To obtain the high level of wavelength tenability necessary in our study, a picosecond OPO (PT257; Ekspla, Vilnius, Lithuania) was used as the excitation source for thermal imaging. This source produced 5-ps pulses at a rate of 88 MHz with a maximum power of 400 mW, and a tunable range from 690 to 990 nm. Measurements were collected from a range of 700 to 950 nm in 50-nm steps, and the residual light from the internal pump laser was used for a 1064-nm data point. The laser radiation was delivered to the microscope via a 400-μm core multimode fiber. The distal tip of the fiber was imaged onto the sample surface at a magnification, producing a 500-μm flat-top excitation spot. In order to control the laser power measured at the image plane, the laser was introduced to a polarizing cube, producing linearly polarized light which could then be attenuated by a half-wave plate. The half-wave plate was adjusted such that of average power () was measured at the image plane for all wavelengths. The thermal profile was recorded at 800 Hz using a subarray on a thermal camera (SC4000; FLIR, Wilsonville, Oregon). The thermal camera imaged the epithelial surface of the tissue sample at magnification, resulting in a field of view of 1.6 mm with 8.6-μm resolution. A dichroic beamsplitter (BSP-DI-50-2; ISP Optics, Irvington, New York) was used to separate the IR radiation needed for thermal imaging (3 to 5 μm) from the excitation radiation. The experimental setup is depicted in Fig. 1. The sample stage was contained inside an acrylic box that was maintained at a constant 34°C, allowing the tissue sample to be held at a physiologically relevant temperature prior to laser exposure. The laser was delivered from the bottom of the box, so tissue samples were placed epithelium down on a 100-μm thick coverslip.