In the classical conversion method, the light that passes the medium is converted to the photosignal. Then, it is amplified by an amplifier having linear characteristics. Considering the amplitude of the amplified signal, the measured parameter is determined. In the proposed conversion method, the intensity of the reference light that falls on the medium is inversely modulated with respect to the absorption function. The intensity of the light used for the measurement is kept constant. The intensity modulated reference light and the measurement light, which are in inverse phases, pass through the medium subsequently. Then, they are detected by a photodetector. At the output of the photodetector, the phase changing instant of the obtained signal and the duration between the starting instant of the reference light and the phase changing instant are determined. Considering this duration, the value of the measured parameter is determined. The proposed conversion method linearizes the conversion phenomenon. The photodetector does only detect the instant at which the reference light and the measurement light are similar. The photodetector operates as an indicator. The measured parameter is converted to the pulse duration or the number of pulses. The value of the measured parameter can be directly obtained at the output of the photodetector. The comparison and the mathematical operations are directly achieved on lights. Analysis of the proposed method and application examples are given in the paper. (c) 2005 Elsevier B.V. All rights reserved.