© Brooke Clarke 2009
How They Work
Bushnell Golf Yardage Pro 400
Laser Technology Criterion
Prexiso X2 Laser Range
1942 Stadimeter (bar type)
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
and the Laser Technology
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.
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
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
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
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.
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,
Dot sight), surveyor's telescope or the ship's
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
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
This method is also known as Tacheometry
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
1942 Stadimeter (bar type) (Mk 1?)
|Fig 2 Adjustment Instructions
|Fig 3 Adjustment Step 2 Peep hole and
Index Glass (3)
(telescope has been removed)
Fig 4 Horizon Glass (4) adjustments step 2 and step 3
& step 4.
Fig 5 Index Glass Adjustment step 3.
|Stadimeter Mk III
|Stadimeter Mk V (photo from Wiki)
Sextant type (see patents below)
S. S. Watkin,
Mar 4, 1879, 356/20 -
Range Finder, Bradley
A. Fiske (Wiki)
Jul 9, 1889, 33/280
- electrical bridge
Method and apparatus for Range Finding, Bradley
Jul 31, 1894, 356/22
- bar type
Range Finder, Bradley
A. Fiske (Western
Oct 10, 1905, 356/22
- two horizontal angle measurements for nautical use
Instrument for Measuring Distances Optically, Bradley
, Dec 12, 1916, 356/22
ship for use with gun
Stadimeter chassis, Douglas
F. Linsley (Schick
Jun 2, 1942, D10/70 -
Stadimeter Gearing, William
), Sep 15, 1942, 74/422
; 356/22; 74/405;
74/406 - bar type
Backlash take-up, Linsley
Douglas F (Schick
Mar 30, 1943, 74/441
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.
Aug 5, 1969, 356/22
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
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
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
Once you have selected the appropriate top menu press the ENTER
button. For Example:
<down> i.e. SURVEY
ENTER i.e. BASIC MEASUREMENTS
ENTER display is:
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.
eBay search term: "Prexiso X2". Price range: $100
Range: up to 98 feet with 1/16" resolution (or) 30m with 1mm
Pressing + or - between distance measurements allows adding or
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
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.
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.
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.
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