The calibration of scanning monochromators for the measurement of absolute irradiance

Certain approximations commonly used in the absolute calibration of a scanning monochromator are examined in terms of the response of the instrument to a monochromatic input. The absolute irradiance due to any spectral feature in the neighborhood of lambda(0) is commonly computed from the expression W = AB(lambda(0))/H(lambda(0)) where A is the area of the spectral feature as recorded by the monochromator output trace, B(lambda) is the spectral irradiance of a standard source, and H(lambda) is the response of the monochromator to B(lambda) when the monochromator corresponds to wavelength lambda. As an example, approximations used in justifying such calculations are examined and applied to an Ebert 0.5-m monochromator. For the case chosen, the approximation is shown to be valid to an accuracy of 1.5% to 2%, depending upon assumptions made in the calculation. It is found that the most serious error for this example is introduced by changes in the sensitivity of the monochromator over a wavelength interval comparable with that of the spectral feature under investigation. A second source of error is found to be the change in the irradiance of the standard source over a wavelength interval comparable to the instrument resolving power.