## 3 Experimental Setup  
### Optical Transmission and Backscattering Setup  
When in vivo tissue is subjected to rapid freezing, the intravascular fluids with its high-water content, form micro-crystals and micro-bubbles, giving a characteristic white appearance when illuminated with white light, attributed primarily to Mie scattering from the frozen microstructures in the tissue volume. Furthermore, in living tissue some transient alterations, such as blistering and inflammation persist, due to the processes described earlier. There is a similar \"whitening\" in ex vivo porcine tissue subjected to freeze thaw cycles, reverting to its original color after thawing without any visible degradation to its structure, making it ideal for our experimental work. If ex vivo skin tissue is illuminated with a monochromatic light source, it will partially transmit, reflect and scatter light from its surface and volume. When tissue is rapidly frozen with the application of liquid nitrogen cryogen, the ratio of the light that is transmitted, reflected and scattered can be measured to determine its temporal and spatial characteristics. For a freezing/thawing cycle, cryogen was applied on side A for several seconds until the temperature of side B reached a temperature around \(-65\)\({}^{\circ}\)C to \(-70\)\({}^{\circ}\)C; the frozen region had a diameter of about 25 to 30 mm. Subsequently the tissue thawed, while taking measurements of the optical signals from both arrays, together with its front and back temperatures. The energy extracted from the tissue, determined by the weight of liquid nitrogen spent and the latent heat of evaporation. Both fiber-arrays were on the same horizontal plane with the central illumination and central detection fiber (T4) in array-B aligned, thus measuring the transmitted light through thickness \(x\) mm of tissue, as shown in Figure 1. Additionally, in order to eliminate transmission losses emanating from surface frozen dew, which appeared during freezing cycles, array-B was set in contact with the skin and index-matching gel (IMG) bridged the small surface anomalies between the skin and the array (Figure 1). During the calibration stage, local transmitted and backscattered intensities, during a freezing/thawing cycle, were measured using a thin cross-section ex vivo porcine skin sample while they were correlated to the corresponding freezing depth using a procedure outlined below. The experimental set up is shown schematically in Figure 1, with a 3 mm thick porcine skin, mounted vertically in a frame, which allowed access from both sides. An optical fiber array with eight detection fibers, (array-A), and a central illumination fiber (source fiber), was positioned in front of tissue surface, to measure backscattered light from the tissue volume. The illumination fiber was mounted on micrometer stage and could be moved freely inside the tissue volume along a small hole of comparable dimensions, emitting at wavelengths of 650 nm. Fiber arrays-A, used multimode fiber (MMF) with core/cladding diameters of 240/250 \(\upmu\)m, with an NA of 0.37 and inter fiber separation was about 1 mm. A second fiber-array-B, with seven fibers, was placed on the back of the tissue and measured the transmitted light, as shown in Figure 1. This array had core/cladding diameters of 450  
Figure 1: Experimental set up of calibration setup. and 500 \(\upmu\)m, respectively, with an NA of 0.43 and inter-fiber separation of 2 mm. A multichannel detector measured the optical signals from each of the fiber in the two arrays and thermocouples indicated the temperature on the tissue's front and back surfaces. It worth noting that the low NA of the optical fibers, limit the illumination and \"viewing\" volume in the tissue as only light within the small acceptance cone can be detected, thus enchasing the resolution of the optical diffusion measurements in both fiber arrays.