To provide a highly stable optical calibration standard which does not require recalibration at a factory or such a design that the optical calibration standard may be maintained constant even after environmental changes and the passage of long time and provides standard information over a wider band than a gas standard.

A first standard 120 is used as a transmission standard for a second standard 140 and provides calibration information for the second standard 140. The first standard 120 is used for calibrating the second standard 140 over the range of the first standard 120. Then the information and response characteristics of the second standard 140 are extrapolated to other object frequencies or wavelengths outside the range of the first standard 120.