Specular variable angle absolute reflectance method and reflectometer

A method of measuring a reflectance of a sample having a sample axis, the method comprising:

• projecting a light beam from a light source towards a mirror system;
• thereafter reflecting the light beam from the mirror system towards the sample, the sample rotated by a first angle about the sample axis;
• thereafter reflecting the light beam from the sample towards an elliptical roof mirror, the elliptical roof mirror having an ellipsoidal reflector surface, the elliptical roof mirror rotated by a second angle about the sample axis, the second angle being about twice the first angle;
• thereafter reflecting the light beam from the ellipsoidal reflector surface of the elliptical roof mirror back towards the sample;
• thereafter reflecting the light beam from the sample back towards the mirror system;
• thereafter reflecting the light beam from the mirror system towards a detector, whereby a modified light beam is generated;
• and
• calculating the reflectance of the sample based on optical properties of the modified light beam as detected by the detector.

A method of using a specular variable angle absolute reflectometer comprising a light source;

• a mirror system in a light path of the light source, the mirror system configured to reflect a light beam from the light source towards a sample that is optically reflective;
• an elliptical roof mirror disposed in the light path after the sample, the elliptical roof mirror having an ellipsoidal reflector surface configured to reflect the light beam back towards the sample;
• a mechanism connected to the elliptical roof mirror, the mechanism being configured to rotate the elliptical roof mirror about an axis of the sample;
• and a detector in the light path after the elliptical roof mirror such that the detector receives light that has been reflected from the elliptical roof mirror, thence back to the sample, thence back to the mirror system, and thence to the detector, the method comprising:
• removing the sample and verify a sample holder does not restrict the light beam;
• aligning the elliptical roof mirror to a 100% configuration;
• measuring a signal at the detector to form a 100% measured value;
• measuring a total light source power at the detector;
• thereafter moving a mirror system so that the light beam is projected into a light trap to interrupt the light source;
• thereafter measuring a background noise at the detector;
• thereafter moving the mirror system back to receive the light beam;
• replacing the sample into the path of the light beam;
• rotating the sample to a desired incident angle;
• rotating the elliptical roof mirror to a complimentary reflection angle;
• measuring a sample reflection at the detector to form a measured value;
• measuring a total light source power at the detector;
• thereafter moving a mirror system so that the light beam is projected into a light trap to interrupt the light source;
• thereafter measuring a background noise at the detector;
• thereafter moving the mirror system back to receive the light beam;
• calculating a reflectance of the sample as a ratio of the 100% measured value with background compensation;
• and
• calculating an absolute reflectance of the sample as a square root of the reflectance to the 100% measured value.