The infrared spectrum

Infrared (IR) light is a type of electromagnetic radiation from approximately 700 nm – 1 mm in wavelengths. We can think of it as an extension of the red color in the electromagnetic spectrum. IR light has three major ranges, the near IR (NIR, 700 – 1400 nm), mid IR (MIR, 1400 – 3000 nm) and far IR (FIR, 3000 nm – 1 mm). IR light is no longer visible for human eyes, it falls out from our visible spectrum (400 – 700 nm) however there are many animals with the ability to see in IR light. CCD and CMOS camera sensors are sensitive to light from 350 – 1100 nm, basically from the edge of the UV to the NIR. For IR photography we use the NIR spectrum around 700 – 1100 nm.

9f2b5-visible-spectrum
The electromagnetic spectrum (source: internet)

The IR photography we are discussing here is not capable of thermal imaging. Conventional thermal imaging sensors have the ability to capture light from around 7000 – 14000 nm that cover the thermal radiation of the comparatively low temperature objects found near to the surface of planet Earth. Those cameras work with an entirely different technique and cannot use silica based sensors nor glass lenses because of the low FIR transparency of glass.

Cameras, sensors and filters

As we know now, CCD and CMOS camera sensors are sensitive to light from 350 – 1100 nm so there shouldn’t be any problem to capture NIR light. However inside the camera, just front of the sensor there is a bandpass filter (hot mirror, cyan on the image) that blocks away all the UV and IR light, leaving only the visible light (VIS) to pass. Why is that? If there wasn’t any filter blocking away UV and IR light, the photos would be very surreal because of the unusual lights and reflections in the UV and IR spectrum. Moreover, light diffraction and refraction is wavelength dependent, therefore the focus point is different for the different regions of light (UV-VIS-IR, see chromatic abberation). Because of this, it is simple impossible to build lenses with the same focal point for such a long spectrum. Therefore with this setup only the VIS spectrum can be imaged through the three main color filters of the Bayern-pattern (RGB, shaded parts on the image). The Bayern mosaic filter is a three color patterned filter on the imaging sensor that creates the color information in the VIS range for all pixels.

949ed-standard_camera
Spectral range of the Bayern-pattern (source: http://ir-photo.net/)

An easy solution to IR photography is buying a 700 nm longpass filter (LP 700) and screwing it to the front of our camera lens so only the IR light can reach the sensor. However there is a problem with this setup, the internal 400-700 nm bandpass filter (hot mirror, BP 400-700) is still front of the sensor and with together the 700 longpass filter, they almost block away all the light. Only a very tiny portion of light can pass through the filters as we can see it on the next image (shaded part at around 700 nm). IR photos still can be made with this setup, but very long exposure times are needed (several minutes) and the focus and exposure cannot be used. Everything has to be done manually and blindly since with the black IR filter on the lens we cannot see a thing in the viewfinder.

9c5d1-unmodifiedcamera_plus720filter
IR photography with IR filter on the camera lens (source: http://ir-photo.net/)

There is another option, converting the camera to IR photography by removing the internal BP 400-700 filter (hot mirror) from the front of the sensor and replacing it to a LP 700 filter. With this setup we don’t need any additional IR filter screwed on the camera lens since it is now built-in. The Bayern-filter still enables us to use the whole range basically till the end of the sensitivity of our camera sensor (1100-1200 nm). Sounds perfect! Our IR camera can be used as a normal DSLR camera. After re-calibrating the focus to the IR spectrum, all the auto and manual functions should be available and composing an IR photo is easy as with a normal camera.

f00e8-moddedcamera_720filter
IR photography with built-in IR filter (source: http://ir-photo.net/)

Note that all color filter of the Bayern-pattern is still active so we will be able to “see” colors on an IR photo. However there are no different colors in the IR range, at least not for humans (ask a fish) thus the colors what we can see on an IR image are virtual, not real colors. They don’t contain so much information since the three colors of the Bayern-filter mainly overlap with each other above 700 nm – in other words – every filter of the three colors “see” mostly the same. It gives some surreal look though, so the color can be kept just for the effect, or the image can be converted to black and white.

Converting DSLR cameras in real

All sounds easy but in reality, converting cameras to IR requires a lot of experience and you really have to know what you are doing. The process is not simple as it looks, and you can ruin your camera beyond repair very easily or hurt/kill yourself by touching metal parts that are connected to the high voltage capacitor inside the camera. This is serious, never open your camera unless you really know what you are doing. Therefore I don’t write any detailed tutorial here about how we converted it, I just include some photos what I shot when we DIY-converted my Nikon D50 to Nikon D50 IR.

Capacitor
The beast. Decharging the 330V/280uF capacitor.

Mirror-backside
Backside of the mirror, sensor is removed

Sensor
The sensor with the BP 400-700 filter on the top

Filters
Changing the built-in BP 400-700 filter (bottom) to the LP 720 filter (top)

 

Re-assembling
Re-assembling the sensor mount with the new LP 720 filter

Interesting topics for infrared photography

  • Leaves, foliage, grass, flowers: chlorophyll reflects high amount of near IR light. Leaves, grasses are turning into white and very highlighted on IR photos, creating a surreal and haunting image.
  • Lakes, water surfaces: not so much infrared light is reflected from water surfaces especially when the sky is covered with clouds. They are dark surfaces on cloudy days, good contrast for swans and other light colored birds on lakes.
  • Animals: Some animals (like black panther) have invisible IR patterns and markings which can only be seen with IR photography
  • Sky: scatters light in the blue region (that’s why the sky is blue, see Rayleigh scattering), but longer wavelengths and IR light isn’t scattered. Dark sky effect can be achieved with IR photography, especially when the sun in on our back. Gives good contrast with trees and foliage.
  • Windows, glasses: not very transparent for IR light thus on an IR photo we cannot see through windows, they appeared to be dark or black surfaces.
  • Sunglasses: interestingly IR can pass through plastic sunglasses as it was just air, which gives us some exciting possibilities for portrait photography. In an IR photo they look like normal glasses.
  • Soft lights: even if we shoot in a very sunny day, IR lights are very soft, again exciting possibilities for (ghostly) portrait photography.
  • Cotton clothes: IR light can go through cotton clothes, yes we can see underneath with IR photography, but don’t get any ideas.
  • Human faces: creepy as hell! Big, black, demonic eyes and ghostly face.

For a gallery of infrared photos, please click here.