What is infrared photography?

It is light, not heat

Your eye works between about 400 and 700 nanometres. Near-infrared begins just past that, roughly 700 to 1000 nanometres — the next step along the same spectrum, no more exotic than the colour red.

This is worth being precise about, because “infrared” usually makes people think of thermal cameras. Thermal imaging reads emitted heat at 8,000 to 14,000 nanometres, ten times further out. These photographs carry no temperature information at all. A cold leaf and a warm leaf look the same here; what differs is whether they are alive.

Every camera can see it. Almost every camera is built not to

Camera sensors are made of silicon, and silicon is naturally sensitive well into the near-infrared. So every ordinary camera has a filter bonded over the sensor — a hot mirror — whose entire job is to throw that light away before it lands. Without it, ordinary daylight photographs come out with a muddy cast, because the infrared contaminates every colour.

A full-spectrum conversion removes that filter and replaces it with clear glass. It is permanent: the camera cannot go back to taking normal photographs afterwards. What it records is then decided by a filter on the front of the lens, and how much visible light that filter lets through is the difference between a picture with colour in it and a picture in black and white.

Why the trees are bright

Chlorophyll absorbs red and blue light very strongly — that absorption is what photosynthesis runs on. But in the near-infrared it is effectively transparent, and the cell structure inside the leaf scatters that light back out rather than soaking it up. A healthy leaf returns most of the near-infrared that hits it.

The physicist Robert Wood photographed this in 1919, and it still carries his name. It is the reason a summer field can read as snow. Water does the opposite — it absorbs near-infrared almost completely — and a clear sky scatters far less of it than it does blue light. So the same frame that makes foliage brilliant drops rivers and skies toward black. That contrast is not added afterwards. It is the subject.

What is real, and what is interpreted

A fair question about any photograph this colourful: how much of it is editing?

The honest answer is that the relationships are real and the hue is a choice. Foliage is bright because leaves genuinely reflect infrared. Water is dark because it genuinely absorbs it. Straight out of a converted camera, though, the whole frame is overwhelmingly red — the infrared floods the sensor’s red channel — so these pictures are graded, and some have their colour channels swapped, which is what turns that red wash into blue sky and gold or white leaves.

In other words: nothing here is a colour filter dropped onto an ordinary photograph, and nothing here came out of the camera looking like this either. The structure is recorded; the palette is decided.

How these were made

Chris Riehm has photographed in near-infrared since 2016 — first on a converted Fujifilm X100, then on converted Nikon bodies, and more recently on a full-spectrum Fujifilm X-T2. Shorter-wavelength filters let more visible light through and give more colour to work with; longer ones give a purer infrared signal and more contrast, and less colour.

One series on this site is not infrared at all: Giverny was photographed with the filter in place, in ordinary visible light.

Questions about the process are welcome — cdriehmart@gmail.com.