Operation Manual

12
Frequently Asked Questions
Must the device run on 5 Vdc? Can it run on 3.3 Vdc
instead?
The device requires 5 Vdc to run properly, so this specication is
recommended and supported.
What is the spread of the laser beam?
At very close distances (less than 1 m) the beam diameter is about the size
of the aperture (lens). For distances greater than 1 m, you can estimate the
beam diameter using this equation:
Distance/100 = beam diameter at that distance (in whatever units you
measured the distance).
The actual spread is ~8 milli-radians or ~1/2 degree.
How do distance, target size, aspect, and reectivity
effect returned signal strength?
The device transmits a focused infrared beam that reects off of a target,
and a portion of that reected signal returns to the receiver. The distance is
calculated by taking the difference between the moment of signal transmission
to the moment of signal reception. Successfully receiving a reected signal is
heavily inuenced by several factors. These factors include:
• Target Distance
The relationship of distance (D) to returned signal strength is an inverse
square. So, with increase in distance, returned signal strength decreases
by 1/D^2 or the square root of the distance.
• Target Size
The relationship of a target’s Cross Section (C) to returned signal strength
is an inverse power of four. The device transmits a focused near-infrared
laser beam that spreads at a rate of approximately 0.5º as distance
increases. Up to 1 m it is approximately the size of the lens. Beyond 1 m,
the approximate beam spread in degrees can be estimated by dividing the
distance by 100, or ~8 milliradians. When the beam overlls (is larger than)
the target, the signal returned decreases by 1/C^4 or the fourth root of the
target’s cross section.
• Aspect
The aspect of the target, or its orientation to the sensor, affects the
observable cross section and, therefore, the amount of returned signal
decreases as the aspect of the target varies from the normal.
• Reectivity
Reectivity characteristics of the target’s surface also affect the amount
of returned signal. In this case, we concern ourselves with reectivity of
near infrared wavelengths (“How does the device work with reective
surfaces?”, page 12).
In summary, a small target can be very difcult to detect if it is distant, poorly
reective, and its aspect is away from the normal. In such cases, the returned
signal strength may be improved by attaching infrared reectors to the target,
increasing the size of the target, modifying its aspect, or reducing distance
from the sensor.
How does the device work with reective surfaces?
Reective characteristics of an object’s surface can be divided into three
categories (in the real world, a combination of characteristics is typically
present):
• Diffuse Reective
• Specular
• Retro-reective
Diffuse Reective Surfaces
Purely diffuse surfaces are found on materials that have a textured quality
that causes reected energy to disperse uniformly. This tendency results in a
relatively predictable percentage of the dispersed laser energy nding its way
back to the device. As a result, these materials tend to read very well.
Materials that fall into this category are paper, matte walls, and granite. It
is important to note that materials that t into this category due to observed
reection at visible light wavelengths may exhibit unexpected results in other
wavelengths. The near infrared range used by the device may detect them
as nearly identical. For example, a black sheet of paper may reect a nearly
identical percentage of the infrared signal back to the receiver as a white
sheet.
Specular Surfaces
Specular surfaces, are found on materials that have a smooth quality that
reect energy instead of dispersing it. It is difcult or impossible for the
device to recognize the distance of many specular surfaces. Reections
off of specular surfaces tend to reect with little dispersion which causes
the reected beam to remain small and, if not reected directly back to the
receiver, to miss the receiver altogether. The device may fail to detect a
specular object in front of it unless viewed from the normal.
Examples of specular surfaces are mirrors and glass viewed off-axis.