Maybe use a mirror, ruler and a camera? Ed: i see @andai made a similar comment. Might indeed be better to use a "long" object to indicate pixel length by div

Maybe use a mirror, ruler and a camera? Ed: i see @andai made a similar comment. Might indeed be better to use a "long" object to indicate pixel length by division (credit card is 5432 px = 5 cm) rather than going by a ruler which might typically only be accurate to 0.5mm at best anyway.