Question

Explain the difference between monochromatic and polychromatic confocal sensing and the optimal range of measurement?

Answer 1

The light consists a only one i.e single wavelength is called monochromatic light and the light consists of many wavelengths or broad range of wavelengths is called polychromatic light.

For example, the white light is polychromatic in nature. Since it is a combination of different wavelengths like red or violet e.tc.

Simply we can say that polychromatic is a combination of many monochromatic lights.

The confocal sensors used to moving car at a high repetition rate. The car’s speed is then
determined from the rate of change of the distance with time.
Time-of-flight techniques can also be used for non-line-of-sight applica-
tions. Radar has already been mentioned. Ground penetrating radar can reveal discontinuities and
objects buried underground or faults in concrete structures.

The distance to the features of interest is determined by ranging from at least
two points, and their position is then determined by triangulation.
In some applications acoustic (ultrasound) time-of-flight sensing may be
favored over optical time-of-flight sensing .This is true for ranges
up to a couple of meters, where optical sensors will have times of
flight comparable with nanosecond pulse widths. Since the speed of
sound is about 6 orders of magnitude smaller than the speed of light,
acoustic sensors’ times-of-flight will be much longer than pulse widths,
facilitating easy measurement. Thus acoustic sensors are often preferred
for applications such as robotic sensing and automobile reverse/parking
sensors, where high accuracy is not required and relatively low cost is
important.
Confocal Sensors
Another type of optical sensor, generally applicable for accurate
measurements of displacements and surface profiles at distances of
millimeters, is the confocal sensor. Two versions will be considered:
monochromatic confocal sensors and polychromatic confocal
sensors.
The confocal principle, used in the well-known confocal microscope, is
to use the same optics to tightly focus light emanating from an aperture
onto the object and to detect the light scattered from the object back into
the aperture.

The maximum signal is found when the
object is situated precisely at the image plane defined by the aperture
and optics . The signal, i.e., the amount of light returning to the
illuminating aperture, varies very sharply with the object position (the
full width at half-maximum may be just a few micrometers), permitting
submicrometer accuracy for tracking displacements of the object about
the image plane .
In a confocal sensor an optical fiber is an excellent choice for an aperture.

A monochromatic confocal sensor—employing a monochromatic light
source—commonly operates in closed loop, employing a feedback
mechanism to ensure that the sensor is positioned to receive the
maximum signal from the object . Either the sensor as a whole
is translated, or the lens position is adjusted with respect to the
fiber. Common applications for such a sensor are minute displacement
measurements and surface profiling—measuring the distance
change when the sensor is translated laterally with respect to the object.
In polychromatic confocal sensor(sometimes simply called chromatic
confocal sensing) multiple wavelengths of light or a highly
broadband source are used.