The limits of human visual acuity

The quality standard asks for “defect free”, but what does that mean in terms of what the inspector can find?

If you Google “human visual acuity” you'll find opinions home in on around 90 microns. That's usually based on some calculations involving the rods and cones in the eye and the field of view and so is somewhat theoretical. It tends to assume perfect contrast, excellent lighting, and of course, a young and healthy human.

The real world isn't like that. Inspectors are often a little more mature in years, factory lighting is usually not that great, and defects offer only subtle contrast variation with a good surface. In fairness though, where humans excel is in seeing patterns or detecting deviations from patterns, so we tend to be good with texture-influenced inspection.

I find a practical limit to what an inspector can find without magnification is about 300 microns, or twelve thousandths of an inch. If you take that as your “defect free” criteria you need a resolution of better than 0.1 mm per pixel. And if the inspection is done under 2X magnification, halve those numbers.

Of course, some readers might ask what the product under inspection needs. Or to turn it around, what size of flaw would cause it to fail? That's a very good question, but I don't hear it asked very often.