The digital signal is compressed and stored in the camera's internal flash memory or built-in hard disk card, so you can easily transfer the data to the computer, and use the processing means of the computer to modify the image as needed. CCD is composed of many photosensitive units, usually in units of megapixels. When the surface of the CCD is exposed to light, each photosensitive unit reflects the charge on the component, and the signals generated by all the photosensitive units are added together to form a complete picture.
Its structure is shown in Figure 1. Taking P-type silicon as an example, a SiO2 layer is formed on the surface by oxidation on a P-type silicon substrate, and then a layer of metal is deposited on the SiO2 as a gate. Most carriers in P-type silicon are positively charged empty holes, minority carriers are negatively charged electrons.
When a positive voltage is applied to the metal electrode, its electric field can repel or attract these carriers through the SiO2 insulating layer. Therefore, the positively charged holes are repelled away from the electrode, leaving the negatively charged minority carriers immobile near the SiO2 layer to form a negative charge layer depletion layer. This phenomenon forms a trap for electrons.
Once the electrons enter the trap, they cannot come back, so it is also called electron potential well. When the device is exposed to light light can be injected through the SiO2 layer between the gaps of the electrodes, or through the thin P-type silicon of the substrate , the energy of the photon is absorbed by the semiconductor, and electron-holes are generated. The electrons are attracted to be stored in the potential well.
These electrons can be conducted. The stronger the light, the more electrons collected in the potential well. This will turn the strength of light into the number of charges. The conversion of light and electricity, and the electrons collected in the potential well are stored, even if the light is stopped for a certain period, it will not be lost, which realizes the memory of the light.
CCD structure and working principle diagram. In short, the above structure is essentially a tiny MOS capacitor, which is used to form a pixel, which can be "photosensitive" and leave a "latent image". The photosensitive function is to accumulate charge by electrons generated by light intensity, and the latent image is formed by the unequal charge of each pixel, which left in each capacitor. If you can manage to transfer the charge in each capacitor to another place in turn, then form lines and frames, the image transfer can be achieved.
It can detect incident light with very low light intensity. By the way, its signal will not be covered, making the application of CCD less constrained by the weather. Wide dynamic range : CCD can simultaneously detect and distinguish strong light and weak light to improve the using range of the system environment.
Therefore, signal contrast caused by a large difference in brightness will not occur. Good linear characteristic curve Linearity : The intensity of the incident light source and the size of the output signal have a good proportional relationship.
The object information will not be lost, and the signal compensation processing cost will be reduced. High Quantum Efficiency : The very weak incident light irradiation can be recorded. If it is combined with an image intensifier tube and light projector, in the night, distant scenes can still be detected. A 35mm CCD that is equivalent to the size of a traditional film has begun to be used in digital cameras, becoming a key component to replace professional optical cameras.
Broad Spectral Response : It can detect light in a wide wavelength range, increase the flexibility of the system, and expand the system application area. Low Image Distortion : Using the CCD sensor, the image processing will not be distorted, and the original object information will be faithfully reflected. Small size and light weight : CCD has the characteristics of small size and light weight, so it can be easily installed on artificial satellites and various navigation systems.
Good charge transfer efficiency : This efficiency factor affects the signal-to-noise ratio and resolution. If the charge transfer efficiency is not good, the image will become blurred;. The manufacturing technology of CMOS is no different from that of general computer chips.
It is mainly a semiconductor made of silicon and germanium so that it has a negatively charged N pole and a positively charged P pole semiconductor coexisting on the CMOS. The currents generated by the two complementary effects can be recorded and converted into images by the processing chip.
Later, it was discovered that CMOS can also be used as an image sensor in digital photography after processing. CMOS is a complementary metal-oxide-semiconductor, which is mainly a semiconductor made of two elements of silicon and germanium and realizes basic functions through negatively and positively charged transistors on CMOS. The current generated by these two complementary effects can be recorded and interpreted as an image by the processing chip.
CMOS image sensors are usually composed of image sensor cell array, row driver, column driver, timing control logic, AD converter, data bus output interface, control interface, etc.
These parts are usually integrated on the same silicon chip. The working process can be divided into reset, photoelectric conversion, integration, and readout. Composition of CMOS sensor. The difference is that the information transmission method of the two sensors after photoelectric conversion is different. The MOS transistor and photodiode constitute a structural section equivalent to one pixel. MOS tube pixel structure. When the integration period ends, the scan pulse is applied to the gate of the MOS transistor to turn it on.
The photodiode resets the reference potential and the video current flow on the load. MOS transistor source PN junction functions as photoelectric conversion and carrier storage. When a pulse signal is applied to the gate, the video signal is read out.
Here is the beginning of the CMOS image sensor sensing light. The CMOS image sensor element array structure is composed of a horizontal shift register, a vertical shift register, and a CMOS image element sensitive element array. CMOS image sensitive element array structure. As mentioned above, each MOS transistor functions as a switch under the pulse drive of the horizontal and vertical scanning circuits. The horizontal shift register sequentially turns on the MOS transistors that perform the horizontal scanning function from left to right, that is, the function of addressing columns, and the vertical shift register sequentially address each row of the array.
Each pixel is composed of a photodiode and a MOS transistor that acts as a vertical switch. The horizontal switch is sequentially turned on under the pulse generated by the horizontal shift register. And the vertical switch is turned on under the pulse generated by the vertical shift register. So we can apply the reference voltage bias to the photodiode of the pixel in sequence. CMOS image sensor array working diagram. The illuminated diode generates carriers to discharge the junction capacitance, which is the accumulation process of the signal during the integration.
The above-mentioned process of turning on the bias voltage is also a signal reading process. The size of the video signal formed on the load is proportional to the intensity of the light on the pixel. According to the functional block diagram of the CMOS image sensor, we can find that the workflow of the CMOS image sensor is mainly divided into the following three steps.
A control circuit causes each capacitor to transfer its contents to its neighbor, and the last capacitor in the array dumps its charge into a charge amplifier. The bucket-brigade style of data transfer is characteristic of CCD sensors.
In contrast, a complementary metal oxide semiconductor CMOS image sensor has a photodiode and a CMOS transistor switch for each pixel, allowing the pixel signals to be amplified individually. By operating the matrix of switches, the pixel signals can be accessed directly and sequentially, and at a much higher speed than a CCD sensor.
Having an amplifier for each pixel also gives another advantage: it reduces the noise that occurs when reading the electrical signals converted from captured light. CMOS image sensors cost less to produce than CCD image sensors, because existing semiconductor manufacturing equipment can be repurposed for their production. Unlike CCD sensors that use high-voltage analog circuits, CMOS sensors employ a smaller digital circuitry that uses less power, and are in principle free from smear vertical white streak in the image taken under bright light and blooming corruption of images such as white spots.
Here is an example of an old video camera tube. A CMOS pixel consists of a photodiode, an amplifier, reset gate, transfer gate and floating diffusion. These elements however may not always be within each pixel as they can also be shared between pixels.
The diagram below shows a simplified layout of a CMOS mono and color pixel. Typically a larger pixel size is better for increased light sensitivity because there is more area of the photodiode to receive light.
If the sensor format stays the same but the resolution increases the pixel size must decrease. While this might decrease sensor sensitivity, improvements in pixel structure, noise reduction technology, and image processing have helped mitigate this.
The 2 charts below are the mono and color versions of the same sensor model. The left shows the spectral response of a mono sensor and the right of a color sensor. The majority of machine vision color cameras have IR cut filters installed to block near-IR wavelengths.
This removes IR noise and color cross-over from the image, best matching how the human eye interprets color. However, in a number of applications it can be beneficial to image without the IR cut filter.
Whether or not an IR cut filter is installed a color sensor will never be as sensitive as the mono sensor. The higher the quantum efficiency the better the sensor is at sensing light. The above charts are one of many performance results based on the measurement standards of EMVA The EMVA standard dictates how to test and display performance results so that users can better compare and contrast models across vendors. Visit the EMVA site for more information.
An important function of the sensor is its shutter type. The two main electronic shutter types are global shutter and rolling shutter. These shutter types are different in their operation and final imaging results, especially when the camera or target is in motion. The diagram to the left shows the exposure timing of a global shutter sensor. All pixels begin and end exposure at the same time but readout still happens line by line. This timing produces non-distorted images without wobble or skewing.
Global shutter sensors are essential for imaging high speed moving objects. The diagram to the left shows the exposure timing of a rolling shutter sensor. Exposure timing is different line by line with reset and readout happening at shifted times.
This row by row exposure produces image distortion if either the target or camera are in motion. Rolling shutter sensors offer excellent sensitivity for imaging static or slow moving objects. Understanding the terms and technology in digital sensors will allow you to better pinpoint the appropriate camera for your application.
For example, certain sensor specifications, such as pixel size and sensor format, will play an important role in choosing the correct lens. If you are ready to discuss your camera requirements, please contact our knowledgeable Lucid sales staff. Buy Online!
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