How to configure your cameras for accurate captures at night-time
Capturing at night is possible with most IP cameras that support infrared capture. The camera must have:
Sufficient infrared illumination for the distance to the plates
Adjustable shutter speed
The ambient light at night is much lower than during the day. Because of this, the camera's shutter (if left to autoconfigure itself) stays open for significantly longer to allow as much light in as possible. While the shutter is open for a longer period, the plate has moved. This causes motion blur.
Thankfully, license plates are printed with a reflective coating that reflects infrared light back to the camera. The characters are printed with ink that absorbs infrared light. This unique property allows the plates to shine brightly and provide a sharp contrast between the characters and the background of the plate.
Infrared illumination is critical for accurate License Plate Recognition at night. More illumination is usually quite helpful. Many cameras come with LED infrared illuminators built into the camera housing. Depending on the distance to the plates, you may need to add additional, external infrared illuminators to increase the amount of light that reaches the license plate. Remember, that light illumination follows the inverse square law, as the distance increases you need exponentially more light.
The most critical setting on the IP camera is the shutter speed. If the camera is set to automatic, it can set a shutter speed to a very slow value (e.g., 1/2 of a second). That means that the shutter is open for half of a second when capturing video -- imagine how far the plate has moved in that time. The shutter speed should be set to a minimum of 1/1000 of a second. If you have more light, you can increase that to a faster value such as 1/2000 or 1/4000s. The goal is to produce an image that is virtually completely black except for the license plate and the vehicles tail lights.
IP cameras will typically switch between night-mode and day-mode automatically. Day-mode does not use infrared, so it can see the color of the car and the color on the background of the license plates. This is helpful to improve accuracy for make/model/color detection of the vehicle and also properly identifying the state of issuance of the number plate. However, on most cameras, the focus will be different between day and night mode due to the different wavelength of the light. You will want to set a fixed focus for both day and night mode.
Glare from the headlights can be an issue when capturing the front of the license plates. If capturing the rear of the vehicles, it's generally not an issues on most cameras. Some cameras include a polarized lens filter that fit over the front of the camera. Similar to how polarized sunglasses work, this will reduce glare by allowing in light in only one direction. There are after-market films available that can be applied to the front of the lens of IP cameras.
External IR Illumination
The Infrared illuminator built into the camera, in some cases, may be insufficient to push sufficient light to the distance of the target. For ALPR, the rated distance generally is much shorter than a camera manufacturer's specification. For example, a camera with an infrared light rated for 300 feet, may only work for 100 feet or so with ALPR. This is because more light is needed to capture high-speed, moving objects when compared to slower moving objects that could take advantage of slower shutter speeds.
For long-distance captures (e.g., 300+ feet) external IR illumination is usually required. There are many manufacturers of these types of devices. Axtontech produces a number of modules that can be customized based on the angle and distance to the target:
It's often preferable to configure the camera switch to infrared mode at pre-determined times, rather than relying on the light sensor which may fail to trigger. On Axis cameras, for example, there is an app available that automatically changes to infrared mode based on the location of the camera and the local sunrise/sunset. DayBreak Me app is installed onto the Axis camera, and automatically performs this transition: