d e t e k t o r i n t e r n a t i o n a l · 1 3
Quality images are a necessity for
any security system or transport
monitoring application, including
perimeter surveillance applications.
Not only does the performance of
the system rely on the camera and
lens, but also on the quantity, qual-
ity, and distribution of available
light.
"Light determines whether a
subject can be viewed at all, and at
what distance. The quality and di-
rection of the light also controls the
appearance of the subject. However,
there are lots of different types and
wavelengths of light available and
to consider", states Callum Ryder.
White-light
White-light is one of the most
widely used wavelengths of light
and is something we're all used to
seeing and using in day-to-day life.
A main advantage of using white-
light as part of a security system is
its ability to provide colour images,
if being used with a colour camera.
Identifying the colour of someone's
clothes, hair, or other features,
could provide crucial evidence for
prosecution.
The defining characteristic of
white-light illumination is that
it is highly visible to the human
eye, hence it's use for general
area illumination. For perimeter
surveillance applications, this is
also useful when using white-light
as a deterrent to ward off potential
intruders. On the contrary, if covert
surveillance is the aim, white-light
illumination is unlikely to be the
answer.
Using constant white-light
at night may not be suitable for
all applications, due to the fact
that the target area is lit up, and
therefore, potentially assisting an
intruder in undertaking a crime.
When using white-light, con-
sideration also needs to be given
towards light pollution.
"While the use of LED technol-
ogy means the light can be targeted
accurately, some applications may
be restricted in their ability to use
white-light for environmental
reasons such as protecting the night
sky or wildlife," comments Callum
Ryder.
Infra-red (850nm)
850nm is a semi-covert light and
the preferred choice for most
CCTV systems. Security cameras
are most receptive to this wave-
length of IR and, therefore, it gives
the best night-time images. Being
semi-covert, the illuminator does
give off a slight red glow, visible up
close or when looking directly into
the light.
"However, for most applica-
tions this is not an issue and we
would always recommend the use
of 850nm over other wavelengths
of IR. 850nm will provide the best
night-time images and allow the
camera to perform at it's optimum,"
says Callum Ryder.
Infra-red (940nm)
940nm is a fully covert wavelength
of IR light. A 940nm illuminator
does not emit any red glow, making
it an option for applications where
the red glow from an 850nm light
is deemed unacceptable. However,
the use of 940nm will mean a
significant drop in image quality
and achievable distances when
compared to an equivalent level
of 850nm light. The performance
sensitivity of the camera will drop
by as much as 50% compared to
850nm.
"If possible, we would always
recommend the use of 850nm over
940nm. And when using 940nm,
always ensure a suitable camera
setup is being used," comments
Callum Ryder.
Hybrid
Another option which specifiers
should consider is the use of hybrid
illuminators. Hybrid illuminators
provide an all-in-one solution,
combining both IR (850nm) and
white-light into a single unit.
"This means one illumina-
tor can be used for both covert
surveillance, and as a white-light
deterrent. This is commonly used
for perimeter surveillance systems
where the end user would like to
ward off an intruder before they
commit the crime", explains Cal-
lum Ryder.
FAR-RED (730NM)
While unlikely to be relevant
for perimeter applications, a
wavelength of 730nm (known
as `Far-Red'), is becoming an
increasingly popular choice for
transport applications where
driver or passenger identification
inside the vehicle is required.
Modern windscreens are designed
to block out certain wavelengths
of light. This is mainly to protect
the occupants of the vehicle from
ultraviolet (UV) rays but is also an
attempt to keep the cabin cool by
blocking a proportion of infrared
(IR) light.730nm is commonly
used as less light is blocked by the
windscreen.
Seeing through
windscreen-test
In tests conducted by Raytec for
the purpose of seeing through
windscreens, it is clear that 730nm
light provides the clearest images
to identify the individual within
the vehicle, compared to the
other wavelengths discussed above.
However, a 730nm illuminator
does give off an obvious red glow,
which could become distracting
for drivers in certain cases.
" In an ideal world, the qual-
ity of images produced using
730nm could be achieved using
an illuminator which gives off
no visible light and, therefore,
offers no distraction to the driver.
However, in reality, the further
down the spectrum one moves, the
more light becomes visible to the
driver,"concludes Callum Ryder.
Raytec is a world leader in LED lighting for secu-
rity and safety applications. In this article, Callum
Ryder, Raytec marketing manager, offers his views
on lighting requirements for perimeter surveil-
lance systems.
Comparing light types for
perimeter surveillance systems
Callum Ryder, Raytec on:
In tests conducted by Raytec for the purpose of seeing through windscreens, it is clear
that 730nm light provides the clearest images to identify the individual within
the vehicle, compared to the other tested wavelengths.
Perimeter surveillance installation with
Vario2 white-light illuminators from
Raytec.
Special Feature: Perimeter Surveillance
Editor: Lennart Alexandrie