CCTV Basics
The first thing you should ask yourself before purchasing a
security surveillance system is: What do I need this system to do? Do
you need detection of an incident only, or do you need to
identify the object in question. Keep your answer in mind when
reading this document.
The second thing you should ask yourself before
purchasing a system is: What is my application? The most common
applications are: security applications, safety applications, and
management applications.
Scene and
Lighting
The scene refers to the objects or area to be observed as well as the
environment in which it will be observed. One important aspect that must
be considered is the environment. The environment contains multiple
colors, materials, reflective surfaces, and varying degrees of light
within the picture. To select the proper equipment you must determine
the amount of light present during peak times of operation. The amount
of light on the scene determines everything from picture clarity to
focus. What is the minimum light that will be available? Will it be more
cost efficient to go with a better night-view camera or adding more
artificial lighting to the scene?
Area of illumination in the camera field of view . A minimum
illumination of 70 percent of the camera field of view is recommended. A
camera is an averaging device. If too little of the field of view is
illuminated, the camera will average between the illuminated areas and
the non-illuminated areas, resulting in blooming and loss of picture
detail in the illuminated area.
Lighting position . The position of lighting in relation to
the camera field of view is also important. As much as possible, light
sources must be kept out of the camera's field of view. Lights that are
illuminating a camera scene should be mounted higher than the cameras.
When determining a location and field of view for a camera, extraneous
light sources, such as building-mounted lighting for pedestrians that
will be in the camera view, must be considered. Extraneous light sources
can cause blooming and streaking in a camera, rendering portions of the
field of view unusable. Distant light sources that are relatively dim
are usually not a problem.
Other lighting . Another type of lighting is
known as infrared (IR) or near infrared. The spectrum for this lighting
is just below red and is not visible to the human eye. Most
black-and-white cameras have sensitivity into the infrared. A
black-and-white camera can be used with this type of lighting to observe
areas at night without having lighting that is visible to humans. To
make use of IR lighting, the camera must not have an IR cut filter.
Cameras can be ordered without IR cut filters; be sure to specify no IR
cut filter when ordering.
The Cameras
A camera's performance largely depends on the amount of light present,
as well as the imager used. When the level of light changes
dramatically, usually a camera equipped with automatic iris control can
help ensure consistent image quality. Auto iris enables a camera to open
or close its lens accordingly to the varying levels of light, limiting
or increasing the amount of light passing onto the sensor. Cameras are
available in several imaging formats expressed as 1/2, 1/3, 1/4 inches.
These are the sizes of the imager used. Generally speaking, you should
match the camera's format with the lens format (ex. 1/3" sensor with
1/3" lens). It is crucial to understand a camera's format, resolution,
and corresponding lens focal length when determining what camera will
best suit your needs. Without proper equipment and studying all these
variables, you may be deceived on what really is causing poor imaging.
For example, what might look like a poor camera, may be a poor monitor.
You can't simply take a high resolution camera and expect its high
performance to be visible on a poor monitor or display. Each component
in the system will affect the overall performance. Your overall quality
is only as good as the weakest component in your system.
The Lens
The lens plays a large role in a system's design. The primary function
of the lens is to collect light from a scene, focus the image to produce
a sharp image on the camera's imager. Selection of a lens is critical.
The lens directly affects the size, shape, and sharpness of the image to
be displayed on the imager. Factors such as distance to the scene, focal
length, desired field of view, lighting, and format affect the size and
clarity of the image.
The field of view (FOV) is the actual picture size
(height/width) produced by a specific lens. If the view is not suitable,
consider a different lens to increase or decrease the field of view.
Camera lenses are divided into two major categories:
fixed and varifocal (manual zoom). A fixed lens obviously has a fixed
focal length, while a varifocal lens enables the user to change its
focal length to produce a zooming effect (narrowing the FOV). Focal
length is the distance from the optical center of the lens to the focal
point near the back of the lens. This focal length distance is displayed
on the lens (in millimeters). A lens with a focal length of 8mm on a
1/3" camera produces a field of view similar to the view produced by the
human eye. A wide-angle lens has a short focal length, while a telephoto
lens has a long focal length. In order to change the field of view, you
must change the lens.
The ability of a lens to gather light depends on the
relationship between the lens opening (aperture) and focal length. This
relationship is symbolized as the letter F, also know as F-stop. The
lower the F-stop number, the larger the aperture, thus the greater
ability to pass light through the lens to the camera's imaging device.
For example, a lens with an F-stop of F1.2 can gather much more light
than a lens with an F-stop of F4.0. A lens with a low F-stop number is
sometimes referred as a "fast" lens.
Depth of field is another consideration when determining
the proper lens. Depth of field is the area in focus ahead of and behind
the main object. When you focus on a particular object there is an
amount of area behind the object and in front of the object that will
still be in focus, although not as sharp. Depth of field increases or
decreases based on the length of the lens, the len's aperture, and
distance from the camera to the subject.
