These are used for cans that have a compact fluorescent coating that sticks straight down. They are quite common in kitchens and bathrooms and are installed in an energy-efficient manner. They are often too dark or just take too long to warm up to their true brightness. That is the answer! They turn on at full brightness. This rating is for 4000k, they also come in 3000k which I think is better suited for residential applications but my client wanted a cooler color than his fluorescent lights. I also bought a box of 3000K to use elsewhere. My only issue is that I bought a whole box of 10 and went past the return window before pulling out a broken one. It is divided at the base. (actually it works, but it shouldn't get stuck). I recommend anyone buying a whole box to check out the whole box right away. I'm stuck with $10. a lamp I can't use.