The CCD image sensor imaging device is analyzed in an all-round way
Charge-coupled device (charge coupled device, CCD) It is a commonly used image sensor of the camera at present. With CCD technical constant development and application, people have proposed more high requirement to characteristic and digitization of CCD. As to the development and use of CCD device, it is researcher’s eternal subjects to make the characteristic of CCD device further strengthened and improved, reflect in several following respects specifically: Make the speed of the work of the device faster; The dynamic range is wider; Integrate more picture element on the single device; Low grade of the noise. Subject for research these will make by someone similar research make studying nonsensical new. People need more quick device, wider dynamic range, more picture element and lower noise forever.
Concept and classification of 1 CCD imaging device
The charge-coupled device is a structural new device of a kind of metal oxide semiconductor, its basic structure is a mos capacitor of a kind of solid matter, can be stored and looked like the optical signal electric charge that the quick unit stimulated out in CCD by the incident light, and can be driven by clock pulse of appropriate phase sequence and shift the beam transmission in the form of charge packet of electric charge that is stored, it is self-scanning to realize, finish the conversion from optical signal to electric signal. Usually this kind of electric signal accords with the canonial video signal of TV, can recover into a visible light image of the object on the video screen, can store until tape built-in signal, or input computer, carry on, image enhancement, discern, store, punish. So CCD one kind ideal to make a video recording device in device.
The ordinary scientific CCD device makes detectivity in the dim light and is limited, is not fit for the low-light level imaging. With the development of photoelectric imaging technology, the following dim light CCD imaging devices appear: Enhanced CCD (ICCD) ,Electron impact CCD (EBCCD) And electronic multiplication CCD (EMCCD) .
Noise analysis of 2 CCD imaging device
Charge-coupled device (CCD) Have already generally applied to scientific imaging. It has the following several respect advantages in imaging:
(1)Have than can use the intersection of probe and higher, more widely quantum efficiency while being other;
(2)Only by the intersection of picture element and high-resolution very much that magnitude determine;
(3)In fact there are no cross talk and resolution defect like aureola.
But no matter under what kind of situation it is, it is accompanying that the signal always has a noise when appears, it is no exception to certainly use CCD imaging to couple the device. The noise is a important factor to determine image quality.
The outcoming signal of CCD image sensor is a dispersed analog signal of spatial sampling, among them is mixed with various noises and interfere with, and the noise will influence CCD imaging device to survey a little dyssophotic ability.
To photoelectric device, it could detect the radiation power small enough, it is the essential problem. Can survey radiation power, classify all the intersection of photo-electric detection and important parameter of device as not being minimum generally. Define the radiation power that Pmin incidents for the probe when the signal voltage of the output is equal to the effective value of noise voltage of the output. So must incident the radiation power is:
In the type: P radiation power in order to incident; Un is the mean square value of the noise voltage; Us is the signal voltage of the output; Pmin can survey the power for being minimum. Pmin the smaller, the stronger the detectivity of the device is, it is the stronger to that is to survey a little dyssophotic ability. By formula (1) Knowing, under the situation of certain signal voltage of the output, un the smaller the noise is, it is Pmin the smaller. The stronger the device surveys a little dyssophotic ability. So, the purpose that is dealt with to CCD signal is on the premise of not losing the picture detail, as much as possible to denoise and interference, in order to improve the signal to noise ratio, obtain the high-quality picture. For this reason, must understand the kind, characteristic of CCD noise to some extent, carry on the corresponding one to make an uproar and is dealt with to various noises. So the study on CCD imaging device noise part, help to raise the definition of CCD imaging device, also can improve and survey a little dyssophotic ability.
2.1 Shift the noise
When the charge packet transfers to another potential pit from a potential pit, for various reasons, know in the past and get some electric charges in a charge packet, will also leave some electric charges to the latter charge packet at the same time. The quantity of these electric charges is stochastic, so will present fluctuations, form the noise. Consider including getting the electric charge and losing 2 pieces of course of electric charge in each transformation, so the fluctuations noise should be fetched for the sum of the two. If there are complete charge transfer models, then the electric charge of gain and loss is equal, so single course that the noise is is double. Act as Nε <<1 , , N :
In the type: Ns counts for the signal carrier in each charge packet; Ns0 counts for the base carrier in each charge packet.
2.2 Reset the noise
Reset the noise and get involved in the voltage that the electric capacity brings because of the switch, the noise appears after the switch is cut to OFF, also a noise that must take place in the sampling circuit. Even CCD image sensor, before the signal charge is measured, must reset FD mains voltage,etc., resume FD mains voltage of the basic reference after resetting, will add the noise kTC.
After the diode outcoming signal, in order to receive the next charge packet continuously, need to reset its voltage. The reset circuit formed by MOST, there must be noises while working to feed to the output circuit. This kind of noise is restored to the throne the noise and can be dispelled through the relevant pair of taking a sample laws promptly. Can regard resetting the course as passing the resistance R photoelectric process to the electric capacity C.
QC(t) Noises produced are:
2.3 Shot noise
Even luminous intensity will certainly, because of the particle characteristic with photon of light, the light storing and incidenting the photod in time once, it can not be the same that its photon each time is counted, such a change characteristic initiates the photic shot noise. Mere shot noise the same as shot noise of dark current, photon of photod count the square root of Ns, regard as the mere shot noise Nn incident. For example, suppose light it forces one photoelectric dipoles to be intraductal to take place under the 10 000 terms of signal charge, 100 pieces of mere shot noise take place, the signal to noise ratio is 40 dB.
Generally speaking, the shot noise formula of the device is as follows:
According to formula (4) Can obtain, if you want to improve the shot noise, make it become main noise source, the most direct method is to improve the electronic gain to noise temperature ratio G.
2.4 Dark current noise
As to CCD image sensor, it is because of the dark current of the photod to cause dark fish eye, white damaging or reason of the particle, this too determines the most important factor of the image quality. The cause of the fixed pattern noise is that every picture element hits the dark current heterogeneously, even there is no incident light will happen equally, it is proportional to storage time of the signal voltage of the noise, and have the properties closely related to temperature, such as formula (5) Shown:
By formula (5) Can find out, the magnitude of the dark current noise is in close relation with temperature. In addition, the time accident that it still stores in the potential pit with some small bags has something to do, the longer the memory time is, the greater the dark current noise is. [1][2]
Tags: the camera, The capacitor, the coupler, The semiconductor, The sensor
