We describe a single beam device that records continuously as a function of wavelength, simple quantities related to the reflectance and transmittance, (1 - R)/(1 + R) and (1 - T)/(1 + T). The unit is easily attached to any spectrometer. It is sensitive to changes in these quantities of about 1 x 10(-4), while systematic errors in alignment set an absolute accuracy of about 1 x 10(-3).