Lighting Details
Focal Length/Field Of View
Tips/Guides
Need more
information on
video
surveillance?
Can't decide
what camera will
best fit your
application?
Among other
information,
this section
contains
excerpts from a
research report
from the
National
Criminal Justice
Reference
Service that
may assist you
in your
decisions.
A
more technical
discussion of
formats,
resolution, and
pixels.
A basic
familiarity with
camera
terminology is
probably
adequate for
most school
administrators
who plan to go
out on bid for a
CCTV system.
However, for the
benefit of those
who might be
responsible for
choosing or
upgrading camera
equipment, the
following
discussion
presents these
technical
specifications
in more depth.
Formats
. Camera
format relates
to the size of
the camera
imaging device.
Most solid-state
cameras used in
security
applications
today are
1/2-inch or
1/3-inch format.
There are some
2/3-inch cameras
still in use,
and some
1/4-inch format
cameras are
beginning to
appear on the
market. The
trend has been
to make camera
formats smaller
as picture
element
densities have
increased,
giving the
manufacturer
more imaging
devices per
production run,
reducing costs,
and allowing for
smaller cameras.
Resolution
. Resolution is
the ability to
resolve or see
small details in
an image.
Resolution for
CCTV cameras (as
well as for TV
monitors and
recorders) is
usually
specified in
terms of
horizontal lines
of resolution.
Horizontal lines
of resolution
relates to the
number of
independently
resolvable
elements (small
details) in
three-fourths of
the picture
width. CCTV
cameras range
from 200 to more
than 1,000 lines
of horizontal
resolution.
Higher
resolution
cameras
generally cost
more than lower
resolution
cameras. For a
typical color
security camera
system (system
includes camera,
cabling,
recorder, and TV
monitor) that
uses a standard
National
Television
Systems
Committee (NTSC)
color video
signal format,
300 to 400 lines
of horizontal
resolution are
common.
Black-and-white
systems for
tighter security
applications
typically range
from 500 to 700
lines of
resolution.
Cameras with
more than 800
lines of
resolution are
commonly used in
broadcast TV,
medical, or
industrial
applications.
Pixels
. Active
picture
elements,
sometimes
referred to as
pixels, is a
term used
specifically
with cameras and
is directly
related to
horizontal lines
of resolution.
Active picture
elements are the
actual number of
light-sensitive
elements that
are within the
camera imaging
device. Active
picture elements
are expressed
with a
horizontal
number (the
number of
elements
horizontally
across the
imager device)
and a vertical
number (the
number of
elements
vertically on
the imager). A
camera specified
with 768H by
494V picture
elements has 494
rows of picture
elements
vertically, with
each row having
768 elements
horizontally.
For
black-and-white
cameras,
horizontal lines
of resolution
relate to
picture elements
by a
three-fourths
factor (by
definition of
horizontal lines
of resolution)
so a
black-and-white
camera with 768
active picture
elements will
have 576
horizontal lines
of resolution.
This would hold
true for color
cameras as well,
except that the
NTSC format
limits signal
bandwidth which
reduces
resolution.
Lines of
resolution,
camera format,
and lens focal
length
(discussed
later) are the
camera-specific
part of what
determines if a
camera scene
will be useful
for a particular
application.
Other items to
consider include
lighting,
shadowing,
camera aiming,
and camera
sensitivity.
Before selecting
a camera and
lens combination
for an
application, one
must determine
what is desired
to be seen in
the image. Just
being able to
see a person in
a specific area,
such as a
parking lot,
will require one
set of minimum
criteria for
camera and lens
selection. Being
able to identify
a person by
facial features
(if the person
faces the
camera) will
require a
different set of
criteria. For
identification
purposes, a
person must be
much larger in a
scene than for
the purpose of
just determining
if a human is
present.
Because a
camera scene is
observed on the
TV monitor, the
entire CCTV
system
resolution must
be considered.
This includes
the camera and
lens
combination, the
camera signal
transmission
equipment (such
as coaxial cable
and amplifiers),
the TV monitor,
and the
recorder. All
components of
the system must
have adequate
resolution for
the application
desired.
For
observation of a
camera scene to
determine only
if a human is in
the scene (or to
be able to
distinguish
between a person
and an animal),
a minimum
criteria of 6
horizontal TV
lines across a
1-foot-wide
object within
the scene is
used. (In terms
of active
picture
elements, this
means that a
1-foot-wide
object would
cover 8
horizontal
active picture
elements for
each row of
picture elements
for the height
of the object on
the camera
imager.) For
identification
of a person by
facial features,
16 horizontal
lines (21
pixels) of
resolution
subtending a
1-foot-wide
object is
needed.
The lens
focal length
(discussed in
the next
section), camera
format, and how
far an object is
from the camera
will determine
how large an
object is within
the scene, as
well as how many
active picture
elements the
object covers on
the camera
imaging device.
Higher
resolution
cameras (for
example, 576
horizontal lines
or higher) can
be used to
distinguish
objects farther
away (smaller in
the scene) than
a lower
resolution
camera
(approximately
250 horizontal
lines) allows.
In other words,
an object can be
smaller in the
scene for higher
resolution
cameras and
still meet the
minimum
horizontal
resolution
criteria. The
significance of
this is that
fewer higher
resolution
cameras will be
needed than
low-resolution
cameras in some
interior and
many exterior
applications.
