Do HID mercury vapor lamps have to be mounted at a certain angle?

I read somewhere that HID lights had to be mounted at a certain angle. I don't know if this is the case for mercury vapor or not.

Thanks

Scott

Edit: From what I read, it looks like only Metal Halide (MH) lamps are affected by burn position.

Quote from this site:

What effect does burning position have on metal halide lamps?

The position in which metal halide (MH) lamps are installed makes a difference in the color variation and color shift of the lamps. Position can also affect lamp life. Installing MH lamps in a base-up position generally gives the best results in terms of minimizing color variation and maximizing lamp life, but some lamps perform equally well when installed base down. Some lamps should only be installed horizontally (see Figure 3). The manufacturer's literature should be consulted to determine the best installation position for a MH lamp. Most manufacturers' catalogs give the lumen output for lamps installed in specific burning positions.

Some HID's are sensitive to base orientation (Metal halide,multi-vapor,multi-vapor II, and low pressure sodium, SOX), and even when properly oriented Horizontal and vertical burning versions with the same wattage often don't have the same output. Usually the part number of the lamp will clue you in about burning position.
MXR175/VBD is an MXR175, Vertical burn, base down
MXR175/VBU is an MXR175, Vertical Burn, base up,
MXR175/HOR is an MXR175, Horizontal (+/-1 15 degrees) burn.
If you run them in the wrong configuration, they either don't light properly, have low output and/or short life.


Base down is 17,200 lumen, Base up is 16,300, Horizontal is 15,000 for an MXR175 (Metal Halide 175 watt).

HPS (high Pressure Sodium), and True Mercury Vapor don't care much about vertical/horizontal.

I had to switch to more expensive HOR rated metal halides to get decent life and lumen maintenance compared to Universal rated for one installation.