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Automatic Focus Checking for Optical Inspection System

IP.com Disclosure Number: IPCOM000035919D
Original Publication Date: 1989-Aug-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 2 page(s) / 50K

Publishing Venue

IBM

Related People

Parker, DT: AUTHOR [+3]

Abstract

The mechanical focus position of video cameras in automated optical inspection systems must be maintained to ensure proper defect detection. Optimum focus is the mechanical camera position providing the steepest video rise time.

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Automatic Focus Checking for Optical Inspection System

The mechanical focus position of video cameras in automated optical inspection systems must be maintained to ensure proper defect detection. Optimum focus is the mechanical camera position providing the steepest video rise time.

Referring to the figure, the procedure for setting and checking the focus is as follows: 1. Set a predetermined threshold for the camera.

This threshold should provide a reasonable image

of the circuitry being measured and, if required,

might be changed to accommodate variations in

reflectivity. All subsequent imaging and

measurements should be performed using this same

threshold.

2. Image a set of circuit lines and measure the

imaged line width.

3. Adjust the vertical camera position in both

directions and repeat imaging and measuring. The

result will be a table of values representing the

average line width at each camera position.

The address with the lowest average measurements is the point of optimum focus. This value is compared to the current camera focus address, and adjustments are made, if necessary.

To find the physical location for the camera and lens that represents the shortest rise time from black level to white level, place the camera over a sample circuit line and move the vertical position of the camera and lens. The analog voltage from the camera is monitored by attaching an oscilloscope to the camera output. The point at which the circuit line appears at its most na...