A combined cavity ring-down (CRD) and photometry technique is employed to measure the transmittance of optical laser components in a range extending from below 0.01% to over 99.99%. In this combined technique, the conventional photometric configuration is used to measure, by ratioing the transmitted light power to the input power, the transmittance ranging from below 0.01% to over 99% with a typical relative uncertainty below 0.3%, and the CRD configuration is used to measure the transmittance higher than 99% with an uncertainty below 0.01%. Eight test samples with transmittance in the range of nearly 99.99% to approximately 0.013% are experimentally measured. Uncertainties of approximately 0.0001% for the transmittance of 99.9877% and of 0.003% for the transmittance of 0.013% are achieved with respectively the CRD and photometric schemes of a simple experimental apparatus. The experimental results showed that the combined technique is capable of measuring the transmittance of any practically fabricated optical laser components.