# Matching Scores and Accept Thresholds

For every character key it examines, an OCVMax tool generates a matching score to indicate how well the actual character key in the image matches the expected character key for this position, taking any key sets into consideration. Matching scores range from 0.0 to 1.0, with 1.0 the score for a perfect match.

The tool compares this matching score against an accept threshold, and gives the character key a preliminary passing result if the matching score exceeds the accept threshold. By default, the OCVMax tool uses an accept threshold of 0.50, but you can raise or lower the value as necessary for your vision application.

The accept threshold allows you to vary the sensitivity of the OCVMax tool to character and image variation. As you increase the accept threshold, the OCVMax tool will require a better match between the expected character key and the actual character key in the run-time image.

Raise the accept threshold when your vision application acquires good images of the character strings you want to verify, with character keys of consistently good print-quality, and that exhibit little change in various degrees of freedom. A high accept threshold can improve the execution speed of the OCVMax tool. If you set the threshold too high, however, the OCVMax tool will reject most or all valid character keys in the image.

Lower the accept threshold when your production environment cannot ensure a constant appearance for each character key or when character keys can undergo a change in one or more degrees of freedom. If, as you are developing your application, the OCVMax tool fails to verify correct character keys, try lowering the accept threshold.

An OCVMax tool uses two accept thresholds. The first is an accept threshold for each character key, and the second is an accept threshold for the entire string. In general, the two thresholds use the same value. If you enable the tuning feature, an OCVMax tool can generate a good value for each accept threshold automatically. You can further modify the accept thresholds to improve performance or improve reliability.