THz near-field images in this work show the THz absorbance () of the measured samples. Most of the tissue samples were not homogeneous and contained a mixture of tissue types. To extract the properties of the constituent tissue types, the absorbance was averaged linearly by assuming that any reflections and scattering caused by heterogeneities within samples were negligible.9,10,15 The absorbance was calculated according to the Beer–Lambert law , where is the transmitted power of the THz wave through the slices and the coverglass, is the background (transmission power of THz wave through the blank coverglass), and is the thickness of the slice, which was measured and averaged according to 3 to 5 different positions of each slice using an optical microscope. For the poor section quality condition induced by man-made or machine causes, the viscous tissues will adhere to the blade and make the slices with uneven thickness. With higher transmission in these specific thin spots, thus obtained value will be underestimated. We found that if the sample thickness fluctuation exceeded , the calculated would be fallacious. All samples selected had a thickness fluctuation lower than . The absorbance spectra we used for analyzing images are shown in Fig. 4, which was measured in far-field and calculated according to the mean absorbance based on 30 different human colonic tissue specimens (14 healthy and 16 with tumor). Samples for the THz spectra measurement were prepared by cutting slices of tissue with a uniform thickness of approximately . The error was induced by the differences in water content and tissue density of human tissues.10 From Fig. 4, the mean absorbance of healthy colonic tissue was 9.3 to , while tissue with tumor was 11.4 to .8 It is clear that cancerous tissue absorbed much more THz waves than healthy tissues. The distinguishable absorbance contrast between healthy and cancerous tissue was the key to realizing THz imaging examination. Therefore, as our THz image show, we defined the color bar as follows: if the absorbance inside a certain region of the THz image was about 11.4 to , we marked the regions as tumor and showed the region in red color, while the healthy tissue was in green color. The acquired THz near-field images and the corresponding pathologic photomicrograph of H&E stained sections are shown in Fig. 5. From the corresponding THz near-field images, we successfully identified the healthy tissue and cancerous tissue, which agreed well with the identification by pathological H&E stain examination in terms of size and shape.