Specifications

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KNOW-HOW: Camera Technology

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S39

Operating voltage

Today, 12V DC, 24AC, and 230V AC are customary as operating

voltage. The majority of units are supplied with 12V DC. Regulated

PSUs are required to operate these cameras.

If ordinary plug-in PSUs are used, it has to be ensured that these are

only charged with half of the max. current as otherwise they may have

a short life.

There are also camera systems which already feature a built-in PSU

and thus can directly be connected to the 230V mains. This facilitates

the insertion into an outdoor housing, because in most cases a 230V

heating is provided and in this case no additional PSU has to be ap-

plied for the camera.

Cameras with a 24V AC voltage supply are quite often used for in-

dustrial applications. Current cameras often feature the facility to be

operated with both 12V DC and 24V AC.

Minimum illumination

With this indication stated in LUX, the sensitivity of a camera can be

rated. This value is the luminous intensity which has to reach the image

sensor to produce a picture which can still be recognised. The value for

a colour camera compared to a B/W camera is generally higher by

the factor 10. Unfortunately, the phrase "All cats are grey at night"

also applies to video surveillance technology.

The light quantity actually required at the object to be observed must

be many times higher, as only a certain part of the light striking the ob-

ject is reflected. It is also the lens used which further reduces the quan-

tity of light reaching the image sensor.

Regarded as a mere value, this indication allows to compare the dif-

ferent sensitivities of camera systems with each other.

B/W cameras feature a certain sensitivity in the invisible infrared

range as opposed to colour cameras. At standard colour cameras, this

area is blocked by a suppression filter, so that the colour reproduction

is not affected.

Cameras with an Exview CCD image sensor are very popular. These

image sensors offer a higher sensitivity and an extended sensitivity

range into the near infrared range.

Day/night switching

These colour cameras switch to B/W operation if the luminous inten-

sity falls below a minimum value. To simplify matters, the brightness

information of the three colour pixels are now added. Thus, the sensi-

tivity is increased. Additionally, some cameras do not feature a sup-

pression filter so that the camera is then suitable for IR in B/W

oper-ation. At top-class products (e.g. TVCCD-800COL), this filter is

mechanically moved in front of the image sensor in order to provide a

perfect picture in both types of operation.

Automatic gain control (AGC)

Within the video output signal, the range of 0.25 to 1Vpp is available

for the actual video signal. If the luminous intensity is reduced, the max.

output level is no longer reached. Only a part of this range is used for

the video signal. The picture gets darker and becomes poorer in con-

trast.

The automatic gain control (AGC) tries to keep a constant output

level taking the illumination into account. The AGC is able to com-

pensate brightness fluctuations as they occur in normal rooms. The

brightness of the picture is largely kept at a constant level. The high-

dynamic AGC of the camera top models is capable of regulating even

great brightness fluctuations.

With poor luminous intensity, the amplification factor becomes very

high, and picture noise may occur.

Electronic shutter (ES)

The picture information is transmitted for signal processing 50 times

per second. The sensitive image sensor is exposed for a max. of

20 milliseconds

for each picture. With a very high luminous intensity,

so much light has already fallen on the chip that overexposure will

occur.

The trick is to read the chip beforehand and to transmit the video in-

formation earlier. This is done by the electronic shutter. Even short shut-

ter times of 1/50,000 seconds are possible. With basic cameras the

shutter oeprates automatically. With high-quality cameras the function

can be switched off or the shutter times can even manually be preset.

This is e.g. important for process monitoring.

The automatic ES is used in module and compact cameras to match

these cameras to lighting conditions of great fluctuations. Only thus it

is possible to use this type of camera for outside applications.

Backlight compensation (BLC)

If a bright point of light falls into the camera aperture, usually the

darker areas next to it are no longer visible.

For outside monitoring e.g. a dark door and a white wall next to it,

either the door is visible and the wall is too bright, or the wall is vis-

ible and the door is too dark. The BLC reduces the differences in con-

trast of these areas so that ideally both areas are perfectly visible.

Output for iris control

While AGC and ES are mere electronic procedures to match a cam-

era to different lighting conditions, the iris control is the ideal method

to reduce the light quantity already at the lens.

For this purpose, special lenses are required which have corre-

sponding control connections.

In this case, the advantage is the increase of the depth of focus. Only

by reducing the aperture, it is possible to increase the area where the

picture is clear.

Depending on the camera model, connections for lenses with auto-

matic iris or DC-controlled iris or both are available.

TVCCD-820DNR see page S 35