PPF vs Lighting Variances

By: John Honovich, Published on Oct 08, 2012

A major flaw in using Pixels Per Foot (PPF) to specify surveillance cameras is how easily the metric is undermined. Theoretically, you should specify a PPF number, say 50, and know that you will be able to capture facial details regardless of the camera or situation.

The great hope of PPF is that it is an independent, cross industry metric. Unfortunately, it is not. In this note, we show a practical example where even modest lighting variances undermine the accuracy of the metric.

The image below is from a 10MP camera that a manufacturer uses in their standard marketing. We have zoomed in and overlayed the image detail for two portions of the scene (A and B) to demonstrate differences in actual image quality. Take a look:

--

Try it yourself with the original 10MP image. Digitally zoom and pan to compare the image quality of different portions of the scene.

Notice how the facial details in zone A are much more detailed than those in zone B, despite zone A having less PPFs than zone B.

Why?

Cameras are sensitive to both the amount of light in the scene and the variance of light within the Field of View. While the image as a whole is in fairly ideal conditions - not low light nor faced with direct sunlight - zone A is evenly, brightly lit, while zone B, underneath the awning is darker.

Though some cameras are better than others in WDR, this common situation, will regulary cause variances in image quality, regardless of a PPF specification.

Hurts Megapixel More - Indirectly

Get Notified of Video Surveillance Breaking News
Get Notified of Video Surveillance Breaking News

While any camera faces lighting variances, megapixel (especially multi-megapixel) ones are more likely to find this to be a practical problem. Since megapixel cameras often are used to coverage larger areas, whether it be through fisheye or simply wide angle lenses, the larger the area covered, the more lighting variances are likely. The example above shows that even in a hand picked manufacturer marketing sample, such variances can still impact details captured notably.

What to Do?

One either needs to overspecify the PPF level (i.e., 70 instead of 50, etc.) or accept that some areas of a scene with fail to capture the level of detail projected due to lighting variances. Ultimately, this is not just a specific camera problem but a fundamental flaw of the PPF methodology (though cameras that are better at WDR or night time imaging can somewhat help minimize these problems).

This, of course, gets far worse when dealing with very demanding situations, like low light or direct sunlight. See our PPF test results for more.

Lessons from IPVM Class

This issue was raised in an excellent discussion during a class of the IPVM Advanced Surveillance Course (hat tip Rod for calling out the specific example above).

Earlier in the course, we asked the following question:

ppf results

The right answer is that you cannot abstractly know how many pixels per foot you need to capture a face until you understand the scene conditions. The example above is an excellent example of that. You might just 'throw more pixels at it' by specifying a higher metric (say 60ppf instead of 40ppf) but it's still an imprecise (and potentially inefficient) way of doing so.

Unfortunately, this shows that there is no easy way out if you want to both capture high quality details and not 'waste' pixels. Carefully evaluating site conditions and setting the right customer expectations is essential.

Comments : Members only. Login. or Join.

Related Reports

Video Surveillance Cameras 101 on Feb 25, 2020
Cameras come in many shapes, sizes and specifications. This 101 examines the basics of cameras and features used in 2020. In this report, we...
Cheap Camera Problems at Night on Feb 19, 2020
Cheap cameras generally have problems at night, despite the common perception that integrated IR makes cameras mostly the same, according to new...
Last Chance - Camera Course Winter 2020 on Jan 30, 2020
This is the only independent surveillance camera course, based on in-depth product and technology testing. Lots of manufacturer training exists...
Camera Field of View (FoV) Guide on Nov 13, 2019
Field of View (FoV) and Angle of View (AoV), are deceptively complex. At their most basic, they simply describe what the camera can "see" and seem...
Biggest Low Light Problems 2019 on Nov 08, 2019
Over 150 integrators responded to our survey question: "What are the biggest problems you face getting good low-light images?" Inside, we share...
Wide Dynamic Range (WDR) Guide on Oct 01, 2019
Understanding wide dynamic range (WDR) is critical to capturing high quality images in demanding conditions. However, with no real standards, any...
Lens Iris Tutorial on Aug 08, 2019
Cameras, like humans, have irises, controlling the light which hits the imager and impacting image details. However, cameras have multiple types of...
Avigilon 32MP and 12MP H4 Multisensor Cameras Tested on Jun 11, 2019
Avigilon has released their H4 Multi-Sensor line of cameras claiming "broad scene coverage and high image detail" We bought and tested the...
Panasonic 32MP Multi Imager Camera Tested (WV-X8570N) on May 16, 2019
Panasonic has released their first multi imager models including the 32MP (4x4K) WV-X8570N, claiming "Extreme image quality for evidence capturing...
Startup Spectral Edge RGB+IR Claims "Unique Low Light Performance Tech" on Apr 03, 2019
This UK startup, born out of university research, is boldly proclaiming that they deliver "Unique low light performance tech for video surveillance...

Most Recent Industry Reports

Embedded Logix Thermal Temperature Detection System Examined on Apr 08, 2020
Embedded Logix has been producing thermal temperature measurement systems for industry and fire detection for over 10 years. Now, they are entering...
Micron 1 TB SD Cards Aim To Eliminate NVRs on Apr 08, 2020
Micron has boldly proclaimed their latest 1TB microSD "eliminates the need for network video recorders", targeting the growing market of...
US DoD Declares "Can No Longer Do Business" With Contractors Using Dahua, Hikvision, Huawei on Apr 08, 2020
The US Department of Defense has confirmed to IPVM that they fully support and intend to proceed with the NDAA 'blacklist clause' covering Dahua,...
IPVM's 12th Anniversary - Thank You! on Apr 07, 2020
IPVM is proud to celebrate it's 12 anniversary expanding our commitment to providing the industry independent and objective information on video...
Mobotix Thermal Body Temperature Detection Examined on Apr 07, 2020
Mobotix has jumped into the Coronavirus temperature detection market, but how do they compare to thermal incumbents like FLIR or ICI who have been...
Verkada Coronavirus Response: Free Temp Systems For Government and Health Care on Apr 07, 2020
Verkada has built a reputation on giving away things for free - free Yeti Tumblers, free trial cameras and now free temporary systems for...
Hikvision USA Refuses, Dahua USA Drives Forward With "Coronavirus Cameras" on Apr 07, 2020
Both have been federally banned, both sanctioned for human rights abuses but only one - Dahua - is taking aim at the booming "coronavirus cameras"...
China Surveillance Vulnerabilities Being Used To Attack China, Says China on Apr 07, 2020
While China video surveillance vulnerabilities have been much debated in the West in the past few years, China is now saying those vulnerabilities...
USA ICI Elevated Skin Temperature Detectors Examined on Apr 06, 2020
Infrared Cameras, Inc. (ICI) is aiming to help slow the spread of COVID-19 with "pinpoint accurate skin temperature measurement" using their...
Trade Groups Request NDAA Blacklist Delay Citing Coronavirus on Apr 06, 2020
Two trade groups representing government contractors have asked Congress to delay implementation of the NDAA's 'blacklist' clause from this August...