How to measure light intensity in a particular place using iphone

Thanks in advance. I want to find out light intensity at some place using iphone. I searched开发者_JAVA技巧 in google and find some apps also But didn't get how to measure. Links to that apps are

• http://itunes.apple.com/in/app/light-meter-free/id410228606?mt=8
• http://itunes.apple.com/in/app/pocket-light-meter/id381698089?mt=8
• http://itunes.apple.com/in/app/claplight/id321594648?mt=8
• http://itunes.apple.com/in/app/lightmeter/id320768222?mt=8

Do any one have an idea to find out please help me.

if you want to make one yourself:

i won't be able to help you with code specifics but i do know that the image values are going to be meaningless unless you know the exposure and gain of the camera. at the very least what you need to do is have a way to reliably generate light levels and then calibrate your camera for all exposure times and gain using that reference source. also you need to deal with variations across the image. you could average all pixels or crop out relevant parts. you also might want to take account of non-linearities of the cmos sensor and dependency on wavelength. i.e. the colour value of a pixel is not linearly proportional to the light that is hitting it and all this changes with the wavelength ( colour ) of the light, the latter which you can't really measure. although that is an approximation that might work. depending on how accurate you need to be.

may be the StackOverflow answer: getting-exposure-values-from-camera-on-iphone-os-4-0 might help.