Examining CCD Sensors & the Photoelectric Effect

Charge-coupled device (CCD) sensors are utilized in several advanced imaging systems and digital cameras today. CCDs were first created in 1969 by physicists George Smith and Willard Boyle.

CCD technology is based on Albert Einstein’s concept of the photoelectric effect whereby light is converted into electrons. A CCD sensor acquires those electron signals as image points, or pixels, allowing them to be digitally interpreted.

CCD sensor mounted on a circuit board.

CCD sensor mounted on a circuit board.

Boyle and Smith, frequently called ‘the fathers of digital photography’, were awarded the Nobel Prize in 2009 for Physics in recognition of the creation of the CCD and its widespread impact on technology.

Willard S. Boyle (left) and George E. Smith, winners of the Nobel Prize in Physics for their work in digital imaging and their invention of the CCD sensor.1

Willard S. Boyle (left) and George E. Smith, winners of the Nobel Prize in Physics for their work in digital imaging and their invention of the CCD sensor.1

Albert Einstein

Albert Einstein is the ‘Grandfather’ of the CCD camera if Willard and Boyle are the ‘Fathers’. Einstein is one of Radiant’s ‘Physics Heroes’ and the company has a conference room in the corporate headquarters named after him.

Albert Einstein

The important thing is to not stop questioning. Curiosity has its own reason for existing.

Albert Einstein

Einstein was born on March 14th, 1879 in Germany. He is known as the most influential scientist in the 20th century. His general theory of relativity relates Newtonian laws of mechanics to the laws of electromagnetics.

In 1921, he was awarded the Nobel Prize in Physics as a result of the great impact of his work. In 1933, Einstein left Europe after the Nazis targeted him, he left to become a Professor of Theoretical Physics at Princeton University, where he stayed until his death in 1955.

Einstein suggested in a seminal 1905 paper on the relationship between energy and matter that the energy of a body (E) is the same as the mass (M) of that body multiplied by the speed of light (C) squared, or E=MC2.

This equation proposes that very small particles can be changed into vast amounts of energy as the source of atomic power.

Einstein also published literature on Relativity, the Special and General Theory in 1905. He is most known for his work, the Theory of Relativity, but his work on the photoelectric effect is what earned Einstein the Nobel Prize.

The Photoelectric Effect

The first scientists believed that light was comprised of particles (Sir Isaac Newton, 1643 to 1727), whereas others suggested that light was a wave (Robert Hooke, 1635 to 1703, and James Maxwell, 1831 to 1879).

Einstein established that light energy is moved in quantized, discrete wave packets (photons) and light could be a particle and a wave at the same time.

This theory solved questions regarding the relationship between light of varying frequencies (colors) and the energy (electromagnetic radiation) of such light.

The photoelectric effect is how optical signals are changed into electrical signals, and it is the critical phenomenon behind CCD cameras.

When light shines on a metal (or in the example of a CCD sensor, a silicon metalloid) surface, normally the energy of that light (the energy enclosed in the photon packets) will dislodge electrons from the surface in a process called the photoelectric effect, also known as photoemission.

This occurs for every wavelength of photoelectric energy from 190 to 1100 nanometers (nm), making up the entire spectrum of visible light which is around 380 to 700 nm.

The photoelectric effect occurs when light shines on a metal surface, causing it to eject electrons (photo electrons).

The photoelectric effect occurs when light shines on a metal surface, causing it to eject electrons (photo electrons).2

CCD Technology

Electron-hole pairs are created when incident photons carrying a charge are absorbed by the material of the CCD detector.

The ejected electrons gather in distinct elements of the detector during photographic exposure, known as the photosites or pixels of the CCD. (It is important to note that these pixels are not the same as those that produce light in a display screen).

The number of electrons ejected is higher depending on the brightness of the light shining on the metal of the CCD surface.

A CCD sensor is essentially a slab of silicon layers or silicon, which are normally ‘doped’ with the introduction of different elements, such as phosphorus, in order to adjust its conductive characteristics.

The silicon is then coated with a layer of metal oxide for insulation, which allows light energy to pass through ‘gates’. These gates contain a charge that enables only a one-way energy transfer to trap the electrons.

The CCD sensor is sectioned into rows by channel stops, and thin aluminum strips are positioned on top to create a grid. Every square in the grid created is a pixel.

Electrons travel towards the surface of the silicon layer and are then trapped within the pixel grid as they are released under light exposure.

A simplified 3D cross-section of a CCD sensor with various layers of doped silicon. The shaded area on top is the pixel size, the green arrow indicates the charge transfer direction.

A simplified 3D cross-section of a CCD sensor with various layers of doped silicon. The shaded area on top is the pixel size, the green arrow indicates the charge transfer direction.

The sensor can interpret the value (accumulated charge) within every pixel throughout the CCD once it has captured the electrons of the light.

The total amount of charge (number of electrons) that gather in every pixel is linearly proportional to the amount of light incident upon it. The more light intensity generating from the photographic subject, the more charge that ends up being kept in the pixel.

An analog-to-digital converter (ADC) then translates the value of each pixel into a digital value by quantifying the amount of charge in every photosite or pixel and performing a measurement conversion to binary form.

This makes what is practically a machine-readable, digital copy of the patterns of light that have fallen on the device, reproducing the original image.

The currents that alternate across the strips of aluminum direct the stored electrons row by row to the sensor’s edge, where the charge is noted and recorded in the memory of the camera. This means that the CCD sensor is empty and prepared for the next photographic exposure.

A detailed discussion of the science of camera sensors, including silicon doping, the electromagnetic processes of a CCD, and various operating considerations.

Recording light intensity with a CCD sensor creates black and white images. Filters are employed for color imaging to divide the ingoing light into its separate color wavelengths, red, green, and blue, so each can be captured and analyzed to reproduce a photographic image in full color.

Radiant colorimeters utilize a tristimulus filter wheel system suited to human perception (‘tri’-stimulus referring to the three primary filters that are employed to match the response of the three alternate cones in the human eye), as defined in the CIE color space.

A typical Bayer-pattern RGB filter (right) provides information about intensity of light in red, green, and blue wavelength regions—but does not have the sensitivity to discriminate each individual wavelength like our eyes do. Radiant’s filter wheel system (left) uses CIE-matched color filters to replicate the human eye’s perception of light and color.

A typical Bayer-pattern RGB filter (right) provides information about intensity of light in red, green, and blue wavelength regions—but does not have the sensitivity to discriminate each individual wavelength like our eyes do. Radiant’s filter wheel system (left) uses CIE-matched color filters to replicate the human eye’s perception of light and color.

There are some simple forms of CCD architecture with several variations and sizes for particular applications, from smartphone cameras to scientific analysis. For example, CCD sensors are utilized for medical imaging, and in the Hubble Space Telescope.

CCD sensors are reputable for their color accuracy, light sensitivity, and ability to include a high amount of pixels with greater electron capacities, which produce images of a high-resolution.

These features mean that CCDs are especially successful when precision detail is necessary to illuminate the details in an image.

Radiant’s ProMetric® imaging photometers and colorimeters employ scientific-grade CCDs that provide exceptionally high performance, with low noise features, high resolution, and an incredibly fast speed for data transfer.

To understand more about the use of CCD-based systems for precision color and light measurement, read the whitepaper, a ‘Guide to CCD-Based Imaging Colorimeters’.

Image Credits

  1. Image credit for Boyle and Smith photos: National Academy of Engineering.
  2. Image credit for Photoelectric Effect: licensed under Creative Commons Attribution-Share Alike 3.0 Unported.

Acknowledgments

Produced from materials originally authored by Anne Corning from Radiant Vision Systems.

This information has been sourced, reviewed and adapted from materials provided by Radiant Vision Systems.

For more information on this source, please visit Radiant Vision Systems.

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