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INF 5440 - CMOS Image Sensors AO 10V 9.1 Characterization CHARACTERIZATION
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  • 0 - CMOS Image Sensors

    AO 9.1

    Cha

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    racterization

    CHARACTERIZATION

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    AO 9.2

    Cha

    Tes

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    t systems

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    AO 9.3

    Cha

    Op

    Sensor under test

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    racterization

    toliner

    Pattern plate (optional)

    Diffuser

    Optics (optional)

    Sensor

    Control

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    AO 9.4

    Cha

    Lig

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    ht Box

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    AO 9.5

    Cha

    Mo

    Sensorunder test

    s the sensor

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    racterization

    nochromator

    GratingLight source

    Mirror

    Mirror

    Mirror

    The angle of the grating determine the wave length that enter

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    AO 9.6

    Cha

    Me

    WheEo isR isTL isF is

    Alte

    Ref: Nakamura

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    10V

    racterization

    asurement of intensity

    re the source intensity,

    the box reflectance, the optic’s transmission factor, and the aperture.

    rnative: Use of optoliner without optics (no attenuation)

    EpEoRTL

    4F2 m 1+( )----------------------------=

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    AO 9.7

    Cha

    ResThelightMeainten

    Repleas

    Res

    IncopropconvconvgainTheampThe

    F Amp ADC

    V/lux-s

    LSBs/lux-s

    Ref: Nakamura

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    racterization

    ponsivity gradient of the output signal as a function of the intensity. sured by output signal as a function of the sity and the integration time.

    resented in volt per lux-second or in number of t significant bits per lux-second.

    ponsivity: V/lux-s, LSB/lux-s

    ming light is reduced due to the aperture ortion to 1/4F2, erted to charge in the photo diode,erted to voltage according to the conversion , (capacity of the diode, 1/Cpd). voltage is reduced in SF and gained up in the lifier. voltage is converted to LSBs in the ADC.

    Aperture/LensS

    I [lux]

    I/4F2

    t

    V/lux-s V/lux-s

    lux s

    V

    lux s

    V

    lux s

    Vlux s

    LSB

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    AO 9.8

    Cha

    Me

    Swe

    Theis V

    Ref: Nakamura

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    asurement of Responsivity

    ep integration time

    • For each integration time, one image and one dark frame, with same integration time, are stored.

    • The dark frame is subtracted from the image.

    • The mean value is calculated for section if the image (centre) for all colour plans (4).

    • Adapt a straight line output value as a function of the exposure.

    interesting measure for the responsivity, /lux-s after gain or LSBs/lux-s.

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    AO 9.9

    Cha

    Spe

    Use

    Line

    Coll

    Subdark

    Confeed

    All c

    Ref: Nakamura

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    ctral response

    of monochromator

    ar part of the response curve

    imated light

    traction of dark frames to eliminate the current.

    stant light, or constant output signal and back to the light source (control loop).

    olour plans

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    AO 9.10

    Cha

    Un

    Ligh

    CRA ptics and the sensor surface. The . l

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    racterization

    iformity

    t angle

    (Chief Ray Angle) is determined by the optics and the distance between the o responsivity depends on how the light is hitting the pixel. The result is shading

    φ

    φ

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    AO 9.11

    Cha

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    racterization

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    AO 9.12

    Cha

    DarIntecarr

    At aa dablac

    Blaccurr

    Darknon

    DSNsign

    Exa

    stogram

    rk image gained digitally

    M

    D

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    racterization

    k Signalgrated dark current, due to thermal generated charge iers, results in dark signal.

    given temperature, the dark signal appears as a pedestal, rk level, above the zero level. Thus “black” is not totally k.

    k levels are temperature dependent and, typically the dark ent doubles per 6-8 °C.

    current vary form pixel to pixel. The result is a -uniform dark level.

    U (dark signal non uniformity) is given in % of maximum al for a given integration time

    mple: Dark current 200-1000 e/s @ 37 ºC, DSNU ~ 1%

    hiData: G Agranov

    Example of a da

    Output signal

    Pixels, along one row or column

    ax level(white)

    ark level

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    AO 9.13

    Cha

    TemVari

    Nois p*GSF):

    Give

    C⁄

    Gtot2⁄

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    poral Noisees over time - from one image to the other

    Photon noise ,

    Noise in dark current

    Reset noise

    Sampling noise

    Thermal noise in SF

    Thermal noise in amplifier

    Quantization noise

    e contributions, rms values, are added. Referred to the photo diode (where Gtot=Gam

    n in number of electrons:

    vph σnq C⁄= q N C⁄= vph in( )t C⁄ t 2qIph∆f==

    vdrk idrk( )t C⁄ t 2qIdrk∆f C⁄==

    vnr kT C⁄=

    vns kT C⁄=

    vsf 4kT23--- 1

    gm-------∆f=

    vamp 4kT23--- 1

    gm-------∆f=

    vq LSB( )2 12⁄=

    vn vph2 vdrk

    2 vnr2 vns

    2 vsf2 vamp

    2 inn( ) G⁄ sf2 vq

    2+ + + + + +=

    erms vn CG⁄vnCpd

    q---------------= =

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    AO 9.14

    Cha

    Tem

    Nois

    (9.1)

    rons,]

    Nois

    Eas d by photon noise, we know:

    The

    (9.2)

    We gnal (at sufficiently high light leve

    INF 544

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    racterization

    poral noise (cont.)

    e in dark images

    where [N = number of generated elect

    e in illuminated images

    y way to determine Conversion Gain. At bright images where noise is dominate

    refore we can write the Conversion Gain:

    find the total conversion factor by the ratio variance to the mean value of the sils so that the photon noise dominates the noise).

    σdark2 σshot dark,

    2 σreset2 σread

    2+ +=

    σshot dark, Ndark=

    σillumin2 σdark

    2 σshot signal,2+=

    vsignal CG( )Nsignal=

    vn shot, CG( ) Nsignal=

    vn shot,2 CG( )2Nsignal CG( ) vsignal= =

    CGvn shot,

    2

    vsignal-----------------=

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    AO 9.15

    Cha

    TemMeafromThemea

    Thefrom

    Opt

    ManTheremDivitemunco

    Typ

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    poral Noise (cont.)sured by taking the rms value several frames, pixel by pixel.

    mean value of the rms values is a sure of the temporal noise.

    method eliminates contributions FPN.

    ional method

    y pixels within the same frame. difference between 2 frames oves the FPN. des the results by 21/2 because the poral noise in the two frames is rrelated.

    ical values: 10-50 erms

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    AO 9.16

    Cha

    FixVari

    Give rray. Tak

    Typ

    σ

    ixels

    Output valueDark level

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    ed Pattern Noise - FPN (DSNU)ations pixel to pixel for a uniformly illuminated sensor.

    • Variations in dark signal (DSNU - Dark Signal Non Uniformity)

    • Variations in the photo diode’s reset level

    • Variations in threshold voltage of the SF-transistor

    n as standard deviation in the output signal (Volt or LSBs) across the sensor ae the average of several frames to eliminate temporal noise.

    ical values: < 5 LSB (8 bit), < 2 mV

    Output signal

    Pixels, along one row or column

    Max level(white)

    Dark level

    Number of p

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    AO 9.17

    Cha

    Dat

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    a filtering can be used to find the cause of FPN.

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    AO 9.18

    Cha

    PhoOutppixe

    PRN(full

    Typ

    Mea

    Usaseve50%

    FPN

    Colo

    Whiope

    Signal

    ration time

    σ

    d contrast

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    to response non-uniformity (PRNU)ut signal as a function of light, varies from

    l to pixel. This is caused by variations in:

    • Pixel conversion gain (capacitance)

    • Source follower threshold voltage and quiescent point.

    • Variations in sampling capacitors (column), which determines the gain (with the feedback capacitor).

    U rms value of the spread is given in % of full scale well).

    ical values are 1-2%

    surement method

    ge of light box or collimated light. Mean value form ral frames removes the temporal noise. Exposure to or 80% of saturation. Measure for all colour planes.

    in light sensitivity: Green pixels G1, G2ur FPN: B-G, R-G or B/G, R/G

    te balance and colour processing should be in ration during recording.

    Integ

    Full scale

    black

    Enhance

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    AO 9.19

    Cha

    LAUsa

    Fist

    (9.3)

    e no.

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    Gge of fast shutter or light diodes

    image close to saturation, the succeeding frames in dark.

    Imag1 2 3 4

    Signal level

    S1

    S2S3

    LAG S2S1------100 %[ ]=

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    AO 9.20

    Cha

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    IMAGE QUALITYSome examples

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    AO 9.21

    Cha

    Res

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    olution• A measure of the smallest detail

    • Highest spatial frequency

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    AO 9.22

    Cha

    Imp

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    Frequency response

    ortant for sharpness, but is different from resolution.

  • 0 - CMOS Image Sensors

    AO 9.23

    Cha

    No

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    ise

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    AO 9.24

    Cha

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  • 0 - CMOS Image Sensors

    AO 9.25

    Cha

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    Dynamic Range

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    AO 9.26

    Cha

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  • 0 - CMOS Image Sensors

    AO 9.27

    Cha

    Und

    Corr in the colour reproduction.

    Tes

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    Aliasing - Moiré

    er sampled image results in interference patterns.

    ectly sampled luminance, but under sampled chrominance, gives interference

    t chart: CZP (circular zone-plate)

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    AO 9.28

    Cha

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    Example of a real image

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    AO 9.29

    Cha

    Ref

    rmats

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    erences:

    NakamuraImage Sensors and Signal Processing for Digital CamerasJunichi Nakamura (editor)CRC - Taylor & Francis

    Wikipediahttp://en.wikipedia.org/wiki/Comparison_of_graphics_file_fohttp://en.wikipedia.org/wiki/Jpeg#Encoding

    CharacterizationTest systemsOptolinerLight BoxMonochromatorMeasurement of intensityWhere Eo is the source intensity, R is the box reflectance, TL is the optic’s transmission factor...Alternative: Use of optoliner without optics (no attenuation)

    ResponsivityThe gradient of the output signal as a function of the light intensity. Measured by output signal...Represented in volt per lux-second or in number of least significant bits per lux-second.Responsivity: V/lux-s, LSB/lux-sIncoming light is reduced due to the aperture proportion to 1/4F2, converted to charge in the pho...

    Measurement of ResponsivitySweep integration time• For each integration time, one image and one dark frame, with same integration time, are stored.• The dark frame is subtracted from the image.• The mean value is calculated for section if the image (centre) for all colour plans (4).• Adapt a straight line output value as a function of the exposure.

    The interesting measure for the responsivity, is V/lux-s after gain or LSBs/lux-s.

    Spectral responseUse of monochromatorLinear part of the response curveCollimated lightSubtraction of dark frames to eliminate the dark current.Constant light, or constant output signal and feedback to the light source (control loop).All colour plans

    UniformityLight angleCRA (Chief Ray Angle) is determined by the optics and the distance between the optics and the sen...

    Dark SignalIntegrated dark current, due to thermal generated charge carriers, results in dark signal.At a given temperature, the dark signal appears as a pedestal, a dark level, above the zero level...Black levels are temperature dependent and, typically the dark current doubles per 6-8 ˚C.Dark current vary form pixel to pixel. The result is a non-uniform dark level.DSNU (dark signal non uniformity) is given in % of maximum signal for a given integration timeExample: Dark current 200-1000 e/s @ 37 ºC, DSNU ~ 1%

    Temporal NoiseVaries over time - from one image to the other• Photon noise ,• Noise in dark current• Reset noise• Sampling noise• Thermal noise in SF• Thermal noise in amplifier• Quantization noise

    Noise contributions, rms values, are added. Referred to the photo diode (where Gtot=Gamp*GSF):Given in number of electrons:

    Temporal noise (cont.)Noise in dark images(9.1)where

    [N = number of generated electrons,]Noise in illuminated imagesEasy way to determine Conversion Gain. At bright images where noise is dominated by photon noise,...Therefore we can write the Conversion Gain:(9.2)We find the total conversion factor by the ratio variance to the mean value of the signal (at suf...

    Temporal Noise (cont.)Measured by taking the rms value from several frames, pixel by pixel. The mean value of the rms v...The method eliminates contributions from FPN.Optional methodMany pixels within the same frame. The difference between 2 frames removes the FPN. Divides the r...Typical values: 10-50 erms

    Fixed Pattern Noise - FPN (DSNU)Variations pixel to pixel for a uniformly illuminated sensor.• Variations in dark signal (DSNU - Dark Signal Non Uniformity)• Variations in the photo diode’s reset level• Variations in threshold voltage of the SF-transistor

    Given as standard deviation in the output signal (Volt or LSBs) across the sensor array. Take the...Typical values: < 5 LSB (8 bit), < 2 mV

    Data filtering can be used to find the cause of FPN.Photo response non-uniformity (PRNU)Output signal as a function of light, varies from pixel to pixel. This is caused by variations in:• Pixel conversion gain (capacitance)• Source follower threshold voltage and quiescent point.• Variations in sampling capacitors (column), which determines the gain (with the feedback capaci...

    PRNU rms value of the spread is given in % of full scale (full well).Typical values are 1-2%Measurement methodUsage of light box or collimated light. Mean value form several frames removes the temporal noise...FPN in light sensitivity: Green pixels G1, G2Colour FPN: B-G, R-G or B/G, R/GWhite balance and colour processing should be in operation during recording.

    LAGUsage of fast shutter or light diodesFist image close to saturation, the succeeding frames in dark.(9.3)Some examples

    Resolution• A measure of the smallest detail• Highest spatial frequencyFrequency responseImportant for sharpness, but is different from resolution.

    NoiseDynamic RangeAliasing - MoiréUnder sampled image results in interference patterns.Correctly sampled luminance, but under sampled chrominance, gives interference in the colour repr...Test chart: CZP (circular zone-plate)

    References:Image Sensors and Signal Processing for Digital Cameras Junichi Nakamura (editor) CRC - Taylor & ...http://en.wikipedia.org/wiki/Comparison_of_graphics_file_formatshttp://en.wikipedia.org/wiki/Jpeg#Encoding