You want to download "Pocket Light Meter." It is free to download and a small fee to remove the ads and/or buy the developer a pint. From everything that I have read, it is the best iPhone Meter available. I believe there was a thread on it and the alternatives here at one point. It does not work well in low-light, but I am very happy with my results. I don't have a bonafide light meter to compare its readings to though...
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