Infrared Explained


What exactly is infrared light? And how is photography with it different than normal photography?

As I said before, I'm not an expert on photography in any part of the electromagnetic spectrum, but I did study physics for a really long time, so I think I'm qualified to explain this.


Basics of Light

All light is just an electromagnetic wave. Visible light is light that our eyes happen to be able to respond to. Since light is a wave, it has a wavelength, which is the distance between crests of the wave, and a frequency, which is the rate at which the crests of the wave pass by as they travel through space. Waves also have a speed, which is how fast a crest (or trough, or any other point on the wave) travels. The speed of a wave is equal to the wavelength times the frequency. (This formula is written c = l n, where c = speed, l = wavelength, andn = frequency.)

Light, of course, travels at the speed of light, which is very nearly 3x108 m/sec, or 3x1010 cm/sec, or about 186,000 miles per second.

The wavelength of light is really really small, so it is measured in nanometers, abbreviated nm. That's one billionth of a meter, or one millionth of a millimeter. A nanometer is roughly ten times larger than a typical atom. Pretty small.

The range of wavelengths of visible light is around 400 nm, which is violet, to 700 nm, which is red. Wavelengths shorter than this are called ultraviolet; wavelengths longer than this are called infrared. Some people can see light with a wavelength as short as about 350 nm, but almost no one can see much past about 700 nm.

The frequency of light is not something we generally use when describing light, but if you apply the rule that c = l n, then you can calculate n =  545 THz, or 540,000 GHz.

Infrared light is all around us all the time, but we can't see it. The sun emits a very broad spectrum of light, from really long wavelengths hundreds or thousands of times longer than visible light, all the way up to x-rays and gamma rays, which have wavelengths many thousands of times shorter than visible light. The light from most TV and VCR remote controls is infrared, usually around 850 nm.


Infrared Photography

Infrared photography is just taking pictures using some kind of medium that is sensitive to infrared light. Most standard photographic film is not at all sensitive to IR. This is desirable for most purposes since there is so much IR around. If normal film were able to detect it, then most pictures would be washed out by it or have weird color casts. There is film which is specifically designed to detect infrared light, but I don't know much about it.

Digital images sensors, on the other hand, both CMOS and CCD, are actually quite sensitive to IR. When they are used in digital cameras for normal photography, an IR-blocking filter has to be inserted somewhere in the optic path before the sensor. This is a piece of glass or plastic which has a dye in it that absorbs the infrared light and allows normal visible light to pass through. They work quite well but are not perfect, and they do allow a small amount, usually around 1-2%, of the incoming infrared light to pass through.

So what is needed is a filter which blocks visible light and allows IR to pass (often called an "infrared filter," even though it is really filtering out visible light and allowing IR to pass through). This is placed in front of a digital camera's lens, and if it blocks enough of the visible light, then what will make it to the image sensor will have a much higher proportion of infrared than visible.

Then the only issue is blocking the visible light, which requires a special filter. If this is not done, even after removing the IR-blocking filter, then the visible light will overwhelm the infrared light, and the image will look nearly normal. This is because even digital image sensors are much more sensitive to visible light than they to IR.

The exposure time will have to be very long, because the IR-pass filter will block about 98% of the visible light, and the IR-block filter in the camera will block about 98% of the infrared, so only a very small amount of light will be making it to the sensor.

To really take good infrared pictures with a digital camera, the internal IR-blocking filter has to be removed. This is tricky and requires opening up the camera and breaking the filter off. I did this to our old Fuji FinePix2400, but only after we were sure we didn't want it anymore because it had multiple problems.


Infrared Filters

A typical infrared filter is the Hoya R72. This filter blocks visible light and allows infrared to pass through. The wavelength where the transition happens is 720 nm, hence the "72" in the name. At 720 nm, 50% of the visible light is blocked. This is called the "cutoff" wavelength. There are many different types of visible-blocking filters with different cutoff wavelenths and different slopes of cutoff. What I would like to get is a filter with a higher cutoff, so that much more visible light is blocked, probably something around 800 nm or even 850 nm. This will not work on the Canon S2 since it still has its IR-blocking filter in place, but on the modified Fuji it should be very nice.


Conclusion

Now, how is infrared photography different? What's so great about it?

Well, it's certainly not like being transported to another dimension or something. But it given a different view to certain things. The sky, for example, is much darker because very little infrared is reflected by the water in the air that makes the sky blue. Leaves appear very bright because they reflect nearly all the IR light that hits them.  So it's really just a minor artistic variation.

Some materials, however, are transparent to IR light that are not transparent to visible light. This should be obvious since there we've been talking about photographic filters that block visible light and allow IR to pass. The dark red covers on the end of many remote controls are like this too -- they are nearly transparent to IR but nearly opaque to visible light, mainly for aesthetic reasons. But some fabrics are transparent to IR. This was well-publicized a while ago when Sony made a video camera that could remove the IR-blocking filter and sometimes you could see through certain fabrics. I have indeed observed this with my Fuji + Hoya setup, but I don't have any pictures of this that I can post right now. Maybe someday I'll figure out an interesting application for this.

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