"For use in fixtures which do not redirect a substantial portion of the energy toward the arc tube; otherwise very early failure is anticipated."
What types of HPS lamps are affected by this restriction and what is the mechanism of action for reflection into arc tube damaging the lamp?
What circumsntaces/fixtures cause the excessive reflection to occur?
Reflected light caused the arc tube to heat up. This results in a higher lamp voltage and also increased power consumption.
During lamp life the lamp voltage increases gradually. The lamp life is determined by the lamp voltage. As soon as a certain lamp voltage is reached, the lamps starts to "cycle" (burn intermittently).
The increased power consumption accelerates the lamp voltage rise.
In principle the effect stronger with a compact fixture and also with fixtures with a reflector that reflects the light onto the arc tube.