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Colorimetry. Silja Holopainen 29.3.2006. Outline. Introduction to colorimetry Colorimetry in general Measuring diffuse reflectance Measuring fluorescence Measuring transmittance Measurement geometry and special cases Conclusion. Introduction.

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Colorimetry


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colorimetry

Colorimetry

Silja Holopainen

29.3.2006

outline
Outline
  • Introduction to colorimetry
  • Colorimetry in general
  • Measuring diffuse reflectance
  • Measuring fluorescence
  • Measuring transmittance
  • Measurement geometry and special cases
  • Conclusion
introduction
Introduction
  • Color has always been important in art, religion and clothing
  • At present color is also used for signs, safety cloths, paper whitening etc.
  • It is often important to be able to measure color accurately
what is color
What is color?
  • Electromagnetic radiation between 380-780 nm
  • Color is one aspect of appearance
  • Color = light source + object properties + eye + brain
  • The human eye is most sensitive at 555 nm
the three dimensions of color
The three dimensions of color

White

  • Hue distinguishes blue from green from yellow etc.
  • Lightness distinguishes light colors from dark colors
  • Chroma describes how different a color is from grey

Lightness

Chroma

Hue

Black

Hue

Lightness

Chroma

colorimetry6
Colorimetry
  • Two objects may appear the same when viewed under one light source, but different under another = metamerism
  • Metamerism is one of the major industrial problems in color matching
  • Colorimetry attempts to quantify the perception of color
  • CIE is a voluntary organization giving recommendations concerning modern colorimetry
sources and illuminants
Sources and illuminants
  • Source = physical entity that produces radiation
  • Illuminant = table of values of spectral power distribution
  • Illuminant D65 represents average daylight. D50 represents typical indoor light
color perception
Color perception
  • 92 % of men and 99,5 % of women have “normal” color vision
  • The retina comprises rod cells (night vision) and cone cells (color vision)
  • Majority of the cells are rod cells
  • There are three types of cone cells: one has peak sensitivity to blue light, one to green light and one to red light
tristimulus values
Tristimulus values
  • All colors can be matched by varying amounts of red, green and blue lights (X, Y and Z)
  • The amounts of X, Y and Z that must be mixed to match a color are called the tristimulus values
  • The tristimulus values depend on the reflectance or transmittance of the object, the illuminant and the observer
  • Pairs of objects are said to match when their tristimulus values are the same
the cie standard observers

Test Side

Matching

Side

Spectral

Light

Red +

Green +

Blue

The CIE Standard Observers
  • In the CIE experiment one half of a circular field is illuminated with spectrum color and the other with a mixture of red, green and blue
  • The observer adjusts the red, green and blue until it matches the spectrum color
  • The result is a set of color matching functions used to calculate the tristimulus values
color difference
Color difference
  • Color measurements are mostly made to determine quantitatively whether or not the color of two objects are the same
  • The total color difference ∆E and its coponents: lightness ∆L, chroma ∆C and hue ∆H can be numerically calculated
  • The color difference is calculated using the tristimulus values
  • Numerical color differences may be used for setting tolerances for quality control
objects

Incident

Light

Reflected

Light

Absorbed

Light

Objects
  • Objects are characterized by the amount of light they emit and reflect or transmit at each wavelength of interest
  • When light is incident on an object a part of it is absorbed, a part is reflected and a part may be trasmitted
  • The object may also emit light
  • All these characteristics contribute to the observed color

Transmitted

Light

reflectance
Reflectance
  • Specular (regular) reflectance = mirror like reflectance
  • Diffuse reflectance = reflectance in all directions
  • Gloss = combination of specular and diffuse reflectance

Specular

Diffuse

Glossy

definitions
Definitions
  • Reflectance ρ is the ratio of the total radiant flux reflected by the surface to the flux incident on the surface
  • Reflectance factor R is the radiant flux reflected in the direction delimited by a given cone to that reflected in the same direction by a perfectly reflecting diffuser identically irradiated
  • If the solid angle of the cone approaches a limit of 0 or 2π sr, reflectance factor R approaches radiance factor  or reflectance ρ
spectrophotometers and colorimeters
Spectrophotometers and colorimeters
  • Spectrophotometers are used to measure an object’s reflection characteristics
  • Colorimeters measure directly tristimulus values or related color coordinates
  • Colorimeters are less expensive and simple to use but less accurate for determining tristimulus values
  • Colorimeters determine the color difference between two samples better than tristimulus values
  • Colorimeters can not determine metamerism
measuring diffuse reflectance
Measuring diffuse reflectance
  • Instruments measuring the color of reflecting objects consist of an illuminator, a sample holder, and a receiver
  • The CIE recommends four illuminating and viewing geometries for making reflectance measurements: 45/0, 0/45, d/0, and 0/d
  • The most common instrument for measuring diffuse reflectance is the integrating sphere
  • Another type of technique, which is getting more popular, is the angular integration of gonioreflectometric measurement results
integrating sphere based techniques
Integrating sphere-based techniques
  • An integrating sphere is coated from the inside with uniformly diffusing material
  • It has openings for the sample, light source and the receiver
  • The idea is to either create a diffuse geometry of illumination or to collect light scattered diffusely by the sample
d 0 geometry

Photometer

Specular port

Light

source

Sample

d/0 geometry
  • The light is incident on the sphere wall and is reflected in all directions
  • As the result of multiple reflections the sample is illuminated from all directions
  • The sample is viewed in a near normal angle
  • The specular reflection is directed back to the source and is not measured
0 d geometry

Photometer

Baffle

Light

source

Sample

0/d geometry
  • The light is incident on the sample
  • The sample scatters the light and after multiple reflections it illuminates the detector from all directions
  • The 0/d geometry is equivalent to the d/0 geometry
absolute and relative measurement methods
Absolute and relative measurement methods
  • Relative measurement methods produce values that are relative to reference standards
  • Absolute measurement methods relate the reflectance values of a standard to that of the perfect reflecting diffuser
  • The relative methods are commonly used in industry, whereas the absolute methods are commonly realized in national standards laboratories
example of relative method
Example of relative method
  • Signals are measured from the sample, the reference, and the light trap (light incident on the trap)
  • The light trap gives the dark signal which is subtracted from the results
  • The sample and reference readings are compared and corrected by the known values of the reference

Sample

Holder

Reference

Entrance

Reference

Holder

Light

Trap

Sample

Entrance

example of absolute method

C

A

Light

source

Sample, Cap

or Light trap

B

Example of absolute method
  • Taylor’s method: Detector readings when the sample port is not covered (a), it is covered with sphere material (b), and it is covered by the sample (c)
  • Increase from a to b is proportional to the reflectance of the sphere
  • The reflectance of the sample is calculated from the ratio of a and c
goniometric techniques
Goniometric techniques
  • Gonia = angle
  • The idea is to illuminate the sample in a certain angle and measure reflectance on the surface of a hemisphere around the sample (or vice versa)
  • In practice this can be realized with a two-axis goniometer or with a one-axis goniometer by integrating over the polar angles
  • Enables bidirectional measurements
gonioreflectometer at tkk
Gonioreflectometer at TKK
  • One-axis goniometer
  • The idea is to illuminate the sample in one direction and measure reflectance over the semiarch
  • Total diffuse reflectance is obtained by integrating the measured values over the whole hemisphere
things to be considered
Things to be considered
  • The major source of uncertainty in the system is isochromatic stray light
  • The biggest contribution is light scattered about the main beam
  • To compensate the effect a significant correction factor must be used
  • In our previous system the correction factor was much greater than today due to the more complicated optics
gonio vs sphere
Gonio vs. sphere
  • Goniometric technique provides bidirectional measurements which are not possible with a sphere
  • The scattering of light about the main beam is clearly a problem for the gonio but not for the sphere
  • Systematic deviations have been reported earlier between goniometric and sphere-based techniques
  • The scattering of light about the main beam is a strong candidate for causing these discrepancies
fluorescence
Fluorescence
  • A fluorescent material absorbs some of the light incident on it and emits it on higher wavelengths
  • Part of the energy of the incident photon is lost in internal vibrations and heat
  • Fluorescence is used e.g. in paper whitening, safety signs and textiles
commercial fluorescent colorants
Commercial fluorescent colorants
  • Inorganic fluorophors: stable but toxic, used in security markings and fluorescent lamps
  • Optical whiteners: organic compounds, with excitation at 340-400 nm and emission at 430-460 nm, used heavily in textile, paper and plastic industries to whiten materials
  • Daylight fluorescent materials: organic compounds, emission and excitation in the visible part of the spectrum, used to color papers and plastics and especially in safety applications
measuring fluorescence
Measuring fluorescence
  • Polychromatic illumination → appearance and color
  • Monochromatic illumination → fluorescence separated from reflectance
  • Often we want to measure fluorescence quantum yield of a material
  • Fluorescence quantum yield = the number of emitted photons relative to the number of absorbed photons
  • Quantum yield measurements require monochromatic illumination and viewing
the principle of a ccd
The principle of a CCD
  • CCD = charge-coupled device
  • The CCD comprises a two-dimensional array of pixels
  • Every pixel gathers radiation from a different spatial position → large area of spectrum (~200 nm) measured in one picture
problems related to fluorescence
Problems related to fluorescence
  • Stability of the fluorescent standards
  • No universally recognized method for characterization of fluorescent instruments
  • Different instruments give different results
  • Even the same instrument can give different results over time
  • Comparing different fluorescent samples is difficult even with the same device
transmittance measurements
Transmittance measurements
  • Similarly to reflectance, we can have regular, diffuse or glossy transmittance
  • Transmittance is utilized e.g. in interference filters and glass filters
  • The most common measurement geometry is 0/0

Regular

Diffuse

Glossy

fabry perot filter and interference filters

Input

signal

Transmitted

waves add

In phase

Reflections

Fabry-Perot filter and interference filters
  • The cavity length determines the passed wavelength
  • MDI filter: thin partially transmitting metal layers
  • ADI filters: alternating layers of substances with differing refractive indices
  • Sensitive to temperature and angle

Fabry-Perot

cavity

a double beam transfer standard spectrometer at tkk

Light

Source

Reference

Detector

Unit

MC

Sample

A double-beam transfer standard spectrometer at TKK
  • Used to calibrate filters
  • The idea is to measure similar beams through the filter and through air
  • Detector readings from both sample and reference are compared to yield transmittance
a single beam reference spectrometer at tkk

Averaging

Sphere

Detector

A

OPM

Filter-holder

Unit

A single-beam reference spectrometer at TKK
  • Detector readings are taken through the filter, through air and dark reading
  • The filter and light trap can be moved into the beam by a linear translator
  • The measurement system can be modified to measure e.g. diffuse transmittance

Light

Source

MC

choosing measurement geometry
Choosing measurement geometry
  • Bidirectional illuminating and viewing geometries can be very sensitive to surface texture and polarization
  • Bidirectional geometries are similar to the way a person evaluates color visually
  • Diffuse geometries minimize the effect of a sample’s texture and gloss
special cases
Special cases
  • Metallic and pearlescent samples
  • Retroreflecting samples
  • Lamps, light sources and displays
conclusion
Conclusion
  • Color and appearance are important quantities in several branches of industry e.g. paper, textile and plastic industry
  • The color and appearance of a material are effected by the light source, observer and spectral properties of the material
  • Reflectance, transmittance and fluorescence measurements all require special instruments
  • Fluorescence measurements still present severe problems due to the instability of standards and lack of universal calibration methods of instruments