Absolute specular reflectance

Absolute specular reflectance measurements are defined as the measurement of a material in which the only change from the measurement of the baseline to the measurement of the sample is that the sample changed in the optical chain. All angles of incidence and the total path length of the measurement are maintained throughout the measurement. This concept means that no artifact standard is necessary for the measurement, thus canceling a major source of uncertainty in the measurement of specular materials. 

The second geometry to measure absolute specular reflectance is the V-N geometry (Fig. 6). In this measurement geometry, the only difference is a single bounce off the sample at a particular angle. Although obviously an... 

Universal reflectance accessories to measure the absolute specular reflectance of polished surfaces and films are also available. 

For this modality of measurements, when collecting the dark current we take the sample out interrupting the beam path by the lack of a reflecting element and for collecting the baseline, we use a known reflectance standard mirror. No standard mirror is perfect, so to calculate the absolute specular reflection of a sample, we need to take the true reflectance curve for the standard mirror used to acquire the baseline into account. For that reason, these accessories are called relative specular reflectance accessories. 

Compared to the PAS spectra of cotton cloth, in the Specular Reflectance spectra, the signal-to-noiae is higher and any noise is low enough to show clearly the spectral features described here. Even though there are differences due to the change in sampling conditions, the spectra can be compared for a one-to-one comparison and identification of absorption bands. Mote however that in the PAS spectrum relative intensities are displayed and that the Specular Reflectance spectra show an absolute response and measurement which leads to a better possibility for quantification and a more rigorous treatment of the data. 

Since even a few percent error in R((o) is crucial to the Kramers-Kronig (K-K) analysis, extra care was taken in all procedures for obtaining absolute values for R(d)). Surface quality is essential for accurate reflectance measurements. Therefore, the surface morphology of each film was checked, both optically and using scanning electron microscopy [269], All the sample surfaces were of excellent optical quality and exhibited specular reflection. Thus, any scattering... 

This holds for the diffuse reflecting outer cylinder. The heat flow has decreased significantly. The ratio Q(N = 1 )/Q(N = 0) has the value 0.487/2.368 = 0.206. With a mirrorlike reflecting outer cylinder d2 is replaced by dg. This then gives Q(N = 1 )/L = —0.480W/m and Q(N = 1 )/Q(N = 0) = 0.480/1.948 = 0.246. In specular reflection of the outer cylinder, the relative decrease of the heat flow caused by the protective shield is somewhat lower than that for a diffuse reflecting outer cylinder. However, the smallest absolute value of Q/L is yielded when both the shield and the outer cylinder reflect mirrorlike. 

Both Carr and Brice, Halwer and Speiser have studied carefully the calibration of absolute intensity of scattering by comparison with the reflected intensity from diffusely reflecting surfaces, such as magnesium oxide or casein paint. Apparently none of these surfaces acts as an ideal diffuse reflector there is always at least a small fraction of the light which undergoes specular reflection, and this must be allowed for in the calibration of light scattering measurements. 

In order to avoid the problems coimected with measurement of absolute reflectivities, changes in reflectivity are detected as the potential of the electrode is modulated in the case of the so-called modulated specular reflectance spectroscopy. ... 

Specular reflectance measurements fall into two categories. Relative reflectance requires the use of a reference mirror to produce accurate measurements. Absolute specular measurements do not require a reference mirror but instead use various optical techniques to bring the reflected beam to a detector. Both of these measurements have their advantages and pitfalls that must be considered in choosing the measurement technique. 

Clarice, F. J. J. (1992). Absolute Regular Reflectance Standards for the Thermal Infrared Region, N.P.L. Publication No. DES h 076. Description of measurement techniques for mid-lR specular samples. 

Another problem arises in integrating sphere measurements when the sample cannot be placed flush against the port of the sphere, ff the sample is specular, the reference should be recessed by an identical distance as the sample to be measured. For materials that are primarily diffuse in character, we have determined the measured reflectance for a lambertian material decreases by approximately 3% (absolute) per millimeter of distance of the sample from the sphere up to approximately 4 mm. Beyond that, it is difficult to predict. Measurement of the reference at a similar distance helps but is still very inexact. 

Hanssen (2001) Integrating-sphere system and method for absolute measurement of transmittance, reflectance, and absorptance of specular samples by L. Hanssen, App/. Optics 40(19), 3196-3204. 

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