Camera Test: PPF Needed For IDs, Text, MoneyBy Derek Ward, Published Feb 19, 2014, 12:00am EST (Research)
Need to see the fine print of a dollar bill, euro, driver's license or text on a document?
We tested 3 different types of print.
Currency, both the US dollar and the Euro:
Identification card, a US state driver's license
And text, with point size ranging from 8 to 40, like so:
The tests were done with an overhead shot (like for cash counting):
And an angled shot (like at a bank teller):
This test answers the questions of:
- What resolution do I need?
- How wide of a FoV can I cover?
- What impact do angles have on capturing this?
- How many PPF do I ned for text, money and IDs?
- Placing cameras directly above the target (such as cash registers) may result in more light reflecting directly into the camera, overexposing the image.
- Using an angled view of ~45° reduces reflectance issues while still providing similar details in most cases. Only the driver's license in our test was more difficult to read when the cameras were angled, due to its construction.
- Credit card numbers are especially difficult to discern at any resolution and FOV, since most modern credit cards use solid colors for both numbers and background to protect card holder information. Even at 1000+ PPF, numbers are only partially legible.
- Lux levels on target for objects lying on a table or counter are much higher than expected compared to subjects elsewhere in the room, since more overhead light is reflected onto these objects. For example, subjects in our test environment are typically illuminated to ~160 lux on target, while objects facing up read 700+.
Users seeking best detail of papers, currency, IDs, or other objects are best served by angling the camera, instead of looking straight down from above. Where this is not possible (due to obstructions or existing cable pathways, etc.) lights should be as diffuse as possible to reduce reflectance issues. Finally, WDR cameras may reduce some light handling issues, mitigating overexposure problems.
The chart below provides our PPF recommendations for capturing currency, IDs, and other objects in this test:
Here are our key findings from this test:
Overhead Image Comparisons
The image comparisons below complie overhead shots with the cameras mounted directly above the target, varying the horizontal field of view at 3', 5', and 9'. We take a look at varying font sizes, a Pennsylvania drivers license, €5 note, and $20 USD dollar bill.
This image shows an overview of the scene at these FOVs:
Varying Font Sizes
We begin with a 3' wide field of view. In this case, 720p was only able to reliably discern 14 pt. font and above, while 12 pt. was legible in our 1080p camera. 5 and 10 MP cameras were able to discern 10 and 9 pt. fonts, respectively.
Widening the FOV to 5', the 720p camera is only able to make out our largest font, 40 pt, and 1080p is barely able to discern 20 pt. At this FOV and light level, light handling begins to become an issue for the 5 and 10 MP cameras, with text smaller than 20 pt. impossible to read due to overexposure of the white page.
Finally, at a 9' HFOV, both 720p and 1080p are able to discern only 40 pt. font, while the 5 and 10 MP cameras are so washed out no text is recognizable.
At a 3' HFOV, 720p and 1080p resolutions are unable to make out any of the text details of the driver's license, and lack detail in the subject's photo. Both the 5 and 10 megapixel cameras are able to read address, license number, and some personal details (greyed out below), and provide better detail of the subject photo.
Moving to 5' FOV, none of the cameras are able to read text on the license, with overexposure becoming an issue due to the high reflectence of the plastic license.
Finally, at 9', details are entirely lacking.
Looking at a €5 note, we can distinguish the denomination at all resolutions, but serial number (seen top center) is not possible to read.
At 5' FOV, the €5 denomination is still easy to make out on all cameras.
Finally, widening to 9', denomination is still visible, though with difficulty in the 720p camera. The 5 and 10MP cameras become overexposed here, but €5 is still visible.
$20 Dollar Bill
Looking a US $20 bill, all cameras are able to discern bill value and serial number at a 3' HFOV.
Moving to 5' HFOV, the 720p and 1080p cameras no longer can make out the serial number due to lowered resolution. The 5 and 10MP cameras provide enough PPF for legibility, but exposure problems start to arise.
Finally, at a 9' HFOV, the serial number is obscured in all cameras. The 10MP camera is so overexposed at this point that even the $20 mark is difficult to read.
45° Viewing Angle Image Comparisons
We moved our cameras back from the table and angled them at about °45 to see what effects this had on text legibility and exposure issues seen in our overhead shots. Again, we take a look at varying font sizes, a Pennsylvania driver's license, €5 note, and $20 USD dollar bill, seen in this FOV:
At 3', details are essentially the same as the overhead shot, with 720p and 1080p discerning 12 and 14 pt. font, respectively. The 5MP camera is able to read down to 9 pt., and 10MP down to 10 pt.
Widening the FOV to 5', only 40 pt. font is legible at 720p, with 20 pt. visible in the 1080p camera. Overexposure in the 10MP image is modestly reduced here, with 14 pt. font marginally visible (previously 20 pt. was the smallest visible).
Finally, at a 9' HFOV, overexposure is once again a problem for the 5 and 10 MP cameras, with fonts smaller than 40 pt. illegible.
The driver's license becomes more difficult to read when the caemras are angled. At a 3' HFOV, even 5 and 10 MP have difficulty discerning the largest letters on this ID.
Widening the FOV to 5', details are near completely lost.
Finally, widening the FOV to 9', no details are discernible, with the state barely legible in even the 10MP camera.
At a 3' HFOV, the €5 note is still easily visible in all cameras.
Exposure issues are moderately reduced on the Euro, seen in this comparison. All cameras are still able to make out the denomination without issue.
Finally, where the Euro was previously so overexposed the 10 MP camera had difficulty discerning the €5 mark, angling the cameras reduces this, making details of the note fully visible.
$20 Dollar Bill
Using a 3' HFOV, bill denomination and serial number is visible on all cameras with no issues.
At a 5' wide HFOV, the serial number is partially visible in only the 10 MP camera, with others unable to make it out. Bill value is clearly visible in all cameras.
Finally, at 9' wide, serial number is impossible to discern. However, exposure issues are reduced due to camera angle, making the bill denomination visible in all cameras.
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