As Chuck Berry has been reported to have said:
"Keep it simple ...."
For a totally blind person, this app allows detection of light at various levels. If a light has been left on or not turned on, the detector indicates this by its tone: low for low or no light and high for bright light, with gradations according to the light source. The app lets the user know if a bulb has been burnt out or whether someone in the house has, or has not, forgotten to open or close blinds or curtains.
Works every time without fail on my IPhone 4.
If you want to play with it, you can engage the app, scan the phone across a solid object, such as a standing person and the tones will warble and oscillate. Just tell your subject that they have confirmation that they have not been transformed into an alien. (smile)