The Metis MY39 with 3.95 µm spectral response was developed especially to take advantage of unusual absorption or transmission features of certain materials.
At a wavelength of 3.95 µm, hot furnace gases and clean burning gas flames are quite transparent, therefore allow the Metis Model MY39 to measure temperature accurately of materials ( metals /composites) engulfed inside a flame or inside a furnace containing clean hot combustion gases. Applications include flame treating of metals or metal heat treating in an industrial gas fired, reheat furnace.
Among other reasons, some windows (used as viewports) provide a high level of IR transmission within this spectral band. A sapphire window is a perfect example as it offers a high IR transmission and is rated for very high ambient temperatures as well as high pressure levels. Also a window can reduce the amount of hot incoming IR energy reaching the IR sensor as the typical ambient rating for an IR temperature sensor is 150°F.
The infrared energy radiated by the target is centered via a focusable or fixed focus lens directly onto the detector. The focusing feature allows control of the cone of vision and offers the possibility to measure either a small spot (focused) or the average of a bigger spot (out of focus). Fixed focus lenses with larger diameter collect more infrared energy and therefore provide smaller spot sizes. The lenses are made of calcium fluoride CaF2 which is highly transparent in visible and infrared regions up to 10 µm. If additional windows are necessary they must offer similar optical characteristics.
The detector is sensitive to infrared radiation in an area called the cone of vision. For the spot size diameter Ø of the cone of vision at shortest, medium and widest focal distances, please see Chart 2. For full scale temperatures up to 1300°C the cone of vision diameter in front of the lens is about 16 mm and about 9 mm for full scale temperatures above. This area has to be kept free from any intervening objects. The spot size diameter for distances not given in the chart can be calculated by interpolation.