CCD and CMOS sensors

Electronic image sensors work because light can displace electrons in silicon. Every incoming photon causes a valence electron to jump into the conduction band. In that state, the electron is free to move around, and the image sensor traps it in a capacitive cell. The number of electrons in the cell, and hence the voltage on it, is an accurate indication of how many photons arrived during the exposure. Modern sensors achieve a quantum efficiency near 100%, which means they capture an electron for nearly every photon.

The difference between CCD and CMOS sensors has to do with how the electrons are read out. CCD stands for charge-coupled device, a circuit in which the electrons are shifted from cell to cell one by one until they arrive at the output (Figures 11.1, 11.2); then the voltage is amplified, digitized, and sent to the computer. The digital readout is not the electron count, of course, but is exactly proportional to it.

CMOS sensors do not shift the electrons from cell to cell. Instead, each cell has its own small amplifier, along with row-by-column connections so that each cell can be read out individually.There is of course a main amplifier along with other control circuitry at the output.1

Which is better? Originally, CCDs had the advantage; CMOS image sensors were designed to be made more cheaply, with lower-grade silicon. CMOS sensors were noisy and had low quantum efficiency because most of the photons fell on the amplifiers rather than the photosensors.

1 The term CMOS means complementary metal-oxide semiconductor and describes the way the integrated circuit is made, not the way the image sensor works. Most modern ICs are CMOS, regardless of their function.

LIGHT

INPUT (IF ANY)

OUTPUT

Figure 11.1. Charge-coupled device (CCD) consists of cells (pixels) in which electrons can be stored, then shifted from cell to cell and retrieved. Light makes electrons enter the cells and slowly raise the voltage during an exposure. (From Astro-photography for the Amateur.)

Figure 11.1. Charge-coupled device (CCD) consists of cells (pixels) in which electrons can be stored, then shifted from cell to cell and retrieved. Light makes electrons enter the cells and slowly raise the voltage during an exposure. (From Astro-photography for the Amateur.)

IMAGING AREA

Figure 11.2. CCD array shifts the contents of each cell, one after another, through other cells to the output for readout. CMOS arrays have an amplifier for each pixel, with row and column connections to read each pixel directly. (From Astrophotography for the Amateur.)

Today, however, CCD and CMOS sensors compete on equal ground. The CMOS amplifiers are under the pixels, not between them, and the quantum efficiency is comparable to CCDs. So are all other aspects of performance, including noise. Canon DSLRs use CMOS sensors (to which Canon has devoted a lot of work), most others use CCDs, and neither one has a consistent advantage over the other. Nikon adopted a Sony CMOS sensor for the high-end D2X camera.

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