For a long time continuous wave near infrared instruments have been used to detect oxygenation changes in tissue. These instruments have proven to be reliable. The new generation of instruments, such as phase-modulated systems, or time-of-flight instruments, is not yet reliable enough for clinical applications. Most available continuous wave near infrared instruments have low temporal resolution and low signal-to-noise ratio. For functional brain imaging, for example, a sensitive and fast instrument is needed. Therefore we developed the OXYMON, an instrument with a sample frequency up to 50 Hz and for optical densities up to 9 OD. The instrument uses 3 laser diodes, and is equipped with 1 or 2 avalanche photo detectors. Modular building techniques make maintenance easy.