Thursday, 16 January 2020

Filtering a full-spectrum camera

My full-spectrum camera, a FujiFilm IS-Pro, has no built-in filter and shoots from near UV to near IR unimpeded. This also means that when I put a filter over the lens it also affects what I see through the viewfinder. With, say, a 720nm filter I am unable to see through the viewfinder so would have to use a tripod. Sadly the Fuji's live-view is pretty-well useless.

Many people replace the high-pass IR-blocking filter over the sensor with a low-pass filter such as a 720 or 820 nm to get around this problem ... at the small expense of loss of versatility.

In recent years I have used a deep blue filter, which I can see through to frame. Blue filters usually pass a lot of near IR. You can use a red filter of course, as you might have done with film, but I find the blue filter sometimes produces an interesting colour balance with minimal post-processing. Interestingly, the auto-focus works most of the time, which helps.

I recently bought a specially-designed filter for a type of colour infrared photography from the American company, Kolari Vision. It's called their IR Chrome Lens Filter, which I'll come back to in a moment. This nudged me into looking at results of a number of filters with the full-spectrum camera.

First, here is the camera output without any filter. This is basically a 'normal' colour image but with infrared contamination. You can click on the images to make them larger.


Next is a minus-blue (ie yellow) filter. This can be used to emulate the old infrared Ektachrome film ... see this blog page for more details.


Next comes the red (#25) filter.


Now the blue filter. Different black and white results can be achieved by either removing saturation or by selecting individual channels. (This also applies with other filters of course.) The green channel is useful because, with a Beyer filter camera the green channel has twice as many pixels as the red or blue. With this filter I find I need to under expose (according to the camera) by 3 or 4 stops.


This is Kolari's IR Chrome Lens Filter, which gives a good approximation of the old Ektachrome images. However, it is not exactly the same so is not as useful for foliage health analysis: but it's not a bad approximation.


I had achieved good results using neutral density filters in the past, with Sony's Night Shot, since the ND doesn't apply at IR wavelengths. So I bought a variable ND filter, which is basically two polarising filters together. You rotate one with respect to the other in order to reduce the amount of visible light going through. In this case once I had frames with minimal density I simply rotated the outer filter until I could only just see anything then fired the shutter. Autofocus worked and by trial and error found the exposure change: in this case under by 4 stops. There is a little (false) colour information left but this method works best for a monochrome result.


Finally, for comparison, here is a 720 nm filter result.


One thing this experiment also showed me was how bad the chromatic aberrations are around the edges in the lens I am using, which are quite noticeable with colour shots but usually vanish when reducing to monochrome.

For more on this subject, here is Kolari's page outlining the characteristics of their various filters.

Monday, 2 December 2019

James Jarché and early infrared photography

The legendary press photographer, James Jarché (coincidentally the grandfather of the Suchet brothers), was also a pioneer of infrared photography ... or infra-red as it was denoted in those days.

He tells in his memoirs that in 1932 ...
... the Editor of the Daily Herald, Mr Spooner, suggested that I should try to get a picture of an audience in a theatre, during the showing of a film, when the whole house was in darkness. [...] So I went one evening to the Carlton Theatre in the Haymarket, during the performance of 'The Sign of the Cross, to see what could be done. Mr.Short, an expert from the Ilford Photographic Plate Paper and Film Manufacturers, came with me. By a priwous arrangement with the management of the theatre, six infra-red lamps had been fitted to the balcony. [...] Hidden by the darkness, and wthout attracting any one’s attention, I shot an audience I could not see, giving nine seconds' exposure. That is less than is required to take a phbtograph in a lighted room. [...] I made several experiments that evening with different exposures, varying from six seconds to ten seconds. I was very doubtful whether they would be successful, but when I developed the plate, they were as clear and sharp as though the shots had been taken in broad daylight.
He also describes using the heat of a hot clothes iron to take an infrared image in his darkroom, with an exposure of an hour, and being asked by Ilford to photograph the plate manufacturing process (in the dark).

Some of his photos are held in the Getty collection, which you can find with this search.

He also mentions an infrared photograph of the 1932 Armistice Day commemoration at the cenotaph in London, which was taken (but not by him) using an infrared plate because of the poor weather. The image appeared in the Manchester Guardian on November 12th that year.

Jarchés autobiography, titled 'People I have Shot' is available on the internet archive, which claims that it is out of copyright. That may be the case in the USA but since he died in 1965 it will still be in copyright in the UK.

Monday, 7 October 2019

Blue Mountains in Infrared

Photographer Steven Saphore has used his modified DSLR to take some nice shots of the Blue Mountains national park in New South Wales, Australia. A set of the images has appeared in the Guardian newspaper. The photographs are faux-colour but Stephen has applied the colour subtly, which makes for an interesting variation.

The description of the technique is slightly misleading, in that it implies that the whiteness of the foliage is due to chlorophyll being reflective at these wavelengths, whereas it's the chlorophyl itself being transparent and letting 'light' bounce around in the plant cells. But that's a nit-pick ... the photos are the important thing.

Tuesday, 9 July 2019

Solar panels and 'reflective' grass

I was interested to read in today's Guardian that BP are researching the most reflective kinds of grass to plant underneath solar panels; panels that pick up on top and underneath. The story points out that "bifacial panels can increase electricity output by almost 15% – but this can be much higher if the ground beneath the panel is particularly reflective".

Although the story doesn't mention it, regular readers and fans of infrared photography will know that grass (as well as other foliage) strongly reflects near-infrared light due to the retro-reflective effect of those wavelengths travelling through plant cells. Chlorophyl is transparent to near-infrared radiation. The effect is the same one that makes snow appear white, which is why infrared photos and snowy scenes can be confused.

The Solarquotes blog in 2017 looked at the proportion of solar radiation that a solar panel can exploit. Their context was about UV but if you scroll down the page you'll see a diagram that shows that a silicon solar panel will make use of radiation between 400 and 1100 nanometres. Since visible light extends from about 400 nm to about 650 nm you can see that including near-infrared more than doubles the available energy bandwidth.


All this makes planting grass underneath bi-directional solar cells a logical thing to do. That reflective grass is not just fun for infrared photography then ... or grazing!

Tuesday, 18 June 2019

Infrared astronomy faces a gap

In late 2016 I noted that the James Web infrared space telescope was two years away (in this post) but it now seems that the launch has been pushed back to March 2021 at the earliest. This delay means that there will be a gap in infrared observation capabilities, as the Spitzer is set to cease operation over a year before that.

More information on this can be found on the Scientific American web site.