Use a iPhone to check the efficiency of a fluorescent light

An interesting use of an iPhone or probably any other smart phone is to use it to see easily if a fluorescent light is controlled by an efficient HF digital ballast or a conventional ballast (these should really be banned I think as they are very energy innecficient). You do not need to download and App. Simply use the inbuilt camera and point it directly towards the tube when the light is on. The image you will see on the screen will be a constant image of the light if an HF ballast and if it is a conventioanl ballast the image will flicker. The reason is quite simple, the HF ballast operates at 70KHz, far higher that the camera while the conventional ballast operates at 50Hz, similar to that of the camera causing interferences. If you see the scrolling image then consider upgrading to a more energy efficient High Frequency unit and of course control it properly, it could halve the energy consumption!