In an examination room shorter than 20 feet, which system may be used to create an optical 20-foot distance?

• A. Mirror system
• B. Telephoto lens
• C. Prism bar
• D. Ruler method

Answer: (A)

When you need the standard 20-foot viewing distance but the room is shorter, you must create the same angular size at the eye. A mirror system does this by giving an extended optical path: the chart light travels to a mirror at the end of the room and back to the patient’s eye. The light’s round-trip distance simulates a much longer distance, so the letters subtend the same angle as they would at 20 feet, preserving the validity of the acuity test in a small space.

A telephoto lens changes magnification, not the testing distance. A prism bar is used to measure prism corrections, not to establish viewing distance. The ruler method involves physically measuring distance, which isn’t feasible to alter the optical distance to 20 feet in a short room. So, the mirror system is the appropriate solution.