reflector in a metal halide high bay may only be 70 to 75% efficient while most T5-HO and T8 fixtures efficiency ratings will be above 90%. Quality of lighting is important. T5-HO..., two issues will occur with HID lighting systems: First, more fixtures are required to compensate for the lumen depreciation, which raises the initial foot-candles, energy output... and the installation cost. Or, the system depreciates down to an undesirable lighting level that requires a re-lamping cycle sooner than expected, therefore increasing the cost...