11 Key Parameters You Should Know About Image Sensors

An image sensor is the key component of any kind of camera and vision system. Our primary consideration when selecting a PTZ camera is its image sensor. An image sensor is a device that detects and converts light signals into electrical signals to make an image. This article discusses 11 Key Parameters when choosing an image sensor.

What is an Image Sensor

 Generally speaking, image sensors are typically used in standalone or embedded digital cameras to capture light through the lens, convert it into an electronic signal, and then send that signal to a processor, which converts the electronic signal into digital images.

sensor

1. Resolution

An image sensor’s photosensitive area consists of a one- or two-dimensional matrix arranged by many pixels, in which a pixel (pixel) is a single photosensitive unit. The resolution of the sensor is determined by the number of pixels in the horizontal and vertical direction of the matrix, that is, the number of pixels of the image sensor = resolution.

2. Pixel Size and Target size

  • Pixel Size:The larger the pixel size, the more photons can be collected. The more valid signals the image sensor can receive under the same light and exposure time. The size of pixels and the number of pixels (resolution) together determine the target size of the camera.
  • Target size: The size of the image sensor’s light sensing part, usually expressed in inches and refers to the diagonal length of the sensor. For example, a 1-inch sensor has a diagonal length of 16mm (instead of 25.4mm). With larger target size, more light will be transmitted and more light information will be recorded. The larger the target size of the sensor, the more photons are captured, the better the sensitivity, and the lower the signal-to-noise ratio. Today, CMOS sensors are commonly available in sizes of 1 inch, 2/3”, 1/2”, 1/3”, and 1/4”.

3. Frame Rate

The frequency of images recorded or captured per second. The frame rate represents the number of times per second that the sensor is able to update while processing the image. Higher frame rates provide a smoother, more lifelike visual experience.

frame rate

4. Sensitivity (ISO)

Sensitivity is an important indicator of the photoelectric performance of sensors. High sensitivity means that a clear image can be obtained when the light is dim or the exposure time is short.

5. Dynamic range

The dynamic range is determined by the signal processing capability and noise of the CMOS sensor, which reflects its working range. Referring to the dynamic range of the CCD, its value is the ratio of the signal peak voltage at the output to the root mean square noise voltage, usually represented by DB.

6. Quantum efficiency

Quantum efficiency is the absorption and conversion efficiency of a device in different bands of light. It is an important parameter describing the photoelectric conversion capability of an optoelectronic device, which is the ratio of the average number of photoelectrons produced per unit time to the number of incident photons at a specific wavelength. As the surface state (rough or smooth) of the photoelectric surface changes, so does the amount of photoelectron escaping. One photon is converted into an electron for every two that hit the photosensitive area if the quantum efficiency is 50%.

7. Spectral response

The signal voltage Vs and signal current Is of the sensor are functions of the incident light wavelength λ. The spectral response characteristic refers to the relationship between the response capability of the sensor and the wavelength, which determines the spectral range of the CMOS sensor.

8. Noise

  1. Noise is the number one issue that affects sensor performance. This noise includes FPN (Fixed pattern noise), dark current noise, thermal noise, etc. Fixed pattern noise occurs because the output signals generated by two different pixels illuminated by the same beam are not exactly the same. For fixed pattern noise, double sampling or associated double sampling techniques can be used. Double sampling involves reading out the charge integration signal generated by the illumination, temporarily storing it, resetting the pixel unit, and then reading its output signal.

9. Dark current

  1. Due to the influence of device impurities, temperature, external interference, etc., even if no light irradiation to the sensor, the pixel unit also will generate charge that generate dark current. It is difficult to distinguish between dark current and light-generated charge. Dark current is also not the same in the pixel array and causes fixed graphics noise. For pixel unit with integration, the fixed pattern noise caused by the dark current is proportional to the integration time. The generation of dark current is also a random process, which is a source of shot noise. Therefore, the magnitude of the dark current generated by the thermal noise element is equal to the square root of the number of electrons in the pixel. Long time integration units cause this type of noise to play an important role in affecting the quality of the image signal.

10. Signal-to-noise ratio

This is the ratio of signal voltage to noise voltage. The unit of the signal-to-noise ratio is expressed in dB. The higher the signal-to-noise ratio, the better the noise control. This parameter is related to the number of noise in the image, the higher the signal-to-noise ratio, the clarity the picture, and the less noise in the night vision picture. The signal-to-noise ratio given in the manual is generally a parameter at a gain of zero.

11. Shutter mode

 

  • Electronic shutter: As the ISO value of a sensor is determined by the accumulation of signal charges, the longer the photosensitive time is, the longer the charge accumulation is, and the larger the output signal current. The slower the electronic shutter, the longer the light sensitivity, and is ideal for shooting in low-light conditions.
  • Global shutter: Achieved by exposing the entire scene at the same time. Sensor All pixel dots simultaneously collect light and expose it at the same time. That is, at the beginning of the exposure, the sensor starts to collect light; At the end of the exposure, the light collection circuit is switched off. As a result, a photograph is taken of the sensor value.
  • Rolling shutter: It is achieved by the way of progressive exposure through the Sensor. At the beginning of the exposure, the sensor scans and exposes line by line until all pixels are exposed.

Conclusion

This blog explains what an image sensor is and outlines 11 parameters that can be used to select the right image sensor for you.

With great advancement of technology, China’s PTZ camera is also developing rapidly, PTZ cameras for live streaming use world-class imaging sensors, including but not limited to Sony and Panasonic. If there is a need to source PTZ cameras, feel free to contact Telycam.

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