When should light guides be replaced?

• Illumination is low and replacing the lamp does not improve brightness
  
• Dark or uneven areas become visible in the field of view
• A section of the light guide becomes noticeably warmer than the rest of the guide
• It is 2-3 years old or has been in use 4000+ hours or 2 lamp changes

Liquid light guides are universally exposed to UV light which causes slow degradation in transmission properties.  It is always a good idea to check your light guide if you notice a lower signal than normal.