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ELECTRONICS' 2005

21 ­ 23 September, Sozopol, BULGARIA

CONTROL CIRCUIT FOR AUTO-IRIS LENSES USED IN DIGITAL VIDEO CAMERAS

Peter Milkov Gugutkov

Department of Electronics, Technical University , Sofia, Bulgaria, e-mail: [email protected]

Todor Stoyanov Djamiykov

Department of Electronics, Technical University , Sofia, Bulgaria, e-mail: [email protected]

Nikita Stoyanov Dimitrov

Space Research Institute, Bulgarian Academy of Sciences, Sofia, e-mail: [email protected] This paper describes the structure and the theory of operation of auto-iris control circuit which can be used in almost all nowadays lenses with DC iris control. The method and circuit is simple and very reliable, designed to take as small place as possible, which is very important in applications where space limit is significant. This circuit is a part of digital camera system designed to work in industrial environment in heavy conditions.

Keywords: DC auto iris, lens, iris control, image processing, digital camera 1. INTRODUCTION Today we can see cameras everywhere ­ at the street, in home, in supermarket. Camera development and manufacturing industry grows everyday and we use different cameras in more and more applications and control systems. There are a lot of different cameras and accessories and when we started our project it was very difficult to choose the base platform from which we should start the development. The main problem was that many companies offered different components and accessories but they were either very expensive for our system requirements or without enough good support like technical documentation and terms of delivery. One of the difficulties was how to choose the lens type and model. Our camera should work in heavy conditions ­ with variable focal distance and in very large range of illumination so we had to use varifocal lenses ( a lens which has a variable focal length that can be set manually. It is ideal for applications, where the required view falls between the focal lengths of two monofocal lenses. The disadvantage is that any adjustment in the focal length will require the lens to be re-focused.) with auto iris (an electronic circuit fitted to a lens that enables the iris to automatically adjust to control the amount of light falling upon the image device ). There are two main types of auto iris - Video and DC Iris. A video iris lens requires a video signal input from the camera and converts it to a DC input through its built in amplifier circuit. A DC iris lens, often referred to as direct drive, does not have this circuitry and requires simply a DC input from the camera. In the result of some specific requirements of our camera we had to use DC auto iris type so we had to make the

ELECTRONICS' 2005

21 ­ 23 September, Sozopol, BULGARIA

auto iris circuitry ourselves. This circuit had to be extremely reliable according to requirements to whole system, to work in industrial standards requirements and to give enough flexibility to be used with different types of lenses with DC auto iris control and the main ­ it had to be designed in such a way to permit adjustment of iris levels according the specific environment conditions ­ open air, room, dust, light and etc. 2. PROBLEM STATEMENT After we analyzed the market and last technology innovations and after we choose base components like image sensor, processor unit, lenses, power supply we separate the problem in two points. 2.1. HOW TO COVER FULL DYNAMIC RANGE Table 1 give us an idea of dynamic range of illumination in outdoor environment.

Illumination (LUX) 100000 10000 1000 100 10 1

Condition Sunlight Full Daylight Overcast Day Very Dark Day Twilight Deep Twilight

Daylight Range

Full Moon Quarter Moon Starlight Overcast Night

.1 .01 .001 .0001

Low Light Level Range

Table 1. As we see if we want to have good quality images all day and night and independently from weather conditions camera must have dynamic range around 108 . Direct sunlight produces the highest-contrast scene, allowing maximum identification of objects. On a cloudy or overcast day, less light is received by the objects in the scene, resulting in less contrast. So we decided that to produce an optimum camera picture under the wide variation in light level (such as occurs when the sun is in clouds), an autoiris camera system is mandatory because there is no image sensor, which can cover such a dynamic range and to give us high quality video images ­ what was our main purpose.

ELECTRONICS' 2005

21 ­ 23 September, Sozopol, BULGARIA

2.2. IRIS CONTROL CIRCUIT Our camera structure is shown at Fig. 1. Lens with DC 1 autoiris Image sensor

2

S(t)

3

ADC Video Processor

D(13..0)

4

CPU block

D(y..x) SDC(t)

5

Sir(t) Iris control Fig. 1

6

DAC

S(t) D(0..15) D(0..x) Sir(t) SDC(t)

- raw analog signal from Image sensor - digital video data bus after AD conversion - part of digital video bus, which we use for back loop to control autoiris - analog video signal used like input for iris control circuit - output control signal for DC iris drive

The scopes of the main problem are blocks 1, 5 and 6. Because of fully digital design of the camera there is no standard analog video signal and this was one of the main reasons in which we choose DC auto iris control lens. To use a DC autoiris lens in a video camera provided with a function to obtain a wide dynamic range we have to detect changing in the brightness for each frame from the video sequence and give this signal to lens circuit. We do it in following way: · We take 6 or 7 bits (D(y..x)) from 14 bits digitized signal which we have after video processor block and convert it to "pseudo video" analog signal ­ Sir(t). Which exactly bits have to be taken depends on the desired autoiris circuit speed. For fast changes we must take LSB but for slow speeds we have to take MSB. · Because of DC autoiris circuits implemented in lenses usually are galvanometer with driving and dumping coil SDC(t) signal consist of two signals ­ driving and dumping. Driving signal is used to move the iris diaphragm and dumping signal is loopback used to calm the speed of iris diaphragm. Now we shall look in details at block 5 ­ Iris Control. It converts "pseudo" video signal in levels suitable for galvanic autoiris type. Detailed structure of this block is given in Fig. 2. Sir(t) is very detailed signal it changes every pixel so we put integrator which cuts high frequencies and make signal smoother. This integrator determines the speed of iris diaphragm and in such way we can avoid unsteady brightness effects in video

ELECTRONICS' 2005

21 ­ 23 September, Sozopol, BULGARIA

image. The integration time must be over minimum one frame and sometimes it is good to be even over 20 ­ 30 frames. Sir(t) Integrator S1(t) Peak Detector S2(t) Sum Integrator speed adjustment White balance adjustment Fig. 2. After integrator we have filtered video signal S1(t) and we bring it into peak detector. This block detects peak values in video signal and depending on their value and the value of coefficient from white balance adjustment (WBA) forms signal S2(t). White balance is needed for fine tuning of video image brightness depends on environment in which the camera shall be installed. The sum of S2(t) and SDMP(t) give us driving signal for autoiris system. 3. RESULTS We made some tests by taking video images of a PCB and lighting it with powerful lamp. Results can be seen at Fig.3 ­ without autoiris ( with fixed iris ) and Fig.4 ­ with autoiris circuit. Images are 4 and taken in interval 100ms between 2 sequential. + + SDR(t) SDMP(t) Driving coil Dumping coil

Fig.3.

ELECTRONICS' 2005

21 ­ 23 September, Sozopol, BULGARIA

Fig.4. 4. CONCLUSIONS Obviously as is seen from results our efforts were rewarded. This autoiris control circuit was tested with few lenses and gave very good results with all of the lenses. We can say that we have developed universal, small-sized and very reliable circuit. 5. REFERENCES

[1] www.analog.com [2] www.ti.com [3] www.tokina.co.jp

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