Not only measure the difference in the
brightness of the lights, you should document the consistency of the new lights (as fluorescents age, their color changes). Other metrics that should be recorded are the amp draw reduction in the lighting panels, as well as the temperature drop.
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This is a factory I swapped out for LEDs. You can see the difference in both the brightness as well as the consistency of the new LEDs. The old mercury-vapors had colors all over the place.
The amperage used by the new lights dropped by 80.9 amps, and the temperature in the lighting panels dropped 21°F!
Important note: Unless you're truly familiar with shooting bracketed RAW photographs in full manual mode and editing them to the same white balance, it's hard to truly document a side-by-side image like this. These are not just cell phone photos.