What size pinhole can be reliably detected using visual inspection?

John Predmore

Involved In Discussions
What size pinhole can be reliably detected using visual inspection? Our customer wants hole defects logged with cartesian coordinates, any hole larger than 0.010 inch diameter. A missed defect is grounds to reject the lot. Our initial response was this requirement is not humanly possible using visual inspection, even with backlighting. Our firm has not budgeted for a camera-based inspection system, and if we tried to adopt a turnkey vision system, implementation and validation would likely exceed project lead-time constraints. The customer’s response was what size hole can be reliably identified? Can you help me find published data on visual inspection for pinholes?

Some important details: we are working with 0.002 inch thick paper, 36 inches wide, moving at 6 feet/min roll-to-roll. At 20 inches viewing distance, I calculated a 1.5 inch field of focus (2°). For baseline data, I found 8 thin spots >0.010” per square yard using ImageJ software (unless the hole is circular and perpendicular to the paper, the true size is difficult to discern). For reference, I am saying 0.010 inches is the size of the period (full stop) at the end of this sentence.


Quality Manager
When they say cartesian are they wanting actual X, Y, Z locations or do they just care about diameter size?

Edit: Seeing the cartesian made me think of true position and I don't know if that's what they mean or if they just want ID sizes.

John Predmore

Involved In Discussions
actual X, Y, Z locations
s this the customer spec on the agreed-upon contractual documentation?
The customer spec is being negotiated prior to purchase order placement. Our initial response was 0.010" is not humanly possible.
The material is flat, 2-dimensional, and continuous length, but yes, I understood they want location in the downweb and crossweb directions.

Bev D

Heretical Statistician
Staff member
Super Moderator
The only way to truly know is to perform a MSA. (NOT a guage R&R but a full MSA.
minimum 30 "inspection areas" both with pinholes of varying sizes and without, under production conditions, 'inspected' twice by at least 2 inspectors (your best and worst?)

anything else is opinion.


Looking for Reality
FWIW, we visually inspected all items (2D, flat/flexible/thin, and continuous in length). Hundreds of thousands of feet per year.

With strong backlight (assuming your product is dark or semi-opaque), 0.010 is easy to see...we were finding 0.002" reliably with strong backlight.

But don't ever forget that humans miss things...control the high demand on inspector against lot size (potential cost of fail) against cost of setting up a line scan camera...


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