Do speed cameras use interferometers, and how do they work?

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If they work like I assume they do, by comparing the wavelength emitted with the wavelength reflected back, they might use an interferometer. If so, how does it work?
 

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DrClaude
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I should also add that hand-held lidar speed guns are also quite common. These are based on the timing of light pulses. The thread title mentionned speed cameras, and to the best of my knowledge they still use Doppler radars.
 
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anorlunda
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I should also add that hand-held lidar speed guns are also quite common. These are based on the timing of light pulses. The thread title mentionned speed cameras, and to the best of my knowledge they still use Doppler radars.
Both radar and lidar have been used, but it occurs to me that a video camera could do it for less money using image analysis.

Just use image analysis to measure the time for the car to pass two marks with a known distance between them. The same video images could capture the plate number and a photo of the driver.

You could even do it with a single still frame. Take a shot of the plate with a known shutter speed. The image will be blurred. Apply de-blurring to the image. The degree of de-blurring needed gives an estimate of vehicle speed.

Since digital cameras and image processing have become so cheap and ubiquitous, they could become competition for radar and lidar.
 
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DrGreg
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Just use image analysis to measure the time for the car to pass two marks with a known distance between them. The same video images could capture the plate number and a photo of the driver.
In the UK, where I live, speed cameras that are used for prosecution are required by law to use an additional secondary method to measure speed as well as the primary method that triggers the camera. The two methods must give the same answer (within specified error bars) for a prosecution to succeed. In the case of Doppler radar cameras, the secondary method is photographic, pretty much as you described.

480px-Gatso.camera.arp.jpg

Example of UK road markings at a Doppler radar speed camera
 
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anorlunda
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In the UK, where I live, speed cameras that are used for prosecution are required by law to use an additional secondary method to measure speed as well as the primary method that triggers the camera. The two methods must give the same answer (within specified error bars) for a prosecution to succeed. In the case of Doppler radar cameras, the secondary method is photographic, pretty much as you described.
That sounds pretty sensible for reasons of prosecution.

Looking to the future, I expect such devices to be also used for taxation. Motorists could be assessed micropayments every 100 meters or so using an algorithm based on time of day, lane used, and speed. Even inspection and registration violations could be addressed administratively by increased fees instead of criminal fines. If millions of such devices are deployed, the cost per device would become a major factor.

It can be fun to speculate on the social engineering made possible by technology. For example, a 55 mph national speed limit to save energy would be heavy handed and hated. But a use fee per mile that grows exponentially with each additional mph speed would be more subtle, especially if the exponent coefficient grew incrementally year by year. Politically, a scheme like that might succeed where a speed limit or a carbon tax would fail.
 

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