In order to conserve space, rooms shorter than the standard 20 foot viewing distance are often used. In rooms with less than a 10 to 15 foot viewing distance mirrors are used to give the equivalent to at least a 15 to 20 foot viewing distance. Commonly the projector is pointed toward a mirror on the wall the patient is facing and the image is reflected back onto a screen on the wall behind the patient. The patient then views the image of the screen in a second mirror.
The image on the screen behind the patient will be reversed since it is reflected off the mirror. When the patient views the screen in the second mirror it will appear correct.
To calibrate the chart, the viewing distance is the distance from the screen on the back wall to the patient viewing mirror plus the distance from the mirror to the patientΓÇÖs eyes. By using this distance the correct size of the 20/200 (6/60) letter can be determined. The size of the 20/200 (6/60) letter is measured and adjusted on the screen behind the patient just as previously described.
