UV-Vis Frequently Asked Questions - Relative Total Reflectance
What is relative total reflectance?
Determine relative total reflectance by measuring all the reflected light, including both specular and diffuse reflected light, as shown here. This is effective for samples with either rough or shiny surfaces.
Relative total reflectance is measured using an integrating sphere attachment. Relative total reflectance is measured by shining light on the sample at an incident angle of about 10 degrees or less (8 degrees for most integrating spheres) and using an integrating sphere to measure not only the diffuse reflected light, but also the specular reflected light. Typically, a white plate with a diffuse surface, such as barium sulfate, is used as the reference plate. However, unlike relative diffuse reflectance measurements that do not include specular reflected light, aluminum-coated mirrors are also often used as the reference plate if the sample has a high specular component.
An integrating sphere can make an absolute measurement if a “calibrated” reference white plate is employed for the baseline correction. The known values of the calibrated white plate can then be used to mathematically “normalize” the relative values measured by the accessory.
The figure here shows a diagram of the measurement.
Reverse the roles of sample and reference beams in measurement parameter settings.
In Shimadzu software, this means setting the S/R switching function to reverse.
Attach a white reference plate at position (2) of the integrating sphere. Perform baseline correction. The sample beam shines on the white reference plate.
Remove the white reference plate from position (2) and attach the measurement sample in its place. For Shimadzu integrating spheres, the beam shines on the sample at an incident angle of 8 degrees.
The graphs here show the results from using a barium sulfate reference plate to measure the relative total reflectance of an opaque plastic material. Relative Diffuse Reflectance Measurement. The graph at right shows a comparison of the relative diffuse reflectance spectrum and relative total reflectance spectrum. Because the relative total reflectance includes the specular reflectance components, the reflectance level is higher than the relative diffuse reflectance. The difference between is the approximate specular contribution.