| ||Passive Remote Sensing: The eyes passively senses the radiation reflected or emitted from the object. The sensing system depends on an external source of illumination. |
The human visual system is an example
of a remote sensing system in the general sense. The
sensors in this example are the two types of photosensitive
cells, known as the cones and the rods, at the
retina of the eyes. The cones are responsible for colour vision.
There are three types of cones, each being sensitive
to one of the red, green, and blue regions of the visible
spectrum. Thus, it is not coincidental that the modern computer
display monitors make use of the same three primary colours to
generate a multitude of colours for displaying colour images.
The cones are insensitive under low light illumination
condition, when their jobs are taken over by the rods. The rods
are sensitive only to the total light intensity. Hence, everything
appears in shades of grey when there is insufficient light.
As the objects/events being observed are located far away from the eyes, the information
needs a carrier to travel from the object to the eyes. In this
case, the information carrier is the visible light, a part of
the electromagnetic spectrum. The objects reflect/scatter the
ambient light falling onto them. Part of the scattered light is
intercepted by the eyes, forming an image on the retina after
passing through the optical system of the eyes. The signals generated
at the retina are carried via the nerve fibres to the brain, the
central processing unit (CPU) of the visual system.
These signals are processed and interpreted at the brain,
with the aid of previous experiences.
When operating in this mode, the visual system is an example of a "Passive
Remote Sensing" system which depends on an external source
of energy to operate. We all know that this system won't work
in darkness. However, we can still see at night if we provide
our own source of illumination by carrying a flashlight and shining
the beam towards the object we want to observe. In this case,
we are performing "Active Remote Sensing", by supplying
our own source of energy for illuminating the objects.
| ||Active Remote Sensing: The sensing system provides its own source of illumination. |
What is Remote Sensing?
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