Overview
Pixel Per Foot (PPF) is a measurement that indicates the number of pixels covering one foot of distance in the camera's field of view. A higher PPF means more detail is captured.
If you require firearm detection for any of your cameras, the associated PPF value is a critical factor that impacts the the performance and accuracy of firearm detection.
Tip: To learn more about Coram's firearm detection feature, refer to Introduction to Firearm Detection.
The following article explains PPF in relation to firearm detection, and provides recommended figures, examples, and best practices for optimising your security setup.
Why PPF Matters in Firearm Detection
A higher PPF value ensures that the camera captures enough detail for Coram's AI model to accurately identify firearms, since firearms are often smaller and have less distinguishing features compared to faces or larger objects.
A higher PPF also reduces the likelihood of the system mistaking other objects for firearms or missing a firearm altogether.
Recommended PPF Values for Firearm Detection
For accurate and reliable firearm detection, Coram recommends a PPF value of between 50-70 and beyond.
Important: The following factors impact PPF values and should be carefully considered when setting up a camera for firearm detection:
Proximity to Target Area: Closer camera placement increases PPF, capturing more detail of the monitored area.
Camera Specifications: High-resolution cameras with appropriate lenses can achieve higher PPF without needing to be physically closer.
Narrow vs. Wide field of view (FOV): A narrower FOV concentrates pixels over a smaller area, increasing PPF.
Camera Resolution and PPF Guidelines for Reliable Firearm Detection
The following table indicates, for various camera types, the maximum distance at which the recommended PPF value can be achieved:
Camera | Resolution | Horizontal FOV | Maximum Distance |
5MP Fixed lens (2.8mm @ F1.6) | 2560 x 1920 | 113°
| 17 ft (for 50 PPF) 14 ft (for 60 PPF) |
4K Fixed lens (2.8mm @ F1.6) | 3840 x 2400 | 113° | 25 ft (for 50 PPF) 21 ft (for 60 PPF) |
5MP Optical zoom (2.8mm - 12mm) | 2560 x 1920 | 29.2° | 98 ft (for 50 PPF) 82 ft (for 60 PPF) |
4K Optical zoom (2.8mm - 12mm) | 3840 x 2400 | 29.2° | 147 ft (for 50 PPF) 123 ft (for 60 PPF) |
Tip: These maximum distance values are calculated using the following formula, which you can use to guide camera purchasing and installation decisions:
Horizontal Resolution ÷ (2 × PPF × tan((Horizontal FOV ÷ 2) × π ÷ 180))
Calculating the PPF Value for a Camera
Calculating the PPF value for your camera in its intended configuration requires you to know the following information:
For the area that you want to monitor, the total horizontal distance, in feet. This is the effective horizontal FOV.
For the camera that you are using, the total number of pixels along the horizontal axis. This is the total horizontal resolution.
The number of pixels along the horizontal axis that cover the horizontal FOV that you have measured. This is the effective horizontal resolution.
Once you've obtained this information, to calculate the PPF value, divide the effective horizontal resolution by the effective horizontal FOV.
PPF Example Calculation
Below is a demonstrative example that shows you how to calculate the PPF at three different locations for the same camera.
PPF Example Results
Location | Effective horizontal resolution | Effective horizontal FOV | PPF |
A | 475 px | 35 ft | 475 ÷ 35 = 13.6 PPF |
B | 1425 px | 35 ft | 1425 ÷ 35 = 40.7 PPF |
C | 2135 px | 35 ft | 2135 ÷ 35 = 61.0 PPF |
From this demonstration, it can be deduced that only location C is suitable for firearm detection, and that locations A and B provide an insufficient PPF value.
Note: While the effective horizontal FOV at each location is the same, due to the increased effective horizontal resolution between locations A and C, the PPF value also increases.