What is the Titmus Fly test?

This is a test that opticians use, normally with children, to detect stereopsis (the ability to perceive depth).

The optician asks the person to put on a pair of specially tinted glasses and look at a picture. The optician will ask questions and from the responses can work out the level of stereopsis and therefore if the patient has a lack of it, e.g. if they have amblyopia.