noise in image sensors - tu berlin · noise in image sensors sensorscharacteristics of ideal image...

Noise in Image Sensors

Processing Digital Camera Images Alexander HilgarthProcessing Digital Camera Images - Alexander Hilgarth

Noise in Image SensorsCharacteristics of ideal Image SensorsCharacteristics of ideal Image Sensors

• No sensor noise• Infinite dynamic range• High (spectral) responsivityg ( ) y• Infinite high frame rate• Infinite number of pixels• Zero energy consumption• Low manufacturing cost


Noise in Image SensorsCharacteristics of ideal and real Pixel CellsCharacteristics of ideal and real Pixel Cells

Ideal RealIdeal Real

Electrons Electrons

Saturation LevelResponsivity

c R

ange

Noise Floor

Dyn

ami

Minimum Resolvable

PhotonsPhotons

Processing Digital Camera Images Alexander Hilgarth

Signal

Processing Digital Camera Images - Alexander Hilgarth

Noise in Image SensorsHow does Noise effect Dynamic RangeHow does Noise effect Dynamic Range

• Ratio of maximum signal to minimumdetectable signal.

Electrons

• What is the maximum signal?• Full-well capacity (electrons)• Saturation signal (mV)• Saturation illumination

at a specific exposure time

Saturation LevelResponsivity

c R

ange

at a specific exposure time

• What is the minimum signal?• Noise floor electrons• Noise equivalent illumination

Noise Floor

Dyn

ami

• Minimum illumination

Minimum Resolvable

Photons


Signal


Noise in Image SensorsPerception of NoisePerception of Noise

• Perception based differentiation of noise in image sensors:

• Random noise: Location and time of occurrence not predictableRandom noise: Location and time of occurrence not predictable• Fixed pattern noise (FPN): Occurrence is location based due to underlying structure

• Perception in still imaging:

• Underexposed images contain more visible noise due to proximity to noise floor

•

• Random noise averaged by eye (perceived amplification at 5fps)St ti i ti bl (d k FPN i l FPN l FPN)• Stationary noise more noticeable (dark FPN, pixel FPN, column FPN)

• Line noise or column FPN level noticeable at above 0.5% RMS• Random noise of 5% RMS barely visible• Noise less noticeable at high temporal frequency• Noise less noticeable at high spatial frequency


Noise in Image SensorsPerception of Noise – Random NoisePerception of Noise Random Noise


10% Pixel Random Noise [1]


Noise in Image SensorsPerception of Noise – Random NoisePerception of Noise Random Noise


25% Pixel Random Noise [1]


Noise in Image SensorsPerception of Noise – Fixed Pattern NoisePerception of Noise Fixed Pattern Noise


0.5% Column FIxed Pattern Noise [1]


Noise in Image SensorsPerception of Noise – Fixed Pattern NoisePerception of Noise Fixed Pattern Noise


2% Column FIxed Pattern Noise [1]


Noise in Image SensorsPerception of NoisePerception of Noise

NoiseNoise:

• Random noise

• Fixed pattern noise:

• DSNU - Dark Signal Non-Uniformity

• DSNU is seen as an offset between pixels in darkDSNU is seen as an offset between pixels in dark• Corrected by subtracting a dark frame• Measured in the absence of light

• PRNU - Pixel Response Non-Uniformity

• PRNU is seen as a responsivity variation between pixels under illumination• Corrected by offset and gain for each pixel (which is mostly done off-chip)


Noise in Image SensorsRecapitulation Structure of Image Sensors (CMOS)Recapitulation Structure of Image Sensors (CMOS)

Underlying structure determines parameter value of noise

dres

seg

iste

r

R

1 T passive pixel 3 T active pixel 4 T pinned photodiodeactive pixel

Line

A

Column Bufferg

Master Clock

G B G BR G R

BG

Column Buffer

Horizontal ShiftRegister

Video Amp ADC Image Recovery and

Processing

Tim

ing

Adr

essi

ngClock

Reset

Data


Gain

Offset

[1]


Noise in Image SensorsUnderlying Structure determines Parameter Value of Noise (CMOS)Underlying Structure determines Parameter Value of Noise (CMOS)

1 T passive pixel

Advantages:

3 T active pixel

Advantages:

4 T pinned photodiode active pixelAdvantages:Advantages:

• Simple addressing• High fill factor (cheap sensor

high pixel count)

Advantages:

• High Signal-Noise-Ratio

Disadvantages:

• Smaller fill factor

g

• High SIgnal-Noise-Ratio• No kT/C noise• Low dark current

Disadvantages:Disadvantages:

• Small output swing (bigger noise)• Low SIgnal Noise Ratio

• Smaller fill factor• More complicated addressing• Source follower adds FPN and

Vt drop

Disadvantages:

• Smaller fill factor• More complicated addressing• Source follower FPN and Vt drop


Pixel cell designs are often patent pending, above examples refer to VLSI Online (left), Lucent (center), Kodak (right). [1]


Noise in Image SensorsOverviewOverview

• Temporal Noisep

• Photon shot noise• Dark shot noise (leakage current, maximum at hot pixels)• kT/C reset noise (thermal noise in capacitors)• Thermal noise

• Spatial Noise

• Dark FPN• Pixel source-follower FPN (offset and gain)• Pixel source-follower FPN (offset and gain)• Column FPN (offset & gain)

• System Noise

• ADC quantization noise• ADC quantization noise• Line noise / crosstalk


Noise in Image SensorsSummary sources of interferenceSummary sources of interference


Noise in Image SensorsQuestions?Questions?


Noise in Image SensorsLiteratureLiterature

[1] CMOS Image Sensor Design, Robert Henderson (2001)[ ] g g , ( )

[2] Noise in Image Sensors, R.I. Hornsey


noise in image sensors - tu berlin · noise in image sensors sensorscharacteristics of ideal image...

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