Encyclopedia of Color Science and Technology

2016 Edition
| Editors: Ming Ronnier Luo

Reflectance Standards

Reference work entry
DOI: https://doi.org/10.1007/978-1-4419-8071-7_360

Definition

A reflectance standard is a physical reference sample that includes ratio values between the total amount of radiation, as of light, reflected by a surface, and the total amount of radiation incident on the surface across the visible spectrum. Reflectance standards are used for the calibration and verification of spectrometers. There are two kinds of reflectance standards: diffuse and specular. In practice, white and black reflectance standards are utilized for calibration. They require a similar specular or diffuse surface property to the device under test (DUT).

Applications

Diffuse and specular reflectance standards are used to calibrate colorimeters, reflectometers, spectroradiometers, bidirectional reflectance distribution function (BRDF) scatterometers [1], ultraviolet–visible (UV–VIS) spectrophotometers, and Fourier transform infrared (FTIR) spectrometers.

Diffuse white reflectance standard samples can be obtained with diffuse reflectance of 98 % or more. Some materials can be carefully sanded (some require water with the sanding) or cleaned to refresh the surface to its maximum reflectance as the surface becomes soiled or contaminated. Such reflectance standards can be used for making illuminance from a luminance measurement of the standard. Moreover, with the proper measurement geometry, luminance factor can also be obtained. Therefore, the measurement geometry is used to calibrate the standard. If the reflectance (or diffuse reflectance) of the standard is around 98 % or 99 %, it usually does refer to the reflectance; that value can then only be used for a uniform hemispherical illumination. For industrial applications, white ceramic tiles are often used for a reflectance of 90 %. If we use an isolated source at a particular angle, there is no reason to expect that the 99 % value is even close to the proper value of the luminance factor for that geometrical configuration [1, 2, 3].

For calibration of the spectral reflection, one reference white standard and one reference black standard of calibrated spectral reflectance factor R WS (λ) and R BS (λ) respectively for the same measurement geometry as in the test configuration can be used [4]. Figure 1 depicts the white and black specular reflectance standards. In many commercial systems, black wedges are widely used for black specular reflectance standards of optical measurements.
Reflectance Standards, Fig. 1

(a) White and (b) Black specular reflectance standards

To measure the spectral reflection of the device under test (DUT), set up the measurement geometry and warm up the light source. First, measure the spectral reflectance factors of the reference white standard, R WS ′(λ), with a spectrophotometer. Second, measure the spectral reflectance factors of the reference black standard, R BS ′(λ). Third, measure the spectral reflectance factor of the DUT, R′(λ). The spectral reflectance factor, R(λ), is calculated in Eq. 1.
$$ R\left(\lambda \right)=\left[{R}^{\prime}\left(\lambda \right)-{R_{BS}}^{\prime}\left(\lambda \right)\right]\cdot \left[\frac{R_{WS}\left(\lambda \right)-{R}_{BS}\left(\lambda \right)}{{R_{WS}}^{\prime}\left(\lambda \right)-{R_{BS}}^{\prime}\left(\lambda \right)}\right]+{R}_{BS}\left(\lambda \right) $$
(1)
where R WS (λ) and R BS (λ) are the calibrated spectral reflectance factors of the reference white standard and the reference black standard respectively. Then, International Commission on Illumination (CIE) tristimulus values, X, Y, and Z are obtained as
$$ \left\{\begin{array}{c}\hfill X=k\cdot {\displaystyle \sum_{\lambda }R\left(\lambda \right)\cdot S\left(\lambda \right)}\cdot \overline{x}\left(\lambda \right)\cdot \Delta \lambda \hfill \\ {}\hfill Y=k\cdot {\displaystyle \sum_{\lambda }R\left(\lambda \right)\cdot S\left(\lambda \right)}\cdot \overline{y}\left(\lambda \right)\cdot \Delta \lambda \hfill \\ {}\hfill Z=k\cdot {\displaystyle \sum_{\lambda }R\left(\lambda \right)\cdot S\left(\lambda \right)}\cdot \overline{z}\left(\lambda \right)\cdot \Delta \lambda \hfill \end{array}\right. $$
(2)
where
$$ k=\frac{100}{{\displaystyle \sum_{\lambda }S\left(\lambda \right)}\cdot \overline{y}\left(\lambda \right)\cdot \Delta \lambda } $$
  • S(λ): the relative spectral power distribution of the illuminant considered

  • \( \overline{x}\left(\lambda \right) \), \( \overline{y}\left(\lambda \right) \), \( \overline{z}\left(\lambda \right) \): color-matching functions of CIE 1931 standard colorimetric observer

  • R(λ): spectral reflectance factor of the DUT

  • Δλ: wavelength interval

  • λ: wavelength range from 380 to 780 nm

For the calibration of the filter photometric reflection, one reference white standard and one reference black standard of calibrated CIE tristimulus values (X WS , Y WS , Z WS ) and (X BS , Y BS , Z BS ) respectively for the same measurement geometry as in the test configuration shall be used [4]. The filter photometric reflection procedure of the DUT is as follows.

First, set up the measurement geometry and warm up the light source. Second, measure the CIE tristimulus values of the reference white standard, X WS ′, Y WS ′, and Z WS ′ using a colorimeter. Third, measure the CIE tristimulus values of the reference black standard, X BS ′, Y BS ′, and Z BS ′. Finally, measure the CIE tristimulus values of the DUT, X′, Y′, and Z′. CIE tristimulus values, X, Y, Z are obtained using Eq. 3.
$$ \left\{\begin{array}{l}X=\left[{X}^{\prime }-{X_{BS}}^{\prime}\right]\cdot \left[\frac{X_{WS}-{X}_{BS}}{{X_{WS}}^{\prime }-{X_{BS}}^{\prime }}\right]+{X}_{BS}\hfill \\ {}Y=\left[{Y}^{\prime }-{Y_{BS}}^{\prime}\right]\cdot \left[\frac{Y_{WS}-{Y}_{BS}}{{Y_{WS}}^{\prime }-{Y_{BS}}^{\prime }}\right]+{Y}_{BS}\hfill \\ {}Z=\left[{Z}^{\prime }-{Z_{BS}}^{\prime}\right]\cdot \left[\frac{Z_{WS}-{Z}_{BS}}{{Z_{WS}}^{\prime }-{Z_{BS}}^{\prime }}\right]+{Z}_{BS}\hfill \end{array}\right. $$
(3)
where X WS , Y WS , Z WS and X BS , Y BS , Z BS are the calibrated CIE tristimulus values of the reference white standard and the reference black standard respectively.

The measurement method of spectral or filtered photometric reflection is performed for the reflection of the DUT by using a spectrophotometer or a colorimeter. Compared with the filter photometric reflection, spectral reflection measurement results include wavelength information. However, a spectrophotometer is more expensive than a colorimeter. According to practical applications, the measurement method of spectral or filter photometric reflection is appropriately utilized. Typically, color, fluorescence, grayscale, and wavelength reflectance standards are usually used in the color industry. They are used to check the reliability of the instruments used. Various types of standard according to their application are introduced here.

Color Reflectance Standards

Diffuse color standards are used to calibrate colorimeters and spectrophotometers within the range 360–830 nm. Compared with standards for gloss surfaces, the diffuse nature of the standards simplifies measurements by removing the effects of viewing or illumination geometries.

Fluorescence Reflectance Standards

Fluorescence standards aid in the development of optically brightened materials, such as paper and textiles, and are widely used in the cosmetics industry. The appearance of an illuminated object is dependent upon the illumination and viewing conditions. For example, the total radiance factor of paper and board products containing fluorescent whitening agents (FWAs) is the sum of the reflected radiance factor and the luminescent radiance factor [5]. As the luminescent radiance factor originates from fluorescence of the FWAs, its magnitude depends on the amount of UV radiation of the illumination. It is therefore essential to calibrate not only the radiance (reflectance) factor scale, but also the UV content of the illumination used in the measurement apparatus. The adjustment of the UV content is achieved by altering the position of the UV filter in the apparatus so that the light incident upon the sample has an effective UV content corresponding to that in the CIE illuminant C. The UV filter adjustment is based on the ISO brightness (C/2°) value [5].

Grayscale Reflectance Standards

Grayscale diffuse reflectance standards are used to establish the linearity and accuracy of reflectance spectrophotometers and colorimeters. The standards are suitable for users who need a wide dynamic range of reflectance values that are abbreviated in range, but not in number of calibration steps. Industries such as clay and processed minerals, paper, and paints typically require such a range, which accurately reproduces the range of reflectance values exhibited by those products.

Wavelength Reflectance Standards

Wavelength calibration standards provide stable absorption spectra for validating the wavelength calibration of spectrophotometers in the ultraviolet–visible-near infrared (UV–VIS-NIR) region. Complete absorption spectral data are supplied with each standard. A holmium oxide standard is available for UV–VIS-NIR calibrations, while a dysprosium oxide standard is offered for NIR calibrations. Meanwhile, an erbium oxide standard is provided for VIS-NIR calibrations. Alternatively, a mixed oxide standard is available, which is doped with all three rare earth oxides for use over the entire UV–VIS-NIR region.

Cross-References

References

  1. 1.
    VESA 2.0: Flat Panel Display Measurements Standard (2001)Google Scholar
  2. 2.
    CIE 15: Colorimetry. 3rd edn (2004)Google Scholar
  3. 3.
    SID IDMS Information Display Measurements Standard, v1.03, 1 June 2012Google Scholar
  4. 4.
    SEMI D68-0512: Test Methods for Optical Properties of Electronic Paper Displays (2012)Google Scholar
  5. 5.
    ISO 2470–1:2009: Paper, board and pulps – measurement of diffuse blue reflectance factor – Part 1: indoor daylight conditions (ISO brightness) (2009)Google Scholar

Copyright information

© Springer Science+Business Media New York 2016

Authors and Affiliations

  1. 1.Department of Mechanical and Mechatronic EngineeringNational Taiwan Ocean UniversityKeelungTaiwan