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Infrared filter types: pass and cut

Longpass, shortpass, bandpass, the hot mirror, and full-spectrum clear glass. What each filter type passes, what it blocks, and where IR filters fit.

Updated

Every infrared setup comes down to one question: which wavelengths reach the sensor. Filters are how you control that. Here are the types you will run into and what each one passes or blocks.

Hot mirror (IR-cut) pass block UVVisIR In every normal camera: passes visible, blocks UV and IR. Full spectrum (clear) pass block UVVisIR A converted sensor with no filter: passes UV, visible, and IR. Infrared longpass pass block UVVisIR An IR filter: blocks below its cutoff, passes infrared above it. Bandpass pass block UVVisIR Passes only a narrow slice and blocks both ends.
What each filter type passes (high) and blocks (low). Schematic, not measured data.

How filters pass and cut

Filters are named for the part of the spectrum they let through.

  • A longpass filter passes long wavelengths and cuts the short ones.
  • A shortpass filter does the opposite, passing short wavelengths and cutting the long ones.
  • A bandpass filter passes only a slice in the middle and blocks both ends.

The cutoff is the wavelength where a filter switches from blocking to passing, and it is usually the number printed on the filter.

Hot mirror (the IR-cut filter)

Every normal camera has one of these glued over the sensor. It passes visible light and blocks ultraviolet and infrared, which keeps everyday photos looking natural. It is really just a shortpass filter aimed at cutting IR. It is also the reason a stock camera can barely see infrared, and it is the first thing removed in a conversion.

Full spectrum (clear)

A full-spectrum conversion swaps the hot mirror for clear glass, so the sensor records ultraviolet, visible, and infrared all at once. On its own that is not an “infrared look,” it is a blank slate. You decide what to capture by putting a filter in front of the lens.

Infrared longpass filters

These are the IR filters most people mean. Each one blocks everything below its cutoff and passes infrared above it. The cutoff is the nm number: 590, 665, 720, 850, and up. A lower number lets some visible color through for false-color work, a higher number pushes toward pure black and white. For which cutoff gives which look, see the filter reference.

Bandpass

Less common for everyday IR. A bandpass filter passes only a narrow slice and blocks both ends, which is handy when you want to isolate one part of the spectrum, such as a specific ultraviolet band or a narrowband scientific use.

New to the spectrum? Start with the basics. To pick a cutoff, see the filter reference.