Luxi App is available in the iOS App Store. (Link)
Luxi is a diffusion dome that fits securely over your iPhone’s front-facing camera. When attached and used with a light meter app, Luxi will help you determine the best settings for your DSLR or other camera so that you can take perfectly exposed pictures.
Teachers: write firstname.lastname@example.org to inquire about education discounts.
Distributors / Resellers: click here.