principles of light and color measurement...the measurement of color as perceived by the human eye....

Radiant Vision Systems | A Konica Minolta Company

Presented By Shannon Roberts | March 10, 2020PRINCIPLES OF LIGHT AND COLOR MEASUREMENT

Global SupportAutomated Visual InspectionLight & Color

3Radiant Vision Systems | A Konica Minolta Company

© Radiant Vis ion Sys tems, LLC.

TODAY’S AGENDA

• Light & Color Theory• What is light?

• Quantifying Human Visual Perception• Photometry and colorimetry

• CIE Color Matching Functions

• How to calculate CIE-matched color values

• Light & Color Measurement Systems• Optical metrology devices for light measurement

• Benefits of imaging technology

• Example Applications



LIGHT & COLOR THEORY



Ultraviolet Infrared(UV) (IR)Visible Light

WHAT IS LIGHT?

Visible light is a tiny range of the electromagnetic spectrum to which our eyes respond.

Gamma Rays< 0.01nm

X-Rays0.01 - 10nm

UV10 - 390nm

IR 700nm - 0.0003m

Microwaves0.0003 - 1mRadio Waves

>1m

Light is made of photons: an electric field combined with a magnetic field, moving through space.



SPECTRAL POWER DISTRIBUTION

390 430 470 510 550 590 630 670 710

Wavelength λ (nm)

D65 (Sunlight)Illuminant ALCD BlueLED RedHeNe Laser

Every light source is defined by its uniqueSpectral Power Distribution (SPD)

SPD = Light power (Watts) at each wavelength



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



DIFFERENT SPD CAN LOOK THE SAME

Sunlight

LED

Incandescent

CFL

All of these light sources appear ‘white’ to the eye

MetamerismDifferent in spectral power distributions that appear as the same color to the human eye, but do not actually match.



QUANTIFYING HUMAN VISUAL PERCEPTION



If the eye is the judge and target market for your products…

Would you design and test your products against values that do not match human visual perception?




Photometry is the science of the measurement of as perceived by the human eye.

• Visible light spectrum only

Colorimetry is the science of the measurement of color as perceived by the human eye.

• How 3 cones in the human eye respond (in terms of power) to different wavelengths of light

Visible Light

Wavelength λ

Pow

er

350 450 550 650 750

2⁰ Cone Response

L

M

S




COLORA mathematical language is needed to quantify the brightness

and the variations of color that are perceived by a standard human observer, in order to be used for measurement.

Formula to quantify human eye’s response to each SPD.

Mathematical model to define visible colors (CIE color chart).

Human photopic response at each wavelength.

Quantify light based on spatial area and direction of travel.



390

400

410

420

430

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510

520

530

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560

570

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600

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630

640

650

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Wavelength λ (nm)

LIGHT: 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



COLOR: CIE COLOR-MATCHING FUNCTIONS

Mathematical way to determine human perception of color and brightness from different SPDs

�𝒙𝒙�𝒚𝒚

�𝒛𝒛 CIE Color Matching Functions



JAMES MAXWELL’S COLOR MATCHING TEST

Maxwell knew the additive nature of light in which mixing three primaries in equal proportions creates white. To test the eye’s response to colors, Maxwell presented various colored samples to an observer. The observer then adjusted the brightness levels of two primaries shining on the target until the resulting white matched a white target.

ColorSample

WhiteTarget

Red (615 nm)

Green (525 nm)

Blue (445 nm)



CIE COLOR MATCHING FUNCTIONS IN USE

= X, Y, Z⨯CIE Color Matching

Functions�̅�𝑥 𝜆𝜆�𝑦𝑦 𝜆𝜆�̅�𝑧 𝜆𝜆

� 𝐒𝐒𝐒𝐒𝐒𝐒 𝑑𝑑(𝜆𝜆) of Source



0.0

0.5

1.0

1.5

2.0

380 430 480 530 580 630 680 730 780


�𝒛𝒛Shade of Blue

CALCULATING CX & CY

Cx = ↓X↓X+↓↓Y+↑↑Z = Cy = ↓↓Y

↓X+↓↓Y+↑↑Z =𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠

SPD 1 (Blue LED)

𝐂𝐂𝐂𝐂 > 𝐂𝐂𝐂𝐂



CALCULATING CX & CY

0.0

0.5

1.0

1.5

2.0

380 430 480 530 580 630 680 730 780


�𝒛𝒛 Shade of Green

SPD 2 (Green LED)

Cx = ↓X↓X+↑↑Y+↓↓Z = Cy = ↑↑Y

↓X+↑↑Y+↓↓Z =𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝑠𝑠𝑠𝑠𝑠𝑠𝑙𝑙𝑠𝑠 𝐂𝐂𝐂𝐂 < 𝐂𝐂𝐂𝐂



CALCULATING CX & CY

0.0

0.5

1.0

1.5

2.0

380 430 480 530 580 630 680 730 780


�𝒛𝒛Shade of Red

SPD 3 (Red LED)

Cx = ↑↑X↑↑X+↑Y+↓↓Z = Cy = ↑Y

↑↑X+↑Y+↓↓Z =𝑠𝑠𝑠𝑠𝑠𝑠𝑙𝑙𝑠𝑠 𝑠𝑠𝑠𝑠𝑑𝑑𝑚𝑚𝑚𝑚𝑠𝑠 𝐂𝐂𝐂𝐂 > 𝐂𝐂𝐂𝐂



CALCULATING CX & CY

0.0

0.5

1.0

1.5

2.0

380 430 480 530 580 630 680 730 780


�𝒛𝒛

Mostly White

SPD 4 (Sunlight)

Cx = ↑↑X↑↑X+↑↑Y+↑↑Z = Cy = ↑↑Y

↑↑X+↑↑Y+↑↑Z =𝑠𝑠𝑠𝑠𝑑𝑑𝑚𝑚𝑚𝑚𝑠𝑠 𝑠𝑠𝑠𝑠𝑑𝑑𝑚𝑚𝑚𝑚𝑠𝑠 𝐂𝐂𝐂𝐂 = 𝐂𝐂𝐂𝐂



1931 CIE CHROMATICITY CHART

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



COLOR SPACES: 1931 VS. 1976CIE 1931 (x,y) CIE 1976 (u’,v’)



MACADAM ELLIPSES

A region on a chromaticity diagram which contains all colors which are indistinguishable, to the average human eye, from the color at the center of the ellipse.

*Ellipses at left are 10x their actual sizeImages source: Wikipedia.org



MACADAM ELLIPSESCIE 1931 (x,y) CIE 1976 (u’,v’)



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



THE LANGUAGE OF LIGHT



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



LIGHT & COLOR MEASUREMENT SYSTEMS



HUMAN-CENTRIC MEASUREMENT

Light & color measurement systems aim toreplicate and quantify human visual perception of:

color



TECHNOLOGY COMPARISON

Light measurements systems include: Illuminance Meters

Luminance Meters

Spot Colorimeters

Spectroradiometers

Imaging Colorimeters



TECHNOLOGY COMPARISON: COLORCol

orim

eter

Spec

trom

eter

ImagingSpot



COLORIMETER: TRISTIMULUS FILTER SYSTEM


�𝒛𝒛CIE Color Matching Functions



390 430 470 510 550 590 630 670 710 750 790 830

Wavelength λ (nm)

PHOTOMETER: PHOTOPIC FILTER SYSTEM

Y-bar filter is used to accurately match the human photopic response to brightness.

SpectralSensitivity of

the Human Eye



PHOTOMETER: PHOTOPIC FILTER SYSTEM

Y-bar filter is used to accurately match the human photopic response to brightness.

0.0

0.5

1.0

1.5

2.0

380 430 480 530 580 630 680 730 780

��̅�𝒙��̅�𝒚

�𝒛𝒛



TECHNOLOGY COMPARISON: IMAGINGCol

orim

eter

Spec

trom

eter

ImagingSpot



TECHNOLOGY COMPARISON: IMAGINGCol

orim

eter

Spec

trom

eter

ImagingSpot

1698

2

4

3

7

5



ADVANTAGES OF IMAGING

Imaging devices excel at: Contextual evaluation Measuring uniformity Identifying defects (pixels,

blobs, artifacts) Measuring multiple spots

(LED arrays) Rapid collection of multiple

data points Advanced analysis



IMAGING SYSTEM SENSORS

Representation ofSensor Pixels

One Sensor Pixel



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



THE IMPACT OF RESOLUTION

Higher-resolution imaging enables: Easier detection of

contrast variations Pixel-level defect

detection and correction (displays)

Detection of subtle defects



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



THE IMPACT OF NOISE

217Δ0O%

216Δ1

O.4%

215Δ2

O.8%

214Δ3

1.2%

213Δ4

1.6%

212Δ5

2.0%

211Δ6

2.4%

Original Image

Small Amount of Noise Added

A Little More Noise Added



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) NOISEDark Current vs Temperature (logarithmic)

Addressing Thermal Noise• Cool camera to reduce dark

current

• 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



IMAGING COLORIMETER DESIGN

1

2

1. Electronically-controlled lens for any working distance

2. Precision tristimulus and neutral density filters



ZY X

Xb

TRISTIMULUS & NEUTRAL DENSITY FILTERS

Each proprietary Radiant color filter precisely matches a CIE curve; which allows for NIST traceable results.

Color images are a combination of images taken through each filter.

Color Filter Wheel Precision made filters Xb Filter option Radiometric option

ND Filter Wheel Bright Lights Modulated Lights ND3 option

Greyscale Image Sensor Scientific-grade with Large Pixels Cooled to 5⁰C & Precision Calibrated 29-megapixel option

Lens



IMAGING COLORIMETER DESIGN

1

4

2

3

1. Electronically-controlled lens for any working distance

2. Precision tristimulus and neutral density filters

3. Cooled image sensors reduce thermal noise and improve accuracy

4. Scientific-grade image sensors simultaneously capture millions of data points



Displays Backlit Symbols Lighting

WHAT CAN WE MEASURE?



Non-Uniformity (Color &

Brightness)

Pixel & Line Defects

Mura Image Sticking View AngleLight Leakage

Color OffsetBrightness Offset

Lv

COMMON DISPLAY TESTS

Sparkle



Auto-detection of dark & bright blobs (mura)

EXAMPLE: IDENTIFYING MURA



Contrasting anomalies like dead pixels, particles, and lines

EXAMPLE: IDENTIFYING DEFECTS



UNIQUE APPLICATIONS

AR/VR HeadsetsView Angle OLED Correction

Head-up Displays Near-IR Sensing Anti-Glare Layers



BACKLIT SYMBOLS, PANELS, AND SIGNS

(Above) Luminance and CIE color coordinates of a point on a panel under analysis.

(Right) Luminance and CIE color coordinates for all backlit symbols on panel.



LIGHT SOURCE MEASUREMENT

Lighting design file generation (IES) using luminous intensity distribution.

Light source evaluation using rotating imaging goniometer to measure light from all angles in lab.



SUMMARY

Imaging photometers and colorimeters have significant advantages for light & color and precise spatial measurement.

By quantifying human visual perception, process improvements can be driven to ensure quality that accurately reflects customer experience.

Imaging photometers and colorimeters meet a range of measurement applications; one system can meet multiple needs.

Radiant Vision Systems | A Konica Minolta Company

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principles of light and color measurement...the measurement of color as perceived by the human eye....

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