It is noteworthy that, despite all the attention, a precise in situ determination of several important optical properties of intralipid emulsions, such as the particle size, refractive index, and attenuation coefficient (which is commonly expressed as the imaginary part of a complex refractive index13–21), has continued to elude researchers.28 The chief difficulty arises from the fact that intralipid emulsions, being highly turbid, have a large attenuation coefficient. From the point of view of particle sizing, standard methods such as optical microscopy and dynamic light scattering (DLS) require heavy sample dilution and are hence not in situ. Dilution inevitably leads to errors and, hence, wide variations in measured particle size. For example, reports of measurements of the average particle diameter in intralipid and liposyn have ranged anywhere from 97 nm (26) to 185 nm (25) to values between 350 and 660 nm.23,24,28 Clearly particle sizing in intravenous nutrients is critically important in order to eliminate the possibility of thrombosis.23,28 From the point of view of in situ complex refractive index measurement of highly attenuating tissue, it is attractive to consider reflectance-based methods since very little light transmits through. However, the most widely used reflectance-based method, which equates the point of maximum slope of the reflectance-versus-incident-angle curve with an effective critical angle,3,13–15,21 has been shown to be significantly inaccurate even after error-correction is attempted.29 Other attempts to extract the complex refractive index by modeling the reflectance data either introduce extraneous fitting parameters (beyond the two parameters of interest, namely, the real and imaginary parts of the refractive index) resulting in overfitting of the data19,20 or focus on only the critical angle region, which is just a small subset of the reflectance data.14–18. Both approaches have built-in arbitrariness causing wide variation in extracted values. Finally, note that the particle size is typically extracted from a measurement of the complex refractive index using a Mie calculation. A critical assumption for standard Mie theory to be valid is that no multiple scattering occurs.30 For a highly turbid medium such as an intralipid emulsion, this criterion is only well satisfied when the sample length is no more than a micron (for light in the 400- to 1100-nm regime), a condition not met in previous work with intralipids to the best of our knowledge.