This chapter discusses the mechanism of steady-state relaxation spectrophotometry. The design of a spectrophotometric system is concerned with maximizing the information-to-noise ratio, by increasing the information, minimizing misinformation, and minimizing noise. In steady-state relaxation of sinusoidally driven systems, the photosynthetic apparatus is driven by actinic light, which is modulated to give 50% on and 50% off time of variable frequency. The wave form of the actinic light is preferably sinusoidal. This forces the electron carriers to respond in a sinusoidal response at the frequency of the actinic light. The amplitude and phase relationship of the carrier's response is governed by the appropriate kinetic parameters. It allows noise reduction through extreme narrow banding. The main parts in the spectrophotometer instrument includes (1) a source of monochromatic light and suitable detecting system, (2) a source of strong modulated actinic light, and (3) a rotating sector disk which causes the sample to be alternately illuminated by the measuring and actinic lights.