Here is an experiment you can run tonight at your Mac. Take two monitors from different manufacturers, set both to 50%, and turn the room lights off. They will not match. One will glow like a reading lamp while the other sulks. Nudge the dim one up until they agree and you might land at 50 and 71: numbers that look wrong and light that looks right.
This is the whole reason this app exists, so it’s worth spelling out why it happens and why the obvious fixes don’t work.
Percent of what? Why 50% isn’t a brightness
A brightness percentage is not a unit of light. It’s a position on that particular panel’s dial, and every panel’s dial covers a different range. One monitor’s backlight might span 80 to 400 nits; the laptop next to it might span 2 to 500. At “50%” those are very different amounts of light, and the relationship isn’t even linear, because manufacturers shape the response curve differently, panels age, and coatings differ.
So “set everything to the same number” fails, and so does the slightly smarter version: “set everything to the same number plus a fixed offset per monitor.” An offset that matches two panels at the bright end drifts apart at the dim end, because the two dials don’t just start at different places, they travel at different rates.
Matching isn’t a number you set. It’s a relationship between panels, and it changes along the range.
Two points, one line: calibrating by eye
The fix this app uses is old-fashioned surveying: measure twice. In calibrated mode you set every screen by eye while the room is dim, then again when it’s bright. Those two passes give each display its own line: this panel at this master position should sit at this level. The master slider walks all the lines at once.
The app refuses the second pass until the ambient light has genuinely changed. That feels bureaucratic the first time it stops you, but two readings taken in the same light are one reading. Any line drawn through them is a guess, and this app does not guess about the thing it exists to get right.
Why by eye, rather than a sensor or a formula? Because your eye in your chair is the only instrument that experiences your monitors the way you do: through their coatings, at their angles, against your wall. A perceived-luminosity formula is somebody’s average observer in somebody’s average room. You are not average, and neither is the glare on your left-hand screen.
The travel is honest too: where monitors stop agreeing
Once each panel has a line, an uncomfortable truth appears: there are master positions where the set can’t stay together. Push dim enough and your least-dimmable monitor bottoms out while the others keep sinking; push bright enough and your weakest panel pins at 100 while the rest sail on. Either way the set is no longer matched, which was the whole point.
So the master simply doesn’t go there. Its travel is the intersection of what your particular panels can do, and the Displays tab tells you what that range is. On the rig this site’s screenshots came from, it’s 16.7 to 77.1. A slider that runs 0 to 100 anyway would just be lying at both ends.
78 · 72 · 83
That’s what the three numbers in the screenshot on the front page mean. Three panels, three different percentages, one apparent light level. If those numbers were all 78, the screens would look different. Because they read 78, 72 and 83, they look the same.
Matched, not equal. It fits in a menu bar.