The Coefficient of Utilization is an indication of a fixture's efficiency. In other words, just how well the fixture gets the lamp lumens onto the horizontal surface to be lit.
Remember, some of the light exiting the fixture leaves at high angles or may exit toward the back of the fixture. The result is that some lumens don't end up on the parking lot, but in the night sky or on adjacent land.
For exterior fixtures, we're actually interested in the beam utilization. This is the percentage of lamp lumens that eventually reach the area. For directional flood-type fixtures, the CU ranges from about 65 to 90%. For non-directional fixtures, most manufacturers provide utilization curves. In general, the larger the area to be lit, the higher the beam utilization. Most fixture catalogs don't have this data, so a call to the manufacturer will be necessary.
