1. Mick West

    Mick West Administrator Staff Member


    Source: https://www.youtube.com/watch?v=ookTfBP5sUU


    When you shine a laser beam across the surface of a lake, it is very prone to refraction, but often this is ignored, and the result is declared a victory. Laser tests are a very poor choice of a test of curvature, when much clearer (and easier) tests are available, such as looking at large objects (like mountains) over water.

    In the video above, I describe the problem, and answer this question:
    Metabunk 2019-04-07 11-50-51.

    With a resounding "YES, OF COURSE". It's actually really common for conditions to exist where a low-level laser is visible to a low-level camera many miles away. In fact, we are generally seeing this type of curvature most of the time during the day when the ocean is cooler than the air. We just don't really notice the very thin strip of compression near the horizon. The laser makes it stand out.
     
  2. Mick West

    Mick West Administrator Staff Member

  3. Bryan Olson

    Bryan Olson New Member

    Great, but I do not see how a constant angle of beam divergence helps get over the curve. With one milliradian divergence, if only the bottom of the beam reaches the camera, we could just aim the laser half a milliradian lower and the middle of the beam would then reach the camera.
    Would more divergence help get over the curve? A light bulb has the full 2 pi radians, but I don't think it's any more visible given the same positions.
    Refraction is certainly a factor, and perhaps could increase the angle of beam divergence with distance, or even the diffraction effect from being partly blocked by water could, but I don't think the constant angle helps.
     
  4. Mick West

    Mick West Administrator Staff Member

    The main contribution it has to the results being misleading is that it extends the beam downward. So if they have a beam that is parallel to the ground, there's always going to be a light path that goes directly to just skimming the horizon, the region of greatest refraction. The yellow line in the video
    Metabunk 2019-04-09 08-07-11.

    This misleads because they think the center beam should not be visible because not only is the laser hidden by the hidden value, the center of the beam is the "drop" above the surface - which is even more.

    The Brighton test folk say:
    Metabunk 2019-04-09 08-11-55.

    Since their laser is at 1.5 m and "level", they (seemingly) are not considering the air below 0.5m. However, the beam WILL spread down to that region, and their temperature reading show a quite significant temperature gradient, with the air above the water 0.5°C cooler than the air at 1.0m.

    Of course this effect is less if you have a less divergent beam. But the Brighton tests use a standard $15 green laser, like mine, which has a 1.0 mrad divergence.

    Having LESS divergence will not make it impossible to see the laser, it just makes it more dependant on the angle.

    For example, here I've reduced the divergence to 0.05 mrad. The beam still diverges, but now I have to tilt it down from level by 0.026° to make it visible. Notice it's a lot brighter too, as it diverges less.
    Metabunk 2019-04-09 08-18-25.


    (I accidentally put the boat with the laser at 7 miles here, but still no problem seeing it)
     
  5. Bryan Olson

    Bryan Olson New Member

    Did the Brighton testers claim to precisely level the laser? I thought they were testing solely for visibility over the curve, and were free to play with the aim.

    Having LESS divergence will not make it impossible to see the laser, it just makes it more dependant on the angle.​

    O.K. From how the video named beam divergence along with refraction I had thought you were suggesting that divergence helps get around a curve as refraction does.
     
  6. Mick West

    Mick West Administrator Staff Member

    Not directly, but note they say: "between 0.5m and 1.5m there was no density gradient measurably", so they are not considering the air below 0.5m, which would indicate they don't think the beam goes into that air. They had the lasers and cameras at 1.0m.
    Metabunk 2019-04-09 14-08-16.