You searched for: “lumen depreciation
lumen depreciation (s) (noun), lumen depreciations (pl)
The gradual loss of efficacy of a lamp over time, typically caused by the loss of chemical additives in the lamp and wall darkening in the arc tube: Lumen depreciation can be indicated by a curve depicting the pattern of decreasing lucid output.

The florescent illumination in Mildred's kitchen started to flicker, apparently because of the lumen depreciation in the tubes.