Rangefinders

Brooke Clarke 2009





Background
How They Work
Equipment
    Bushnell Golf Yardage Pro 400 Binoculars
    Laser Technology Criterion
    Prexiso X2 Laser Range Finder
    1942 Stadimeter (bar type)
    Stadimeter Mk III (Circular type)
    Retroreflector
Links

Background

There are many applications for range finders (Wiki) such as: photography (to adjust the lens focus), golf (to determine what club to use), military (adjust weapon for range and target designation), rifle scope graticule (known height) etc.

How They Work

Radar, Sonar & Optical Pulse

These are "time of flight" systems where the round trip propagation time is measured and divided by twice the propagation velocity.  Time of flight is used in modern digital optical range finders like the Bushnell golf binoculars and the Laser Technology Criterion units.  These typically have one lens for the outgoing pulse of IR light and a separate lens to focus the light on a fast photo sensor.

Optical

Coincidence rangefinder (Wiki)

A single point on the target is brought into alignment to get the range, so the point of the triangle is at the target.  This has the advantage that you know need to know
any dimensions of the target, but the disadvantage that the instrument gets very large if you need to range up to1,000 yards or longer.

The range finder works by having a known separation of the two sight lines in the instrument.  So, the separation between the sight line needs to increase as the range increases.
For example a camera focusing rangefinder may have an inch or so separation to range to maybe 30 feet, but to range to 100 yards (300 feet) requires about 6 feet separation.
The Edscorp Field Range Finder has a separation of a couple of inches and can range to about 100 feet.

Image fuzziness

Image property range finders are used in modern digital cameras.  One way these work is to look at the image contrast as the lens is set a various focal lengths.  The highest contrast image is deemed to be the best focused.  The position of the lens is calibrated in distance to the subject.  There are a number of other methods employed in digital cameras that analyze the pixels and the focus motor changes the lens focus.  When they are done the best focus has (most of the time) been achieved and the camera and the EXIF data (Wiki) in the photograph data file reports the distance.

Stadimeter (Wiki)

A single point in the instrument forms a triangle where the other two points are at the target.  The disadvantage is that you need to know some dimension of the target, such as the height of the top of a ships. main mast above the water line, the height of a man's head, the height or length of a vehicle, etc.  The advantage is that the instrument can be small like a rifle telescopic sight (Wiki: angular mil, Mil Dot sight), surveyor's telescope or the ship's Stadimeter.

The surveyor's method of stadia depends to tick marks on the telescope reticule that include an angle of 10 milli radians (Wiki) so that if there's a separation on the rod of 1 foot (between the top and bottom stadia marks) then the rod is 100 feet away.

One of the oldest optical ranging systems is based on the concept of stadia.  The idea is that you can determine distance by measuring the included angle of an object of known height.
Examples: K&E 76 0000 Alidade, Leitz 115A transit, Stadimeter (Wiki)

This method is also known as Tacheometry (Wiki).

The early surveying telescopes that implemented the method of stadia had a problem in their optical system that made the formula for the distance from the center of the instrument equal to 100 times the rod intercept PLUS a correction term (maybe 1 foot).  All the newer instruments have a correction term of zero.

1942 Stadimeter (bar type) (Mk 1?)


Fig 1
1942
                    Stadimeter (bar type)

Fig 2 Adjustment Instructions
1942
                    Stadimeter (bar type)
Fig 3 Adjustment Step 2 Peep hole and Index Glass (3)
(telescope has been removed)

1942 Stadimeter (bar type)

Fig 4 Horizon Glass (4) adjustments step 2 and step 3 & step 4.

1942
                    Stadimeter (bar type)
Fig 5   Index Glass Adjustment step 3.
1942
                    Stadimeter (bar type)





Stadimeter Mk III
Stadimeter Mk III Range Finder
Stadimeter Mk V (photo from Wiki)
Sextant type (see patents below)
Stadimeter Mark V Range Finder Sextant type

Stadimeter Mk III (Circular type)


Fig III-1
Stadimeter Mk III (Circular type)
Fig III-2
Stadimeter Mk III (Circular type)
Fig III-3
Stadimeter Mk III (Circular type)

Stadimeter
U.S. Navy-Bu. Ships
Mark III-Mod.[ ]-1942
-Instructions-
Rotate knob until object is spanned
between line on glass and movable sight.
Read Range opposite known height.
(If length instead of height is known,
use instrument on its side).
Patented

Fig III-5
Stadimeter Mk III (Circular type)
Rotating knob rotates cam that raises post sight.
Glass with line is fixed.  At top there's an adjustment screw to set zero.
Fig III-4
Near top you can see line on glass just above dot of light from
pinhole.  Near bottom you can see post sight.
Stadimeter Mk III (Circular type)


213018 Surveying Instrument, Henry S. S. Watkin, Mar 4, 1879, 356/20 - complex
406829 Range Finder, Bradley A. Fiske (Wiki), Jul 9, 1889, 33/280 - electrical bridge
523721 Method and apparatus for Range Finding, Bradley A. Fiske, Jul 31, 1894, 356/22 - bar type
801578 Range Finder, Bradley A. Fiske (Western Electric Co.), Oct 10, 1905, 356/22 - two horizontal angle measurements for nautical use
1208120 Instrument for Measuring Distances Optically, Bradley A. Fiske, Dec 12, 1916, 356/22 - ship for use with gun
D132595 Stadimeter chassis, Douglas F. Linsley (Schick Inc.), Jun 2, 1942, D10/70 - bar type
2295877 Stadimeter Gearing, William A. Thomas (Schick Inc.), Sep 15, 1942, 74/422 ; 356/22; 74/405; 74/406 - bar type
2315343 Backlash take-up, Linsley Douglas F (Schick Inc.), Mar 30, 1943, 74/441
2403965 Stadimeter, Brandon Thomas O, Jul 16, 1946, 356/22 - sextant type - lower cost to manufacture since it uses common sextant design
3459478 Stadiametric rangefinder including a transversely movable lens, Walter W Hollis, Paul M Marasco (U.S. Army), Aug 5, 1969, 356/22 -



Equipment

Bushnell Golf Yardage Pro 400 Binoculars

One objective lens is for the IR transmitter and the other is for the IR receiver.  There is a small visual telescope in the center that's for the operator to observe where the spot is pointing.  The two large lens are black in color, not clear like optical binoculars.  The range is displayed in units of either meters or yards, a little to coarse for surveying.

Bushnell Golf
                Yardage Pro 400 Binoculars
Bushnell Golf
                Yardage Pro 400 Binoculars



Laser Technology Criterion

This is the model 400 that includes tree measuring functions.  The range is displayed in 1/100 of a meter, i.e. a cm which is less than 1/2" so are excellent for surveying.  In fact part of the set is a fork mount that fits a standard surveying tripod.

Operation

After power on you will see one of the top menus.  Use the up or down arrows to scroll to the desired menu.  In my case they are:
TREE
CONDUCTOR CLEARANCE
SURVEY
SYSTEM
TREE
Once you have selected the appropriate top menu press the ENTER button. For Example:

POWER (on)
<down>
<down> i.e. SURVEY
ENTER i.e. BASIC MEASUREMENTS
ENTER display is:
HD:-------.-- M
AZ:------.- DEG
<press trigger>
display blanks change into data and pressing up or down scroll arrow shows more data like inclination, slope distance, etc.

When in Survey measurement mode and the trigger is pressed you hear a buzzing sound as the laser range finder and other sensors are working.  When aimed at a wall about 4 feet away the spot size as viewed using the PAS-6 IR scope is about an inch high and half an inch wide.

Laser
                Technology Criterion
Laser
                Technology Criterion
Laser
                Technology Criterion




Prexiso X2 Laser Range Finder (Manual, Specs)

Prexiso X2 Laser Range
          Finder

eBay search term: "Prexiso X2".  Price range: $100
Range: up to 98 feet with 1/16" resolution (or) 30m with 1mm resolution.
Pressing + or - between distance measurements allows adding or subtracting them.
To measure [Area - Volume - Triangle] press the key [once, twice or three] times prior to making a measurement.
The upper left of the LCD shows the reference plane: either the front or back (shown in photo).  Pressing the units key toggles between front and back.
Pressing and holding the units key gets you into the units mode.

Outside in the daytime it's difficult to locate the red laser pointer spot on natural vegetation.  Getting the spot onto a 5 gallon white plastic paint bucket at 30 feet is difficult.  But when you press and hold the "Laser On" button for a few seconds to turn on continuous measure mode it's easy to start close to the bucket and back away while maintaining the spot.  It might be easier to use outside at dawn or dusk.  Need white or reflective targets, not trees or bushes.

Retroreflector

A retroreflector (Wiki) is a device that returns incident light back to the source.  A simple one can be made by taping three flat mirrors at right angles to each other making a corner reflector.  Another type is a half silvered sphere, this is the cause of "cat's eye" (Wiki) or "red eye" (Wiki) or Raised Pavement Markers (Wiki).

The range of surveying total stations is limited when the target is non cooperative, like a tree or building, but when a retroreflector is used the range is much greater because of the efficiency of light return.  The below reflector is about 60 mm (2.36") in diameter and is what's called a "Zero Offset" design.  That means the range you measure is the same as if the target was on the point shown in Fig 2 and Fig 3.  Other designs have some offset distance that needs to be backed out of the measured range.

Fig 1
60mm dia.
                  Surveying Retroreflector
Fig 2
60mm dia.
                  Surveying Retroreflector
Fig 3
60mm dia.
                  Surveying Retroreflector

The retroreflector prisms are specified by how close they come to sending the light back to it's source.
Here are some Hollow Retroreflectors - not chromatic aberrations - between 1 and 20 arc seconds of angle.

Links


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