IMPS (TM) : Definition of Performance Measures

Determining the performance of an LPR system is not a straight forward exercise. The lack of clear definition of the performance measure compounds the difficulty. Following is IMPS defintion and performance.

Vehicle Detection Rate ( or Detection Rate )

This is the actual number of vehicles that is detected by the system. It does not include repetitions of the same vehicle. For inductive loop sensors, bumper to bumper traffic flow will result in a lower detection rate. This is because tailgating vehicle often moves into the loop sensor before the preceding vehicle has left thereby going undetected. Using inductive loop sensors, performance of 95% and above is attained. This is highly dependent on traffic flow type.

Related: Missed Detection Rate, Multiple Detection Rate, False Detection Rate

Good Image

During our audit process, good images are those we judge as those that should be accepted and OCR correctly by the LPR system. Usually this means that the entire license plate is within the image; the plates are not damaged and the plates are not obscured by strong reflections or any occlusions.

Related: Bad Image

Acceptance Rate

Acceptance Rate is the actual number of good images accepted by the LPR system. Accepted images are those where the LPR system have read the plate numbers sufficiently confidently to output a number. This should be in the range of 95 to 99 %.

Rejection Rate

This is the number of plates that are rejected by the LPR system.

Accuracy

The number of correct plate numbers read out of those accepted by the LPR system. For Singapore plates IMPS accuracy is 99.7%.

Error rate

Accepted plates that are read wrongly. Hence accuracy + error = 100%.

Rejection Error Rate

Plates that are rejected wrongly. Sometimes the license plate number read is rejected due to low confidence even though it is correct.

Response Time

Time taken to read the plate number. This ranges from 0.4 to 2 seconds, configurable. 

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We are constantly improving our system, hence all specifications subject to change without prior notice.
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