FUJI & CO.(Piezo Science)
The window placed over the elements of a detector has several functions. It provides a way for the detector elements to view the outside world while providing a barrier that can be hermetically sealed. Hermetic sealing in very important because the element impedance is so very high that it would probably fail to function with the introduction of any humidity. The window material must be something that will transmit the wavelengths of interest to the sensor designer. For the infrared bands this material is commonly silicon. Other materials such as germanium have been used but are more expensive.
The window is also used to deposit a wave filter which can reject wavelengths that are not wanted and pass those which are. The filter is constructed of multiple layers of metallic compounds each layer being a fraction of the wavelength it is designed to deal with. they are created by multiple, 50 or more, evaporations in a vacuum chamber.
The Optics of Windows
The window on the detector has optical properties associated with the laws of refraction. The figure shows the effect of refraction on the incoming rays and how they are deflected from their straight lines by the refractive index of the window. Because the refractive index of silicon, or germanium, is very high (3.5 and 4.0) the bending through the window is extreme. The window is also relatively thick so its effect is to move the focal plane of the optical system. This must be allowed for in the design. It also improves the angle of view of the detector by deflecting rays sharply upward to the rim of the case. Both of these factors can be calculated using Snell's Law.
N1 sin I1 = N2 sin I2
Another kind of window used in sensor using reflective optics is the protective one used to keep drafts and dust out. Polyethylene is the only inexpensive material that will transmit far infrared effectively. Even so it has losses and the thicker the material the higher the loss. Losses in this component is a loss of optical efficiency and spoils the signal-to-noise ratio. The thinner the better from a loss standpoint but thicker is more robust.
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