Part III: Sensors

Published

September 22, 2025

Work in Progress

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Last updated: September 22, 2025

This part of the book explains how the analog light image formed by the optics is converted into a digital signal. We begin with the fundamental physics of light detection—the photoelectric effect—and explore how the quantum nature of light dictates the fundamental limits of sensor performance. From there, we delve into the engineering of a modern CMOS pixel, defining the key parameters that characterize its performance, such as quantum efficiency, read noise, and dynamic range. The final chapters describe the complete sensor system, including color filter arrays and control electronics, and survey the landscape of modern sensor innovations.

Sensor topics

Photons to Electrons

This chapter covers the initial and most critical step in digital imaging: the conversion of light particles (photons) into electrical charge (electrons).

  • Explains the photoelectric effect and the quantum nature of light.
  • Introduces the concept of shot noise, arising from the Poisson distribution of photon arrivals, which sets a fundamental limit on signal quality.
  • Describes the process of converting photons into photoelectrons within a silicon photodiode.

Pixel Design

Here, we examine the architecture of a single pixel, the building block of the image sensor.

  • Details the architecture of a modern CMOS pixel (e.g., the 4T pixel).
  • Explains the function of key components: photodiode, transfer gate, floating diffusion, and source follower amplifier.
  • Discusses the readout process and the origin of different noise sources within the pixel.

Sensor Parameters

This chapter defines the essential metrics used to quantify and compare the performance of image sensors.

  • Defines quantum efficiency (QE), full well capacity, and conversion gain.
  • Explains different noise sources, particularly read noise and dark noise.
  • Derives key performance figures like Signal-to-Noise Ratio (SNR) and Dynamic Range (DR).

Sensor Components

This chapter is planned. It will describe the components that make up a complete sensor package beyond the individual pixels.

  • Explains the role of microlens arrays in improving light collection efficiency.
  • Describes the design and function of Color Filter Arrays (CFAs), particularly the Bayer pattern.
  • Covers the Analog-to-Digital Converter (ADC) and its role in quantizing the signal.

Sensor Control

This chapter is planned. It will cover the electronics and timing signals required to operate the sensor.

  • Explains the difference between rolling shutter and global shutter readout modes.
  • Discusses exposure control, gain settings (ISO), and frame rate.
  • Introduces the basic principles of sensor timing and control logic.

Sensor Characterization

This chapter is planned. It will describe the practical methods for measuring the performance parameters defined earlier.

  • Introduces standard measurement procedures, such as the EMVA 1288 standard.
  • Explains how to measure key parameters like read noise, QE, and linearity from test charts.
  • Discusses the creation of a noise model for a sensor.

Sensor Innovations

This chapter is planned. It will survey the landscape of modern and emerging sensor technologies.

  • Covers stacked and backside-illuminated (BSI) sensor architectures.
  • Introduces novel sensor types like event cameras, curved sensors, and Single-Photon Avalanche Diodes (SPADs).
  • Discusses trends in computational sensors, such as quaternary and transparent CFAs.