Difference between CCD sensor and CMOS sensor

5) Quantum yield [%]
An image sensor transforms photons into electrons. The conversion rate, quantum yield (QE), depends on the wavelength. The higher the number of photons transformed into electrons, the more a sensor is photosensitive and the more the amount of information emitted in the image is high. The measured values ​​of a camera can differ from data from the manufacturer’s manufacturer P. ex. in the event of using a protective glass or a filter.

Operating mode and characteristics of CMOS sensors

Mode of operation, characteristics and comparison of the performance of cameras and sensors according to the EMVA 1288 standard

Operating principle

Image sensors transform photons into electric load by photoelectric effect. Unlike the CCD (Device-load load-free) sensors), the CMOS sensors (Complementary Metal-Oxide Semiconductor) transform the loads already on the pixel, in tension. This is amplified, quantified and transmitted in digital form.

Current CMOS sensors seduce with their high images pace and their excellent image quality. They allow efficient industrial cameras to carry out a precise analysis of the images. Due to technological progress, they replaced CCD sensors in most applications.

The following presentation gives an overview of the fundamental operating principle and the essential characteristics of CMOS sensors.

1) Full capacity [e – ] and capacity for saturation [e –
Imagine that a pixel is a “bucket” and that full capacity is the maximum number of electrons that can be accumulated in this “bucket”. The saturation capacity really used for the characterization of a camera is directly measured in the image of the camera. The value is generally lower than full capacity, in order to avoid non-linearities. High saturation capacity allows longer exposure times. A superxposed pixel is set at the maximum DN digital value and therefore does not contain any useful information.

2) Absolute sensitivity threshold [e –
The absolute sensitivity threshold (AST, Absolute Sensitivity Threshold) describes the minimum number of photons (minimum detectable radiation) for which the camera can differentiate useful information in the image of noise. This means that the lower the threshold, the more sensitive the camera. The absolute sensitivity threshold includes quantum efficiency, darkness noise as well as photonic noise and it must be taken into account when low light is used, rather than considering quantum yield.
The absolute sensitivity threshold corresponds to the value of the threshold for which SNR is worth 1 (signal equivalent to noise).

3) Timeless noise of darkness [e –
Each pixel produces a signal (dark) even if the sensor is not lit. Electrons are even generated without light in each pixel if the exposure time and the temperature increase. The variation in the darkness signal is described as the noise of darkness (measured in electrons). A low darkness of darkness is advantageous for most applications. The noise of darkness with photonic noise and quantification noise describe the noise of a camera.

4) Dynamics [DB]
Dynamics is the ratio between the maximum and minimal number of measurable electrons of saturation capacity. High dynamic cameras can simultaneously provide detailed information on dark and clear areas of the same image. This is why a high dynamic is especially important when the image has dark and clear areas or the lighting conditions change quickly.

5) Quantum yield [%]
An image sensor transforms photons into electrons. The conversion rate, quantum yield (QE), depends on the wavelength. The higher the number of photons transformed into electrons, the more a sensor is photosensitive and the more the amount of information emitted in the image is high. The measured values ​​of a camera can differ from data from the manufacturer’s manufacturer P. ex. in the event of using a protective glass or a filter.

5) Maximum signal-outlet (SNRMAX) [DB]
The signal-out (SNR) ratio is the ratio between the value of gray (corrected for dark noise) and the sound of the signal. It is often expressed in DB. The SNR depends mainly on the K coefficient and the noise of darkness and increases with the number of photons. The maximum SNR (SNRMAX) is reached when the pixel has accumulated the maximum number of electrons of the possible saturation capacity.

7) KEFFICT K (Dn/e – ))
A camera transforms the electrons (E -) of the image sensor into numerical value (DN). This conversion is indicated by the general amplification K of the system, expressed in numerical value (DN) by election (E -): K electrons are required to increase the gray value of a DN. The K coefficient depends on the thermal design and the electronics of the camera. A better K coefficient can improve linearity at the expense of saturation capacity.

Performance comparison

With the EMVA 1288 standard, the EMVA (European Machine Vision Association) defines uniform and objective measurement and characterization methods for image sensors and cameras in the industrial image processing, and thus encourages comparability between Cameras distributors.

Difference between CCD sensor and CMOS sensor

The image sensor is one of the major components influence Quality of a camera. It ensures the transformation of light signals into electrical signals. In video surveillance, we find Two technologies: the CCD sensor (Loaded CoupleD Device) and the CMOS sensor (Complementary Metal Oxide Semiconductor).

The CCD sensor

CCD technology has been specially developed, over 20 years ago for cinema, and therefore For the cameras industry.

He’s from better quality than a CMOS sensor especially in terms of its Light sensitivity which allows a better image rendering even in underexposure.

A non -standard manufacturing process and the difficulty in integration into cameras make technology CCD a more complex and therefore more expensive technology.

A CCD sensor consumes more energy and the resulting heat production promotes the appearance of parasitic signals (compensated by cooling systems). We also see a phenomenon called “Smear”, Vertical trail when filming an overly luminous object.

The CMOS sensor

Technology CMOS was created to integrate into computers, It is simpler and more recent.

She comes today maturity and the quality of the rendering is close to CCD technology.

Due to the simplicity of their technology and low energy consumption, The CMOS sensors are less expensive and allow you to have cameras at a lower cost. The current limit of the CMOS lies in their Low light sensitivity. Indeed, as soon as we film unlit scenes, it results in An image either very dark or full of “noise” (parasites). We also see, in some cases, Image distortions during fast movements.

Other factors play on quality

In conclusion, we can say that CMOS technology (more recent) matures But that, in the Particular domain of videosurveillance cameras, it does not (yet) equal CCD technology in terms of sensitivity and qualitative rendering of the image.

Note that the quality of video images is also linked to the quality of the objective and the associated technologies: Amplification control (AGC), white compensation software (AWB), automatic counter -day management (WDR).

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