Yoctopuce Yocto-RangeFinder, Руководство пользователя, Страница: 13 / 497

3. Working principle and limitations
The Yocto-RangeFinder emits laser pulses and measures the time that they take to come back to the
sensor after their reflection against an obstacle. Time intervals for such a measure are so short that
the sensor uses indirect physical principles to evaluate them. Therefore, a number of external
perturbations can hinder the measure.
3.1. Ambient light
As long as it does not hide the reflection of the emitted laser pulses, the ambient light does not
perturb the workings of the sensor. But naturally, the infrared laser reflection in a bright outside
environment is much harder to detect than reflection in an inside environment that is poor in infrared.
Generally speaking, the maximal detection distance is reduced by a factor of two outside (sunlight
about 5000 lux), and accuracy is reduced by half. If the infrared sensor is saturated by a strong
radiation, the measure can even prove impossible.
3.2. Color and reflectivity of the detected objects
The sensor is designed to estimate the distance of objects independently from their color and their
reflectivity. However, these parameters have an influence on the quantity of reflected light, and
therefore on the maximal detection range and on the accuracy. A light object (white 88%) is typically
detected at a distance almost twice as far as a dark object (gray 17%), and with a twice as good an
accuracy.
3.3. Workings behind a protective glass
Even if it is transparent to infrared light, a protection glass necessarily creates glints which interferes
with the measure. In presence of glints, the sensor is faced with two simultaneous reflections to be
measured: an intentional reflection, and a parasitic reflection. It is unfortunately impossible to directly
filter the parasitic reflection. However, you can counter the measure bias that it introduces through a
material calibration of the sensor. Its process is described below.
Even if you perform the calibration with the greatest care, it cannot do anything against the fact the
radiations reflected against the glass are not going to reach the object of which you want to measure
the distance. The presence of glass has therefore also as side effect to decrease the maximal
detection distance and the measure accuracy. To limit this negative effect, you must absolutely use
the thinnest and the most transparent glass possible, and above all to press it against the sensor.
The more space between the sensor and the glass, the more the reflections hinder the measure.
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