Thanks for telling us about the app, brf. I would never have thought to check for an app. If you are too non-tech to go that route, the no-brainer solution is to buy the hand-held, battery powered strobe from KAB. Along with their strobe disc, you will have a very accurate estimate of speed. The others are correct to say that depending upon a steady 60Hz AC line frequency powering the bulb is chancy.
