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CAMERA AND SENSOR TECHNOLOGY

The advent of the charge-coupled device (CCD) sensor was hailed as being the end
of maintenance of cameras.  For a short while, this is actually appeared to be true, but
only with the simple CCD chips of approximately 10 years ago.

CCD CHIPS

What is a CCD Chip?

The CCD chip is actually an array of thousands of photosensitive diodes arranged in
a matrix. Associated with every photodiode is a metal oxide semiconductor (MOS)
capacitor which is used to retain a charge, built up during the field period of the video
signal. An analogue shift register also forms part of the CCD chip and the shift register
forms the other part of what is often known as a bucket brigade device. The buckets are
the MOS capacitors and the brigade refers to the analogue shift register. The charge
is passed from bucket to bucket through the device.

The advertised advantages of CCDs are that they:

(a) are solid state
(b) have no electron beam
(c) are almost immune to magnetic or electric fields
(d) are not susceptible to vibration
(e) have no burn in
(f) need no adjustments
(g) need no changing of tubes
(h) have an unlimited lifetime.

However, the reality of the situation was less than perfect. The CCD chips gave a lot
lower resolution (approximately 240 lines on the first devices) and also had a lack of
ability to deal with the extremes of light level. Thus, all lenses had to have auto iris
control, whereas the Vidicon tubed cameras did not require auto iris controlled lenses,
but used manual iris lenses.

The early CCD chips were all frame transfer devices, which were also less sensitive
than the tubed cameras chosen for outdoor use. Interline transfer devices then
improved the sensitivity, but brought a crop of problems, such as transfer smear (see
below). The unlimited lifetime of the CCD chips is true to some extent, but the life is
limited by the very rapid advances in chip design — making obsolescence the limit of
lifetime of the chips.

Types of CCD Chips

At present, there are both monochrome and color CCD chips, in both single and three
chip combinations. It is possible to achieve more than double the early resolution, ie
up to 570 lines in monochrome and up to a supposed 480 lines in color (only
achievable at the chip, which very few manufacturers mention). The sensitivity of the
CCD chips has improved markedly since their introduction.

During the 1990s, there have been two basic types of CCD chip, ie the frame transfer
and the interline types.

The frame transfer chip has two distinct sections, ie a photosensitive area and a
storage section. The focused image is projected onto the photosensitive area, thus
causing the charges to be defined in the chip. The storage area is masked from the
light using an opaque aluminum optical mask. Every one-fiftieth of a second, a field of
information is transferred from the photosensitive area to the storage area, where it is
then processed during the next one-fiftieth of a second.

The interline transfer chips have shift registers between the columns of pixels. This
enables better processing to be carried out, although light leaks onto the shift registers.
Although screened, light does penetrate and generates electrons deep within the chip
where there are large overloads. These deep electrons add to the charges being
transferred in the registers, an effect which is typified by a vertical red or white line
down the screen, known as transfer smear.
 

For many years the manufacture of CCD chips was dominated by one manufacturer,
but there is now competition. The types that are now practical devices are hyper hole
accumulated diode (hyper HAD), super dynamic, frame interline transfer and the
exwave HAD.

Hyper hole accumulated diode

The hyper HAD was released in 1990 and introduced a number of improvements in the
design of interline transfer devices. These improvements serve to reduce the dark
noise which could be purely from:

dark current
reset noise
optical shot noise
fixed pattern noise
residual point noise.

Due to the improved sensitivity, which is largely a result of the on chip lens, it has
rendered the use of electronically-controlled shutter speeds, to be a far more reliable
method of controlling the exposure. There is also an improvement in dealing with
transfer smear, because of careful profiling in the design of the chip.

Super dynamic

This is a recent refinement in the design and manufacture of interline devices where
the number of vertical shift registers has been doubled, thus enabling charge generated
by the photodiodes to be dealt with in two different ways.

The overall scene is processed and the exposure of the photodiode is determined by
the light falling on the photodiode. For dark parts of the picture, the photodiodes are
exposed for almost one-fiftieth of a second. The parts of the picture with highlights
(where light levels are high) will be exposed for a far shorter period of time, perhaps as
short as 1/2000th of a second. The short charge and the long charge are processed
together which results in the output picture. This technique gives dynamic light
handling, better than 20-40 times that of the conventional hyper HAD.

Frame interline transfer

This type of device combines the benefits of both the frame transfer and the interline
transfer devices.

The chip design features a masked storage area exactly along the lines of the frame
transfer chip. Every one-fiftieth of a second, a field of the generated charges is
transferred straight into this storage area. From here, the information is then the
processed line by line, in the manner of the interline transfer device.

In dealing with the charges in this manner, almost all the problems of generation of
deep electrons within the semiconductor are avoided and this results in an
improvement of the transfer smear by reducing the levels by a factor of approximately
60. This technique was designed a few years ago and was used exclusively in
broadcast cameras. It was considered too expensive to use in CCTV, but it is now
possible to obtain the appropriate chips.

Exwave HAD

Exwave, in this case, means extended wavelength, because the chip has an improved
spectral response. It gives approximately double the sensitivity at 800nm and
approximately four times the sensitivity at 900nm (both being in the infra-red
wavelengths). This diode has greater sensitivity in part because of the on chip lens
design and the improved masking of the vertical shift registers, giving less transfer
smear.

CAMERA FACILITIES

What comes out of the back of a camera? Answer — a mains cable! This is a genuine
answer given by a delegate on a training course, but it does raise the question of what
is the right answer? The real answer could have been any one of the following:

(a) composite video
(b) horizontal synchronization pulses
(c) vertical synchronization pulses
(d) luminance
(e) chrominance
(f) RS 485 data
(g) data, as defined by the Data Protection Registrar.

Is it all necessary? With tube cameras, the controls that were available were all
necessary to enable the maintenance of picture quality over the two year period or so
between tube replacements.  Such controls were electrical focus, target voltage, the
current and pedestal.  For a while CCD cameras were very simple, ie they were MOS
cameras that had no controls that were user configurable. However, many cameras
now contain such features as line phase, genlock, chrominance, peak white blanking,
auto white balance, auto white tracing, text and clocks, auto iris settings, direct drive
iris outputs, peak/average adjustment, electronic iris, manual shutter selection, high
light suppression, Y & C video outputs, picture memory, movement detection, backlight
compensation, kangaroo lens drive, output level adjustment, user memory settings,
motorised back focus, field integration, DSP, security code, remote controllable
functions, on-screen menus, and restore factory default. A brief overview of these
functions is given below.

1. Line phase enables a.c. powered cameras to have the line phase adjusted to enable
multiple cameras to be switched through a simple analogue video switch without the
resultant picture bounce of unsynchronized cameras.
2. Genlock is an input to enable all cameras to be synchronized to a single
synchronization source.
3. Chrominance usually enables the increase and decrease of both red and blue,
manually (there is not usually adjustment of green, because adjustment of red and blue
will give the impression of increasing and the decreasing the level of green, ie red and
blue and green = white).
4. Peak white blanking describes the situation where highlights of approximately 150%
or more will be changed from a peak white to either black or a shade of grey to enable
darker parts of the picture to be seen.
5. Auto white balance means that the camera decides on what is white within a picture
and compensates by adjusting the colors to achieve this white level.
6. Manual white balance is the facility whereby at the commissioning stage, the camera
is provided with a “white target” filling the screen this is set as white, normally by
pressing a set button.
7. Auto white tracing is useful where the color temperature of the lighting may change
with time or as external lighting changes.
8. Text and clocks can be programmed either at the camera only or from the control
room; it should be noted that the remote programming of text can be considered a
potential problem when evidential material is being collected.
9. Auto (video) iris settings involve an output known as a digital auto iris; this output
used to be a video signal, but is now an integrated signal which can cause problems
when used in combination with some lenses — making the setting of iris level a very
twitchy thing to do.
10. Direct drive iris removes the control electronics from within the lens and relocates
them to within the camera, thus allowing the purchase of cheaper, lighter lenses.
11. Peak/average is an adjustment which gives the commissioning engineer the
opportunity to change detection between peak within any part of the picture or an
average over the whole picture.
12. Electronic iris enables the use of a manual iris lens, so the camera now controls the
exposure of the CCD chip by changing the shutter speed continuously to suit the
ambient lighting conditions.
13. Shutter is the equivalent of the shutter adjustment on a photographic camera, which
helps to stop blur due to motion of the object. The higher the shutter speed the greater
the amount of light that is required. It can also be used to overcome the problems with
having a 60Hz mains frequency, but using systems at 50Hz, such as in Saudi Arabia,
produces the effect of a visible pulsing of the lighting.
14. Y-C video outputs enable the highest resolution color signal to be transmitted and
recorded (recording as SVHS). However, it requires two transmission paths and two
matrix inputs per camera.
15. Picture memory can enable one field of video to be stored in the camera in response
to a stimulus.
16. Movement detection uses relatively simple movement detection criteria, such as
video level detection, to determine if there is movement within the field of view; it will
then signal (via telemetry) that there is movement within the picture, but should never
be considered as an alarm device.
17. Backlight compensation is used to deal with the problem of backlighting that causes
people and objects to be silhouetted.
18. A kangaroo lens drive is an output to drive the latest two position iris lenses being
used with electronic iris and field integration.
19. Output level adjustment is used to boost the video signal prior to transmission on
longer lengths of coaxial cable.
20. Memory settings — in some cases, up to four different memories can be available
to memories the multitude of settings required for the operation of the camera.
21. Motorized back focus can simplify the pre-assembly or the installation of cameras
and can aid where infra-red lighting is used, causing a change in focus.
22. Field integration is very useful when lighting levels are very low, but should be used
with caution and (realistically) only with fixed cameras, because of the amount of blur of
even slow motions that can be seen when using long integration periods.
23. Digital signal processing (DSP) can improve the resultant image by defining outlines
by introducing a one pixel wide black line around any objects within the scene to improve
visibility where the levels of contrast are low.
24.Security mode ensures that if a camera has been stolen, it cannot be used unless a
code number is entered to prove ownership.
25. On-screen menus have become more essential with the increase in the number of
features of modern cameras.
26. Restore factory defaults can be an essential item where an engineer has finger
trouble (they all do) and has forgotten where he was when he started; this feature should
only be used as last resort and it is essential for all the settings to have been
documented carefully for each camera within the system.

FUTURE DEVELOPMENTS

Suitability

The above list includes 26 different features, but users should be careful that their
manufacturers give independent views. Who have the marketing strategists targeted?
Have they persuaded the consultants that there are particular sets of features which are
essential for every job they may be specifying? Alternatively, has the control room
manager been given demonstrations of the camera in the controlled environment of a
dark room or perhaps even a demonstration vehicle in his or her own locality, but  with
the highly trained manufacturer’s operator driving the demonstration camera?  At this
stage of the proceedings, it is very unlikely that the control room operators have been
interviewed, let alone had some training and commented which features may or may not
be necessary.

It is inevitable that as camera features become more varied, so the operation of the
system will become more complex. There will be more adjustments to make to the
system on a time-profile basis. This is one particular aspect that can be simplified by the
availability of user configurable memories within the camera to enable different settings
to be chosen, depending upon the ambient conditions. It must be remembered that this
will also involve the operator having a good memory or clear concise documentation
within a busy control room environment.

Controlling the Features

Another word of caution is that it must be clearly remembered that to have all these
advanced features available for your system, that you must have the camera
manufacturer’s telemetry control system. This choice of the control system may lead to
restrictions of hardware that may not (at first) be evident. In other words if we fitted one
camera with remote control of back focus into another manufacturer’s control system,
you cannot, without modification to the system, control the back focus.  This design and
modification can be carried out, but puts a premium on to the price of the control system.

Digital Cameras

There is much talk of digital cameras, and some of the commonly available cameras
actually have the word mentioned on the body of the camera.  However, there are only
a very limited numbers of true digital cameras available and they are attached to a
computer, used in videoconferencing applications. The current restriction is the data
transfer rate; for high-quality video in a serial data stream, transfer rates of greater than
250 Mega bits per second are needed. Clearly, the current transfer rates over ISDN are
totally inadequate, but if there is a parallel data bus system of, say, 32 bits’ width, then
the data rate reduces to approximately eight Mega bits per second.

There is currently much work being done with asynchronous transfer mode (ATM)
networks, Ethernet and the like, for both serial and parallel data transmission. However,
for the immediate future, it is unlikely that digital cameras will be appearing in town
centers until the fiber optic equipment manufacturers can provide an economic bearer
solution.

Similarly, there is no established standard yet for the compression of video for both
transmission and storage. Will it be MPEG, JPEG, wavelet or even fractal compression?
Any digital cameras that purport to use IEE1394 bus are cameras for machine vision and
will feed into a PC and not a video matrix. The transmission distance on copper cable
is rather limited as well — only 4.5m!

Some people have been slightly misled to believe that digital signal processing (DSP)
cameras are digital cameras, but all DSP cameras still have an analogue video output.
The DSP aspect relates to signal processing within the camera, that is used to enhance
various aspects of the video output. DSP can be used to give sophisticated backlight
compensation and the aspects of motion detection that some manufacturers include.
When an area being observed is low contrast between the objects and the background,
DSP cameras can enhance the outlines by drawing a one pixel wide black line around
all of the objects, both foreground and background, thus giving well-defined edges. It
should be noted that some cameras can introduce so much processing that post-
production video enhancement, such as used by courts for evidential purposes, cannot
actually improve the pictures further.

Camera Size

There has been a great tendency to reduce the size of the CCD chip and the cameras,
driven largely by the camcorder market, which has led to the development of the covert
spy-type cameras. With large-scale integration, the CCD chip and lens can be separated
from the rest of the electronics to produce cameras that are fitted into pens and
spectacles, which then combine with miniature video transmitters to enable the remote
recording of those pictures. It should also be remembered that under the Data Protection
Act 1998, all personal data must be fairly obtained.

Specialist Cameras

Another area of development relates to thermal imaging cameras which, only a few
years ago, required the detector devices to be cooled by the liquid nitrogen. It is possible
now to purchase a hand-held thermal imaging camera, with a fixed lens, and no external
cooler, for under 14,000. Similarly, there are now image intensifiers that can be fitted
between the lens and the CCD chip to give performance in lighting conditions down to
starlight (0.0001 lux) and they cost less than 10,000.

There are also cameras that can give up to 700 lines of resolution, but these are  limited
to broadcast applications with cameras being priced from 20,000 upwards (prices and
sizes are dropping fast). Caution is needed when looking for these high resolutions,
because they are obtained using three separate CCD chips and lenses with prisms fitted
to split the light. These cameras give four output signals, as red, green and blue and
synchronization (sometimes three, ie RGB with synch on green) and, therefore, require
four separate cables and four channels per camera in the matrix switch. A standard
industrial timelapse VCR will not be able to record this form of signal.

Users should consider carefully the ramifications of installing increasingly complex
cameras and control systems. The man machine interface or graphical user interface
or keyboard needs to be as simple as possible, to enable operation and management
of the system to be as effective as possible.

 

 

 

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