LEDs in frequency-domain spectroscopy of tissues

Light spectroscopy in the frequency-domain has been used to study the optical properties of biological tissues. We have analyzed the possibility of using light emitting diodes (LEDs) as intensity modulated light sources for frequency-domain spectroscopy. The use of LEDs presents several advantages: one LED's output covers a spectral region of about 80 nm, and commercially available LEDs allow for the coverage of the spectral range from 550 to 900 nm, which is a region of interest in near infrared medical applications; the light output of an LED is stable with respect to that of lasers and lamps; the wide angular disthbution make LEDs safe for in vivo studies. Furthermore, LED frequency-domain spectroscopy is a relatively inexpensive technique. We describe some circuits we used to modulate the intensity of LEDs at radio frequency, and point out the possibility of building a multi-source spectrometer. Some applications of LED frequency-domain spectroscopy, both in vitro and in vivo, are shown.