an AF lens (many of them work quite well in UV) or I can use UV-Nikkor - it only needs adjustment for IR, usually only about 800nm, not with 720 filter.

 

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