Light at wavelengths in the visible and near-infrared (NIR) regions (650 nm – 1,350 nm) is used in numerous medical applications. The NIR region between 650 nm and 950 nm, known as the first optical window, allows for deeper depth penetration in tissue than in the visible region due to a reduction in absorption. There also exists a second NIR optical window with wavelengths from 1,100 nm to 1,350 nm. Longer wavelengths above 1,350 nm were ignored due to major water absorption and lack of 2D photodetectors. In this study, a new therapeutic spectral window with wavelengths between 1,600 nm and 1,870 nm is reported. This third optical window can be used for imaging more deeply into tissue due to a reduction in scattering. In this paper, light attenuation from 400 nm to 2,000 nm, including all three optical windows, was measured. 200 micron slices of normal and benign prostate and breast tissues were studied. The total attenuation lengths (lt) of light were obtained. The attenuation length of malignant and normal tissue in the third optical window was larger than in the first and second therapeutic windows. Optical images of chicken tissue over three black wires were also obtained using the third optical window.