Today Greg emailed me and informed me that:
-My wall lights at Wylam are 175 watt MH wall packs using mercury vapor lamps in a metal halide fixture, but you can not use a metal halide lamp in a mercury vapor fixture.
-Metal Halide puts out a light that has a color temperature that is warmer i.e., the colors look more like they are supposed to. Mercury vapor lamps don’t have that quality, but they put out more lumens (measurable light) and last longer. Regardless, both dim with age and it is a good policy to swap every lamp on both properties every two years.