Infrared Photography with a Color Camera

The following educational material is extracted from the video course “The Infrared Photography Masterclass”.

When a digital camera is modified for use in infrared photography, the hot mirror is removed and replaced with a filter that cuts the visible spectrum. The alternative is to replace the hot mirror with clear glass to allow the full range of radiation to pass to the sensor, in which case lens mounted filters are used to control the radiation spectrum that reaches the sensor. In either case, a camera modified for infrared photography will still perform as a color camera. However, that is okay because we have ways to deal with color when we process our images. It is important that we understand what goes on inside a color digital camera so that we can work with a modified camera when shooting for infrared.

Let's get inside a standard digital color camera to see what's going on that makes infrared photography with a modified camera challenging.

Each photosite of a digital camera's sensor, of which there are typically millions, collects photons for purposes of measuring the cumulative radiation energy that falls into the photosite. The photon energy of each photosite is eventually converted to an electrical potential that corresponds to the intensity of the radiation energy measured by each photosite. If the photosite's bucket is filled to its capacity with photons, it will have an electrical potential at its highest level, which will ultimately become a pure white pixel. If the photosite's bucket receives no photons, it will have an electrical potential at its lowest level, which will ultimately become a pure black pixel.

The internal computer of the digital camera will perform its magic by scanning each of the photosites very quickly, determining the digital values for each of the photosites. Each value then becomes a pixel of the image that has a certain digital value, depending on the scene in the camera's field of view. Pure blacks in the image are pixels with a zero digital value while pure white are pixels with a digital value of 255. Between pure black and pure white tones are various tones. Thus we have tones that vary between black and white. When viewed by a human, when all of these pixels are grouped closely together, an image is perceived.

So far, I've made reference to photon energy, not color. Let's look at the camera sensor a bit deeper.

The sensor of a digital camera is monochromatic. That is, the photosites themselves are not aware of color. They measure photon energy, which is how tones are derived. The higher the photon energy of a photosite, the more white the tone of that pixel will be.  Photosites with photon energy between the highest and lowest amounts represent tones between white and black. This is basically how tones are determined by a digital camera. So, the next question is how is a color image produced given that the sensor is monochromatic?

There is a coating installed on top of the digital camera's sensor called a "Bayer Filter Array", also called a "Color Filter Array", or CFA. This coating forms a matrix of tiny red, green and blue filters above each photosite of the sensor. On top of the array are micro-lenses for each photosite to help concentrate photons into its respective photosite. The array is intended to assist the camera's computer to produce an image with a color arrangement that closely approximates the human visual system. The array is needed because a camera's semi-conductor sensor does not detect or measure color, as I mentioned. As infrared photographers, we are interested in the array because it is part the color camera's color encoding system which is not removed when a digital camera is converted for infrared photography.

As mentioned, the sensor of a digital camera is color-blind. A color image is essentially faked using the Bayer Filter Array and software. In the end, it appears that the camera recorded the color of the scene, when in fact it did not. The array places tiny filters over each photosite to pass green, blue or red of the visible spectrum. As a result some of the photosites will receive more photons in the blue spectrum, some in the green spectrum and some in the red spectrum.

The camera's computer assembles the resulting image from this matrix of color-filtered photosites, each of which have been tuned to respond to the energy of the various portions of the visible spectrum. That process ultimately forms a matrix of tones and color hues that, when packed closely together, create an image that closely resembles the scene, in terms of what the human visually understands.

This histogram illustrates that each color channel is active in an unprocessed infrared capture. And what is interesting is that this image was taken with a digital camera modified with an internal 720nm filter. It is not at all surprising that some red from the visible spectrum is being passed, given the 720 nanometer is very close to the red of the visible spectrum. But, where are the blue and green colors coming from?

There are two factors involved. First, the photosites under the blue and green segments of the Bayer Filter Array are still receiving some photons. Second, the camera's computer is making every effort to produce a full color image. Photons that fall into the photosites are encoded into color channels even though they are not actually from the visible spectrum. The persistence of color encoding is one reason why a digital camera converted for infrared presents certain challenges for infrared photographers. But at the same time, open up creative opportunities. 

This is the image of the histogram shown above. You can see how the pinkish tint dominates. Now, the important point to remember is that even though we might use an infrared filter that may not pass any of the visible spectrum at all, we still get color in our images. And that is because the color camera, even when modified for infrared, still encodes color just as it was designed to do.

And this is not at all a problem situation because you can creatively leverage the color channels of an infrared capture as this processed image illustrates. Here I swapped the red and blue channels, brought the blue channel saturation down and adjusted the yellow channel hue. This example image illustrates that we get color in our images even though we are photographing in colorless radiation.

The challenges working with a color camera for infrared photography is that a digital camera still wants to function like a color camera, even though we modify it for infrared photography. Recognizing this helps us understand why the camera outputs images with color channels. But as it turns out, colors that are encoded in our infrared shots can be put to creative use.

Please refer to my infrared photography course “The Infrared Photography Masterclass” for more information about the digital camera for infrared photography.