Coupled charge device

Sensor CCD.

A charge-coupled device (in English charge-coupled device, also known as CCD>>) is an integrated circuit that contains a certain number of capacitors linked or coupled. Under the control of an internal circuit, each capacitor can transfer its electrical charge to one or more of the capacitors next to it on the printed circuit. The digital alternative to CCDs are the CMOS (complementary metal oxide semiconductor) devices used in some digital cameras and many web cameras. Today CCDs are much more popular in professional applications and digital cameras.

The first CCD devices were invented by Willard Boyle and George E. Smith on October 17, 1969 at Bell Laboratories, both awarded the 2009 Nobel Prize in Physics precisely for this invention.

Digital photography

The term CCD is popularly known as the designation of one of the main elements of digital cameras and video cameras. In these, the CCD is the sensor with tiny photoelectric cells that record the image. From there the image is processed by the camera and recorded on the memory card.

The resolution or detail capacity of the image depends on the number of photoelectric cells in the CCD. This number is expressed in pixels. The greater the number of pixels, the greater the sharpness in relation to the size. Digital cameras today incorporate CCDs with capacities of up to one hundred and sixty million pixels (160 megapixels) in Carl Zeiss cameras.

Bayer filter used in numerous digital cameras.

The pixels on the CCD record gradations of the three basic colors: red, green, and blue (abbreviated "RGB", red, green, blue), whereby three pixels, one for each color, form a set of photoelectric cells capable of capturing any color in the image. To achieve this color separation, most CCD cameras use a Bayer mask that provides a raster for each set of four pixels so that one pixel registers red light, another blue light, and two pixels are reserved for green light (the eye Humans are more sensitive to green light than to red or blue colors). The end result includes information about the brightness in each pixel but with a color resolution less than the illumination resolution. Better color separation can be achieved by using devices with three coupled CCDs and a light separation device such as a dichroic prism that separates the incident light into its red, green and blue components. These systems are much more expensive than those based on color masks on a single CCD. Some high-end professional cameras use a rotating color filter to record high-resolution images of color and brightness, but they are expensive products and can only photograph static objects.

Physical Functioning

3D simplified version of a CCD sensor.

CCD detectors, like photovoltaic cells, are based on the photoelectric effect, the spontaneous conversion of light received into electric current that occurs in some materials. The sensitivity of the CCD detector depends on the quantum efficiency of the chip, the number of photons that must hit each detector to produce an electric current. The number of electrons produced is proportional to the amount of light received (unlike conventional photography on a photochemical negative). At the end of the exposure, the produced electrons are transferred from each individual detector (photosite) by a cyclical variation of an electric potential applied on horizontal semiconductor bands and isolated from each other by a layer of SiO2. In this way, the CCD is read line by line, although there are many different detector designs.

Types of CCDs

There are three types of CCD:

• CCD "Full Box"Full Frame(c): where the whole of the surface contributes to detection. It's the most sensitive, but it has several disadvantages:
  • Polycrystalline silicon electrodes circulate above the photosensitive layer and absorb an important part of the blue spectrum (0.35-0.45 micrometers);
  • An external shutter is required to allow the load transfer cycle to be without lighting;
  • It is very sensitive to dazzlement (blooming). When a "photosit" is overwhelmed, it floods its neighbors. To overcome this inconvenience, it can be equipped with a device known as "load evacuation drainage" (LOD-Lateral Overflow Drain), which eliminates overflowing electrons and limits dazzlement, but decreases sensitivity.
  • In the CCD "full frame" there are "photosites" that can store up to 60,000 electrons with a quantum efficiency of 20%.

A "full frame" 60.5 megapixels (with an effective area of 53.9 × 40.4 mm).

• CCD "box transfer" (full-frame transfer(c): combines two CCD matrices of the same size, one exposed to light, and the other hidden. It can therefore proceed to a rapid transfer of the matrix exposed to the storage matrix, and then digitize it in parallel with the acquisition of a new image.
  • The main drawback is that it reduces by two the size of photosite.
  • Other disadvantages, such as spectral response and dazzlement, remain present.

• CCD intertwined ': It is more complex: it combines a photodium for each CCD cell. It is used mainly in digital cameras.
  • The specialized photodiode allows you to find a spectral response that correctly covers the visible light (0.35 to 0.75 micrometers).
  • In general, they have a drainage of cargo evacuation that limits the spread of dazzlement.
  • On the other hand, it is inherently less sensitive, since the photodiots represent only 25% of the total surface. This defect is partially corrected by a network of convergent micro lenses that improves quantum efficiency from 15% to 35-45%.
  • Recently intertwined CCDs have photosites that can store up to 100,000 electrons.

20-megapixel interlaced CCDs (24 × 36 mm effective area) can be manufactured.

Application to astronomy

In all CCDs, electronic noise increases strongly with temperature and typically doubles every 6 to 8 °C. In astronomical applications of CCD photography it is necessary to cool the detectors so that they can be used during long exposure times.

Historically, CCD photography had a big push in the field of astronomy, where it replaced conventional photography starting in the 1980s. The sensitivity of a typical CCD can be up to 70%, compared to typical sensitivity of photographic films around 2%. For this reason and the ease with which image defects can be corrected by computer, digital photography quickly replaced conventional photography in almost all fields of astronomy. A major disadvantage of CCD cameras compared to conventional film is the reduced area of the CCD, which prevents wide-field photography compared to some shots with classic film. Professional astronomical observatories often use 16-bit cameras that work in black and white. Color images are obtained after computer processing of images of the same field, taken with different filters at various wavelengths.

The images obtained by a CCD camera are subjected to a correction process that consists of subtracting from the image obtained the signal produced spontaneously by the chip due to thermal excitation (dark field), dividing an image from a homogeneous field (flat field or flat field) that allows correcting differences in sensitivity in different regions of the CCD, and partially correcting optical defects of the camera or the objectives of the instrument used.

The first astronomical paper on the use of a CCD was entitled Astronomical imaging applications for CCDs, by B. A. Smith, published in JPL Conf on Charge-Coupled Device Technol. and Appl. pages 135-138 (1976). And, the CCD Surface Photometry of Edge-On Spiral Galaxies, which appeared in the Bulletin of the American Astronomical Society, Vol 8, p. 350 of this same year, obtained greater diffusion.

Manufactured measurements