The specific type of incandescent lamp used and the kind of fixture involved also make a difference. For example, a 75-watt ellipsoidal reflector lamp delivers more light in a stack-baffled down light than a 150-watt R lamp of higher wattage. This is because much of the R lamp's light is trapped in the fixture and converted to heat. The 75-watt ER lamp's shape and reflective interior focuses light down, outside of the fixture, thereby producing more light on the surface. And there are energy efficient versions of the 150-watt R lamps available that use only 120 watts and provide only slightly less light.
The rated life of an incandescent lamp is determined by operating a fixed number of lamps at standard voltage until 50% burn out. The voltage at which a lamp is operated affects its life expectancy. A lamp operating with higher voltage will burn brighter, but with dramatically shortened life, and one operated under voltage will not burn as brightly, but will have extended life.
Look for lamps marked "energy efficient". They use 5% to 13% less energy than standard bulbs. There are energy-efficient replacements for all standard wattages and while they may cost a little more, they more than make up for their extra cost through lower operating costs. Another label that appears on some lamps is "long-life" or "rough service". These bulbs have a thicker filament, which makes them last longer, but it also makes them less efficient and therefore they are not recommended except in locations where changing bulbs is difficult, or where conditions cause standard bulbs to fail prematurely.
The most common types are: the "A" or arbitrary bulb-shaped lamp; the "PS" or pear-shaped lamp; the "R" or reflector lamp; the "PAR" or sealed-beam lamp, and the tungsten-halogen lamp.