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66 2 Image formation

Figure 2.23 Image sensing pipeline, showing the various sources of noise as well as typical digital post-
processing steps.

until they are deposited at the sense amplifiers, which amplify the signal and pass it to
an analog-to-digital converter (ADC). 10 Older CCD sensors were prone to blooming, when
charges from one over-exposed pixel spilled into adjacent ones, but most newer CCDs have
anti-blooming technology (“troughs” into which the excess charge can spill).
In CMOS, the photons hitting the sensor directly affect the conductivity (or gain) of a
photodetector, which can be selectively gated to control exposure duration, and locally am-
plified before being read out using a multiplexing scheme. Traditionally, CCD sensors
outperformed CMOS in quality sensitive applications, such as digital SLRs, while CMOS
was better for low-power applications, but today CMOS is used in most digital cameras.
The main factors affecting the performance of a digital image sensor are the shutter speed,
sampling pitch, fill factor, chip size, analog gain, sensor noise, and the resolution (and quality)
of the analog-to-digital converter. Many of the actual values for these parameters can be read
from the EXIF tags embedded with digital images. while others can be obtained from the
camera manufacturers’ specification sheets or from camera review or calibration Web sites. 11

Shutter speed. The shutter speed (exposure time) directly controls the amount of light
reaching the sensor and, hence, determines if images are under- or over-exposed. (For bright
scenes, where a large aperture or slow shutter speed are desired to get a shallow depth of field
or motion blur, neutral density filters are sometimes used by photographers.) For dynamic
scenes, the shutter speed also determines the amount of motion blur in the resulting picture.

10
In digital still cameras, a complete frame is captured and then read out sequentially at once. However, if video
is being captured, a rolling shutter, which exposes and transfers each line separately, is often used. In older video
cameras, the even fields (lines) were scanned first, followed by the odd fields, in a process that is called interlacing.
11 http://www.clarkvision.com/imagedetail/digital.sensor.performance.summary/ .