CCD Camera Composition, Application and Adjustment

What is CCD?

As a kind of solid-state image sensor, CCD is fully called "Charge coupled device".

CCD develops based on MOS transistor. CCD composition is basically same with MOS (metal—oxide--semiconductor) capacitance structure. Please watch Picture 1. Oxidize the surfaces with P-type silicon underlay, create into 100nm~150nm Sio2, plate a piece of metal material (e.g. aluminum), add a bias voltage (gate voltage) between underlay and metal electrode, and then compose a MOS capacitor. So CCD is composed by rows of MOS capacitor array which closely stand on underlay.

Current CCD mostly adopts photodiode (LED) instead of traditional MOS capacitor, which enlarges a N+ area in order to form a P-N junction diode on P-type Si underlay, sets LED with reverse bias, creates directional charge region (called Exhaustion Region). In exhaustion region, optical electron-holes separate and optical electron is gathered in space charge region. For negative electron, space charge region is a region with low potential energy, so it is also called potential well. Projection light charge is stored in this well. The max capacity of well is called potential well capacity. Well capacity is proportional to grid voltage.

Compared to MOS capacitor, photodiode has advantages of high sensitivity, wide spectral response, good blue response and small dark-current. Putting multi MOS capacitor or photodiode into array, connecting related electrodes each other with biphase, triphase or four-phase mode, adding certain drive-pulse on each electrode group, thus photodiode or MOS capacitor possesses basic functions of CCD sensor.

CCD Camera Composition

B/W CCD camera is mainly composed by CCD sensor, scan drive & synchronous signal produce section, image processor & amplifier, camera lens and power supply.

Color camera has more components than B/W camera, which additionally include color filter array, IR cut-off filter and color signal processing circuit. As for IP camera composition, besides basic functions of the above cameras, it also should add network transmission function which means video compression process and video server.

Correct Application of CCD Camera and Installation & Adjustment of Lens

Install lens

As we know, camera is applied with adopting lens. But whether the lens is suitable relies on practical situation. For example, there are mono-focal lens or zoom lens, manual IRIS lens or auto IRIS lens, standard lens or wide angle lens, telephoto lens as well as some special lens. Moreover, we should care whether size assortment between lens and camera is suitable, whether is C-mount or CS-mount, whether adding 5mm ring. In practical installation, we should first get rid of protection shell of camera and lens, slowly screwing lens in port to set well. If auto IRIS lens, we should also connect lens control line to AI port of camera.

Connect power line with signal line

Lens installed well, is it available connecting power line with signal line. For 12V power camera, it should plug 12V output adaptor (220V AC to 12V DC) in socket of camera. Camera sockets most are pin-type with built-in, so it finishes to simply connect 12V plug with socket. But for some cameras with two wire connecting terminals, it is necessary to unscrew bolt of connecting terminal, open and plug 12V output-line in terminal, finally screw bolt. The matter should be noticed is that care about electrode to avoid destroying camera.

Camera video is putout from BNC adaptor of back panel. So we can adopt 75 Ω BNC cables, one end connecting to video out socket of camera and other end connecting to vide in socket of monitor. Finally, power-on and open monitor, camera video is displayed well.

Some cameras have built-in sound pickups which offer RCA socket. We can adopt screened cables with RCA adaptor, one end connecting to RCA socket, the other end connecting to audio in socket of monitor. Thus we easily hear sound from monitoring spot.

Adjustment for aperture and focus

After installing lens and connecting power with signal line, we can not get ideal image, and now it is necessary to adjust aperture and focus. In such condition, we should first close shutter and backlight compensation, then make lens toward to objects, adjust aperture and focusing ring in order to ensure image in optimum. If in complex illumination environment, we had better adopt auto IRIS lens and keep shutter "OFF" status. If adopting manual IRIS, we should keep shutter "ON" status; moreover, when spot is in max .illumination, try to enlarge aperture and still adjust image to optimum (avoid image overload), thus lens adjustment is over. In result, when illumination low, shutter will auto adjust to slow speed mode and adopt larger aperture, by this way image still can reach ideal effect.

Something should be noticed is that in adjustment process, if adjusting aperture to minimum rather than maximum, shutter will auto adjust to low speed status; although image normally displays on monitor, image maybe a mass of darkness when illumination becomes lower, because the aperture is related smaller and shutter has already been in minimum speed (1/50s).

After adjustment of aperture and focus finished, the last step is installing housing and bracket.

We should care about that for a motorized zoom lens (two-zoom or three-zoom), there must have the space (focal length), where focusing operation needs, between front-lens spherical surface and housing glass surface. Otherwise, lens will extend an excessive section which hits glass window of housing, destroy motor focused. On the other hand, the space (focal length) should not also be overlong in order to avoid limiting light ray and damaging image quality. The best solution is manual adjustment for viewing position, apart and slightly screw to the bottom until can not go on. Thus we can exactly and correctly define the space (focal length).

Adjustment for back focal length of lens

Back focal length means that when standard lens has been installed (standard C/CS mount lens), objects can be just imaged on CCD sensor. When leaving factory, back focal length commonly has been adjusted well, so matching with mono-focal lens need not adjust back focal length again.

But in some applications, even though focusing ring has been adjusted to the utmost, image still is unclear. In such condition, we should step by step check whether C/CS-mount is correct, whether should adjust back focal length.

a). Adjustment for back focal length of mono-focal lens

First step: install lens on camera correctly
Second step: open shutter, adjust aperture to maximum, screw focusing ring to the farmost point.
Third step: aim to an object beyond 20m
Fourth step: release inner-hexagon-screw at camera front-end, adjust focusing ring (if no focusing ring, directly adjust lens in order to adjust back-focusing ring accordingly) to reach optimum image.
Fifth step: tighten inner-hexagon-screw again

b). Adjustment for back focal length of zoom lens

In most applications for cameras adopting motorized zoom lens, we should adjust back-focal-length. Adjustments as follow:

First step: install lens on camera correctly
Second step: open shutter, adjust aperture to maximum, screw focusing ring to the farmost point.
Third step: aim to an object beyond 20m
Fourth step: adjust zoom by lens controller, put object to the farmost point or called "telescope status"
Fifth step: adjust zoom by lens controller, put object to the nearest point
Sixth step p: release inner-hexagon-screw at camera front-end, adjust focusing ring (if no focusing ring, directly adjust lens in order to adjust back-focusing ring accordingly) to reach optimum image.
Seventh step: put lens as telescope status and affirm whether object (captured according to fourth step) is clear or not, which determines whether or not need repeat fourth, fifth and sixth steps. The whole process is for the purpose of keeping clear image constantly.
Eighth step: tighten inner-hexagon-screw again

Adjustment for Color CCD Camera

Color camera adjustments mainly include white & black balance as well as related adjustment and color temperature adjustment. Introductions as follow:

White Balance adjustment

Based on color temperature knowledge, we know that same light source with different color temperature, or different light source with same color temperature, human eyes will incorrectly sense color of white object. E.g. synchronously opening fluorescent lamp and incandescent light, human eyes will sense fluorescent lighting wall with a bit blue, but incandescent lighting wall with a bit red. Therefore, in different color temperature environments, it is necessary for color camera to be adjusted white balance in order to perfectly show color of white wall. White balance directly affects image color effect. If white balance setting is incorrect, image will appear color cast; especially dye objects (e.g. white wall) which should have been colorless.

a). What is Black Balance?

Generally color camera should putout "multicolor" video signal. The "multicolor" involves brightness, hue and saturation. Brightness refers to light intensity; hue refers to color types; saturation refers to color purity or color depth. It is common to call hue and saturation as color chroma. So chrome defines color types and depth. Chrome and brightness together compose "multicolor"

Natural colors are composed by red, green and blue with different proportion, on the other hand, most colors can be decomposed into red, green and blue. In ideal situation, it is equivalent with each for three equivalent light paths (red, green and blue) to get equivalent light quantity, so their (red’s, green’s and blue’s) signal voltages are also equivalent each other, accordingly, standard monitor can perfectly display image of white objects. Thus most people call it White Balance which means camera shoots white objects with equivalent voltages among red, green and blue, or abbreviates as "UR=UG=UB".

Only when kinescope receives equivalent tricolor voltage, it can display standard white color. So when shooting same white object, we should adjust and keep tricolor (red, green and blue) signal voltage equivalence as color temperature changes. Generally speaking, when color temperature changes, people adjust tricolor (red, green and blue) gain to keep UR=UG=UB relationship. The adjustment is called "white balance adjustment".

Commonly, white balance adjustment is operated in certain range. If color temperature changes in a wide range, only adjusting tricolor signal gain can not reach ideal effect. By such conditions, it is necessary to change CCD spectral characteristic by color temperature adjustor. Broadcasting color camera often matches three classes temperature adjustor, but CCTV camera need not match color temperature adjustor.

Something need to point out is that balance performance shows as color restore, so monitor should avoid color cast, otherwise it can not determine whether or not have color cast for camera. By such condition, we can adopt video signal test disc to check whether screen white has color cast by sending white balance signal to monitor.

b). Auto white balance (AWB) adjustment

White balance is finished in process amplifier of camera, so we can adjust red and blue signal gain to keep UR=UG=UB proportion in order to complete white balance adjustment. Adjustment requirements as follow: in any light source condition, before shooting, we should firstly shoot a standard white object (usually is standard white balance test card), adjust all paths gain to reach white balance status. And until now we can shoot other objects. Therefore, white balance should be readjusted as light source changes.

In real CCTV applications, camera usually works for longtime or for 24h, light source temperature and circuit parameter (especially in outdoor applications) will change greatly, so frequent manual adjustments for white balance is unavailable. In current cases, almost all cameras adopt auto white balance (AWB) adjustment skill.AWB mainly includes two methods as follow:

a. Transmit three colors (red, green and blue) signal into white balance circuit, after white balance window (white balance gate) pulse sampling, commutate them, get average direct current level, and then compare red average level and blue average level with green average level. Put green path level as reference, amplify and send back error voltage to gain control section of amplifier in order to change and ensure output signal level of red path and blue path the same with green path. By this way, white balance realizes.

White balance adjustment finishes, AWB circuit should break-off. Then the error voltage should keep constant, and manage gain control section in order to keep white balance.

Error voltage maintaining includes analog mode and digital mode.

Error voltage analog mode means that save error voltage to capacitor (Picture 1: Capacitor C). The capacitor insulating resistant should as high as possible. Within 4h memory level change must be less than 1%, so capacitor C requires link to equivalent resistance with more than 105MΩ. This requires algorithm amplifier should be high resistant, and differential-mode input resistance should be more than 106MΩ.

Error voltage digital mode means that convert error voltage into digital signals and save them, convert them into analog voltage then transmit to gain control level. Unless counter power break-off, digital signal keep constant and error voltage also keep constant.

b. Transmit color error signal (R-Y and B-Y) into AWB circuit, after being integrated by R and C network, compare with zero level. When shoot white objects and reach white balance, the both of color error signals should be zero. So with comparing color error signal with zero level, it is easy to reach white balance adjustment. E.g. when R-Y>0, comparer putout voltage to red path gain control stage in order to reduce gain until to R-Y=0; when R-Y<0, red path gain should amplify. Blue path gain is controlled by B-Y signal. The specific control method is the same with red path.

Black balance adjustment

Black balance is also a parameter of color camera, most broadcasting camera have black balance adjustment circuit. But in actual CCTV applications, mainly adopted are middle or low class CCD cameras, so the black balance adjustment circuit is seldom found. The reason is that the effect for human eye to sense black balance is far less than white balance.

a). What is white balance?

Black balance means that when shooting black objects or closing lens hat, camera output tricolor level should also be equivalent; only by this way monitor can display standard black object, which is called Black Balance. If black balance adjusts improper, there will be black with red or black with green on monitor.

According to above analysis, correct display black & white image is the precondition and basis for correct display multicolor image. So it is important for correct display multicolor to adjust black & white balance of CCD camera.

b). Auto black balance (ABB) adjustment

The dealing method for ABB is similar to AWB, but there are more reasons than white level to cause change imbalance for black level. Considering from circuit structure, black level adjustment and white level adjustment affects each other. Thus the front of the gain control stage of process amplifier should add dark-current circuit, backlight compensation circuit and stray light regulator circuit to ensure black level change along with neither gain nor stray light. Doing by such a way avoids affecting black balance when adjusting white balance.

There are also two methods for adjusting black balance

a. Black level of process amplifier for red, green and blue path is determined by a public reference black level. Once closing lens cap, each path signal automatically keeps accordance between black level and blanking level before mixing them. Thus the black level of last three output video signal must also be accordant.

b. Put green level as reference, compare green level respectively with red level and blue level, the created error voltage respectively controls red black level and blue black level in order to be equivalent with green black level. The theory and introductions about circuit is similar with AWB circuit and no longer introduce.

c). Other parameters adjustment

Except for white and black balance, color camera also has other parameters which include phase adjustment (horizontal and vertical) to ensure image color and stability, external sync input port to ensure multiple cameras keeping "sync adjustment lockup" status.

a. Horizontal Phase (HP) adjustment

Horizontal Phase (HP) has strict lockup relationship with color subcarrier (PAL color subcarrier frequency: fs=4.43361875MHZ). Once phase lockup fails, image will turn to no color or color distortion, e.g. red flag turns to green flag, human face color turns to green.

The lockup relationship of HP and subcarrier phase is finished in sync signal circuit of camera. Usually, circuit designment defines wide tracking range of phase-locking for the lockup relationship, and need not adjust commonly. Just with this reason, most middle, low class CCD cameras do not provide external HP adjustment function.

It is common for high & middle class color cameras to require more functions and more limitations for application environment. So we often add a HP adjustment knob, which can be adjusted to solve color distortion problem, on camera back plate or side plate.

b. Vertical Phase (VP) adjustment

Vertical Phase (VP), or called Field Phase, has also strict lockup relationship with horizontal phase. VP mainly ensures correct television scanning mechanism. When image appears vertical mobility, we can adjust VP knob to remove image mobility.

c. External sync input

For most high & middle CCD color cameras back plate, there are BNC video-out port (written as "VIDEO") and another port (written as "SYNC") with the same shape as first port. Around this port there is a pulling ON/OFF. This BNC port is the just "SYNC port". When applying camera by standalone, SYNC port is unnecessary to connect for getting stable image, but only connecting BNC port with vide devices (monitor or video recorder) via video cable. However, sometimes multiple cameras together use terminal equipment (e.g. video quads processor); we should adopt SYNC port to avoid multi-image with non-sync.

Known from the above analysis, in order to ensure correct television scanning relationship, inner camera must have sync signal circuit. Such sync signal circuit provides needed sync signals for all circuits of camera. This working mode of camera is called "internal sync". When multiple cameras together use terminal equipment, because of internal sync of camera is separate each other, terminal equipment can not determine to track which camera, and images appear non-sync. By the way pointing out that current video quads processors widely adopt phase-locking circuit with wonderful sync tracking ability, so there have no the above problems of non-sync in most actual applications.

Adjust SYNC ON/OFF to set camera as external mode, connect all camera external sync input ports to external signal generator, and camera gets the needed sync signals from external input port, not from internal sync circuits (cameras close all their internal sync circuits). These sync signals come from only one sync signal generator, so image sync relation relationship is well ensured.

d). External sync connection structure

For some special applications (e.g. multi camera survey project), camera must adopt such an external sync mode.