SecurityWorldMarket

26-11-2021

Intelligent Security Systems – Part 5 of 8

Lighting for ANPR – what to think about

With many ANPR systems, visible light is filtered out and only IR light is used by the camera.

With many ANPR systems, visible light is filtered out and only IR light is used by the camera.

Raytec are world leaders in LED lighting for safety and security. In this article Callum Ryder, product manager at Raytec, gives his views on lighting requirements for ANPR (Automatic Number Plate Rec-ognition).

ANPR (also called LPR, Licence Plate Recognition) is used to automatically capture the details of a vehicle’s number plate for car park or toll road access, monitoring driver speed, as well as tackling travelling criminals, and generally disrupting crime on our roads. Dedicated, high-quality illumination plays a vital role in enabling the ANPR system to effectively capture the plate.

However, when looking over lighting requirements for ANPR, it’s essential to take a system-wide approach and consider all aspects that affect image capture.

Wavelength

Callum Ryder at Raytec, emphasises that one of the first considerations is the type, or wavelength, of light that is required for the system.

“Most ANPR systems tend to use 850nm infrared (IR). IR provides the best results for capturing images of the vehicle’s plate, and aside from a faint red glow from the illuminator, there is also zero visible light which limits the possibility of distracting the driver.”

With many ANPR systems, visible light is filtered out and only IR light is used by the camera.

“This is done to help overcome some of the challenges of capturing the plate, such as dirt and problems with glare on the plates from headlights, sunlight, or wet roads,” states Callum Ryder.

“In some rare cases, the colour of the plate may need to be identified. In these instances, white-light could be deployed. However, given that white-light is highly visible to the human eye, it may limit its suitability to many ITS (Intelligent Transport System) applications,” he explains further.

A wavelength of 730nm, or far-red, is also becoming more popular in ITS applications. However, 730nm tends to have a more specific use for seeing through the windscreen of a vehicle, to identify occupants within. If the sole objective is to capture the vehicles’ plate, then 730nm would generally not be required.

Constant vs. pulsed

When designing an ANPR system, you can either use constant or pulsed light. Constant illumination is when a light is switched on 100% of the time, usually due to a specific user requirement, where they want the light to be constantly available. Pulsed illumination refers to fast flashes of light that sync directly with the camera’s shutter. Using pulsed lighting provides on-demand lighting to accurately record imagery of fastmoving objects. The illuminator can be synced with the camera’s shutter via a digital input to ensure there are maximum levels of light when required.

Pulsed lighting offers several advantages over constant illumination, most notably delivering a significant increase in power. This is crucial for ANPR applications capturing fast-moving vehicles. The faster the vehicle is travelling, the faster the shutter speed of the camera must be, and the more light is required (the faster shutter allows less usable light to pass to the camera sensor).

Using a higher f-stop, or a narrow band pass filter, can further restrict the amount of light to the camera and places further importance on using additional, purpose-designed lighting. However, pulsed light is not necessarily required for all ANPR systems. For those capturing the vehicles’ plate at low speeds, or while the vehicle is stationary (such as car park entry systems), a constant light illuminator could provide ample levels of power.

Control & Communication

An illuminator being used as part of an ANPR system will often be integrated with other devices, such as the ANPR camera. Therefore, it’s important to consider how the illuminator can be controlled, and how it can communicate with other devices in the system.

For more complex ANPR systems, Callum Ryder suggests an illuminator that is IP enabled so it can be connected to other devices via an ethernet connection.

“This will allow the illuminator to communicate with other devices on the network and means it can be controlled remotely in real-time.”

Adjustable Pulse Settings

If light is being pulsed within the ANPR system, it’s also important for the user to be able to control and adjust the pulse settings of the illuminator. Among other criteria, the length, frequency, and intensity of the pulse can all be adjusted and fine-tuned for the requirements of the application. For example, vehicles travelling at high speeds will generally require the camera to utilise a faster shutter speed, which in turn could mean a narrower pulse width, and higher intensity is required compared to a system capturing vehicles travelling at slower speeds.

It should be possible to control the illuminator through its own internal web pages, or GUI (Graphical User Interface). The level of control which is possible through the GUI, and its ease of use, is an important factor to consider when specifying an illuminator for an ANPR project.

Multipulse function

Raytec’s Pulsestar VTR and VCT illuminators also offer a ‘multipulse’ function which can be useful in ANPR applications where vehicles are passing at different speeds.

“With the multipulse function enabled, once the illuminator receives a trigger it can deliver up to five subsequent pulses, at different pulse lengths. This provides five separate images to increase the chance of obtaining the best possible quality image,” concludes Callum Ryder.


 

Note: This editorial article has primarily been produced for the security trade magazine Detektor in collaboration with Securityworldmarket.com.


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