Reflectance
Reflectance spectroscopy is the ideal technique to measure the spectral properties of opaque samples in the wavelengths of analysis that are adsorbed on surfaces, powders, or colloidal media. Instead of measuring the amount of transmitted radiation, reflectance measures the amount of radiation that is reflected by the sample. The spectrum obtained from the reflectance measurement is partly similar to that obtained from the transmittance measurement.
Reflectance for Qualitative Analysis
The reflectance spectrum is represented as a plot of reflectance percentage (%R) against the wavelength, where the reflectance (R) is described by the following equation:
Here, Iᵣ corresponds to the reflected radiation intensity by the sample, and I₀ corresponds to the incident radiation intensity, which is obtained from reference material. Although the wavelengths at which the absorption of incident radiation takes place in reflectance are the same as transmittance, the relative intensity of the peaks in each technique differs (peaks with a weak intensity in the transmission spectrum have great intensity in the reflectance spectrum).
How to Apply the Kubelka-Munk Transformation to a Reflectance Spectrum
The basic function from Kubelka-Munk theory established that, for an ideal diffuser and infinitely thick sample, R∞ is related to K and S (K is the absorption coefficient, and S the dispersion coefficient) by the remission function, F(R∞), also named Kubelka-Munk function:
The reflectance of highly concentrated powder samples is usually measured by mixing them with a highly dispersive Barium Sulfate powder.
Differences Between Specular and Diffuse Reflectance
For a mirror-like surface, the angle that the incident beam makes with the normal surface is equal to the angle of the reflected beam, and it’s called specular reflection. The diffuse reflection is more complex because when the beam is incident on the surface, there are multiple beams reflected in various directions due to the irregularities in the surface, see Figure 1.
Figure 1 –Specular and Diffuse Radiation
The radiation reflected by a sample contains a specular and diffuse component, see Figure 1. For rough surfaces, to obtain a well-defined diffuse reflectance spectrum, it is fundamental to reduce the specular component of the radiation. Reducing the specular component and maximizing the diffuse component of the reflectance spectrum can be achieved by choosing the best spectroscopic setup, changing the angle of the incident light, or with actions on the sample side like decreasing the particle size, in the case of a powder or colloidal or improving the sample packing are other important conditions to take into consideration when the diffuse component must be maximized.
Typical Setup for Reflectance Measurements
The light produced by a tungsten-halogen, LS-W, or a high-powered deuterium tungsten-halogen light source LS-DWHP, is guided using a reflectance probe through six illumination fibers to a reflectance standard. The reflected light is collected in the center of the probe tip through a collection fiber that connects to the spectrometer and records the intensity of reflected light in real time. Changing the standard for the sample will result in the detection of a different intensity of reflected light and, therefore, reflectance spectra.
For UV-Vis-NIR reflectance measurements, Sarspec has two light sources available: a high-powered deuterium tungsten-halogen light source (LS-DWHP) for the UV-Vis-NIR range and a tungsten-halogen (LS-W) light source for the Vis-NIR range. Sarspec has also reflectance probes with different core diameters and lengths for illumination and collection purposes. These reflectance probes can be combined with our standard and multi-angle probe holders that yield simple and accurate measurements. Finally, the setup is completed with our FLEX spectrometer.