If you try to take a photo of your patient and all you see is a black screen, it means that you need to change iPad settings to allow Smart Flow access your camera:
To do that, open Settings on your iPad:
Scroll down and on the left column select Smart Flow application:
On the right hand side, you can change the permissions for the Smart Flow. Make sure that the knob for camera is ON (the green part is visible):
Now when you go back to Smart Flow, you should be able to take a photo easily.