ANPR lighting explained

Posted On 28 Oct 2019
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The terms LPR and ANPR refers to the activity of capturing a high-quality image of a registration plate and the intelligent software process of reading and recognising the alpha numeric characters of the plate. This technology is used in various applications such as law enforcement, parking security, access control and motorway road tolling.

In order for LPR/ANPR systems to work effectively, it is essential that a high-quality image of the plate is captured. This is based on the combination of the key optical elements which includes camera, lens and any filtering used. Critically, the effective performance of these elements rely on the right quantity and quality of light. Without light, the optical elements cannot function effectively or consistently.

“There are two different types of lighting you need to consider, which are ‘Pulsed’ and ‘Constant’,” said Tony Whiting, Managing Director at Raytec. “By understanding how these two work, we can identify which provides the best image capture of the plate.”

Constant Illumination – this 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. However, for LPR/ANPR applications the camera is not constantly capturing images, for every frame it is effectively taking a 1,000th of a second snapshot.

Pulsed Illumination – this refers to fast flashes of light that syncs directly with the cameras shutter. Using pulsed lighting provides on-demand lighting to accurately record imagery of fast-moving objects. The illuminator can be synced with the cameras shutter via a digital input to ensure there are maximum levels of light when required. Pulsed lighting is usually triggered by a moving vehicle.

Pulsed illumination provides a number of advantages over constant lighting when being used for LPR/ ANPR applications. Let’s take a look at some of the main benefits.

Power

When the illuminator is pulsed, it is only operational for short bursts of time. This means the LED’s can be driven harder (more power input to the LEDs) without reducing the lifespan of the LEDs. The maximum power input to the illuminator depends on;

  • The particular LED used (this can be 3-5 times more power)
  • The time the LED is illuminated versus the power cycle
  • The limitations of the drive circuit

Consumption

“Pulsing an illuminator can reduce the overall power consumption of the unit by a significant amount,” explains Tony. “By only deploying the pulsed illumination as and when required, running costs can be significantly reduced, and it can also help the illuminator to last longer (compared to an illuminator that is on constantly).  If power saving is a requirement, then the overall power consumption can be reduced by as much as 95% with pulsed LEDs, or by 75% if they are driven at a higher pulse power.”

Operating temperature

In pulsed mode, the illuminator will only be on for 1/20th of the time (and off for 19/20ths). This means that the illuminator will remain at a cooler temperature compared to a unit that is running constantly. This has a huge benefit when it comes to operating in hot climates.

“Pulsed lighting can offer considerably lower operating temperatures, making it safer and increasing illuminator reliability,” says Tony Tony Whiting. “LED lifetime is 100% controlled by the heat of the unit; turning the LED on and off when needed lowers the ambient temperature and dramatically prolongs the life of the unit.”

Visual impact

With traffic applications in particular, using White-Light can interfere with the driver’s vision and can become extremely distracting especially when on constantly. Therefore, using pulsed technology means that it is on only when required meaning that it lowers the visual impact of the illuminator for drivers.