lower end of the range is sufficient for example the threshold for determining whether the probe is on or off tissue can be in the range of 1 measured hemoglobin concentration the linear range of spectral features versus hematocrit concentration e g less than about 25 in fig4 need only be used for this application however in accordance with the present invention the measurement accuracy can be extended to greater percentages of hematocrit by redefining the algorithm to account for nonlinearities the algorithm could for example be redefined as a multiple regression algorithm consisting of multiple slope and second derivative transformations linear transformations examples of nonlinear equations include where mso2 1 mso2 2 are nonlinear slope value coefficients which can be stored in the lookup table the probe scaling factor psf can be empirically determined by collecting second derivative spectral measurements of a chromophore medium preferably having constant scattering and absorption properties with optical probes having variable distances between the optical send and receive fibers the spectral measurements at each probe spacing are then referenced ratioed to one of the fixed probe spacing spectral measurements at a particular wavelength of interest the ratio of one second derivative spectrum value at a probe spacing of interest to the second derivative spectrum value of the reference probe spacing then reflects the probe scaling factor fig8 is a graph of second derivative spectra measured at 4 different probe spacings the medium used to obtain the data in fig8 was a 2 5 aqueous solution of 1 micron polystyrene microspheres fig9 represents the probe scaling factor measured from the ratio of second derivative spectrum values at approximately 725 nm the absorbance peak in fig8 the following equation represents an example calculation of the probe scaling factor from the spectral information in fig9.