Most farms have at least one high intensity discharge lamp (HID) lighting up the yard at night. A mercury vapor (MV) light is one type of HID lamp. However, the mercury vapor lamp is the least efficient of HID type lamps and poses a greater environmental risk than other types such as high pressure sodium (HPS) lamps and metal halide (MH) lamps. A HPS lamp emits about 95 lumens/watt while MH lamps emit about 60 lumens/watt and a MV lamp emits only 32 lumens/watt, the lowest of the HID type lamps. The metal halide lamp provides the best color rendering light and should be considered for replacing MV lamps in areas where color recognition is important such as a dairy freestall barn. Many lamp manufacturers have HPS or MH retrofit lamps that can be used to directly replace a mercury vapor lamp without replacing the fixture, but these are expensive, costing as much as a new fixture with the lamp. A 100-watt HPS lamp can provide 2.5 times more light than a 100-watt MV lamp while a 100-watt MH lamp produces 155% more lumens.

Metal halide lamps are available in a standard version and a pulse-start version. The pulse-start is newer technology which results in up to 50% longer lamp life, about 8% more lumens per watt, faster warm up and restarts and up to 33% less lumen depreciation over the lamp life. They should be considered for any new fixtures. However, the lamps for pulse-start metal halide (PSMH) cannot be used in standard metal halide fixtures and standard metal halide lamps cannot be used in PSMH fixtures. Therefore, if adding new fixtures to an existing installation, it is recommended that all fixtures be upgraded to pulse-start ballasts and lamps to eliminate inventory issues.

Low bay HID fixtures are typically used in freestall barns (see photo at right). They should be a wide distribution type which usually means they will have a diffuser or cover over the lamp to diffuse some of the light horizontally. It is recommended that the fixture be rated for a damp environment with a gasketed diffuser.

How does a HID lamp work?
A HID lamp consists of an arc tube (or sometimes called a discharge tube) filled with a gas. In the case of a metal halide lamp, it contains a starting gas (usually argon), mercury and metal halide salts. When the lamp is operating, a high voltage is applied to the gas mixture in the tube causing it to vaporize and emit light and UV energy. The outer jacket or bulb provides a stable thermal environment for the arc tube. If the HID lamp is turned off, it has to cool before it will re-light or re-strike which can take 5 to 20 minutes.

When is it time to change a HID lamp?
Because HID lights do not have a filament to burn out like an incandescent bulb, they don’t just stop working but instead just fade away as they age. If the lamp lumen output is severely degraded the lamp should be changed. The second indication that lamp replacement is needed is frequent shutting down and re-striking while power is applied to fixture. It is best to change the lamps at approximately the rated life which is usually 20,000 hours or more.

If you have questions about the information on this site, please contact
Scott Sanford, Distinguished Outreach Specialist, University of Wisconsin, sasanford@wisc.edu.