Colorimetry: Understanding Colors and Light Perception

1. Colorimetry Silja Holopainen 29.3.2006

2. Outline • Introduction to colorimetry • Colorimetry in general • Measuring diffuse reflectance • Measuring fluorescence • Measuring transmittance • Measurement geometry and special cases • Conclusion

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. Reference spectrofluorimeter at NRC

31. Gonio-fluorometer at TKK

32. 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

33. 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

34. 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

35. 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

36. 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

37. 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

38. 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

39. Special cases • Metallic and pearlescent samples • Retroreflecting samples • Lamps, light sources and displays

40. 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