Chapter 6 • Key Features
6.1 Calibrated Reflectivity
31
6.2 Laser Return Modes
31
6.2.1 Single Return Modes: Strongest, Last
31
6.2.2 Multiple Returns
32
6.2.3 Dual Return Mode
32
6.3 Phase Locking Multiple Sensors
36
6.1 Calibrated Reflectivity
The VLS-128 measures reflectivity of an object independent of laser power and distances involved. Reflectivity values
returned are based on laser calibration against NIST-calibrated reflectivity reference targets at the factory.
For each laser measurement, a reflectivity byte is returned in addition to distance. Reflectivity byte values are segmented
into two ranges, allowing software to distinguish diffuse reflectors (e.g. tree trunks, clothing) in the low range from retrore-
flectors (e.g. road signs, license plates) in the high range.
A retroreflector reflects light back to its source with a minimum of scattering. The VLS-128 provides its own light, with neg-
ligible separation between transmitting laser and receiving detector, so retroreflecting surfaces
pop
with reflected IR light
compared to diffuse reflectors that tend to scatter reflected energy.
Diffuse reflectors report values from 0 to 110 for reflectivities from 0% to 100%.
Retroreflectors report values from 111 to 255, where 255 represents an ideal reflection.
Note: When a laser pulse doesn't result in a measurement, such as when a laser is shot skyward, both distance and
reflectivity values will be 0. The key is a distance of 0, because 0 is a valid reflectivity value (i.e. one step above noise).
6.2 Laser Return Modes
The VLS-128 supports three laser return modes: Strongest, Last, and Dual. A sensor can be configured to handle laser
returns in one of these ways interactively via the sensor's web interface (where the setting is called Return Type) or pro-
grammatically via curl command. (See
Configuration Screen on page 68
or
Sensor Control with curl on page 75
for addi-
tional information related to setting laser return modes.)
A laser return is a detection of a reflection. Up to two returns per laser shot are supported by the VLS-128.
6.2.1 Single Return Modes: Strongest, Last
As shown in
Figure 6-1 on the next page
, when a laser pulse hits a solid wall a single return or measurement is obtained. In
this situation, the reading is considered both the strongest and the last return. (More on the nature of laser pulses emitted
by your sensor, including the rectangular shape of the pulse, is covered in
Laser Pulse on page 113
.)
Chapter 6 • Key Features 31
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