We present a method for measuring weak absorption in optical materials, using a new diferential
interferrometry technique. Both thermal shift phase and thermal characteristic time may be determined by
fitting the observed time-resolved interferometric signal.
We report on the optical characterization and measurement of oxygen singlet quantum yield of Chlorophyll and Chlorin e<sub>6</sub> in water-ethanol mixtures by direct observation of thermal relaxation using time resolved thermal lens method. The analysis of the time-resolved curve allows the determination of the quantum yield of singlet oxygen formation. The quantum yield is deduced from the relative magnitudes of the fast and slow components.