CATRA is a highly portable, and easy to use accessory that attaches itself to a smart phone and easily detects cataract. Made from off the shelf components, and consisting of no moving parts, CATRA, developed by Ramesh Raskar's team at MIT, is being considered to be ideal for the developing world.
The user is made to look through the accessory and press a few buttons. Doing so allows the app to collect quantifiable data and create an attenuation map, which in turn helps detect cataracts.
Watch this (technical) video to know more about CATRA.
Note: Although the price of the accessory is not mentioned anywhere, the same team designed a similar accessory called NETRA for eye exams which costs $1.
For more technical details, and to read the paper, hit the source link.