HPS (High Pressure Sodium) lamps can burn in any orientation.
MH (Metal Halide) lamps come in three basic types:
*Base up (BU), which must be operated in the base up position.
*Horizontal (HOR), must be operated in the horizontal position.
*Universal (U), which may be operated in any position.
Orienting metal halide lamps in burning positions other than those
specified can result in severe reductions in performance and potential
nonpassive failure. Lamp life,light output and color can be affected by
the burningposition. Some burning positions may need enclosed fixtures
for safety reasons.
When replacing ANY HID (High intensity Discharge) lamp, never touch
the bulb with bare hands. HID lighting gets exceptionally hot and any
oils from your skin left on the bulb will burn and can cause the lamp
Always use soft, clean gloves when handling the lamps.
Observe extreme caution while foliar feeding any plants around
exposed lamps. I suggest turning off any fans and ventilation while
spraying the plants, any water blown onto an exposed, burning lamp can
cause it to explode in your face. Be safe.