Hi Amir,

there is no simple answer to you question since any wavelength interacts with tissue. So if you do not kwow which physical effect you want to study. Diffuse Reflectance is usually due to body scattering. Scattering itselfe is dependent on the wavelength and the size of the targets (and other things, maybe have a look at wikipedia or a textbook). 

You should also consider absoption. If you choose a wavelength that is, e.g. stongly absorbed by water it migth not give you much signal in refectance.

As for the difference between lasers and LED, laser are basically strictly monochromatic, means emitt light with one exact wavelength, LED emitt ligth with an band of wavelengths, means a maximum wavelength and bandwidth.

Hope that gives you some directions.

Agree with Holger. In addition, I'd add that you may want to consider choosing the wavelengths in the biomedical transparency window of the tissue under study. Then, you may want to relate obtained values to spectroscopic signatures of particular chromophores present in the tissue. Regarding lasers and LED, lasers would allow you to include polarization effects in your experiments. Again, these are very general considerations without knowing much about goals of your study.

I suggest you read:

T Durduran, R Choe, W B Baker, and A G Yodh. Diffuse optics for tissue monitoring and tomography. Reports on Progress in Physics, 73(7):076701, June 2010

You want to choose wavelengths in the 600-1000nm range because of the low absorption region there from the chromophores in tissue. Using an LED (as mentioned by others) will broaden you wavelength and will add uncertainty to your quantification.

Common wavelengths are 690, 740, 785, 808, 830 nm. Power required will depend on how deep you want to go. For remission measurements 50-80mW should be more than enough (you can always attenuate).

Dear Holger, Serge, and Han,

Thank you so much for your really helpful advices.

Best