scope:

This LM outlines factors that can cause differences between calculated and measured illuminance and luminance values of outdoor high intensity discharge (HID) and low pressure sodium (LPS) lamps and luminaires. It does not offer solutions, nor does it quantify all of the possible variables that might be encountered. The relevant ANSI/IES Recommended Practice document should be consulted for specific design recommendations.

when comparing field measurements with predicted calculations using photometric data. It is possible that the tolerances for all the components could combine to produce a substantial difference in illuminance or luminance performance between laboratory and field conditions. The variables can be divided into five general categories:

• Power supply and ballast variations

• Lamp variations

• Luminaire optical variations

• Field conditions

• Field measurement

Each category is reviewed and its significance with respect to HID luminaire operation is explained.