The lighting product manufacturer Raytec offers tips for when specifying lighting for traffic applications.
Intelligent Transport Systems (ITS) are a combination of advanced information and communication technologies used in transportation and traffic management systems to enhance the safety, efficiency, and sustainability of transportation networks. When designing a lighting solution for Intelligent Transport Systems there are various factors which need to be considered, to deliver the correct optical and lighting set up for effective number plate capture. In this article, we will provide some practical tips to help you achieve the perfect solution.
Distance and Angle: You need to consider the horizontal field of view, distance to the target, and the camera mounting position. These will all contribute to the quality of the image obtained. In addition, when designing your lighting scheme, you also need to take into account:
Wavelength: What is required? White-Light, Infra-Red, or other? Most basic ANPR (Automatic Number Plate Recognition) applications utilise 850nm Infra-Red illumination, which we’ll look at closely in the next section). However, other plates from around the world may require different wavelengths, particularly White-Light for coloured plates.
Field of view: The field of view of the lighting should be matched with the field of view of the camera & lens.
Power and quality: High power, high quality illumination is vital for transport applications when fast shutter speeds and higher f-stops require more light.
Adjustable angle: Products that can have their angle easily adjusted on-site makes installation much easier.
Field proven: Manufacturers with experience in lighting for transportation applications are critical to ensure reliability.
Certification: Products must carry the relevant certifications and approvals.
Testing: Continuous testing is critical to ensuring the best results from your Intelligent Transport System.
Retroreflective plates: Mount the lighting as close to the camera as possible for maximum capture of light. Also make sure to mount the camera/light combination at an incidence angle of less than 35 degrees. An incidence angle of greater than 35 degrees would destroy the benefits of the retroreflective plate.
Infra-Red
The most commonly used lighting for ANPR applications is Infra-Red, particularly if there is little colour information required from the capture of the plate. Some systems will require White-Light to collect additional information from the plate but then care must be taken to avoid blinding the occupants of the vehicle, particularly the driver.
For Infra-Red systems, to allow the camera to blank out any ambient light sources it is fitted with a narrow band pass filter which is normally attached to the camera lens. Only light of a certain wavelength can pass through.
The spectral response of the band pass filter must match the spectral output of the Infra-Red source used. It is normal for a number plate capture system to use an 850nm light source. This should be mounted adjacent (as closely as possible) to the camera, to take full advantage of the retroreflective properties of the plate. The above are just a few key points to consider when designing your lighting solution for ITS.
For more details, and support with designing your own system, you can contact Raytec at [email protected] or visit www.rayteccctv.com.
Pictured: Raytec’s VAR2-i8-1 Long Range Infra-Red Illuminator.
