Infrared images: how to make them visible?

Why look in the infrared?

A window onto the invisible

The human eye sees only a very narrow part of the light spectrum, from violet to red. Infrared lies just beyond red, with wavelengths longer than those of visible light. The James Webb Space Telescope (JWST) is designed to observe this light, mainly in its near and mid ranges.

Piercing the veils of dust

Forming stars are often wrapped in clouds of dust and gas. Dust grains absorb and scatter visible light more than long infrared wavelengths: infrared therefore makes it possible to pass through these clouds and observe the stars hidden behind.

Thanks to its infrared instruments (NIRCam, NIRSpec, MIRI, FGS/NIRISS), JWST can also detect the very faint thermal emissions of extrasolar planets, and study objects that do not shine brightly enough in the visible to be detected otherwise.

Observing the edges of the Universe

The expansion of the Universe stretches the wavelengths of light emitted by distant objects: their light is redshifted, then moves into the infrared. This shift, called redshift, is greater the farther away the object is.

In 2025, JWST confirmed the galaxy MoM-z14, with a redshift of 14.44, observed about 280 million years after the Big Bang: it is, to date, the most distant galaxy whose distance has been confirmed by spectroscopy. This record has already been broken several times by the telescope itself, and it could be broken again.

A telescope cooled to the limit

Why cold is indispensable

An infrared instrument must be kept at very low temperature, otherwise its own heat would drown out the faint signal coming from space. JWST's instruments operate around 40 K (about −233 °C), and a large sunshield protects the whole assembly from the heat of the Sun and the Earth.

The special case of MIRI

The MIRI instrument, which observes the mid-infrared up to 28 µm, requires even more intense cold: it is maintained below 6.7 K, or about −266 °C, thanks to an onboard cryocooler. Without this precaution, it would be blinded by its own thermal emission.

From infrared to visible colors

Processing raw data

The infrared data captured by the instruments are processed and converted into colors using computer algorithms. These colors do not correspond to the real colors of the observed object, but to intensities measured in precise wavelength bands. JWST actually records grayscale images, one per filter.

STScI, which manages Webb's images, prefers to speak of representative colors rather than false colors: the hues convey a measured reality, without claiming to reproduce what an observer placed near the object would see.

Chromatic ordering

The infrared domain is divided into three subdomains: near-infrared (0.75 to 1.5 µm), mid-infrared (1.5 to 30 µm) and far-infrared (30 µm to 1 mm). JWST mainly covers the first two.

The infrared wavelengths are then converted into the colors of the rainbow, according to a principle called "chromatic ordering": the longest receive red hues, the shortest blue hues. This approach improves the communication of information: the same colors represent the same information from one visualization to another, and the images remain pleasant to look at and understand.

Transposing wavelengths

To do this, imaging developers shift the infrared wavelengths toward the visible part of the spectrum, so that our eyes can distinguish them. It is exactly like the transposition of a tune played on the piano into a different key: the pianist changes the pitch of all the notes by a constant interval, while preserving the relationships between these notes. In both cases, the melody remains the same.

Applying filters

Image processing specialists then apply filters: a red filter on the farthest wavelength, a green filter on the middle wavelength, a blue filter on the nearest wavelength, and sometimes a narrow-band filter colored orange. The captions published by NASA also specify the filter associated with each color: for the Tarantula Nebula seen by NIRCam, F090W is blue, F200W green, F335M orange and F444W red.

After superimposing the filters, the developers obtain a first colorized image, more interesting but not yet aesthetically satisfying. This is where human creativity comes in: imaging developers call on their artistic sense, in dialogue with scientists.

During these manipulations, details and quality improve progressively. The retouches enhance contrast and sharpness without modifying the information: they can remove noise and artifacts, but they do not invent any object absent from the data. The goal remains above all to produce a beautiful image to look at, faithful to the measurements.

The Tarantula dresses in infrared light

The Tarantula Nebula revealed

Located in the Large Magellanic Cloud, about 161,000 light-years away, the Tarantula Nebula (30 Doradus) is the largest and brightest star-forming region in the Local Group, the set of galaxies near the Milky Way. Webb's near-infrared camera (NIRCam) reveals tens of thousands of young stars never observed before, hidden by dust.

When mid-infrared pierces the dust

In the mid-infrared observed by MIRI, the region changes appearance: the hot stars at the center fade, while the colder gas and dust become visible. Points of light signal protostars still buried, which continue to accumulate matter.

A star leaving its cocoon

Observations by the NIRSpec spectrograph, in integral field spectroscopy, made it possible to clarify a bubble-like structure spotted in the NIRCam image. The spectra show that it is not a bubble blown by a young star, but the top of a dense pillar of gas and dust, sculpted by the radiation of neighboring massive stars. The star, still protected by this cocoon, is just beginning to emerge from it.

And tomorrow?

The relay of the Nancy Grace Roman Telescope

NASA's Nancy Grace Roman telescope will also be an infrared telescope, but designed to observe vast portions of the sky rather than isolated targets. Its launch is envisaged as early as the end of 2026. It will complement JWST, which excels in the detailed study of precise objects, with large-scale surveys.

What to remember

A key to modern cosmology

In short, observation in the infrared stands out as an indispensable key to modern cosmology: it reveals regions that visible light cannot reach, piercing the veils of dust that conceal stellar nurseries, and unveiling cold and distant objects that any other method would leave in the shadows.

A window onto the invisible

Thanks to this window onto the invisible, astronomers can now trace the birth of stars, scrutinize the atmospheres of exoplanets, reconstruct the assembly of galaxies over billions of years and approach the very first chapters of the evolution of the Universe.