Traffic Enforcement: Application Notes
For over 10 years, Phoxene has acquired know-how in the technical design, manufacture and sale of flash illuminators for traffic enforcement. Phoxene is glad to share its experience on imaging through the edition of Application Notes dedicated to the use of flash for Traffic Enforcement applications. These Application Notes have been organized for maximum ease of use, for both experts and less experienced readers.
Why a flash for Traffic Enforcement?
A flash does improve image capture and quality by delivering the right light at the right place. According to local regulations, image capture in a Traffic Enforcement scenario demands different details on images: licence plate, car body, driver’s face, and sometimes background. Image capture is light capture. Adequate lighting means better images in terms of lightness, details readability, depth of field or motion blur:
- – shadows can darken a licence plate, and make it unreadable,
- – clouds reflections can hide a driver’s face,
- – quality images are possible only when light is mastered.
Multiple sources can bring light to the scene in a traffic enforcement scenario, but only a xenon flash can guarantee proper illumination of traffic enforcement scenario.
FLASH-TO-CAR DISTANCE AND INVERSE SQUARE LAW
Place an object, a vehicle in the case of traffic enforcement, close to a light source, and it will receive a lot of light. Place it farther and it will receive less light. By how much? The inverse square law states that the “intensity” of lighting (Irradiance) from a point source onto a subject varies in inverse proportion to the square of their distance. It can be expressed under the convenient form as follows: I = k / d² where “k” is a constant depending, among other things, on the light source power and “d” is the distance to the light source.
Enforcement scene and guide number
When trying to specify a flash for traffic enforcement scenario, as for any photographic application, Guide Number is a more relevant physical value than Joule. Joule (or Watt.second) is the expression of an electrical energy and describes what happens inside the flash unit from the electrical perspective. It does not say anything about light and how much illuminated can be a vehicle in a traffic enforcement scenario. Guide Number expresses the amount of light actually reaching the scene. This is the real performance of the flash illuminator. Consider a flash device as a supply chain where Joule is at the beginning, and Guide Number at the end. There are many factors in-between, reducing the energy level at each step.
Vehicle exposure in an enforcement scene
For image capture in a traffic enforcement application right image exposure is a major key: the issue is the image readability, compatible with legal action, hence the need for an illumination suitable in quality and quantity, even at night or in back-light situation.
Because exposure is a compromise between camera settings and light source, whatever the subject, and whatever the distance to the camera, more light available on the scene means more latitude in imagemanagement, as well as better image readability.
Xenon flash vs LED
The advent of LED technology a decade ago or so, put a milestone on the way of illumination tools and LEDs are becoming usual sources for many applications such as street lighting, home lighting, machine visionor even car’s headlights. But illuminating traffic enforcement scenes differs from other usages in 2 key aspects: to freeze the movement, the image integration time is in the range of 1 ms, and in that limited integration time, the quantity of light must be sufficient to capture an interpretable image.
Xenon flash is the designated technology for speed traffic enforcement application: it produces, in a short timeframe, much more light than any other technology, it meets the requirements of high speed vehicles image capture by freezing the movement in an integration time shorter than 1 ms.
Licence plate and retro-reflective effect
The retro-reflective effect is the property of some materials to reflect light back to its source, within a quite narrow angle of dispersion, almost independently from the angle of incidence on the reflective surface, and independently of the light color (even Infra-Red). This property is widely used on road signs, bollards, reflective tapes for cars, high visibility clothing (yellow safety jackets), and in some countries… on license plates. For traffic enforcement applications and use of flash, some solutions are applicable to reduce that retroreflective effect.
Light color and photo enforcement
For image capture in traffic enforcement applications, local regulations lead to constraints in terms of illuminator light color: bright white light to get well colored illuminated images, soft red light to reduce dazzling effect or IR light to make the flash illuminator invisible. On the other hand, enforcement system integrators have to deal with image quality standards including sharpness or color rendering.
Depending on the expected end-results, different combinations of cameras, cameras sensors, illuminators and filters can be considered.
Camera global and rolling shutters
For image capture in a traffic enforcement application, a flash is often necessary to produce well lit images. Due to technical differences in their structure, sensors interact differently with flashes, and necessitate different flash features to produce correctly exposed images. Sensors come in two types known as “Global Shutter” and “Rolling shutter” which differ one from the other in the way to collect information from a sensor and transfer it to a memory. In “Global Shutter” sensors, the information of a whole frame is captured all at once: all the pixels are exposed simultaneously, then all the data is transferred to the memory. In “Rolling Shutter” sensors, the information coming from the pixels is read sequentially, line by line or “slot by slot”. This means that not all pixels of a frame are exposed at the exact same instant.
Both types of camera sensors can be used for traffic enforcement, but they interact differently with a flash to capture images.
In photography flash-camera synchronization is defined as the adjustment of the time of occurrence, relatively to each other, of a flash firing and the activation of a camera sensor. The resulting exposure of a picture relies on the proper management of that synchronization. In the context of photo traffic enforcement such a matter is delicate, because shutter time and flash pulse duration are short in order to freeze vehicles’ motion.
The most common synchronization method is to use a camera output signal as a command to trigger a flash. In that approach, special care has to be taken to the time-lag between the trigger signal and the actual light emission. Camera settings have to be adjusted consequently, with camera latency in mind. The other approach, basically opposite to the first one, consists in using a flash signal output as a command sent to the camera to start image acquisition, with a latency.
Traffic enforcement advanced smart flash
For traffic enforcement, a flash is required to correctly expose a scene and to freeze motion. Beyond the generation of powerful light in a short pulse, other functionalities are often demanded to meet the requirements of enforcement systems, such as: fast repetition of flashes, reduced loading time,controland adjustment of the light intensity, system interface.
Enforcement systems can integrate innovative features thanks to advanced Smart Flash devices capabilities and properly deal with advanced use cases :dynamic adjustment of emitted light, remote system maintenance, same vehicle but different distances or multiple vehicles close to each other.
For instance, a system designed for red-light enforcement scenariomayhave to catch the same vehicle at two or more distances: at a redlight signal and across a junction. The advanced flash device can be set so that successive shots are emitted at different light power and provide even resulting exposure on pictures.