What Is False Color

In astrophotography images, we often use “false color” to present signals of different wavelengths — SHO and HOO palettes, for example. So what exactly is false color?

False color is a method of presentation based on an artificial assignment of colors. First, let’s clear up a word that’s easy to misread: “false.” It actually has several definitions, and in this context it’s more fitting to understand it as the adjective “artificial” rather than “fake” color. In other words, these colors aren’t faked — they’re artificially assigned.

The Chinese Wikipedia has relatively little on this topic, so I’d suggest going straight to the English version. Here’s my brief translated summary:

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False color (or pseudo color) refers to a group of color rendering methods used to display color images recorded in the visible or non-visible parts of the electromagnetic spectrum. A false-color image is an image that depicts an object in colors that differ from those a photograph (a true-color image) would show. In such an image, colors have been assigned to three wavelengths that the human eye normally cannot see.

In addition, false color has some variants, such as pseudocolor, density slicing, and choropleth, which are used to visualize “data collected from a single grayscale channel” or “data that does not depict a part of the electromagnetic spectrum” — for example, elevation in a topographic map, or tissue types in magnetic resonance imaging (MRI).

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Once you understand the definition of false color, look again at the SHO, HOO, and other palettes in astronomical images, and you’ll see: they don’t “capture” the nebula in those colors; rather, they map narrowband signals invisible to the human eye onto visible colors through an artificial assignment, thereby presenting the distribution of different elements.