New design to measure the absolute spectral reflectivity

A new type of the normal incidence scanning reflectometer has been designed and constructed. In the system design, instead of using a reference reflective sample or adjusting the optical path as did in the traditional design, we use a fixed M-type fused quartz prism to split the incidence light source beam into two ones by a total internal reflection configuration. Therefore, these two light beam have the same spot sizes, intensities and spectral response. One beam goes directly to the detector and is used as the incidence reference beam. The second sampling beam goes to the sample first, and then is reflected by the sample with the beam intensity measured finally by the same detector. A metal disk with three holes is driven precisely by a stepping motor and is used to control the reference beam, reflection beam and background signals to be measured in sequence. Afterwards, the absolute reflectivity of the sample at the certain wavelength can be determined immediately by the computer through calculating those three signals. The system is controlled automatically by the computer and worked in a wavelength range from 400 to 800 nm under a 5-degree nearly normal incidence angle condition. In the paper, the details of the system design, optical element configuration and error reduction are given and discussed. The measured reflective spectral results for testing samples are also given and shown in good agreement with those measured by other optical method. The system designed in this work, however, is simpler and more reliable to be used in many optical measurements of the sample.