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Radiant Vision Systems | A Konica Minolta Company
LIGHT & COLOR THEORY OVERVIEW
Automated Visual InspectionLight & Color Global Support
3Radiant Vision Systems | A Konica Minolta Company
© Radiant Vision Systems, LLC
TODAY’S AGENDA
• Introduction to Light & Color• Instrumentation for
Photometry & Colorimetry • Characteristics of Imaging Sensors• Addressing Imaging Limitations
4Radiant Vision Systems | A Konica Minolta Company
© Radiant Vision Systems, LLC
INTRODUCTION TO LIGHT & COLOR
5Radiant Vision Systems | A Konica Minolta Company
© Radiant Vision Systems, LLC
WHAT IS LIGHT?
• Electromagnetic Radiation• EM Spectrum of light extends from UV to IR• Visible light consists of a very narrow band between UV and IR• Spectrum of light can be broken into discreet wavelengths that we
see as colors
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LIGHT SOURCES AND SPECTRAL POWER DISTRIBUTIONS (SPD)
• Light emitted by a source can be defined by the source’s Spectral Power Distribution (SPD)
• SPD is the light output (power) at a given wavelength along the electromagnetic spectrum
• Different sources will have different SPDs
7Radiant Vision Systems | A Konica Minolta Company
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SPD OF COMMON LIGHT SOURCES
390 430 470 510 550 590 630 670 710
Wavelength λ (nm)
D65 (Sunlight)
Illuminant A
LCD Blue
LED Red
HeNe Laser
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HUMAN VISION
The human eye has three types of cones (S, M, and L) that are each sensitive to a range of wavelengths of light.
A cone cannot detect the individual light wavelengths. Rather, a cone sees the sum total of light from all wavelengths under that cone’s spectral sensitivity curve.
350 450 550 650 750
2⁰ Cone Response
L
M
S
The human eye is sensitive to and
9Radiant Vision Systems | A Konica Minolta Company
© Radiant Vision Systems, LLC
390
400
410
420
430
440
450
460
470
480
490
500
510
520
530
540
550
560
570
580
590
600
610
620
630
640
650
660
670
680
690
700
Wavelength λ (nm)
HUMAN PHOTOPIC VISION
SpectralSensitivity of
the Human Eye
The spectral limit of average human sight is:
380 – 830nm
Maximum luminous efficacy is:@ 555 nm (green)
SPD of Sunlight
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PhotometricHuman Visual Perception
RadiometricAll Radiation
Total light outputLUMINOUS FLUX
lumens (lm)1 lm = 1 cd * 1 steradian
RADIANT FLUXWatts (W)
Light from a Direction
LUMINOUS INTENSITYcandela (cd)
1 cd = 1 lm/steradian
RADIANT INTENSITYW/sr
Light incident on a surface
ILLUMINANCE IRRADIANCEW/m2
lux (lx)1 lx = 1 lm/m2
foot-candle (fc)1 fc = 1 lm/ft2
Brightness
LUMINANCE RADIANCEW/sr * m2
cd/m2
1 cd/m2 = 1 nitfoot-lambert (fL)
1 fL = 1 cd/∏*ft2
PHOTOMETRIC VS. RADIOMETRIC UNITS
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THE LANGUAGE OF LIGHT
Amount of light emitted in the range of a three-dimensional angular span
Candela (cd)
Luminous Intensity
Amount of light emitted from a light source or reflected back from a surface in a given direction
Candela per meter squared (cd/m2)
Luminance
Measure of the total luminous flux of a light source by integrating its intensity over its angular span
Lumen (lm)
Luminous Flux
The amount of light incident on a surface per unit area
Illuminance (lm/m2 or lux)
Illuminance
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© Radiant Vision Systems, LLC
THE LANGUAGE OF LIGHT
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THE FOUNDATIONS OF COLORIMETRY
In the mid-1800’s, James Clerk Maxwell demonstrated that mixing three primaries could produce the entire gamut of hues. He presented various colored samples to an observer, who then adjusted the brightness levels of two primaries shining on the target until the resulting white matched a white target.
ColorSample
WhiteTarget
Red
Green
Blue
W. Davis Wright and John Guild built upon this research in the late 1920’s to show how much red, green, or blue light energy was needed for a human observer to perceive any color across the visible spectrum.
14Radiant Vision Systems | A Konica Minolta Company
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CIE COLOR-MATCHING FUNCTIONS
Mathematical way to determine perceivedcolor and brightness from SPDs
CIE Color-Matching Functions
�𝒙𝒙�𝒚𝒚
�𝒛𝒛
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CIE 1931 COLOR SPACE
CIE Color SpaceEvery visible color to the human eye.Colors defined by coordinates (Cx, Cy)
R
B
G
Color GamutProducible colors using a device’s primaries (a tristimulus color system via machine).• Using a linear combination of primary
colors (R,G, B) that occur at each corner of the triangle
GR B+ +
y
x
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COLOR AND CIE TRISTIMULUS CURVES
• Three curves multiply the SPD, and the integrals are Tristimulus X, Y, and Z.
• Combinations of X, Y, and Z define color coordinates in two-dimensional space.
1931 CIE Color Space x = X/(X+Y+Z) y = Y/(X+Y+Z)
1960 CIE Color Space u = 4X/(X+15Y+3Z) v = 6Y/(X+15Y+3Z)
1976 CIE Color Space u’ = 4X/(X+15Y+3Z) v’ = 9Y/(X+15Y+3Z)
17Radiant Vision Systems | A Konica Minolta Company
© Radiant Vision Systems, LLC
0.0
0.5
1.0
1.5
2.0
380 430 480 530 580 630 680 730 780
�𝒙𝒙�𝒚𝒚
�𝒛𝒛Shade of Blue
CALCULATING CX & CY
𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠Cx = XX+Y+Z
= Cy = YX+Y+Z
= 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠
SPD 1 (Blue LED)
𝐂𝐂𝐂𝐂 > 𝐂𝐂𝐂𝐂
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© Radiant Vision Systems, LLC
CALCULATING CX & CY
0.0
0.5
1.0
1.5
2.0
380 430 480 530 580 630 680 730 780
�𝒙𝒙�𝒚𝒚
�𝒛𝒛 Shade of Green
SPD 2 (Green LED)
𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠Cx = XX+Y+Z
= Cy = YX+Y+Z
= 𝑠𝑠𝑠𝑠𝑠𝑠𝑙𝑙𝑠𝑠 𝐂𝐂𝐂𝐂 < 𝐂𝐂𝐂𝐂
19Radiant Vision Systems | A Konica Minolta Company
© Radiant Vision Systems, LLC
CALCULATING CX & CY
0.0
0.5
1.0
1.5
2.0
380 430 480 530 580 630 680 730 780
�𝒙𝒙�𝒚𝒚
�𝒛𝒛Shade of Red
SPD 3 (Red LED)
𝑠𝑠𝑠𝑠𝑠𝑠𝑙𝑙𝑠𝑠Cx = XX+Y+Z
= Cy = YX+Y+Z
= 𝑠𝑠𝑠𝑠𝑚𝑚𝑚𝑚𝑚𝑚𝑠𝑠 𝐂𝐂𝐂𝐂 > 𝐂𝐂𝐂𝐂
20Radiant Vision Systems | A Konica Minolta Company
© Radiant Vision Systems, LLC
CALCULATING CX & CY
0.0
0.5
1.0
1.5
2.0
380 430 480 530 580 630 680 730 780
�𝒙𝒙�𝒚𝒚
�𝒛𝒛
Mostly White
SPD 4 (Sunlight)
𝑠𝑠𝑠𝑠𝑚𝑚𝑚𝑚𝑚𝑚𝑠𝑠Cx = XX+Y+Z
= Cy = YX+Y+Z
= 𝑠𝑠𝑠𝑠𝑚𝑚𝑚𝑚𝑚𝑚𝑠𝑠 𝐂𝐂𝐂𝐂 = 𝐂𝐂𝐂𝐂
21Radiant Vision Systems | A Konica Minolta Company
© Radiant Vision Systems, LLC
INSTRUMENTATION FOR PHOTOMETRY & COLORIMETRY
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TECHNOLOGY COMPARISON
Light measurements systems include:• Illuminance Meters
• Luminance Meters
• Spot Colorimeters
• Spectroradiometers
• Imaging Colorimeters
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TECHNOLOGY COMPARISON: SPOT SYSTEMS
1 698
241 6
98
241 6
98
24
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TECHNOLOGY COMPARISON: IMAGING SYSTEMS
25Radiant Vision Systems | A Konica Minolta Company
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TECHNOLOGY COMPARISON: IMAGING SYSTEMS
1 698
2
4
3
7
5
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TECHNOLOGY COMPARISON
• No single technology is best for all measurement needs.• Imaging (sensor-based) instruments excel at:
• Contextual evaluation• Measuring uniformity• Identifying defects (pixels, blobs, artifacts)• Measuring multiple spots
(LED arrays)• Determining dimensions, distortion and focus quality• Performing advanced analysis
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Imaging Photometers& ColorimetersScientific-grade camera systems with unique optical filters that replicate the human eye’s response to light.
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CHARACTERISTICS OF IMAGING SENSORS
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WHAT IS AN IMAGE SENSOR?
Representation ofSensor Pixels
One Sensor Pixel
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WHAT IS AN IMAGE SENSOR?
• A rectangular matrix of photodetectors• Each detector measures incident flux
Detector matrix Optics form image on sensor
Each detector element (pixel) measures
incident flux
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IMPORTANT IMAGE SENSOR CHARACTERISTICS
Resolution – The number of pixels in an image translates into spatial resolution, which determines the ability to distinguish fine detail within an image.
1MP Resolution 29MP Resolution
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IMPORTANT IMAGE SENSOR CHARACTERISTICS
Pixel count – This translates into spatial resolution, which determines the ability to distinguish fine detail within an image.
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THE IMPACT OF RESOLUTION
Higher-resolution imaging enables:
• Easier detection of contrast variations
• Pixel-level defect detection and correction (displays)
• Detection of subtle defects
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IMPORTANT IMAGE SENSOR CHARACTERISTICS
Dynamic Range – The number of shades of gray. Larger dynamic range yields greater measurement precision and enables higher contrast ratio measurement.
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IMPORTANT IMAGE SENSOR CHARACTERISTICS
Dynamic range:The ratio between the maximum possible signal and the “noise” level at the minimum signal of one measurement
• Measured in dB (decibels)
• Calculates a signal-to-noise ratio (SNR)
Noise
Signal
Background
Signal + Noise
Noise
Signal
Background
Signal + Noise
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IMPORTANT IMAGE SENSOR CHARACTERISTICS
Physical Size – The size of the sensor’s active area determines the field-of-view when working with a given focal length lens.
Sensor
SensorLensLens
θ2θ1
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IMAGING COLORIMETER CONFIGURATIONS
• 3-color commercial sensor• Monochrome, scientific-grade sensor with moving
filter wheel
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3-COLOR SENSOR
• Sensor with color filters physically overlaid on each pixel.
• “Color” of each pixel is determined by using values of surrounding pixels.
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3-COLOR SENSOR
• Advantages• Low Cost• No moving parts• Fast
• Disadvantages• Imperfect CIE match = Reduced color accuracy• Not 100% fill factor = Reduced effective
resolution• Small pixels = Low dynamic range• Low signal/noise ratio affects accuracy
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• Sequential exposuresare made through CIEmatched color filters
• Neutral density filters modify the intensity ofeach wavelength
• Cooled, scientific-grade sensor is used
SCIENTIFIC-GRADE SENSOR WITH FILTER WHEEL
Imaging Lens
Color Filters
ND Filters
Sensor
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ZY X
Xb
TRISTIMULUS & NEUTRAL DENSITY FILTERS
Each color filter precisely matches a CIE tristimulus curve; which allows for NIST traceable results.
Color images are a combination of images taken through each filter.
ND Filter Wheel Bright Lights Modulated Lights Various filters
Greyscale Image Sensor Scientific Grade with Large Pixels Cooled to 5⁰C & Precision Calibrated Sensor with various resolution
Color Filter Wheel Precision made filters Xb Filter option Radiometric option
Lens
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SCIENTIFIC-GRADE SENSOR WITH FILTER WHEEL
• Advantages• Best possible CIE match = highest color
accuracy• High dynamic range • Low noise• High quantum efficiency (for low light)• Higher sensor fill-factor
• Disadvantages• Higher cost• Slower measurement times• Moving parts (filter wheels)
Imaging Lens
Color Filters
ND Filters
Sensor
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USAGE RECOMMENDATION
• 3-color commercial sensoro Not recommended, cannot guarantee color and luminance accuracyo Used only when gross relative color errors are needed (e.g., looking
for an orange LED instead of a red LED)
• Monochrome, scientific-grade sensor with moving filter wheel
• Recommended for R&D when accuracy and precision are important• Recommended for low-contrast display mura detection• Recommended for production applications where speed and
accuracy are both critical
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ADDRESSING IMAGING LIMITATIONS
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MEASUREMENT ERRORS
• Electronic• Optical• Mechanical
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MEASUREMENT ERRORS
• Electronic• Photon (shot) noise• Dark (thermal) noise• Nonlinearity• Non-Uniform Pixel Response
• Optical• Mechanical
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MEASUREMENT ERRORS
• Electronic• Optical
• Lens Distortion• Lens Cosine Fall-off• Lens Vignetting• Lens Spectral Dependency• Off-Axis (Keystone)
Distortion
• Screen Effects• CIE color filter mismatch• Filter wedge and
magnification
• Mechanical
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MEASUREMENT ERRORS
• Electronic• Optical• Mechanical
• F-stop repeatability• Shutter repeatability
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THE IMPACT OF NOISE
217Δ0O%
216Δ1
O.4%
215Δ2
O.8%
214Δ3
1.2%
213Δ4
1.6%
212Δ52.0%
211Δ6
2.4%
Original Image
Small Amount of Noise Added
A Little More Noise Added
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PHOTON (SHOT) NOISE
PPP
WellPixel Size
Photons per μm2
P P
P PP
e-e-e-e- e- e-e-e-e-e- e- e-e-e-e-e- e- e-e-e-e-e- e- e-e-e-e-e- e- e-
e-e-e-e- e- e-e-e-e-e- e- e-e-e-e-e- e- e-e-e-e-e- e- e-e-e-e-e- e- e-
e-e-e-e- e- e-e-e-e-e- e- e-e-e-e-e- e- e-e-e-e-e- e- e-e-e-e-e- e- e-
e-e-e-e- e- e-e-e-e-e- e- e-e-e-e-e- e- e-e-e-e-e- e- e-e-e-e-e- e- e-
e-e-
LIGHT
Saturation(e-)
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PIXEL SIZE / WELL DEPTH COMPARISONSensor Name Resolution Pixel Size
(μm)Electron Well
Depth (e-)
Sony ICX205 1392 x 1040 4.65 x 4.65 10,000
KAI-02050 1600 x 1200 5.5 x 5.5 20,000
KAI-08050 3296 x 2472 5.5 x 5.5 20,000
KAI-16050 4896 x 3264 5.5 x 5.5 20,000
KAF-1602E 1536 x 1024 9 x 9 100,000
KAF-6303E 3072 x 2048 9 x 9 100,000
KAF-0261E 512 x 512 20 x 20 200,000
KAF-1001E 1024 x 1024 24 x 24 240,000
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0.01
0.1
1
10
100
-30 -20 -10 0 10 20 30 40
Temperature (C)
Norm
aliz
ed G
ener
atio
n Ra
te (e
/sec
)DARK (THERMAL) NOISE
Dark Current vs Temperature
(logarithmic)Dark Current over Time
(linear)
0
1
2
3
4
5
6
7
8
9
10
0 1 2 3 4 5 6 7 8 9 10
Time
Ther
mal
Ele
ctro
ns
53Radiant Vision Systems | A Konica Minolta Company
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CORRECTING DARK CURRENT
• Cool camera to reduce dark current• Limit exposure times of uncooled cameras to 10 sec• Capture a dark frame with the optical path blocked,
and then subtract it from subsequent measurements, pixel by pixel
• Average multiple images to reduce noise effects
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CORRECTING FOR PIXEL RESPONSE AND LENS TRANSMISSION (LUMINANCE)
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CORRECTING FOR PIXEL RESPONSE AND LENS TRANSMISSION (LUMINANCE)
Uniform LuminanceLight Source
Camera
• Luminance flat fielding• Acquire image using
uniform light source
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• Create correction matrix from acquired image• Multiply data images by correction matrix
CORRECTING FOR PIXEL RESPONSE AND LENS TRANSMISSION (LUMINANCE)
Uncalibrated image of Uniform Light Source
Calibration image of Uniform Light Source
Corrected image
x =
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CORRECTING FOR PIXEL RESPONSE AND LENS TRANSMISSION (ILLUMINANCE)
• In addition to the problems described before, screens have the following problems:
Screen Blemishes Non-Uniform Reflectivity Keystone Distortion
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CORRECTING FOR PIXEL RESPONSE AND LENS TRANSMISSION (ILLUMINANCE)
• Illuminance flat fielding• Illuminate screen with
known light source and acquire exposure
Projection Screen
Known projectionlight source (AC-4001)
Camera
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• Correct perspective distortion• Create correction matrix from acquired image• Multiply data images by correction matrix
CORRECTING FOR PIXEL RESPONSE AND LENS TRANSMISSION (ILLUMINANCE)
X =
Uncalibrated image of LCD projector with screen defect
Calibration image with calibration lamp
Corrected image
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SENSOR LINEARITY CORRECTION AND Y-INTERCEPT ERROR
• Pixel response to light should be linear with time and amount of pixel well depth filled
• Sensitivity to light is not perfectly linear
• Map the sensor response v. saturation level• Use this map to create a correction table
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CORRECTING LENS DISTORTION
• Software driven
Uncorrected image exhibiting barrel distortion in 7-mm lens.
The same image after a software lens distortion correction.
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STRAY LIGHT CORRECTION
• Stray light affects measured Contrast Ratio
Without stray light correction; Luminance of black squares measured as 2-3 Nits
With stray light correction; Luminance of black squares measured as 0.6 Nits
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MECHANICAL REPEATABILITY
• F-Stop Repeatability• Mechanical backlash in aperture springs
• Small error in aperture size leads to error in absolute luminance scaling.
• Always approach destination F-# from a lower F-#• Use electronically controlled lens – programmed to
eliminate this issue
• Shutter Repeatability• Minimum exposure limit driven by mechanical drive tolerances
• Can see an error in absolute luminance scaling or a luminance gradient over the image.
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CIE COLOR MATCH
• Need to match each color channel response to corresponding CIE color curve in order to calibrate color properly.
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EXAMPLE SENSOR SPECTRAL RESPONSE
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CIE COLOR MATCH: 3-COLOR SENSOR• Response of color filters used for 3-color sensors
isn’t a good match to CIE curves.
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CIE COLOR MATCH: COLOR FILTERS
• Excellent match to CIE color curves can be obtained by using individual color filters.
380 420 460 500 540 580 620 660 700
Imaging Lens
Color Filters
ND Filters
Sensor
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CORRECTING COLOR FILTER RESPONSE MISMATCH
• One-color calibration• Scales each filter for the specific DUT
• Four-color calibration• Accurately measures all colors for the RGB device
• Multi-color calibration• Produces a compromise calibration for differing colors
• Single-filter brightness scaling• For brightness only, scales output to particular device
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FILTER WEDGE AND MAGNIFICATION
• User sees a color shift between pixels in the image• Software driven correction• Applied to each filter
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Error Category Source of Error Countermeasure
Sensor NoisePhoton noise (shot noise) Larger pixel size, deeper pixel wells, full use of available
electrons (75% saturation)
Dark noise (thermal noise) Sensor cooling, dark image subtraction
Sensor ErrorsPixel non-uniformity Flat-field calibrationResponse linearity Linearity testing, linearity correction tableResponse Y-intercept error Linearity correction table
Optical System Errors
Lens vignetting, optical system angular transmission differences Flat-field calibration
Lens distortion (short focal length lenses only) Lens distortion calibrationImage Off-Axis Distortion (keystoning) Geometric software correctionLight Scattering Stray light calibrationScreen defects (illuminance measurements only) Illuminance flat-field calibration
Camera ErrorsF-stop repeatability Approach from low F-stop, rescale, electronic lensShutter repeatability Minimum exposure time
Filter ErrorsCIE curve mismatch Color calibration, luminance scalingFilter wedge and magnification Color shift correction
OVERVIEW OF ERRORS AND COUNTERMEASURES
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THANK YOU!