We've seen some apps for iOS in the past that allow your phone to be used as a light meter, and even though some were very skeptical, there were some tests conducted by Ryan E. Walters showing they can work very well (especially considering their cost). What's the catch though? Well, because of the design, you're only ever getting a reflective reading, not an incident reading, which actually measures the light falling upon the object you're shooting. Using the incident reading can usually be faster and give better and more consistent results, so that's why a new Kickstarter project called Luxi is aiming to turn any iPhone into an incident light meter.

With digital cameras, light meters can be considered somewhat redundant because pretty much every camera gives information on the scene and has tools to help identify the correct exposure (not to mention you can see what you're doing). Of course, light meters are still useful, and there are plenty of DPs that use them even on digital sets, but if you're a little short on cash, they can be an expensive proposition. That's where Luxi comes in, and I think it's a fantastic idea to turn any iPhone into an incident meter. The other great thing is that the physical product is being designed App agnostic. Here is some more info on that:

Luxi works with any light meter app that can use the front-facing camera and can be calibrated with an offset. We have tested Luxi with many of the light meter apps available for the iPhone.

We're excited to tell you that we have been working with the developer who created Pocket Light Meter to create a Luxi mode which is specifically designed to be used with Luxi. Stay tuned for updates on this. We may also work with additional app developers in the future to create more exciting ways to use Luxi.

I also think these kinds of light meters are great as backups if you are on a film set and there is no other way to properly judge exposure. I've been in situations where batteries have died and I had no other solution (since many light meters don't take normal batteries), so leaving something like Luxi in your camera bag or in your car means you'll always have a pretty cheap last resort backup just in case.

As of right now the project has already hit its goal -- and actually surpassed it by a great margin with over a month still left -- but you can get your hands on a Luxi for only $19 that will work with either the iPhone 5 or the iPhone 4/4S. They are not planning an Android device at this time simply because of the sheer number of Android phones out there that would all have to have their own specifically designed Luxi.

Head on over to the Kickstarter site to read more about it.

Link: Luxi Incident Light Meter adapter for iPhone -- Kickstarter