Halogen floods are available in both "heavy" glass and "soft" glass. The heavy glass halogen flood light bulbs are commonly referred to as "PAR" lamps. PAR stands for parabolic aluminized reflector. These halogen light bulbs are used both indoors and outdoors. Halogen light bulb PAR lamps are the only halogen flood that may be directly exposed to inclement weather, as the thick glass prevents cracking. PAR flood halogen light bulbs are available in diameters from 1.75" to 4.75". As a general rule of thumb, if the glass covering the PAR lamp is dimpled then the bulb has a wider (flood) beam spread. PAR lamps with a narrow beam (spots) have smooth glass covering the reflector.
Soft glass halogen floods look identical to standard incandescent floods (generally called either "R shape" or "BR shape". These halogen light bulbs should be used either indoors or in fixtures protected from direct weather exposure. Whether your halogen flood is a PAR, R, or BR the diameter of the bulb may be found in the item number directly following the PAR, R, or BR designation. Halogen light bulbs, like incandescent light bulbs, are measure in 1/8" increments. For example a PAR30 halogen light bulb is 30/8" diameter, or 3.75".