About Viewing Distances
Letters on safety signs should be 100mm tall to be safely read within 25 meters – according to the old Guidance to the Fire Precautions. But the guidelines were set out in the days when signs read just EXIT and FIRE EXIT.
What about the new graphical symbol-based signs as required by the Health and Safety (Safety Signs and Signals) regulations?
Guidance on the correct use of fire safety signs has been omitted by the otherwise excellent BS 5499: Part 4 2000 that so expertly meets all the other requirements of the regulations.
Perhaps I can offer some help.
Legibility Data From Eyesight Testing
Luckily there is plenty of research data available from eyesight tests on millions of people. Indeed this is the basis for the old and widely accepted 100mm for 25 meter observation distance rule. It has been found that people with normal (or corrected to normal) vision can reliably resolve a detail that subtends an angle of 1 minute. Using simple geometry, we can work out that the relationship of the observation distance (D) to the detail height (d) for normal sighted, is given by the relationship ran 1 minute = d/D. This gives D = 3437.75 d.