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Infra-red light
Infra-red
(IR) light, like photopic (visible) light is part of the electromagnetic
spectrum and shares many of the characteristics of visible light.
The "colour" of light is determined by it's various wavelengths
and most light sources (like the sun or light bulb) emit a broad
range of wavelengths. This mixture of wavelengths enables us to
determine the colour of an object by the wavelengths of the light
it reflects and absorbs.
The part of the electromagnetic spectrum visible to humans stretches
from about 400nm (violet/blue) to 760nm (deep red). nm is the standard
abbreviation for nanometres - 1 nanometre = 1 thousand millionth
of a meter).
The Infra-red part of the spectrum is split into four main areas
- Near Infra-red (NIR), Mid Infra-red(MIR), Far Infra-red (FIR),
Extreme Infra-red (XIR) and stretches from about 760nm to 1000 microns.
Most good monochrome CCTV cameras can only "see" from
photopic or"visible" light to at best 1000nm in the NIR.
Colour cameras normally filter out almost all of the IR and restrict
the response to the photopic or visible band.
Daylight contains varying amounts of IR and in order for CCTV cameras
(especially colour) to reproduce accurate images, filters are used
in front of the CCD sensor to block the IR.
Colour "day/night" cameras will employ either a switchable
filter or use a filter designed to block some IR bands this only
lets through a narrow band of IR - that of the wavelength of the
IR lighting to be used. Colour rendition of the cameras has to be
adjusted to compensate for the differences in response of the CCD
to required colour and IR pollution of that colour. The main problems
occur in reproduction of grass, trees and vegetation as the chlorophyl
in the leaves reflects large amounts of IR. This reflectivity makes
vegetation appear white or bright at night as IR light is reflected.
More....Cameras
and Infra-red, Lenses and Infra-red
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