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Using a CW dye laser for intracavity laser absorption spectroscopy (ICLAS) with a controlled time generation and a time-resolved spectrum, we show that intracavity absorption follows the Lambert Beer Law. By comparing the absorption of the very weak $\gamma$ band of $O2b1\Sigma g+(v\prime =2)\leftarrow X3\Sigma g-(v\prime \prime =0)$ in ICLAS and in multi-pass cell $technique1$ we confirm that the intracavity technique gives reproducible results under different experimental conditions and is a highly sensitive quantitative method for measuring absolute absorption coefficients and collision broadening coefficients. This method does not necessitate any previous scaling in contrast with other sensitive methods like photo-acoustic or optogalvonic techniques. $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$J. J. Miller, L. P. Giver, R. W. Boese, J.Q.S.R.T. 16 (1976) 595.