When building an automatized system, no matter for what purpose, the first step according to my approach is optimizing the quality of the input, and trying to minimize the number of challenging situations (or events) as much as possible. The equation is easy: optimum input quality for optimal performance; but what are the implications in case of LPR? The answer depends on the intended purpose of your system. What is the critical success factor of the system? Is it hitrate, maximum accuracy, or maximum processing speed? In the second post of the current series I’ll concentrate on hitrate as the key factor of success.
First of all let’s clarify the concept of hitrate; it is the number of registered vehicles / number of total vehicles that passed the checkpoint. Don’t forget that in this case a “registered vehicle” doesn’t necessarily mean a vehicle with correctly recognized license plate.
For maximum hitrate, you must focus on catching every single vehicle passing by and generating an event out of each. It is imperative that the system is able to store exactly one event per vehicle. The most reliable method is using a hardware device that can detect the presence of a vehicle and can differentiate two different vehicles even if they are tailgating. Such a device is commonly referred to as a trigger device. A trigger device can be anything that detects the event. Let’s see some examples:
- inductive loop
- IR barrier
- laser detector
- radar
- manually operated button
In most cases, inductive loops offer the highest accuracy. Their main disadvantage is that they need to be placed in the concrete, certainly not that handy when searching a suitable trigger for already existing infrastructure. It is mainly for this reason that inductive loops are also often considered “old-school” devices. Still – it delivers.
Other – modern – devices can easily be triggered by smaller objects (animals, pedestrians, tree leaves, etc), and can also merge multiple vehicles into one event. This is why users nowadays turn more and more towards software solutions to find a workaround for this problem. In case of Asura’s ARU it is entirely possible to achieve 100% hit rate leaving out hardware triggers and using advanced video analytic solutions only.
Thank you for your time reading this article,
next time we will continue by taking a look at maximum accuracy.
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