Why do eye exams always have you read the letters off of a mirror?
A mirror is not a necessary part of measuring visual acuity.
Over 100 years ago, as the science of Optometry and Ophthalmology was developing, distance visual acuity was defined as a measurement at 20 feet and beyond. [Near vision actually requires a different process, but is rarely an issue until we reach our mid-40s.] Many exam rooms were built quite long or included an "extension" that put the Snellen eye chart at that exact 20' distance.
But since this was a inefficient use of precious office space, equipment manufacturers developed a "folded" system using a projector and mirrors to work in a smaller size room, yet maintain a simulated 20 foot distance measurement. [There is widespread doubt that this truly fools the eye into thinking the chart is really that far away, though.]
More up-to-date and precise visual acuity measurement systems use digital screens or back-illuminated eye charts at a typically reduced 9-10 feet. The size of the optotypes (letters) are then adjusted proportionately for that distance. This obviates the need for unreliable projectors and mirrors but can potentially induce a little accommodation (near response in focusing) so a red/green overlay is often used to correct for this.
_Written by J. Trevor Woodhams, M.D. - Chief of Surgery, Woodhams Eye Clinic