Silicon (Si) is commonly used as an optical window primarily in the 3 to 5 micron band and as a substrate for production of optical filters. It is frequently used for laser mirrors because of its high thermal conductivity and low density. Silicon is also useful as a transmitter in the 20 micron range. Large blocks with polished faces are also employed as targets in neutron physics experiments. Optical Silicon is grown using the Czochralski method to economically produce silicon that is guaranteed to be 99.99% pure. Silicon windows are available as optical grade in mono-crystalline and polycrystalline for use in infrared systems operating in the 3 - 5 micron range. Optical Silicon is generally lightly doped (5 to 40 ohm-cm) for best transmission above 10 microns. Silicon has a further pass band 30 to 100 microns which is effective only in very high resistivity uncompensated material. Silicon windows are used as spectroscopic cell windows, Silicon substrates for multi-layer infrared filter coatings, IR spectroscopy and windows in infrared systems for infrared transmission. It is also useful for some ATR applications (Attenuated Total reflection).