In this article
Take a logo, place its black version on a light background and its white version on a dark one. Same path, same weight. Yet the white version looks thicker. It is not your screen, it is your eye.
It is a detail few people notice consciously, but everyone perceives. The same symbol, in black on light and white on dark, does not appear to have the same thickness. The white always looks heavier. The phenomenon has a name: the irradiation illusion.
The observation
The path is strictly identical. No weight has been added. And yet, on the light version, the solids seem to swell, the counter-shapes tighten, the whole thing looks heavier. This is not a file error, it is a feature of human vision.
Why the eye is fooled
Your visual system never perceives a perfectly sharp edge. Every contour becomes a small transition zone. And on that zone, bright edges appear to spread outward far more than dark ones.
The result: a light shape on a dark background slightly overflows in perception, while a dark shape on a light background contracts. At strictly equal geometry, white visually gains thickness.
The eye does not see sharp edges, it sees transitions. And a bright transition always swells more than a dark one.
The practical rule
The consequence for a logotype is direct: the inverted version of a logo needs a slightly thinner stroke than its dark version. So you do not simply invert the colours of one file, you prepare a dedicated light version, with a very slightly reduced weight to compensate for irradiation.
It is a subtle adjustment, often a few percent, but it is exactly what separates a sloppy variant from a controlled identity.
What it changes in a real deliverable
In a well-built logo system, the light version is not a simple invert colours button. It is a variant in its own right, optimized for its context, just like the monochrome or outline versions. It is the kind of optical correction that goes unnoticed when done, and jumps out when it is not.
This attention to perceptual detail fits the logic of a logo designed as a system rather than an image: each variant is calibrated to stay right on its medium, not just mechanically derived from the original.
Frequently asked questions
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Is the irradiation illusion a screen problem?
- No, it is a perceptual phenomenon inherent to human vision. It appears both on screen and in print, on any medium.
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By how much should you thin the white version?
- There is no universal value: a few percent less weight is often enough. The adjustment is done by eye, comparing both versions side by side at real size.
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Do you really need a separate file for the light version?
- Yes, for a careful logo. A simple colour inversion carries the problem over. A dedicated light version, slightly thinned, gives a perceived thickness identical to the dark version.
