…continuedHome Lighting for Amateur Astronomers
All Lights Are Not Created Equal
Apart from the obvious differences in wattage and brightness output, some of these bulbs are called floodlights, while others are listed as spotlights. Any thoughts we had that the two terms were simply used interchangeably were dismissed when we found a pair of 90-watt halogen bulbs manufactured by Philips, one labeled "spot," the other "flood." Outwardly the only difference between them is the textured surface on the inside of the bulb's face. We assumed that the spotlight would have a tighter beam and thus be of interest to people trying to control the outward spill from these lights.
While our tests did reveal that the spotlight has a slightly more concentrated overall illumination pattern, the cone of light emerging from the bulb, measured at diameters of equal brightness, is only 12 percent smaller than that of the floodlight. More surprising is that at the center of the beam, the spotlight is actually 40 percent dimmer. Unless these bulbs are mounted side by side, however, it's unlikely that a homeowner will visually note much difference between them, especially when it comes to preventing light spill at large radii from the center of the beam.
Although some of the bulbs we tested projected interesting rings and spikes of light on the ground, the most noteworthy trend involved the level of illumination at increasing distances from the center of the beam. As common sense suggests, the greater an incandescent bulb's wattage, the greater its light output, and the broader its cone of illumination at a given level of brightness. In our test setup we measured the diameter of each bulb's cone of light at a fixed progression of intensities out to a minimum of 0.5 foot-candle. At this level of brightness, which is sufficient to illuminate a yard area adequately in most situations, a 50-watt Sylvania halogen floodlight 7 feet (2.1 meters) above the ground produced a cone of light subtending a 104° angle; a 100-watt General Electric floodlight, 116° and a 150-watt Westinghouse floodlight, 126°.