Laser Testing   Robert Leverett
  Apr 30, 2007 04:56 PDT 


    Saturday PM was calibration time for 3 brands of lasers that I own,
the TruPulse 200, the Nikon Prostaff 440, and the Bushnell Yardage Pro
800. I own 3 others laser rangefinders, two are on loan and one, the
Optilogic, is not worth fiddling with – a severe disappointment. So, it
was a testing of the A-team lasers against taped distances. The tables
below shows in brief how the 3 lasers stack up against one another for a
range of tape-measured distances of 57 to 150 feet.

Average absolute deviation for 10 trials:

     TruePulse: 0.47 feet

      Prostaff: 0.63 feet

      Bushnell: 0.83 feet

Average deviation for 10 trials: (positives can offset negatives)

      TruPulse: -0.37 feet

      Prostaff:   0.63 feet

      Bushnell:   0.03 feet

Standard deviations for the 10 trials:

      TruPulse:   0.42 feet

      Prostaff:   0.63 feet (oddly, same as average)

      Bushnell:   1.16 feet (this instrument is 7 years old)

The Bushnell registers long about as much as it comes up short while
the TruPulse undershoots more than it overshoots and the Nikon is either
dead-on or overshoots. The largest error made by the TruPuse was –1.0
foot. The largest error made by both the Nikon and Bushnell was 2.0
feet. Neither the Bushnell nor the Nikon was moved forward or backward
to click-over points.

All targets were visually very distinct - unmistakable. Oddly, the
TruPuse 200 frequently returns bounces off the same target that differ
0.5 feet from one another. So, one must be wary of just pointing,
shooting, and accepting the result with the TruPulse. When scanning a
crown, one expects a range of returns and, but when you have a clear
target and keep shooting it, you’ll likely get bounces that differ a
half a foot. In the above test, I used the most frequent return to a
target from the TruPulse. In a couple of cases variant readings were a
toss up.

   From the standpoint of average error and consistency, the TruPulse
200 is the winner. My Bushnell is showing its age. However, when there
is brush to shoot through, the TruPulse fails to reach the target more
frequently than either of the other brands even when set to maximum
target acquisition. For a beginner, the Nikon Prostaff 440 is still, by
far, the best choice for the money.


Robert T. Leverett
Cofounder, Eastern Native Tree Society
Laser testing   Robert Leverett
  May 08, 2007 10:31 PDT 


     Ed Frank and I have been having some off list discussions about
testing concepts and protocals for our measuring instruments. Ed will
soon begin testing his Nikon. So, it may be a good time to bring
equipment testing back upon the radar scope for discussion.

     The different laser models and the variety of conditions under
which we measure trees suggests that we, in ENTS, emphasize the need for
frequent testing of our equipment and testing needs to be done for the
concepts of both accuracy and precision. Accuracy is how close you get
to the correct distance. Precision has to do with repeatability of a

     For an instrument to return repeatably accurate results, it needs
to have high precision, but an instrument can have high precision, but
not be accurate. For example, my Bushnell as slightly lower precision,
but on the average is more accurate than my Nikon. This is because the
errors with my Bushnell tend to be more random and average out. However,
my Nikon has a bias in the direction of shooting too long, but forms a
tighter pattern. Here the concept of precision has to do with
repeatability of a result. If my Nikon laser consistently shoots long by
say half a yard, I can easily compensate for the error. I know it almost
will always be there. My Nikon has high precision even though most of
its readings, taken at face value are off by 1.5 feet. Naturally, we
want high precision and high accuracy. Failing that, we want to know how
to compensate for a lack of precision, when the bias can be determined
by experimentation.

    To fully test a laser, one needs to take into account the following

       a. target distance

       b. target color and reflectivity

       c. target shape

       d. foreground lighting

       e. background lighting

       f. temperature and atmospheric conditions

       g. clutter in the vicinity of the target

    In consultation with the Ents who do most of the measuring, I hope
to propose a testing protocal. I think the time is right.


Robert T. Leverett
Cofounder, Eastern Native Tree Society