Image sensors and optical devices

Why do images taken in dark places look grainy (with conspicuous noise)?

  1. ABecause the image sensor's pixel count drops in the dark
  2. BBecause light gets longer wavelengths in the dark
  3. CBecause few photons reach the pixels, so noise is relatively large compared with the signal
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Answer: C. Because few photons reach the pixels, so noise is relatively large compared with the signal

In dark scenes, few photons reach each pixel and the signal is small. Photon-count fluctuation (shot noise) and readout-circuit noise remain, so the signal-to-noise ratio drops and the image looks grainy. Raising the sensitivity (amplification) amplifies the noise as well.

  • A. The pixel count does not change with brightness.
  • B. Darkness does not change the wavelength of light.
  • C. Correct. The signal shrinks and the SNR drops.
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More Image sensors and optical devices questions

  1. Which correctly describes the "conversion gain" of a CMOS image sensor?
  2. What is the main advantage of the scheme used in many CMOS image sensors in which each pixel column has its own AD converter for parallel conversion?
  3. Which correctly describes "pixel binning" in image sensors?
  4. What is the phenomenon called in which charge overflows from a pixel hit by very bright light and spreads white into surrounding pixels?
  5. Which component is sometimes placed in front of the sensor to suppress moiré, false patterns caused by interference between a fine striped subject and the pixel array?