Addloes Lighting & Hydroponics

As the lamps have been used longer, the lumen output decreases.
 

• In spite of the fact that a metal halide or high
• pressure sodium lamp has a lifespan of two years, it is advisable not to wait until the lamp totally breaks down.
• For by then the lumen output will have been reduced by almost 50% of a new lamp.~
  

A good rule to go by is to change the lamps once a year.  Do not forget to dispose of your old lamps properly.