Rangefinders

Optical superimposed/split image rangefinder:

(From: Sam Goldwasser (sam@stdavids.picker.com)).

This is the basic principle used in 35 mm rangefinder cameras and other devices where you view the distance scene and turn a knob to line up two images that are either superimposed or split top/bottom half. In the case of the camera, turning the lens focus ring adjusts the angle of mirror A below.

        To distant scene.
          ^               ^
          |               |
          |       C/------/D    
          |A       |      
          \--------\       (B is partially silvered or a half mirror to
         adjust   B|        permit viewing of both sides from the scene.)
         angle     ^
               view here
          |               |
          |<- baseline -->|
The further apart the mirrors are (size of baseline), the greater the useful range. Adjust the angle of mirror A or D until the images are superimposed. Calibrate the angular setting to distance.

The distance from A to the scene is then: tan(angle A) * baseline.

For long distances, C and D can be eliminated - they compensate for the difference in path lengths of the two views - else the sizes would not be the same. (Even this doens't work perfectly in any case. Can you figure out why?)

You can add telescopes and other optics if you like - this is just the basics.

Look Ma, no electronics :-).

Note that SLR cameras do NOT use this approach as they are entirely optical (meaning that adjusting the focus only controls the lens - nothing else!). With SLRs, a pair of shallow prisms oriented in opposite directions (or many in the case of a 'microscreen' type) are cemented onto a clear area of the ground glass. When the image is precisely focused onto the ground glass, the prisms have no effect. However, when the image is in front or behind, they divert the rays such that the two halves of the image move apart (or the image breaks up in the case of the 'microscreen').

Optical rangefinder using HeNe laser for triangulation:

(From: Mike Cimorosi (mcimoros@hopi.dtcc.edu)).

My students construct a simple laser rangefinder using a few basic parts:

Equipment:

  1. He-Ne laser (1/2-mW will do)
  2. Rotary table (to measure second reflected beam angle)
  3. Beam splitter (a simple glass plate will do)
  4. Optical bench (to put all the optics on)
  5. Flat front-surface mirror (to be mounted on the rotary table)
Basic procedure:

  1. Place the laser to the left of the optical bench. Follow standard safety procedures for using 1/2-mW lasers. You can find these in just about any laser book.
  2. About 3 inches to the right of the laser aperture (opening), place the beam splitter at an angle of 45 degrees with respect to (wrt) the incident beam. This will split the beam into two different paths. Most of the beam will pass through the splitter. Some will be reflected at a right angle wrt the incident beam.
  3. About 6 feet to the right of the splitter, place the rotary table with the mirror on it and face it toward the beam that passes through the splitter.
  4. Now, before you turn on the laser, make sure you have a safe place to aim the beam for the distance you want to determine.
  5. Now fire up the laser. Note where the first reflected beam strikes the target (a wall maybe?). Now, slowly and carefully rotate the rotary table until the beam reflected from the mirror coincides with first reflected beam. You now have formed a right triangle made of laser light! Pretty neat! Remember to respect the beam, especially with respect to your eyes!!!
  6. Finally, you can use the trig relation: distance = 6 ft x tan(angle) to determine the distance. How's your trig?
  7. It's not the most precise rangefinder i.e., the equation is pretty sensitive to the angular precision of the rotary table. However, it does demonstrate the basic principle. Maybe the diagram below will help with setting up the laser rangefinder.
Rough diagram of rangefinder setup:

To wall                       To wall
                 |                             \ 
        distance | first reflected beam          \ second reflected beam
                 |                                 \
                 |                             angle \
    Laser --3"---/------------------------------------/
            Beam splitter                    Rotary table with mirror
                 |<------------- 6 feet ------------->|
Of course, you can make the non-laser version of this type of rangefinder. My students also make that one as well. Both are pretty neat and demonstrate the power of trig to determine distances!

Laser rangefinders:

(From: Andrzej Hanczak).

I am just finishing the development of a range finder based on the pulse-time-of-flight measurement method. There are also different methods like phase-shift method which compares the phase shift between outgoing modulated beam and reflected light.

The Pulse TOF method has some advantages which make it very useful: you can use relatively high pulse power and still be in the class I.

While building such a range finder there are two crucial components which have influence on its accuracy: the time measurement circuits and the receiver. Our aim was to build a laser scanner with the resolution of 1cm which means that you have to be able to measure the time with the resolution of 67ps. The range of the scanner should be approx. 30m. We are not ready yet but there are some results.

For the first prototype we used a 1.25 GHz oscillator and special microstrip design to get the resolution of 70ps. In the current prototype we use a special prototype IC which should deliver 50ps resolution.

The problems are on the receiver side, a relatively large jitter (which I'm fighting now) destroys my high time measurement precision. The jitter on the input results in the distance differences of approx. 10cm (4 inch ;-) ). This can be filtered out by averaging of a number of measurements and that is what we are doing now. Our measurement frequency is at present 100 KHz, but we will probably perform the averaging over 10 measurements so that effective measurement rate will be 10 KHz.

I plan to write a paper about the design and experiences I've had while developing this range finder. There are a lot of things you have to be aware while building such a device and the information about it is strong distributed.

The paper should be ready in the December this year. If you have more questions

(This posting is from 12-October-1994)