Maximum recognition performance in ALPR systems

ALPR (automatic license plate recognition) systems, also known as ANPR (automatic number plate recognition) systems, must work 24/7 under all light conditions and in different environments – from controlled indoor surroundings to the harsh outdoors

The ALPR systems apply optical character recognition (OCR), which is based on either the visible spectrum (VIS) or the near-infrared (NIR). Sometimes, it might even be a combination of these two technologies.

The recognition performance is challenged by many parameters, such as changing light conditions, high vehicle speed, and various number plate inks – not to mention the general performance of the applied software and hardware. To optimize the performance of ALPR systems with all these challenges in mind, the signal-to-noise (SNR) ratio is the most critical metric.

The signal-to-noise ratio can be optimized with a high-quality cover glass solution. Besides being an essential part of the enclosure, a cover glass can function as a wavelength filter to improve the device’s SNR. Furthermore, it can be used as an element to enhance the application’s design and aesthetics since a ‘black panel effect’ can be achieved where the cover glass is virtually completely black. That way, the optics behind the cover glass are entirely concealed.

The choice of base material always depends on the specific application’s requirements, but materials such as PMMA, polycarbonate, and glass are typical base materials for cover glass solutions.

However, PMMA is often the preferred choice for ALPR systems due to several factors:

• High optical quality
• Easy and cost-efficient processing
• Lightweight (compared to glass)
• UV resistance
• Better scratch resistance than polycarbonate
• Many surface treatment options to customize the solution for your specific application

If impact resistance is the most critical requirement, polycarbonate or a thicker PMMA could also be an option. Optical glass filters are of high optical quality, but there are limitations with machining, impact resistance, and the weight of glass.

Therefore, you must have a good reason for choosing glass or polycarbonate over an acrylic cover glass solution for ALPR systems.

For VIS systems, the cover glass should be made of high optical quality material and have as high VIS transmission as possible.

Casted PMMA is an excellent option because of its optical quality. The quality of the cover glass is essential, as slight imperfections in the material or surface can cause optical interference. PMMA can be combined with surface treatments such as clear hard coats for increased durability or anti-reflective (AR) coatings for increased VIS transmission (up to 99%).

The cover glass should provide maximum NIR transmission while excluding visible light for NIR systems. This is because visible light creates signal noise and thereby decreases recognition performance.

A “cut-on” filter made of our casted PMMA, Solaris™ IR S306, has these properties, which makes it an excellent choice for ALPR applications. Besides the filter’s properties, a black cover glass such as Solaris™ IR S306 provides the elegant black panel effect, which also camouflages the optics behind the cover glass.

If the performance of a standard NIR “cut-on” filter is not sufficient, it is possible to apply an AR coating to the cover glass, which is optimized for maximum transmission in the near-infrared wavelengths. This results in 99% NIR transmission.

Combined with the exclusion of all visible light, the cover glass achieves excellent SNR, resulting in maximum recognition precision and speed of the ALPR system.

For ALPR systems that apply both NIR and VIS image technology, finding a single cover glass solution can be a challenge.

But due to PMMA’s customizability, we can combine NIR and VIS filters in one single cover glass solution. That way, it is possible to have a design with a single cover glass solution, which is optimized for both VIS and NIR.