Human vision

• The image is formed on retina (sítnice)– The light is focused on retina by lens (čočka)

– Retina contains two types of receptors:• rods (tyčinky) for grey-scale vision in darkness (about 100 mil.)

• cones (čípky) for color vision (about 6,5 mil.)

– There are 3 types of cones for color vision (red, green, blue),the blue cones are substantially less sensitive than the other two

– Each eye acquires 2D discrete color image

– The final 3D image is formed from two 2D images (stereo vision)

Camera visionBasic features of CCD cameras

http://www.roperscientific.com/ccd/library_encyclopedia.shtml

Camera feature Units Description

resolution pixels x pixels horizontal times vertical number of pixels

pixel size m x m horizontal times vertical size of one pixel

grade quality of the chip (depends on the maximal

number of defective pixels)

full well capacity electrons maximal number of electrons in the well

read-out noise electrons/pixel preamplifier noise root-mean square (RMS)

dark current electrons/pixel/sec noise produced by thermally generated charge

(can be reduced by cooling)

dynamic range bits depth of digitization (bits in A/D converter)

readout rate pixels/sec number of pixels read out per second

binning {pixels x pixels} possibility of grouping pixels together

cooling no/air/water/LN medium used for cooling the chip

spectral response:

quantum efficiency

QE versus nm plot of quantum efficiency against wavelength

(sensitivity of the chip to different colors)

Quantum Efficiency (QE)

• Example of the QE curve for two versions of a given camera

Noise types (druhy šumu)

• NR - Readout noise of the CCD (vyčítací šum)

– NR [e-/pixel] = Nr = uncertainty in quantifying the electronic signal

– Nr [e-/pixel] = readout noise specified by the producer

– Depends on:• pre-amplifier design• post-amplifier electronics• CCD temperature (the lower, the lower the noise)• readout speed (the faster, the higher the noise)

• ND – Dark current (Dark charge noise) (šum náboje ve tmě)

– ND [e-/pixel] = SD1/2 = statistical fluctuations of SD

– SD [e-/pixel] = Nd * time = dark charge signal

– Nd [e-/pixel/sec] = dark charge specified by the producer

– Depends on: CCD temperature (the lower, the lower the noise)

Noise types (druhy šumu)

• NP – Photon shot noise (fotonový šum)

– NP [e-/pixel] = SP1/2 = statistical fluctuations of SP

– SP [e-/pixel] = I * QE * time = measured signal

– I [photons/pixel/sec] = photon flux (light intensity)

– QE [e-/photon] = quantum efficiency specified by the producer

– Depends on: only I, QE, time

• NT – Total noise (celkový šum)

– NT [e-/pixel] = (NR2 + ND

2 + NP2)1/2

• Signal-to-noise ratio (SNR or S/N)

– S/N = measured signal / total noise = SP / NT

SNR Example

• Example of different signal-to-noise ratio for given image

Dynamic range (dynamický rozsah)

• Dynamic range (camera) = number of bits in A/D converter– specified by the producer– 8 bit = 256 : 1– 12 bit = 4096 : 1– 16 bit = 65536 : 1

Dynamic range (dynamický rozsah)

• Dynamic range (real) = full well capacity / readout noise– specifies the real dynamic range– full well capacity depends on pixel size– readout noise depends on readout speed

– high resolution (6.8 m pixel size) & high speed (5 MHz):– 45 000 e- / 20 e- = 2 250 : 1 11,14 bit

– high resolution (6.8 m pixel size) & low speed (500 kHz):– 45 000 e- / 7 e- = 6 429 : 1 12,65 bit

– low resolution (24 m pixel size) & high speed (1 MHz):– 350 000 e- / 19 e- = 18 421 : 1 14,17 bit

– low resolution (24 m pixel size) & low speed (50 kHz):

– 350 000 e- / 5 e- = 70 000 : 1 16,10 bit