The most common HID lamps are metal halide (MH) and high pressure sodium (HPS).

Efficacy in lumens-per-watt (LPW), lamp life, and cost are economic characteristics of a HID lamp which can be analyzed objectively.

Color and color rendering properties, however, are much more subjective. The photo shows a facility with two types of systems.

Left side of picture : 400 watt low bay aluminum HPS fixtures (yellow light source) with a Color Rendering Index (CRI) of 25. Lamp: 50,000 initial lumen. Fixture wattage: 464 Watts.

Right side of picture : 400 watt high bay glass MH fixtures (‘white’ light source) with a CRI of 65. Lamp: 36,000 initial lumen. Fixture wattage: 458 Watts.

Metal halide systems are the preferred HID source for applications where color rendering is important. It is common for workplaces with 400 watt HPS systems to be retrofitted to a 400 watt MH systems