It has previously been recognized that wavelength derivative spectroscopy greatly enhances optical structure as compared to conventional absorption or reflectance methods. However, a practical wavelength derivative spectrometer must have a procedure to remove from the spectra the wavelength derivative of I0, the incident light intensity. A sensitive double beam wavelength modulation spectrometer is described which removes the dI0/dλ spectrum to a high degree of accuracy. The removal of the dI0/dλ spectrum is continuously optimized by means of a servo loop controlling the gain of the reference channel. The primary advantage of this spectrometer is that it does not require closely matched components in order to achieve double beam accuracy.
K. L. Shaklee and J. E. Rowe, "Wavelength Modulation Spectrometer for Studying the Optical Properties of Solids," Appl. Opt. 9, 627-632 (1970)