Chapter 7 Figure 7.2 Optical fiber probe measurement + detail of fiber probe tip Data collection Before specimen measurements, a reflectance baseline was recorded with spectralon (Optical‐grade spectralon reference material; Labsphere, Inc., North Sutton, New Hampshire USA). Spectralon is used very commonly as reference spectrum for baseline correction as it behaves as a nearly perfect cosine‐diffused reflector from 250‐2500 nm. Baseline values were measured with the fiber probe in direct contact with the spectralon surface. During the baseline recordings the light source was set at 50% power capacity. The same fiber probe that was used for the baseline recordings was also used for the ex vivo tissue measurements. Overall tissue reflection was much lower than spectralon reflectance. Hence, the light source was turned up to 100% during spectroscopic measurements on tissue, thus achieving a better dynamic range during tissue measurements (light source linearity was verified separately). The fiber probe was gently brought into direct contact with the tissue surface by hand as demonstrated in Figure 7.2. Between measurements of different tissue types the fiber probe was cleaned with saline. Spectra of all tissue types were acquired from fresh human tissue, immediately after surgical removal while the specimen was still within the operating theatre. Several spectra were recorded for each tissue type: healthy colonic tissue, mesenteric fat, arteries, and veins; if present in the specimen, also: ureter, palpable tumor in lumen of intestine. At least two different locations were measured per tissue type and each location was measured five times. This provided a total of at least ten spectra per tissue 94
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