How to collect a laser beam over a wide distance?

Discussion in 'The Projects Forum' started by bambicomp, Aug 4, 2009.

  1. bambicomp

    Thread Starter Member

    Sep 23, 2008

    I have a laser beam emitter unit and a photodiode receiver unit. I wish to use the unit over a relatively wide distance say 5metres to ten metres apart so i can measure the speed of a person running.STarts a clock when passes through one set and stops when passes through the second set.

    They work correctly for this when they are aligned correctly.

    However the problem i have is that the units are mounted on tripods and the floor is bouncy. So when you past between the units the tripods holding the units shake and thus the beam gets thrown. Is there a lens or something that I can mount on the receiver (say 30mm or 40mm) that will capture the light?

    I know there is a lens to disperse the light at the laser tramsmitter end but Im kinda reluctant to try this for safety reason etc.

    Any help or advice would be greatly appreciated.
  2. Wendy


    Mar 24, 2008
    Try using a telescope lense arrangement, and put a piece of paper where the photodetector (or your eye) will be. Then focus it to a dot, which marks the spot.
  3. jpanhalt


    Jan 18, 2008
    How much movement of the beam are you seeing at the detector due to vibration? The solution might be different if it is only a centimeter vs. several centimeters. What wavelength of light are you using? Can you use UV?

  4. Wendy


    Mar 24, 2008
    A fresnel lense might work well too.
  5. BillB3857

    AAC Fanatic!

    Feb 28, 2009
    We used laser interferometers for machine accuracy checks over long distances. They used a "retro-reflector" to return the beam to the same head as the emitter. Basically, it was a dual right angle prism and the alignment was very forgiving.
  6. steveb

    Senior Member

    Jul 3, 2008
    A collimated laser beam is the most dangerous. If you are worried about the safety of this laser with a lens dispersing the beam, then leaving the beam collimated is certainly too dangerous. You are describing a scenario where accidents are likely. A running person can trip and knock over either tripod, sending the beam into the eye of an bystander.
  7. kubeek


    Sep 20, 2005
    I would suggest using the lens on the laser side, a slightly convex lens to make the beam little wider at the receiving site, so that when the laser moves the light still gets to the sensor.
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Most of those brass laser tubes (red 1w etc) have the collimator lens built in, and it's threaded in the tube or glued in a threaded endcap. You rotate the lens to adjust (de-focus) it.
  9. Wendy


    Mar 24, 2008
    Most solid state lasers (under 5mw) aren't dangerous, though there are hand held lasers that definately fall under the catagory of hazardous. You would be suprised what you can do with a high intensity LED too. Are you wedded to the idea of a laser?
  10. steveb

    Senior Member

    Jul 3, 2008
    Depending on the wavelength, a collimated laser beam at 5 mW can still dangerous to the eyes if viewed directly. There are eye-safe wavelengths such as 1500 nm which would be no problem, and visible lasers are less of an issue because a person will see it and avert their eyes. However a 700-1000 nm laser is hazzardous because you can't see it and the power is enough to cause damage when the beam is focused on one spot of the retina for several seconds.

    I'm concerned because to OP has not given any specifcations on the laser. Very little power is actually needed here, if the system is designed properly, but we just don't know the details.
    Last edited: Aug 6, 2009

    AAC Fanatic!

    May 26, 2009
    The problem is the longer distance you have the bigger the dot will be. So you would have to have a high powered laser to keep the dot focused at the same level over a great distance.
  12. bertus


    Apr 5, 2008
  13. bogalark

    New Member

    Apr 24, 2013
    can you please share the schematic of the receiver part as im also worrking on the same but wanna measure the distance of 70 meters, hope your schematic will through some light on mail address is

    thank you..