Image sensors and optical devices

Check the basics of optical devices — CMOS image sensors, backside illumination (BSI) and what determines LED color — with multiple-choice questions.

—Your accuracy
0 / 50Questions answered
Practice this topicAll questions in this topic, in random order Weak questionsRetry only the questions you last got wrong

Image sensors and optical devices: key points

An image sensor converts light into charge with a photodiode in each pixel and reads it out as an electrical signal. CMOS image sensors are now mainstream and widely used in smartphones and automotive cameras.

Backside illumination, which lets light enter from the side opposite the wiring layers, and stacked sensors, which put pixels and circuits on separate chips, have improved performance. Optical devices that convert electricity into light, such as LEDs and lasers, are also part of this topic.

All Image sensors and optical devices questions (50)

  1. In each pixel of a CMOS image sensor, which element converts light into charge?
  2. What is the main advantage of a backside-illuminated (BSI) CMOS image sensor?
  3. What mainly determines the emission color (wavelength) of an LED?
  4. Compared with CCD image sensors, which is a general advantage of CMOS image sensors?
  5. In a Bayer color filter array, which color appears most often in each 2×2 block of pixels?
  6. Why do fast-moving subjects appear distorted with the rolling shutter method of CMOS image sensors?
  7. What is the role of the microlens placed over each pixel of an image sensor?
  8. Which correctly describes dark current in an image sensor?
  9. Which phenomenon do solar cells use to generate electricity?
  10. Which correctly describes the difference between a semiconductor laser and an LED?
  11. Which correctly describes a stacked CMOS image sensor?
  12. What problem tends to occur when the pixel size of an image sensor is reduced?
  13. What is the measurement principle of a ToF (time-of-flight) sensor?
  14. In the buried (pinned) photodiode of a CMOS image sensor, what is the main purpose of the heavily doped p+ layer at the surface?
  15. What is the principle of the microbolometer used in uncooled infrared image sensors?
  16. Which is a correct technique for HDR (high dynamic range) in image sensors, for scenes containing both bright and dark areas?
  17. What is the definition of the quantum efficiency of an image sensor?
  18. Why is it difficult to increase the near-infrared sensitivity of silicon image sensors?
  19. What is the purpose of correlated double sampling (CDS) used in CMOS image sensor readout?
  20. Which is a characteristic of an OLED (organic EL) display?
  21. In the typical four-transistor pixel of a CMOS image sensor, which transistor moves the charge from the photodiode to the floating diffusion?
  22. What is the noise called that appears the same way at the same positions every time because of pixel-to-pixel variation in characteristics?
  23. How does the size of shot noise, caused by statistical fluctuation in the number of photons, relate to the number of signal electrons N?
  24. What is the main purpose of deep trench isolation (DTI), in which deep trenches filled with insulators are formed between fine pixels?
  25. Which material is suitable for photodiodes that receive light in the 1.55 µm band used in optical fiber communication?
  26. Which correctly describes how a CCD image sensor reads out its signal?
  27. Which is closest to the pixel size of high-resolution image sensors for smartphones?
  28. Which autofocus method uses some pixels of the image sensor to receive light from the left and right sides of the lens separately?
  29. What is the maximum number of electrons that one pixel of an image sensor can store called?
  30. Which semiconductor laser has a built-in diffraction grating so that it oscillates stably at a single wavelength for optical communication?
  31. Which correctly describes a "12-megapixel" image sensor?
  32. In image sensors that capture color images, why must color filters be placed over the pixels?
  33. Which image sensor is mainly used in current smartphone cameras?
  34. When the light falling on a reverse-biased photodiode becomes stronger, how does the current (photocurrent) change?
  35. Why do images taken in dark places look grainy (with conspicuous noise)?
  36. Which correctly describes the "conversion gain" of a CMOS image sensor?
  37. 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?
  38. Which correctly describes "pixel binning" in image sensors?
  39. What is the phenomenon called in which charge overflows from a pixel hit by very bright light and spreads white into surrounding pixels?
  40. 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?
  41. Which infrared wavelength band is mainly detected in thermography, which images the temperature of people and objects?
  42. What does the "fill factor" of an image sensor pixel mean?
  43. From the output of a sensor with color filters such as a Bayer array, what is the process called that estimates all of the red, green and blue values for each pixel from surrounding pixels?
  44. Which correctly describes the "three-chip" imaging method used in broadcast cameras and similar?
  45. Which light detector is operated with a reverse bias above its breakdown voltage so that even a single photon triggers a pulse through avalanche multiplication?
  46. Which image sensor detects only changes in brightness at each pixel and asynchronously outputs information only from pixels that changed?
  47. In a CMOS image sensor, what problem occurs when charge transfer from the photodiode to the floating diffusion is incomplete?
  48. Which evaluation method obtains an image sensor's conversion gain and similar parameters from the relationship between the mean and variance of the signal in images taken at different brightness levels?
  49. Which photosensitive material is being researched and commercialized to detect short-wave infrared (SWIR, about 1 to 2 µm), which silicon cannot detect, with low-cost image sensors?
  50. In indirect-conversion flat-panel detectors used for medical X-ray imaging, how are X-rays detected?

Other topics in Devices