Are they compatible? What do R and PAR prefixes represent in general?
Important decision, need to buy several replacement lightbulbs!
According to this site:
R20 bulbs have reflectors that direct light forward and produce more narrow soft-edged beam that is less precise than PAR20 bulbs. R20 bulbs also produce less shadow than PAR20 bulbs. PAR20 bulbs control light more precisely and produce more concentrated light than R20.
My understanding is that PAR lights have thicker glass to withstand more extreme conditions, and are thus usually chosen for outdoor purposes. R lights have thinner glass and are only meant for indoor use.
PAR stands for Parabolic Aluminized Reflector, by the way. The numeric part of the number is the diameter in eighths of an inch. A PAR38 will be 4-3/4" in diameter and an R20 or PAR R20 should be 2-1/2" in diameter.
Both answers above are correct. Just adding a different look.
Par and R 20's can be incandescent, fluorescent and LED. The most common lamps are the 45R20 and the 50Par20's. The pars have a parabolic glass that deflects the light a little better than the R's. The Par's are halogen, burn at a little cooler color (regular incandescent is aprox 2700 kelvins and Pars burn around 3000K.) Now here's the big difference. The 45R20's puts out around 295 lumens, or 6.6 lumens per watt. The halogen 50PAR20HAL-NFL30 (halogen narrow flood 30 degree beam spread) puts out 550 lumens, or 11 lumens per watt, or 40% more lumens.
Since you get more light out of the halogen you can dim your lamps more and use less wattage by dimming and this converts over to heat. More dimming equals less heat. Also dimming a lamp 10% saves 10% electricity and makes the bulb last twice as long.
Also, lamp life is 2500 hours on PAR20's and 2000 hours on R20's. Not much when you compare to 10K hours of fluorescent and 30k+ on LED.
Adding a couple of points: