A couple of months ago, we did a feature on how you can remove the IR filter from a digital SLR, to enable it for Infra Red photography. All good and well, but surely, that filter was there for a reason?

One of our readers was wondering about just this, and sent me a question: “I was wondering If it was possible to have a external IR filter that threads onto my lenses that will block the IR light to the sensor but still transmit all visible light through it, mimicking the original internal IR filter that was previously attached to the cameras image sensor?”

Well actually, yes, you can! It is even quite easy.

When working with IR photography, you need to have the IR filter removed from inside your camera, but in addition, you need an IR Pass filter. This is short for “Infrared Pass / Visible Light blocking” filter, meaning that the filter blocks out all visible light, and lets IR pass through. The visible light spectrum occupies wavelengths roughly from 380 to 780 nm, and the near-infra-red is directly above (780 nm upwards). As such, you want a filter that blocks out everything up to 780 nm or so, and lets through everything beyond that.

Specifically:

Wratten* filters of numbers 87, 87C, and 88A block visible light and pass IR. The 87C is the most visible-blocking of these, with almost complete blocking of wavelengths below 800 nM, and some significant absorption even into the mid 800′s.

If you are interested in the specifics of all of this, you could do worse than check out infra red and the Visible Light Spectrum on Wikipedia.

Either way, if you have taken your IR filter out of your camera, all you need to do is to get an on-camera filter that does the opposite of an IR pass filter. Unsurprisingly, these are known as an IR Block filter. Unfortunately, these are difficult to come by – it turns out it is easy to block out visible light, and let through IR. The other way around is more tricky. Of course – seeing as how these filters exist in digital cameras already – it must be possible, but there aren’t many of these filters commerially available. In normal photography, people mostly worry about filtering out ultra-violet light rays, as these generally are more disturbing to general photography.

Go to a good photography shop and ask them to sort you out with an IR block filter. If all else fails, you could always replace the IR filter in your camera with a filter with increased frequency pass.

Finally read this article which offers a lot of detail on both IR Pass and Block filters.

*) Find out what Wratten filter numbers are, and see what they classify

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