While they may still light, they are wearing out and will die soon. [substeps] If the darkness only forms on one end of a tube light, flip the tube so the darkened end is on the opposite side of the fixture.
[header] How to test fluorescent bulbs [title] Look for dark areas at the base of the bulb. [step] When bulbs age and have been used for a long time, brown spots will appear. If darkness begins to form at the ends of the tube, the bulb may be near its end.