Sensing and Acquisition (2.3)

• 0D point: a photodiode
• 1D line: desktop scanner, aerial scans, CT imaging. Must scan to get 2D image.
• 2D array: camera (CCD array)

A photodiode can have its voltage provided from the front or behind. Behind is more efficient, but somewhat more expensive to build.

A positive voltage at the gate attracts the electrons that are freed, through the photoelectric effect, when a photon strikes the epitaxial layer (Si + other stuff).

The charge in one photodiode is shifted along a row into a charge amplifier, which outputs a sequence of voltages.

Colour capture uses multiple sensors per pixel. Light is filtered or prismatically refracted.

Sensor noise comes from

• Lack of quantum efficiency
• Thermally generated current
• Charge shifting
• A/D conversion

Noise is often modelled as "additive Guassian noise":

sensed value = true value + noise

noise is random, but follows a zero-mean Gaussian probability with some standard deviation:

$g(x) = {1 \over \sigma \sqrt{2 \; \pi}} e^{- {1 \over 2}( {x-\mu \over \sigma} )^2}$

[Wikipedia]

"Blooming" is the overflow of electrons from a saturated well to an adjacent well.