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IR is sometimes called black light and lies approximately between
the wave lengths 700nM and 1,000nM. The eye cannot efficiently see
beyond 730nM whilst CCD Cameras can see up to l000nM and this
enables the use of semi-covert and covert IR applications.
There is often a requirement for semi-covert or covert
installations and filters are fitted in front of the lamp. The 730nm
filter will glow similar to a traffic light, an 830nM filter will
give off a dull red glow only just visible to the human eye. The
950nm will appear covert, with no glow visible.
The key factors in choosing the type of IR lamp relate to the
required viewing distance and scene. The sensitivity of the camera
is an important element, additionally the lens sensitivity and type
play an important role.
When designing CCTV systems for effective 24-hour surveillance
three issues are central: the camera, the lens and illumination.
Camera
Camera selection is vital to a working system but, but in
essence, without light there can be no picture.
All CCD cameras offer some degree of IR response though some have
enhanced IR performance that make them more suitable for longer
range applications or for use with low power IR sources such as LED.
The latest chip sets offer excellent low noise, high resolution, and
low smear characteristics together with excellent IR response.
Day/night cameras are intended to provide the best solution for
24 hour surveillance - colour by day and monochrome / IR sensitive
by night. There are different forms of day/night cameras with some
incorporating optical filters that are automatically moved over the
CCD sensor for daytime/colour operation - IR optical filters cut
filters) are used as IR light distorts the colour display - and
removed during night-time/ monochrome to maximise the low light
sensitivity by allowing IR light to pass to the CCD. Other camera
designs incorporate specialised filters which do not move but which
have both good colour performance and IR sensitivity.
Lens
At night, it is essential to maximise the light gathering
capability of the lens (i.e. have the lowest 'f' stop) but this will
reduce the depth of field of the picture, which may cause focusing
problems. An auto iris lens will naturally open to its maximum
aperture (lowest f' stop) in low light operation but on a fixed lens
there may need to be a compromise between the low light operation
and its depth of field focussing. Daylight and IR light do not focus
on the CCD at the same point because of the different ways in which
the wavelengths of light pass that through the lens.
This may cause a focus shift between daytime and IR operation.
The degree of focus shift may depend on a variety of factors
depending on the quality of lenses, the wavelength of the IR filter
830nM & 950nM will give a more exaggerated focus shift.
Illumination
The key to a successful night system is having sufficient light,
the right quality of light and the right control over the light. The
best solution is IR lighting at the camera head controlled by either
telemetry (for manual switching on/off) or photocell (for automatic
operation when the light drops below a certain level].
The viewing distances that can be achieved depend upon camera and
lens type and these can affect system performance by as much as 50%
compared with daylight operation. All cameras are less efficient at
850nM and achievable distances can be reduced , whilst at 950nM the
performance of many camera/lens combination can be reduced even more
compared with daylight conditions.
When choosing an IR lamp, consideration should be given to the
lamp's ability to provide even illumination. This ensures that the
returned light to the camera is the same for wherever the subject is
in the scene and avoids white out on the screen. Additional factors
such as bulb life and running costs should also be taken into
account. In general, IR lamps driven via the mains tend to have
considerably lower bulb life and higher running costs than low
voltage IR lamps.
It is also important to match the beam pattern of the infra Red
light to the camera lens. Mismatching in this area can lead to
either poor picture quality or loss of IR efficiency. For example, a
camera with a 45 degree horizontal view used with a spot and flood
would have a resulting picture where there would be a light circle
in the centre of the screen and darkness all around. This is because
a spot lens will only illuminate a small selection of the field of
view. The flood lens may be wasting 30% of its energy outside the
field of view.
Solid-state devices such as LED bring major advantages, including
lower maintenance levels and lower power consumption.
The LED units can provide up to 10 years' life. However, they
provide less power on the scene than halogen IR lamps. LED's are
beginning to make a bigger impact on the CCTV scene because of the
increased IR sensitivity of monochrome cameras.
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