title: Detecting heart rate using the camera
body: <p>I need the same functionality as the application <em>Instant Heart Rate</em>.     </p>
  
  <p>The basic process requires the user to:</p>
  
  <ol>
  <li>Place the tip of the index finger gently on the camera lens.</li>
  <li>Apply even pressure and cover the entire lens.</li>
  <li>Hold it steady for 10 seconds and get the heart rate.</li>
  </ol>
  
  <p>This can be accomplished by turning the flash on and watch the light change as the blood moves through the index finger.</p>
  
  <p>How can I get the light level data from the video capture? Where should I look for this?
  I looked through the class <code>AVCaptureDevice</code> but didn't find anything useful. </p>
  
  <p>I also found <code>AVCaptureDeviceSubjectAreaDidChangeNotification</code>, would that be useful?</p>