A two-photon microscope is an advanced instrument that uses a scanning technique, fluorochromes, and long-wavelength light (such as infrared) to visualize specimens. Because the long-wavelength excitation light has low energy, two photons must strike a location simultaneously to excite the fluorochrome. This low-energy light is less damaging to living cells, and the long wavelengths penetrate deep into thick specimens, resolving the resolution limitations of confocal microscopes. They are exceptionally useful for examining living cells within intact tissues like brain slices, embryos, and whole animals, though their high costs (typically $300,000 to $500,000) restrict their use mostly to advanced clinical and research laboratories.