Thanks so much for replying. I have something like this

http://www.intheswim.com/p/hayward-replacement-lights

this was already there when we moved into 8 years ago but it seems to be in good condition still. I bought a 500 watts from intheswim thinking it will work. It did. For 2-3 weeks only and not it stopped working. I am not sure if the main electrical is one giving the problem or the bulb. Someone said here that a 100 watt flood light from HD or Lowes worked as well for 20 yrs without any problems. It makes sense.. maybe I should have done that instead.