Light can reveal much about tissue without ever damaging tissue. Such remote assessment is a key strength of optical techniques. Light does not penetrate deeply in tissues.
Visible light penetrates only a few mm through tissues.
Near infrared light penetrates only a few cm through tissues. This is both a strength and a weakness. It is a weakness because light can only interrogate limited volumes of tissues. It is a strength because much of the body consists of thin tissue layers, therefore optical techniques are well-suited for localized interrogation of tissue layers. Many medical decisions are based on the status of thin tissue layers.