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IMCO: Measuring Color

Posted Sep 16, 2021 Updated Oct 3, 2021 6 min read views

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Konica Minolta Sensing

How can we measure colors ?

  • Spectrophotometer
    • Response through the entire visible spectrum
    • Relatively small areas (few $cm^2$) - Resolution is 1 point
    • “Falt surfaces”
  • RGB Camera
    • Response in 3 wavelengths (Red, Green Blue)
    • Large areas - High Spatial Resolution ($\lt50$MPixels)
    • Any kind of surfaces
  • Hyperspectral camera
    • Response throughout the entier visible spectrum (and more)
    • Large areas - Low Spatial Resolution ($\le2$MPixels)
    • Any kind of surfaces

When NOT to Measure color

  • Using instrument to measure color and compute differences objectively is not always needed
  • For example: A company has a corporate color (possible $^{TM}$)
    • Tour de France: Pantone 123C
    • Veuve Cliequot: Pantone 137C
    • Louboutin: Pantone 18.1663TP
  • Products carrying the color are sold; however they are manufactured by different providers

Judging by visual assessment

  • Need consistent lightning
  • Need consistent viewing
  • Need to Check for Metamers
  • Sufficient when there are few standard samples to be matched
  • Sufficient when tolerance is judged visually by color experts
  • Requires all manufacturers to have a physical copy of the standard, and to have the same hardware
  • Because there are no measurements, we don’t know to adjust color workflow in case we need to match a color

Measuring with Spectrophotometers

Remember Light interaction

Time for another video

What is a Spectrophotometer?

Light Reflection vs Material

  • Matte
    • Light is reflected in all directions equally
  • Semiglossy
    • Light is reflected in all direction but a small part is reflected orthogonal to the incident angle
  • Glossy
    • Light is reflected in all directions but a big part is reflected orthogonal to the incident angle

Spectrophotometers: In a Nutshell

  • Spectral reflectance
    • The ratio of reflected light ($r$) to the incident light ($i$) under specific geometric conditions
\[R_{\lambda}=\frac{\phi_{\lambda}^r}{\phi_{\lambda}^i}\]
  • Spectral transmittance
    • The ratio of transmitted light ($t$) to the incident light ($i$) under specific geometric conditions
\[T_{\lambda} = \frac{\phi_{\lambda}^t}{\phi_{\lambda}^i}\]
  • All measuring instrument need to be calibrated
    • using White Tile made from Spectralon

Spectrophotometers: reflectances ?

Interlude: fluoresence

  • Use an instrument called a Bispectrometer to measure it

Donaldson matrix obtained from a green sample emitting a more satured green light

Colorimeter vs Spectrophotometers

  • Colorimeters are used generally to calibrate screens
  • They mimic the way our eyes perceive color

They measure reflectance in 3 wavelengths (R, G, B) They do not provide a spectral response

Spectrophotometers

Types

  • Bidirectionnal
    • Non-structured and flat surfaces (paper, plastics)
  • Sphere
    • Structured and glossy surfaces (textiles, metallic)

SPIN vs SPEX

  • SPIN Specular Included (gloss is accounted for)
    • Color is measured independent of the sample’s gloss or surface texture
  • SPEX Specular Excluded

Specifications

Example: Automotive interior plaque (items produced using different materials)

  • SPIN: looks at the material independant of surface texture
  • SPEX: values which depend on gloss and surface conditions

Different spectro models

Specifications

 X-RIte i1Pro 2X-RITE Ci62Barbieri LFP qb
Measurment geometry$45^o$ a:$0$ (ring illumination)$di:8^o$$45^o$c:$0$ (circumferential)
Light sourceGas filled tungsten lamp and UV LEDGas-filled tungsten lamp3 point circle, 7-LED chip

Geometry

Reflectance of a semi-glossy object

  • $di:8^o$
  • $\color{orange}{45^o:0}$: measure that color difference
  • $\color{green}{di:8^o}$ measure the same color in both cases
  • $\color{orange}{45^o:0^o}$ simulates normal behavior
    • e.g. when we read a magazine

Aperture

 X-RIte i1Pro 2X-RITE Ci62Barbieri LFP qb
Measurment aperture$4.5mm$$4$ or $8mm$$2$,$6$ and $8mm$

Small aperture

  • Measures quickly
  • may miss relevant info

Large aperture

  • more accurate
  • measurement takes longer
  • needs larger sample

Conditions

 X-RIte i1Pro 2X-RITE Ci62Barbieri LFP qb
Measurment conditionsM0, M1, M2N/AM0, M1, M2, M3
  • M0
    • legacy measurement (tungsten lamp, no standardization of UV content in illuminat, UV strength changes through time)
  • M1
    • Spectral distribution of illuminant
  • M2
    • UV is excluded
  • M3
    • Polarized light

Spectral range

 X-RIte i1Pro 2X-RITE Ci62Barbieri LFP qb
Spectral range$380-730nm$$700-400nm$$380-750nm$

Repeatability

 X-RIte i1Pro 2X-RITE Ci62Barbieri LFP qb
Short term repeatability$0.1$ $\Delta E_{94}$$0.05$ $\Delta E_{ab}$$0.05$ $\Delta E_{00}$

2 different i1Pro 2 spectro

  • 10 measurements of the same object were taken for each instrument
  • $\Delta E$ between first and other 9 measurements were computed for each instrument
 X-RIte i1Pro 2X-RITE Ci62Barbieri LFP qb
Inter-instrument agreementAverage $0.4$ $\Delta E_{94}$ Max $1.0$ $\Delta E_{94}$Average $0.4$ $\Delta E_{ab}$ Max $1.0$ $\Delta E_{ab}$Average $0.4$ $\Delta E_{00}$ Max $1.0$ $\Delta E_{00}$

Transmittance Measurement

  • When we need transmittance ?
    • Light Filters
    • Printed Ads
    • Food Inspection

Inter-instrument agreement

  • Compared measurements of 16 samples used for printing

Recap

  • Many different (standardized) methods to measure Reflectance (and Transmittance)
  • Unfortunately, measured Reflectance/Transmittance is not unique as it depends on the instrument you sued to measure it
  • Type of instrument to used depends on what you want to measure, and how frequent you want to measure
  • Only measurements tales under the same conditions can be truly compared. Therefore, it is necessary to note the following information in a color measurement report:
    • Color instrument (geometry, aperture, measurement condition)
    • Illuminant/observer standards, if you give $L\times a\times b$ values

Future trends: beyond color

Visual appearance of materials

  • Reflection
  • Transmission
  • Absorbance

BRDF Measurement

  • BRDF allows characterizing the surface appearance at a microscopic level (used in Computer Graphics to render objects)
  • Measurable with Goniophotometers

Sources

Metamerism

What’s that ?

Most important types

  • Illuminant Metamerism
    • Different spectral characteristic and
      • same color when viewed under one light
      • different color when view under another light
  • Observer Metamersim
    • Different spectral characterisic and
      • same color when viewed by one observer
      • different color when view by another observer

Examples:

Car industry

Source

Other

Source

Metamerism vs Color Inconstancy

  • Color inconstancy: A single object changing color with changes in the color of the illumination
  • Metameric pair: Two objects having color inconstancy

Recap

  • Metamerism is an effect we need to consider if a pair of objects will be viewed under more than one type of illuminant
  • In the printing industry, neutral (grayscale) colors are more susceptible to illuminant metamerism as a mix of inks is used
  • In the case of displays, illuminant metamerism is not a problem as they create their own light
Image S9, IMCO
Image S9 IMCO
This post is licensed under CC BY 4.0 by the author.
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