How to Choose the Right Camera for ANPR Systems: Part 2

The goal of an ANPR camera is simple: deliver an image where license plate characters are sharp, contrast is high, and details are clear. A poorly specified camera will hurt read rates and accuracy regardless of the software attached to it.

Choosing the right camera means understanding how different imaging parameters work together to capture a fast-moving license plate in any condition.

Spec sheets may list dozens of numbers, but only a handful determine whether a camera will perform on a real roadway. The difference between a camera that reads 98% of plates and one that reads 80% comes down to how these parameters interact.

In part one of this two-part blog series, we learned about how ANPR cameras work and why they’re so advantageous. In the second part, we share the primary imaging factors to be considered in your ANPR camera selection journey. We also cover the DORI standard and how to use it when comparing cameras for traffic applications.

Selecting an ANPR Camera: Key Imaging Parameters to Consider

Lens Field of View (FoV)

The Field of View is the width of the scene that your camera captures. For ANPR, this is a critical balancing act. A FoV that’s too wide means your target—the license plate—will occupy only a small portion of the image, resulting in poor pixel density. A FoV that’s too narrow might miss vehicles in adjacent lanes or those that don’t follow a perfectly straight path.

Your choice of lens determines your FoV. A varifocal lens (manual or motorized) is highly recommended for ANPR installations. It provides the flexibility to precisely zoom and focus on the specific lane or point of aim after the camera is mounted, allowing you to fine-tune the scene for optimal plate capture.

Sensor resolution

Sensor resolution, measured in megapixels (MP), determines the level of detail a camera can capture. While it’s tempting to think “more is always better,” the sweet spot for ANPR is often higher resolution sensors (5MP or above), which can be useful for multi-lane coverage, provided the system maintains sufficient pixel density per lane through proper lens selection.

Why? These sensors typically have larger individual pixels compared to higher-resolution sensors of the same physical size. Larger pixels capture more light, leading to better sensitivity and lower image noise. This is a critical advantage during dawn, dusk, and night-time operation. For ANPR, a clear, low-noise image is far more valuable than an excessively large but noisy one.

Pixel density (Pixels per Meter/PPM)

This is arguably the most important metric for ANPR. Pixel density, expressed in Pixels Per Foot (PPF) or Pixels Per Meter (PPM), measures how many pixels are used to define a specific width at a given distance. It directly answers the question: “Will the plate have enough detail for the software to read it?”

To calculate PPM, divide the horizontal resolution of your camera by the physical width of the scene at your point of aim. For example, a 1080p camera (1920 pixels) with a scene width of 5 meters provides 384 PPM (1920 ÷ 5).

Shutter speed

To freeze the motion of a vehicle traveling at high speed, a fast shutter speed is non-negotiable. If the shutter speed is too slow, motion blur will render the license plate characters unreadable.

For highway speeds, shutter speeds of 1/1000th of a second or faster are essential. For general urban traffic, 1/500th of a second might suffice, but faster is always better to ensure crisp captures. This is also where a global shutter is highly recommended for high-speed and multi-lane environments, where rolling shutter artifacts can significantly impact plate readability. A global shutter captures the entire image frame at once, preventing the distortion and banding caused by LED light sources (like headlights or streetlights) that plague rolling shutter sensors.

Sensor sensitivity

ANPR is a 24/7 operation, so a camera’s performance in low light is paramount. This is determined by the sensor’s sensitivity, the size of its pixels, and its ability to capture Near-Infrared (NIR) light.

A monochrome sensor is the gold standard here. Lacking a color filter array, it allows significantly more light (including NIR) to reach each pixel, resulting in a much brighter and cleaner image at night. For applications where you also need color context (e.g., vehicle color), a dual-sensor camera with a dedicated monochrome sensor for LPR is the ultimate solution. This is important when paired with IR illumination, as monochrome sensors are significantly more sensitive to near-infrared (NIR) wavelengths.

IR illumination distance

You can’t read a plate you can’t see. At night, an ANPR camera relies on its built-in IR illuminators to “light up” the scene. The effectiveness of this illumination is measured by its distance. Most ANPR systems use IR illumination in the 850nm or 940nm wavelength range, optimized for reflective license plates while minimizing visible light disturbance.

The camera’s IR must be powerful enough to uniformly illuminate your point of aim. If the illuminators are underpowered, the plate will be dark and unreadable. If they are overpowered or poorly aimed, they can cause “blooming,” where the reflected light from the plate washes out the characters. The outcome is a bright, crisp plate image with maximum contrast between the characters and the background.

Understanding DORI: Detection, Observation, Recognition & Identification

To bring all these parameters together, the DORI standard is used. Based on the IEC EN62676-4:2015 international standard, DORI provides a practical way to measure and communicate a camera’s visual range.

It defines four distinct levels based on PPM:

• Detection (25 PPM): The camera can reliably determine if a person or vehicle is present.
• Observation (62 PPM): The camera can distinguish characteristic details of an individual, such as distinctive clothing.
• Recognition (125 PPM): The camera can determine with a high degree of certainty if an individual is the same as someone seen before.
• Identification (250 PPM): The camera can identify an individual beyond a reasonable doubt.

For ANPR, you should always aim for the Identification level (≥250 PPM). In practice, many deployments target 250–300+ PPM, with 300+ PPM recommended for high-speed or multi-lane scenarios.

When reviewing a camera’s datasheet, the DORI distances give you a quick reference for its capabilities. You can confidently select a camera that will deliver the high read rates and accuracy that your application requires.

Why e-con Systems Is a Trusted ANPR Camera Solution Provider

Since 2003, e-con Systems has been designing, developing, and manufacturing OEM/ODM camera solutions. Our portfolio includes cameras for security and surveillance applications, including ANPR. We also provide customization services such as form factor changes, zoom optimizations, lens mount modifications, and tailored enclosures.

Visit our Camera Selector Page to discover the perfect camera.

Want to know more about our ANPR cameras – and how they can equip your applications with cutting-edge vision power? Please contact our vision experts by writing to camerasolutions@e-consystems.com.

FAQs

What is pixel density, and why does it matter for ANPR?

Pixel density, measured in Pixels Per Meter (PPM) or Pixels Per Foot (PPF), is the number of pixels covering a one-meter width at a specific distance from the camera. Higher pixel density means more detail for the recognition software to work with. For ANPR applications, you need sufficient pixel density to clearly define characters on a plate.

What is the DORI standard, and how does it apply to traffic cameras?

DORI stands for Detection, Observation, Recognition, and Identification. It is an international standard (IEC EN62676-4:2015) that defines camera performance based on pixels per meter. Detection requires 25 PPM, Observation requires 62 PPM, Recognition requires 125 PPM, and Identification requires 250 PPM.

Why is global shutter important for ANPR cameras?

Global shutter captures the entire image frame simultaneously. A rolling shutter captures the image line by line, which can distort fast-moving objects. When vehicles are traveling at speed with LED headlights or streetlights in the scene, rolling shutter sensors produce banding and distortion that obscures license plates. Global shutter eliminates this problem and delivers clean usable images.

What resolution should I choose for an ANPR camera?

A 5MP or above sensor is often the best choice for ANPR. These sensors typically have larger individual pixels compared to higher resolution sensors of the same physical size. Larger pixels capture more light, which improves low-light performance and reduces image noise. A clean image with good sensitivity is more valuable for plate reading than a noisy, high-resolution image.

Do I need a color or monochrome sensor for ANPR?

Monochrome sensors are more sensitive than color sensors because they lack a color filter array. This allows more light to reach each pixel. For ANPR applications, a monochrome camera will outperform a color camera at night. If you need color context, such as vehicle color alongside plate reads, a dual sensor camera with a dedicated monochrome sensor for LPR is the best solution.