光学工程讲义17-Imaging Detectors

1、September 2004Prof. John WatsonSeptember 2004Prof. John WatsonMost detectors integrate, in space and time, the total amount of light falling on themIn many applications it is essential to preserve any spatial character that the light wave may haveSome detectors respond differentially to the light fa

2、lling on a particular point on the detector surface These are known as Typical imaging detectors: photographic emulsion vidicon type TV cameras photodiode arrays charge-coupled device (CCD) arraysSeptember 2004Prof. John Watson Spectral response range of optical wavelengths to which detector respond

3、s Bandwidth effective range of electrical frequency response Resolution ability of detector to resolve two side-by-side image points as separate Sensitivity ratio of electrical output to optical input Dynamic range ratio of highest to lowest signal which can be linearly recorded Signal-to-noise rati

4、o highest to lowest intensity just above noise Lag susceptibility to retention of some of the signal after readout Blooming susceptibility to leakage of the signal from intensely illuminated areas into neighbouring areasSeptember 2004Prof. John WatsonFuture of real time image detection lies with sol

5、id state imagersAdvantages high resolution high sensitivity small size low power consumption easy interfacing for computer control and data handlingSuch cameras are rapidly gaining favour for image processing, machine vision and industrial surveillance and control.Types of Solid State Imager Photodi

6、ode arrays Charge-coupled devices (CCD) Chargeinjection devices (CID) Hybrid detectors: charge-coupled photodiodes (CCPD) CMOS devicesSeptember 2004Prof. John WatsonMany semiconducting materials like silicon are photosensitive. CCDs are based on metal-oxide-semiconductor field effect transistor (MOS

7、FET) MOSFETs are light sensitive Can store accumulated charge in “potential wells” below the surfaceIncident light generates e-h pairs in the material. The electrons are trapped in the potential well The holes disappear into the substrate the number of electrons produced over a given period of timea

8、nd accumulated charge is proportional to incident light intensity.In an array of interconnected MOSFETs The pattern of charge accumulated across the device replicates the variation in light intensity of the original image.September 2004Prof. John WatsonApplied voltage pushes holes away from +ve elec

9、trode Depletion layer formed under oxideElectrons are attracted to surface Thin dense layer of ve charge forms at top of depletion layerCHARGE STORAGE n-channel CCD “surface” or “buried” channelAll practical devices are buried channel structure charge is stored below the surface for more efficient c

10、harge transferSeptember 2004Prof. John WatsonThe charge packet in the well is moved across device by successive applications of voltage pulse to electrodesInitially charge held under first electrode When voltage applied to second electrode the width of well increases in region under both electrodes.

11、 Charge is shared between two electrodes.Removing the voltage from the first electrode causes that well to collapse Pushes available charge into second well The well and its charge have moved through the semiconductorAccumulated charge can be stored for several hours efficiency of charge transfer pr

12、ocess approaches 99.999 %!September 2004Prof. John WatsonReadout of the signal is accomplished by transferring packets of charge to adjacent CCDs to be read out as a video image.In principle, each sensor is fed from a separate clock pulse but electrodes are grouped together in sets of three or four

13、phases.Each phase is connected to a separate clocked voltage. Here a three-phase clocking system allows the charge to be moved entirely across the device. Clock rates up to 10 MHz are possible for line transfer.September 2004Prof. John Watson Simple line imaging arrangement not suitable for area det

14、ectors time taken to transfer the charge across the device too long For area sensors individual pixels are arranged in a matrix of M x N elements The packets of charge are switched to a series of stores and readout sequentially while a second image is being recorded Area imagers should be able to tr

15、ansfer data at rates compatible with conventional TV systems. Two principal ways in which information is organized on a CCD chip: Frame transfer Interline transferSeptember 2004Prof. John WatsonArray is divided into vertical columns with channel stops in betweenElectrodes run across the array at rig

16、ht angles to the columnsArray is divided into two sections storage region which is shielded from light an optically sensitive regionEntire sensitive area is exposed to light at one time then the developed charge is transferred to the storage area for readout.Necessary for the fast readout rates need

17、ed in TV systems and because of the time taken for readout, not for data transfer.The data is read out sequentially while the next frame is being recorded.September 2004Prof. John WatsonReadout sections are interspersed with sensitive elements Output from each column is taken in parallel using a com

18、mon register.Although potentially faster than frame transfer, because the light sensitive regions are interleaved with non-sensitive regions half the image points are lost.September 2004Prof. John WatsonLarge industrial investment in CMOS (complementary MOS) technology for digital logic Now attention turning to exploitation of imaging proper