Marking Legibility (7.1.2 of IEC 60601-1): Size of Letters

Peter Selvey

Staff member
Super Moderator
#11
Eye charts use the same width (strokes and gaps), so originally it was just a starting point to get some kind of context for what 20/20 means.

For larger fonts I would agree, the assumption is not correct. Clearly the gaps are bigger than the strokes for a typical font. But as the font gets smaller (especially around 1.5mm) it's necessary to maintain a minimum line thickness, and as such the stroke and gap become similar.

Anyway, the final criteria is a gap of 0.29mm. As long as the font maintains this, it should be visible for 20/20 vision.
 
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NickM

#13
A useful resource on this topic that I reference often is Annex B of ANSI Z535.4. It has a bunch of good info on font styles, types, sizes, layouts, etc. It is specific to safety labels/signs but also a good reference for other types of labels. There is a table (Table B1) that gives recommended font size based viewing distance and whether the reading condition is favorable or unfavorable (not clearly defined in the standard). It appears to be a bit more conservative than the references in this thread. For example, at 3ft (90cm), the min letter height is 3.0mm under favorable reading conditions and 6.4mm for unfavorable reading conditions.

Nick
 
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