[magnetometer]

Yes,measuring the energy consumption and efficiency of the conversion from electricity to light is part of the label. The light bulb example is not as unsuitable as you seem to assume. Incandescent light bulbs only convert about 5 percent of the energy consumed to light, the rest is converted to heat. LEDs, in contrast, are 6 to 8 times more efficient.

Your "does image processing..." part is also important. Different processors consume vastly different amounts of energy. A hobbyist example would be trying to run something on a battery powered ESP32 with or without deep sleep.

[alnwlsn]

I meant it's it's physics job to produce light. If it does that efficiently, great! That's one thing you should want.

It's human job is to convey information. If you're measuring efficiency that way, it would be most efficient to have the dimmest screen possible (using the least energy) as long as you can still tell what's happening on screen. If that's already happening, then surely a screen twice as bright as you need it to be would use twice the energy and be half as efficient, even if it physically is equally as efficient making light at any brightness.

You could be using half the energy to do the "same" job.