Ian writes in:
I was looking at buying a digital camera, and read that there are two kinds of sensors that they can use to take a picture- CMOS and CCD. Can you tell me what the difference is, and if one is better to get?
Sure! It’s actually a pretty topical question, as the inventors of the CCD just won this year’s Nobel Prize! As you mentioned, there are two basic kinds of image sensor that are used in today’s digital cameras, CCD (charge-coupled device) and CMOS (complementary metal oxide semiconductor). They both work by converting light energy (photons) into electric charge (electrons), and the difference is in how this charge is read out.
To start, both kinds of sensor are made of a grid of ‘buckets’ placed evenly across a flat semiconductor surface. Each bucket acts as an individual sensor, which only sees a tiny portion of the image. By displaying a bunch of these tiny points in a grid (using a computer monitor or printer), we see the image.
Making color images is a bit more complicated. Because the buckets are sensitive to any wavelength of visible light, if we just looked at the results we would see a monochrome image. To get color information, we arrange the sensors into groups of four, and place tiny red, green, and blue color filters over them. Each group of four sensors is what we call a pixel, and it is interesting to note that modern cameras have millions of them.
Ok, so both CCD and CMOS sensors are basically just big arrays of individual sensors, so how are they different? The difference is in how the charges are collected and read out. In a CCD, the ‘bucket’ that collects charge is just a capacitor. To read the image data out of the CCD, the charge in each bucket is pumped individually over to an ADC (analog to digital converter), which actually measures charge. In a CMOS sensor, each bucket contains a photodiode and some amplifier circuitry. To read the image data out, the output of each amplifier is connected to an ADC through a multiplexer, which measures the voltage at each cell.
I don’t think that either technology is necessarily better, but each has its own quirks. There is an interesting site at dvxuser which talks about the different kinds of sensor artifacts associated with each kind of sensor. For most cases, though, I think that other specifications, such as ease of use and sensitivity to light, are probably more important to think about when choosing a digital camera. Good luck!
[photo by SarahCartwright]