Many factors will influence the distance factor Z of a safety sign which is expressed as the ratio of the observation distance D and the size of the symbol or text d as follows :
Z = D/d
the Z factor ( distance factor Z) is not the same for all graphical symbols
the Z factor is influenced by the luminance of the sign (brightness appearance)
the Z factor is influence by the luminance contrast between the symbol elements or text and the background
the Z factor is influenced by the visual acuity of the audience
the Z factor is influenced by the level of familiarity that the audience has with the symbol or text
The revision of ISO 3864-1 should provide good guidance for the Exit sign of ISO 7010.
UL924 legibility testing for Exit signs provides excellent data for the Z factor of photoluminescent signs of this quality illuminated with only 50 lux of ambient lighting after 90 minutes in complete darkness,
It must be remembered that Z factor is always determined when the sign is observed "head on". For a sign to be legible and recognizeable it must of course be detected in the field of view within peripheral vision. Sign size is of course essential to making a sign conspicuous as is luminance and luminous contrast with the surroundings. However, it is essential the sign be mounted in a position facing the observer so that it can be clearly detected. The sign should also be positioned in the correct context to function effectively. Useful guidance is obtainable from JALITE and JALITE Authorised distributors which distinguishes professionals from printers.