Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

  1. Nov 14, 2016 #1
    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?
     
  2. jcsd
  3. Nov 14, 2016 #2

    DrClaude

    User Avatar

    Staff: Mentor

    They don't use intergerometers. Speed radars use the Doppler shift to measure speed.
     
  4. Nov 14, 2016 #3
  5. Nov 14, 2016 #4

    DrClaude

    User Avatar

    Staff: Mentor

    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.
     
  6. Nov 14, 2016 #5

    anorlunda

    User Avatar
    Science Advisor
    Gold Member

    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.
     
  7. Nov 14, 2016 #6

    DrGreg

    User Avatar
    Science Advisor
    Gold Member

    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
     
  8. Nov 14, 2016 #7

    anorlunda

    User Avatar
    Science Advisor
    Gold Member

    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Do speed cameras use interferometers, and how do they work?
  1. How do springs work? (Replies: 5)

Loading...