This nifty little app lets you know if your phone is detecting any harmful radiation.
How does it do that? The science behind the app is pretty simple - if you cover your cellphone camera with a dark piece of paper or tape over the cell phone, light won't pass through, but radiation will still hit the camera's CMOS censor, causing pixels to "light up".
|My "Radiation" reading after calibration|
NOTE: Using a piece of a floppy disk may generate false positives if your phone is pointed at a light source, or used in a very bright area.
While I don't have a radioactive source available to me, the science behind this is sound, and reinforced by this YouTube video, which shows what a cellphone camera looks like when exposed to radiation.
When you install and calibrate this app for the first time, you should be away from any known radiation sources. For example, if you live in a brick house, you should actually calibrate your app in a location away from any brick buildings. After all, brick houses do give off low levels of radiation. Of course, if you live anywhere near Fukushima or Chernobyl, you might have a hard time calibrating this.