The sensors were then taken to a facility, which had several 600 watt-per-inch or wpi (240 watts per centimeter or wpcm) , microwave excited, medium pressure mercury arc lamps in continuous, 24 hour per day operation. The sensors were mounted in a position near the elliptical reflector. The optical port was aligned directly with the bulb. This is a severe test; as noted earlier, there are advantages to directing the sensor to reflected energy; continuous monitoring by direct view of the lamp is not always desired, or necessary.