Diagnostics in medicine plays a critical role in helping medical professionals deliver proper diagnostic decisions. Most samples in this trade are of the human origin and a great portion of methodologies practiced in biology labs is shared in clinical diagnostic laboratories as well. Most clinical tests are quantitative in nature and recent increase in interests in preventive medicine requires the determination of minimal concentration of target analyte: they exist in small quantities at the early stage of various diseases. Radiometry or the use of optical radiation is the most trusted and reliable means of converting biologic concentrations into quantitative physical quantities. Since optical energy is readily available in varying energies (or wavelengths), the appropriate combination of light and the sample absorption properties provides reliable information about the sample concentration through Beer-Lambert law to a decent precision. In this article, the commonly practiced techniques in clinical and biology labs are reviewed from the standpoint of radiometry.