Steve Cushing Impresionist Fine Art Photography

Steve Cushing Impresionist Fine Art Photography

Embracing imperfection, recording emotions, one impression at a time…

Infra Red Photography

It’s easy to forget that the world looks the way it does simply because our eyes can only perceive an extremely narrow band of electromagnetic radiation emanating from (or reflected off of) objects around us. Beneath the worlds apparent disorder there lies a subtler order. Infrared helps penetrate behind the veil.

To understand Infra-red photography you need first to understand light. The human eye can only see a very small band of light waves called the visible spectrum.

The visual focus of a normal lens is not correct for UV or IR. In general the focal length increases for these regions). Two remedies for this is either to extend the lens further by about 0.3 per cent of its marked focal length or to use a bellows extension by about
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Many old lenses in focusing mounts have a sup­plementary index mark, usually in red, which is for IR work. The lens is focused visually and the distance setting transferred to the secondary index. This index may also serve for the UV focus, but modern lenses using high refractive index glass pass very little beyond 400 nm.

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If we take photographs using infrared-equipped film or cameras, we experience a world that looks very different from that we are accustomed to seeing. Colours, textures, leaves and plants, human skin, and all other manner of objects can reflect the IR spectrum in unique and interesting ways, ones that cannot be mimicked with software tools. Vegetation appears white or near white. Skin takes on a very milky, smooth texture, although veins close to the skin surface can be accentuated and take on a rather ghoulish appearance. Eyes can appear a bit ghostly with the irises registering very dark tones and the whites of the eye taking on a greyish hue. Black clothing can appear grey or white depending on the fabric. The IR spectrum can pass through sunglasses that, to the eye, appear extremely dark or mirror-like (see image below). Blue skies take on a much more dramatic appearance as well.

Photographers started experimenting with IR in early part of the 20th century. During WWI, IR photography proved extremely valuable, as images using the IR spectrum were not affected as much by atmospheric haze as normal photos. IR images were also able to show stark distinctions between vegetation and buildings, better identifying potential enemy targets such as camouflaged munitions factories and other key sites. Rivers, streams, lakes, and other waterways were depicted in a very dark hue, making them much more obvious.

During the 1960s, IR photography saw a number of photographic uses, as some of the leading musicians of the day, such as the Grateful Dead and Jimmy Hendrix, popularised its use via their psychedelic album covers. With the advent of the digital camera in the late 1990s, both regular and IR photography were about to change substantially. In addition to professional and amateur photographers, law enforcement officials rely on IR photography to detect forensic evidence not discerned through normal eyesight.

I have used a full spectrum camera to take the images on this site. That is a converted camera where the filter that excludes light outside of the visible spectrum has been removed from the sensor. Of course this would mean all spectrum including visible colours would be recorded if it were not for the fact that I add filters called IR pass filters to control what spectrum I wish to record and what I wish to block. Filters are called block filters or band pass filters. Block filters block certain frequencies and pass all other frequencies and band pass filters (BP) pass certain frequencies and block the other frequencies. You maybe wondering why we need two types of filter in this way, the main reason is that some photographers, for example Astro photographers need to pass some spectrums but specifically block others.

Reflected IR light produces a fascinating array of surreal effects. With the right filters Infrared photography produces some very distinct effects which make them aesthetically pleasing.

Common Filters

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The orange line in the chart shows the band pass filter in a normal camera which passes the visible spectrum band but blocks all, but a small part of the non viable spectrum above 700nm. As a result of this if you use an IR filter on unmodified camera you will need very long exposure times and even then will only capture a small amount of IR spectrum, as most of the spectrum above 700nm will be blocked by this internal filter in the camera.

Once a camera has been modified to remove this non-visible spectrum band pass filter, a new filter needs to be added to block everything but the spectrum you require and to pass the spectrum you do require.

The chart also shows the effect of some the IR band pass filters used on this site. Note that they block most of the visible spectrum below the stated value but generally pass all the spectrum above the stated value, although some I use also block the higher IR bands. Some filters allow some of the visible spectrum and this allows for what is called "false colour" during processing as this requires a mix of the IR spectrum with some of the visible spectrum. It all depends upon what you wish to achieve.

  • 550nm Inspired by "false colour" IR film, as the 550nm lets in the most visible light of all IR usable band pass filters creating deep blue skies and blood crimson reds.
  • 642nm This lets some visible light in, producing the most vibrant colours when processing for "false colours". Leaves are golden yellow, and skies are bright blue. B+W contrast is lower than the deeper IR filters.
  • 720nm This is the tried and true classic IR filter. It allows some limited visible light for "false colour", and good contrast for black and white.
  • 850nm This filter is good for a dedicated black and white IR. The camera and will produce bright whites and pronounced darks. With a custom white balance in camera, the picture is close to pure B+W without any processing.
  • 950nm This filter is good for extreme contrast black and white, all visible light is blocked.

I used a 642 "BP" short for "Bandpass" for the images HERE. The filter gives a 200nm spectral window from 642nm to 842nm. It blocks the longer infrared.
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As can be seen from the images the BP creates very deep red and gives all light up to 842nm. You get images which are perfectly suited for channel-changeing, and which will show an incredible level of colours and great contrast do to the blocking of the longer IR: A great set of data to start creative image processing!

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I used a 742nm for the images HERE. It removes almost all visible light.

The filter cuts off the part of the spectrum where Chlorophyll looks green and shows its high reflectively in the near infrared. If trees are photographed in spring and summer under blue skies you get stunning images with white trees and clouds in front of a near black background.
Darkens the background during twilight.

This video shows the effects of a range of filters on a full spectrum camera.

Selective Spectrum Filters

So far we have looked at filters that block the visible spectrum to different degrees, but it is also possible to create filters that allow just parts of the visible spectrum and just parts of the non visible spectrum.
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Some filters that do this have a fascinating story of themselves. This is true of Aerochrome. See video.

The filter’s developer, French photographer Yann Philippe, says that he began working on digital emulations for Kodak’s Aerochrome since 2008 and “more actively” since 2015. “My goal was to obtain ‘in camera’ colour IR result[s] without having to mix channels in post-production. I tried to go the other way round the colour wheel using the complementary colour of the yellow/orange filter (used in EIR photography): dark blueish cyan. After trying and failing a lot during a long run of numerous tests, I finally came up with a precise transmission curve that could be produced as a glass filter.”

How Aerochrome Works

This most important thing you have to understand about Aerochrome is that it doesn't record red green & blue like a normal film stock does. Instead, it records infrared, red & green. However, this leads to the question of how to display the infrared information. We obviously can't see infrared light, so instead the film creates a "false colour" image by displaying infrared as red, red as green, and green as blue.
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Trees have the rather unique characteristic of being very reflective of infrared light (ie. if you take a black & white infrared image of a tree, it would show up as bright white). Trees also reflect green light (trees are green remember), so therefore trees reflect green and IR. The green is converted to blue and the infrared is converted to red. If we mix blue and red, we get pink!

From this graph we can see that the blue colour is sensitive from 500-600nm, green from 550-700nm and red from 600-900nm. Using common color filters, we can recreate these ranges.

From the diagram above, we can see that we need to:

  • combine green, yellow & IR cut filters to give us 500–600nm (blue channel)
  • Combine red & IR cut filters to get a 600–700nm (green channel)
  • use a 720nm IR filter to gives us a 720nm–∞ (red channel)

Using ND Filters

Whilst less sophisticated than filters designed to pass only certain spectrum bandwidths, ND filters do block the visible spectrum to different degrees based upon their strength. They also allow the IR spectrum. So one can use them to experiment with IR and false colour photography on a full spectrum camera.

Other Filters

Must photographic filters block certain bandwidths to various degrees. They are all good for experimentation with a full spectrum camera.

Video on Infrared Photography


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