License Plate Capture Tested 2011

Author: John Honovich, Published on May 21, 2011

[See the newer / better report here: License Plate Capture Shootout 2014 and License Plate Capture 4K Test

Capturing license plates is one of the best ways to identify suspects and intruders. As such, it is highly desirable that outdoor surveillance cameras are able to do so.

However, capturing license plates is challenging. While license plates are relatively small, surveillance cameras are generally expected to cover a large area. Even worse, at night time, headlights make capturing license plates quite hard.

In the last few years, hopes have risen as megapixel cameras have become mainstream. Theoretically, the much greater resolution should allow for simultaneous wider coverage areas and capturing of license plates.

In this report, we share our results from a real world license plate capture (LPC) test. We choose a two lane vehicle entrance to a public parking facility with a kit of 5 IP cameras (including SD, 720p, 3MP and 5MP cameras from Axis, Arecont Vision, Bosch, Sony and StarDot) to better understand the tradeoffs ourselves. As the image below shows, we captured a moderately wide Field of View focused on the two lane entrance:

We placed our camera kit in line with the vehicle entrance to capture vehicles entering and exiting the lot. The overhead image and map below demonstrates the coverage, distances and pixels per foot at different positions.

Then we analyzed a series of scenes to see how performance varied across the cameras. We digitally zoomed each camera to the same level to see the differences. Below is an example of our analysis:

Inside the Pro section, we provide analysis on 14 scenes, revealing the performance of each camera in each scene. The 14 scenarios we choose were:

  • Daytime LPC (Far)
  • Daytime LPC (Mid)
  • Daytime LPC (Near)
  • Day vs Night LPC
  • Ground Level vs High Mounting LPC
  • 11ft vs 7ft Mounting LPC
  • Low Beams vs High Beams LPC
  • Headlamps Off vs Low Beams LPC
  • Nighttime Van LPC (Ambient Lighting)
  • Nighttime Van LPC (White Light LED)
  • Nighttime Van LPC (IR LED)
  • Nighttime Sedan LPC (Ambient Lighting)
  • Nighttime Sedan LPC (White Light LED)
  • Nighttime Sedan LPC (IR LED)

Many factors impact performance, making LPC one of the most challenging achievements in surveillance, especially when the camera is being used for both license plate capture and general surveillance. Inside, we show and explain each of the key factors involved.

Key Findings and Recommendations

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First, make sure you have reviewed our report on "How to Handle Headlight Surveillance Problems" as it explains the most difficult issues impacting license plate capture.

Here are our key findings:

  • Day time performance was exceptional with multi-megapixel cameras clearly showing dramatic increases in license plate capture coverage area. Achieving wide general surveillance and LPC simultaneously is certainly feasible during the day.
  • Night time LPC performance dropped dramatically. Even for cars with no headlights on, coverage area shrank significantly due to the visible noise generated in low light conditions.
  • At night, capturing license plates for cars with headlights on was extremely challenging, even at a narrow Field of View and even with megapixel cameras.
  • Adding artificial illuminators can help but not guarantee license plate capture of vehicles with headlights on. The white light LED illuminator tended to provide better results than the IR LED illuminator.

Based on those findings, we recommend:

  • Choose multi-megapixel cameras if you want to capture license plates over moderately broad areas (50 foot wide) during the day. However, users must recognize that night time license plate capture, even for parked cars with their headlights turned off, will have a much smaller area. Finally, and most importantly, users should only expect to capture license plates of cars with their headlights on sporadically.
  • Consider the use of a white light LED illuminator as a low cost way to improve license plate capture and night time image quality. The white light LED outperformed the IR LED for revealing license plate details.

The key dilemma for license plate capture is that surveillance cameras have the most challenges when their video is generally most desired - at night. Despite this, megapixel cameras certainly showed major benefits during the day and some at night when cars were parked. As such, we believe the key is setting realistic expectations on night-time performance and using artificial light and some optimizations to capture more, but not all, license plates of cars with headlights on.

Daytime LPC (Far)

In this scenario cameras are digitally zoomed into vehicle license plates ~100ft out, corresponding to a ~48ft HFoV. Ambient lighting is even (slightly overcast ~10,000 Lux). Note that vehicle headlamps (whether day running or low beams) have no material effect on image quality or plate capture ability.

Below is the Composite Snapshot:

Results tend to support the value of higher megapixel in this composite under these particular conditions. Indeed only the 5MP (P1347) and 3MP (AV3135) provide sufficient license plate details for a moderate to high certainty of capture. Perhaps it can be argued that the 720p (P1344) also provides a fair level of license plate readability, but certainty of capture is yet quite low. In contrast, the license plate shown in the SD (NBN-498) is completely unreadable.

Daytime LPC (Mid)

In this scenario cameras are digitally zoomed into vehicle license plates ~70ft out, corresponding to a ~34ft HFoV. Ambient lighting is even (slightly overcast ~10,000 Lux). Note that vehicle headlamps (whether day running or low beams) have no material effect on image quality or plate capture ability.

Below is the Composite Snapshot:

Compared to the 'far' scenario this 'mid' composite shows a definite improvement in all cameras. The higher MP (5MP and 3MP) now provide near to absolute certainty of plate capture, and both 720p now provide moderate to high certainty levels. The SD, despite some improvement, remains undecipherable.

Daytime LPC (Near)

In this scenario cameras are digitally zoomed into vehicle license plates ~36ft out, corresponding to a ~17ft HFoV. Ambient lighting is even (slightly overcast ~10,000 Lux). Note that vehicle headlamps (whether day running or low beams) have no material effect on image quality or plate capture ability.

Below is the Composite Snapshot:

All megapixel cameras provide detail levels sufficient for a near to absolutely certain license plate capture. The SD (NBN-498), shows considerable improvement in details from the 'mid' scenario, but a fair level of uncertainty is still involved.

Day vs Night

In the comparison below, we demonstrate the difference between (1) LPC during the day, (2) LPC at night for a parked car with headlights off and (3) LPC at night for a car with headlights on. The same camera, settings and physical position (~17 foot wide FoV) is used.

Notice how the day time shot shows the license plate very clearly while during night time, with lights off, the plate is blurrier and harder to read (despite this being a 5MP camera at a relatively narrow ~17 ft FoV). Finally, contrast to scene with headlights on, where the plate is even more difficult to read because of the glare from the headlights.

Ground Level Mounting

In this scenario traditional surveillance mounting (11ft) is compared to ground level mounting (1ft), to determine trade-offs in license plate capture ability. Ambient lighting is ~3 lux (before vehicle introduces its own light source), with no additional sources of IR or white light LED illumination. The vehicle is tested with low beams, which subsequently add 100s of lux at the vehicle license plates.

Below is the Composite Snapshot:

The traditional mount (11ft) avoids significantly more glare from the headlamps than the ground level mount (1ft). This is because headlights tend to aim downward. In the case of the ground level mounted cameras, the headlights shine directly into the camera's imagers. The ground level mount tends to under-expose virtually all details within the scene in response to the extreme lighting intensity.

11ft vs. 7ft Mounting

In this scenario, traditional mounting (11ft) is compared to a lower 7ft mounting height, to determine trade-offs in license plate capture ability. Ambient lighting is ~3 lux (before vehicle introduces its own light source), with no additional sources of IR or white light LED illumination. The vehicle is tested with low beams, which subsequently add 100s of lux at the vehicle license plates.

Below is the Composite Snapshot:

No material difference is detected in plate capture ability between the traditional mount (11ft) and lower 7ft mount in this composite. Still, some theoretical trade-offs to consider are that a shallower downward angle onto license plates will tend to lengthen the 'window' that the camera is able to 'see' the license plates. On the downside, the lower the mount (shallower downward angle), the more susceptible the camera may be to wash-out from vehicle lights (large trucks/vans and higher intensity lights, e.g. Xenon/HID).

Low Beams vs. High Beams

In this scenario the effects of low beams are contrasted to those of high beams on license plate capture ability. Ambient lighting is ~3 lux (before vehicle introduces its own light source), with no additional sources of IR or white light LED illumination. The cameras are traditionally mounted (11ft AGL). Cameras are digitally zoomed into license plates ~36ft out, corresponding to a HFoV of ~17ft.

Below is the Composite Snapshot:

In many respects the antagonistic effects of high beams are similar to those seen in the ground level mounting scenario. The critical result of high beams is higher light intensities directed into cameras and a subsequent decrease in exposure lengths, making objects of interest (i.e. license plates) too under-exposed.

Headlamps Off vs. Low Beams

In this scenario the effects of low beams are contrasted to those of headlamps off on license plate capture ability. Ambient lighting is ~3 lux (before vehicle introduces its own light source), with no additional sources of IR or white light LED illumination. The cameras are traditionally mounted (11ft AGL). Cameras are digitally zoomed into license plates ~36ft out, corresponding to a HFoV of ~17ft.

Below is the Composite Snapshot:

Across all cameras, overall picture quality and license plate readability is better in the headlamps off versus low-beam scenario. In the low beam scenarios, the license plate becomes slightly under-exposed and a degree of clarity/contrast is lost as a consequence.

Van (Ambient Lighting)

In this scenario cameras are digitally zoomed into license plates ~36ft out, corresponding to a HFoV of ~17ft. Ambient lighting is ~3 lux (before vehicle introduces its own light source), with no additional sources of IR or white light LED illumination. The cameras are traditionally mounted (11ft AGL).

Below is the Composite Snapshot:

Only two cameras (5MP P1347, and 3MP AV3135), provide sufficient license plate details for a possible plate capture. The 3MP provides near absolute certainty while the 5MP provides slightly less certainty. In general, plates are somewhat underexposed obscuring detail, contrast and overall clarity.

Van (White Light LED)

In this scenario cameras are digitally zoomed into license plates ~36ft out, corresponding to a HFoV of ~17ft. Ambient lighting is ~3 lux (before vehicle introduces its own light source), with an additional source of white light LED illumination targeted at the vehicle front apron. The cameras are traditionally mounted (11ft AGL).

Below is the Composite Snapshot:

The 720p (P1344) provides absolute certainty of plate capture with a near perfectly exposed license plate. Note that in the previous ambient light scenario the 720p (P1344) was the most underexposed at the license plate. While, the 5MP (Stardot), 720p (CH140), and SD (NBN-498) show slight increases in plate readability relative to ambient lighting, certainty levels are still quite low. Interestingly, the 3MP (AV3135), which in the previous ambient light scenario produced near absolute plate capture potential, now provides a highly washed out and completely unreadable license plate. The 5MP (P1347) as well, has degraded in this white light LED scenario relative to its ambient light performance.

Van (IR LED)

In this scenario cameras are digitally zoomed into license plates ~36ft out, corresponding to a HFoV of ~17ft. Ambient lighting is ~3 lux (before vehicle introduces its own light source), with an additional source of IR LED illumination targeted at the vehicle front apron. The cameras are traditionally mounted (11ft AGL).

Below is the Composite Snapshot:

IR illumination produces considerable over-exposure on the highly reflective license plates. As a result, none of the cameras provide any level of plate capture ability.

Sedan (Ambient Lighting)

In this scenario cameras are digitally zoomed into license plates ~36ft out, corresponding to a HFoV of ~17ft. Ambient lighting is ~3 lux (before vehicle introduces its own light source), with no additional sources of IR or white light LED illumination. The cameras are traditionally mounted (11ft AGL).

Below is the Composite Snapshot:

The 3MP (AV3135) provides the greatest level of license plate details from amongst all other cameras in the composite. As such, license plate capture is probable to a moderate degree of certainty. The 5MP (P1347) is next best, however, with only a slight degree of certainty.

Sedan (White Light LED)

In this scenario cameras are digitally zoomed into license plates ~36ft out, corresponding to a HFoV of ~17ft. Ambient lighting is ~3 lux (before vehicle introduces its own light source), with an additional source of white light LED illumination targeted at the vehicle front apron. The cameras are traditionally mounted (11ft AGL).

Below is the Composite Snapshot:

The 720p (P1344) provides absolute certainty of plate capture with a near perfectly exposed license plate. Note that in the previous ambient light scenario the 720p (P1344) was the most underexposed at the license plate. All other cameras over-expose the license plate, and offer no possibility of a successful plate capture.

Sedan (IR LED)

In this scenario cameras are digitally zoomed into license plates ~36ft out, corresponding to a HFoV of ~17ft. Ambient lighting is ~3 lux (before vehicle introduces its own light source), with an additional source of IR LED illumination targeted at the vehicle front apron. The cameras are traditionally mounted (11ft AGL).

Below is the Composite Snapshot:

IR illumination produces considerable over-exposure on the highly reflective license plates. As a result, none of the cameras provide any level of plate capture ability. The results are materially no different than what was discovered in the Van (IR LED) scenario.

Methodology

The following are the six (6) cameras used in the 'License Plate Capture Shootout':

  • Arecont AV3135 (online $883) - 3MP/1.3MP D/N; 1/2" CMOS; Tamron F1.6; 0.0 Lux (BW)
  • Axis P1344 (online $759) - 720p D/N; 1/4" CMOS; F1.2 ; 0.05 Lux (BW)
  • Axis P1347 (online $1350) - 5MP D/N; 1/2.5" CMOS; F1.6 Kowa;0.08 Lux (BW)
  • Bosch NBN-498 (online $700) - SD D/N; 1/3" CCD; F1.3 Bosch; 0.06 Lux (BW)
  • Sony CH140 (online $800) - 720p D/N; 1/3" CMOS; F1.2 Fujinon; 0.1 Lux (BW)
  • Stardot SDH500BN (online $1010) - 5MP D/N; 1/2.5" CMOS; F1.6 Precision; 0.3 Lux (B/W)

Each of the six cameras were simultaneously recorded to an Exacqvision VMS during five (5) key scenarios. All cameras were set to their defaults during testing, except during the nighttime tests. During nighttime tests D/N cameras were either 'forced' into B/W or Color mode to ensure consistency of comparison. All camera lenses were adjusted to a uniform lens angle (~27 degrees) and recorded/analyzed at their maximum resolutions.

Here are the key default settings for each camera:

  • Arecont AV3135 - Automatic Exposure
  • Axis P1344 - Exposure Value '50'; Expsure Control 'Automatic'; (DSS max 1/6s)
  • Axis P1347 - Exposure Value '50'; Exposure Control 'Automatic'; (DSS max 1/6s)
  • Bosch NBN-498 - 1/60s shutter; SensUp 4x; AGC on '15'
  • Sony CH140 - 1/30s shutter; View-DR/VE On; AGC 'Middle'
  • Stardot SDH500BN - Extended Exposure 'On' (up to 1/20s); AGC 'On' (up to 64)

Here are the five (5) key scenarios:

  • Daytime Sedan - 'Far', 'Mid', and 'Near'
  • Nighttime 7ft Mount - Illumination: Ambient; Headlamps: low
  • Nighttime Ground Level Mount (1ft) - Illumination: Ambient; Headlamps: low
  • Nighttime Van - Illumination: Ambient, White Light, IR; Headlamps: off, low, high
  • Nighttime Sedan - Illumination: Ambient, White Light, IR; Headlamps: off, low, high

Note that for the Nighttime Van and Nighttime Sedan scenarios, a 3x3 testing strategy was employed. i.e., illumination was cycled through ambient, white light, and IR, and for each illumination scenario headlamps are cycled through off, low and high. For all nighttime scenarios, the vehicle is stationary and positioned at a distance ~36ft from the camera setup. With all lenses uniformly adjusted to the same lens angle the corresponding HFoV is ~17ft.

2 reports cite this report:

Directory of Camera Shootout Series on Dec 27, 2011
The following directory lists all of our camera shootouts. These shootouts pit 4 - 8 different surveillance cameras in simultaneous tests on real...
Top 12 Reports for 2011 on Dec 25, 2011
Here are our top 12 reports for 2011, starting with the state of the surveillance industry: 2012 Video Surveillance Guide - Key trends reviewed...
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