The conversion significantly alters the response of the color receptors of the R, G, and B channels: instead of reflecting the red, green, and blue components of a scene, the combination of the IR-pass filter and the dye inside of the receptors leads to the following spectral sensitivity (my measurements were inspired by Samir Kharusi):
The response of the R, G, and B-channels are red, green and blue shaded curves, respectively. First off, we see that R, G, and B channels have different spectral response curves, i.e. we are actually able to capture with this setup something like color (the thin, light red and green lines are examples for spectral separation calculated from a linear combination of the R, G, and B channels). Most notable, the R channel has a response from 720 nm to the limit due to the bandgap of the Silicon CCD; the B channel is sensitive from 800 nm on. Note the dominance of the R-channel: any light in the near IR that hits the sensor and triggers a response of G or B channel will also trigger a response of the R channel. Hence, we will probably want to use a custom white balance: if we tell the camera to use a build-in white balance (designed for visible light, not near-IR) we'll get a photo that looks very red. We also see that the R channel is most sensitive in the shorter wavelengths while the B channels is most sensitive in the longer wavelengths. Hence, we may want to swap color channels and display the R-channel as blue and the B-channel as red.
Camera white balanceIt seems that most people set a custom white balance in IR by taking a photo of green foliage (e.g. grass) instead of something white [in D60: Shooting Menu -> White balance -> Preset manual -> Measure]. This will render foliage white on a JPG straight-out the camera. I like this choice: foliage tends to have a flat reflectivity spectrum in the near IR, i.e. even if we were able to see colors in the near IR we would probably see foliage as gray or white.
Post-processing of JPGs
My program of choice for basic image manipulations is the free IrfanView. The very minimum amount of post-processing I do is swapping the R and B channels of my JPGs (Image -> Swap Colors -> RGB to BGR). Hence, we can interpret the resulting image as follows: if an object looks reddish it has an infrared color, and if an object looks blueish it has more of a visible (deep-red) than infrared tone. I also like to increase the saturation a bit to make the photo a more dramatic (Image -> Color corrections...). Here's an example:
Colors in near-IRIf we apply a white balance based on a green surface, and map the R and B channels to blue and red, respectively (as outlined above), then we tend to get the following colors:
- white/gray: thick, translucent clouds; snow; leaves; grass
- blue: clear ice; water; clear sky; skin
- red: bark; dry grass; some (often black) synthetic materials; some sun glasses
JPG vs. raw image formatI usually save near-IR images in RAW format because I usually want to adjust color balance and exposure. The RAW image (Nikon .nef file) contains a thumb nail jpg which can be displayed and extracted with IrfanView. Of course it doesn't hurt to have the camera save both JPG and RAW at the same time.
Exposure settingThis is tricky with most DSLRs because light is metered independently of the CCD and filter in front of it. Hence, the converted D60 continues to meter light in the visible, and the brightness of a scene in the visible tells us little about the brightness in the near-IR. The amount of IR light increases significantly as we move from overcast skies to clear skies (it is even higher under tungsten light, and much lower under fluorescent light). In order to prevent the R-channel from saturating, I set the exposure compensation on my camera to something like 0 to -1 EV under overcast skies, 0 to -2 EV under clear skies, much lower under indoor tungsten lighting, and much higher (e.g. +3 EV or more) under fluorescent light. Actually, forget these numbers and try for yourself: it depends on the scene. Experimentation is my friend.
Optimal exposure settings depend on what's going to happen with the photo: if I am only interested in near-IR black-and-white photography then I may be most interested in wavelengths above 850 nm. In this case I will expose appropriately for the B channel and deliberately overexpose R and G channels.
Unfortunately, it is very difficult to tell from the display of the D60 whether an image is overexposed (it may be impossible). I find that I have to look at the RAW image data to tell for sure; so I always underexpose when in doubt. Underexposure does not cause artifacts although the picture is dark and there is more noise in the frame. (The RAW images of the D60 are 12 bit.)
In order to analyze the raw image file I use dcraw (free) to convert Nikon's raw NEF image to the well-documented 16-bit PGM format [dcraw -D -4 filename.nef
FocusThe focal point of a lens depends on the wavelength. Quality lenses are designed to compensate for this from 400 nm to 700 nm; however, we use them around 900 nm. Lifepixel performs an adjustment of the focal point for a specified lens set to a specific focal length (I think they do 50 mm by default). However, we can expect to get slightly out-of-focus (soft) images when we use different equipment. Usually, I use a 18-55 mm lens and didn't notice serious issues here. However, with a 200 mm lens I have to manually adjust the focus slightly after autofocussing since the subject is obviously out of focus. (This is easy to detect in playback with the zoom function.)
Hot spot artifactI understand that there is reflection of light between the "polished" CCD and the lens in the camera. Manufacturers make sure that this does not affect the image for visible light. However, a big ugly red spot may appear if we use an unfortunate combination of camera and lens for infrared photography. I use the following Nikon lenses without problems on my converted Nikon D60 body:
AF-S DX Zoom-NIKKOR 18-55mm f/3.5-5.6G ED II (bundled with D40) AF-S DX NIKKOR 18-55mm f/3.5-5.6G VR (bundled with D60) AF-S DX VR Zoom-NIKKOR 18-200mm f/3.5-5.6G IF-ED
Choice of camera and lensI was looking for a system with the following key features
- low noise
- not too expensive
- free of hot spots
- does white balance of IR images properly
Problem with Canon was that their lenses of suitable focal lengths happened to produce hot spots unless I was willing to pay big bucks. If I were to buy a new system today I'd take into account LiveView capabilities in today's DSLRs.