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Full-colour/Tri-colour UV and IR

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#1 Bernard Foot

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Posted 23 June 2020 - 19:53

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):

Attached Image: UVFC 378 Vis.jpg

Attached Image: UVFC 378 .jpg

Attached Image: IRFC 378 .jpg

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.

Attached Image: UVFC 379 Vis.jpg

Attached Image: UVFC 379 .jpg

Attached Image: IRFC 379 .jpg

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.

Attached Image: UVFC 380 Vis.jpg

Attached Image: UVFC 380 .jpg

Attached Image: IRFC 380 .jpg

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

Attached Image: UVFC 382 Vis.jpg

Attached Image: UVFC 382 .jpg

Attached Image: IRFC 382 .jpg

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):

Attached Image: UVFC 390 Vis.jpg

Attached Image: UVFC 390 .jpg

Attached Image: IRFC 390 .jpg

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

Attached Image: UVFC 383 Vis.jpg

Attached Image: UVFC 383 .jpg

Attached Image: IRFC 383 .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.

Attached Image: UVFC 381 Vis.jpg

Attached Image: UVFC 381 .jpg

Attached Image: IRFC 381 .jpg

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):

Attached Image: UVFC 384 Vis.jpg

Attached Image: UVFC 384 .jpg

Attached Image: IRFC 384 .jpg

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):

Attached Image: UVFC 368 Focotar Vis.jpg

Attached Image: UVFC 368 Focotar .jpg

Attached Image: IRFC 368 Focotar .jpg

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

Attached Image: UVFC 365 Focotar Vis.jpg

Attached Image: UVFC 365 Focotar .jpg

Attached Image: IRFC 365 Focotar .jpg

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):

Attached Image: UVFC 362 Focotar Vis.jpg

Attached Image: UVFC 362 Focotar .jpg

Attached Image: IRFC 362 Focotar .jpg


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):

Attached Image: IRFC 372 Pano Focotar Vis.jpg

Attached Image: IRFC 372 Pano Focotar .jpg

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):

Attached Image: UVFC 360 Soligor Vis.jpg

Attached Image: UVFC 360 Soligor .jpg

Attached Image: IRFC 360 Soligor .jpg

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):

Attached Image: UVFC 355 El Nik 105 Vis.jpg

Attached Image: UVFC 355 El Nik 105 .jpg

Attached Image: IRFC 355A El Nik 105 .jpg

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):

Attached Image: UVFC 350 El Nik 105 Vis.jpg

Attached Image: UVFC 350 El Nik 105 .jpg

Attached Image: IRFC 350 El Nik 105 .jpg

Attached Image: IRFC 000.jpg

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):

Attached Image: UVFC 356 El Nik 105 Vis.jpg

Attached Image: UVFC 356 El Nik 105 .jpg

Attached Image: IRFC 356A El Nik 105 .jpg

Attached Image: IRFC 356 El Nik 105 .jpg

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


Attached Image: IRFC 358 Focotar Vis.jpg

Attached Image: IRFC 358A Focotar .jpg

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):

Attached Image: UVFC 385 Vis.jpg

Attached Image: UVFC 385  El Nik 105 .jpg

Attached Image: IRFC 385  El Nik 105 .jpg

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):

Attached Image: UVFC 351 Focotar Vis.jpg

Attached Image: UVFC 351 Focotar .jpg

Attached Image: IRFC 351 Focotar .jpg

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):

Attached Image: UVFC 372 Wild Strawberry Vis.jpg

Attached Image: UVFC 372 Wild Strawberry .jpg

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

Attached Image: UVFC 388 Cornflower Vis.jpg

Attached Image: UVFC 388 Cornflower .jpg

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

Attached Image: UVFC 329 Bindweed Vis.jpg

Attached Image: UVFC 329 Bindweed  New Method.jpg

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

Attached Image: UVFC 336 Mock Orange Vis.jpg

Attached Image: UVFC 336 Mock Orange  New Method .jpg

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

Attached Image: UVFC 334 Campanula Vis.jpg

Attached Image: UVFC 334 Campanula  New Method .jpg

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

Attached Image: UVFC 335 Margerite Vis.jpg

Attached Image: UVFC 335 Margerite  New Method .jpg

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

Attached Image: UVFC 371 Sweet Pea Vis.jpg

Attached Image: UVFC 371 Sweet Pea .jpg

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

Attached Image: UVFC 337 White Dead-nettle Vis.jpg

Attached Image: UVFC 337 White Dead-nettle  New Method.jpg

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

Attached Image: UVFC 342 Dog Rose Vis.jpg

Attached Image: UVFC 342  Dog Rose .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):

Attached Image: UVFC 374 St Johns Wort Vis.jpg

Attached Image: UVFC 374 St Johns Wort .jpg

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

Attached Image: UVFC 333 Buttercup Vis.jpg

Attached Image: UVFC 333 Buttercup .jpg

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

Attached Image: UVFC 332 Dandelion Vis.jpg

Attached Image: UVFC 332 Dandelion  New Method.jpg

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

Attached Image: UVFC 331 Hawksbeard Vis.jpg

Attached Image: UVFC 331 Hawksbeard  New Method.jpg

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

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

Attached Image: UVFC 324A Geranium Vis.jpg

Attached Image: UVFC 324A Geranium  New Method.jpg

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

Attached Image: UVFC 320 Iris Vis.jpg

Attached Image: UVFC 320A Iris New Method .jpg

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

Attached Image: UVFC 326 Pansy Vis.jpg

Attached Image: UVFC 326 Pansy New  Method.jpg

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

Attached Image: UVFC 340 Mallow Vis.jpg

Attached Image: UVFC 340 Mallow  New Method.jpg

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

Attached Image: UVFC 373 Clematis Vis.jpg

Attached Image: UVFC 373 Clematis .jpg

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

Attached Image: UVFC 377 Loosestrife Vis.jpg

Attached Image: UVFC 377 Loosestrife .jpg

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

Attached Image: UVFC 258 Aquilegia Vis.jpg

Attached Image: UVFC 258 Aquilegia .jpg

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

Attached Image: UVFC 387 Snapdragon Vis.jpg

Attached Image: UVFC 387  Snapdragon Vis .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):

Attached Image: UVFC 247 Vis.jpg

Attached Image: UVFC 247 .jpg
Bernard Foot

#2 Cadmium

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Posted 23 June 2020 - 20:00

Bernard, Very nice set of images!

#3 Andy Perrin

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Posted 23 June 2020 - 21:25

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.

#4 Bernard Foot

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Posted 23 June 2020 - 21:36

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.
Bernard Foot

#5 colinbm

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Posted 24 June 2020 - 01:26

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.

#6 dabateman

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Posted 24 June 2020 - 04:17

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.

#7 Andy Perrin

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Posted 24 June 2020 - 04:43

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.

#8 Bernard Foot

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Posted 24 June 2020 - 09:09

View PostAndy Perrin, on 24 June 2020 - 04:43, said:

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.n...raphy/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.
Bernard Foot

#9 GaryR

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Posted 24 June 2020 - 23:44

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.