Module 4: Heat transfer calculations, miscellaneous topics and carbon credit
Lecture 37: Temperature Measurement: Radiation Pyrometry
A number of semiconductors are developed to sense the radiation. These are materials of Si, PbS, indium antimonides etc. Their response is though instantaneous but it is selective to wavelength.
Silicon is suitable only around and lead sulphide around 1 to.
It is important that gases like and dust should not obstruct the path of radiation. The dust particles scatter the radiation, whereas and water vapor selectivity absorbs radiation.
Any instrument built to sense the radiation has to be in an enclosure to avoid dirt, dust and gases present in industrial environment. Normally a window is provided with some optical materials to see the radiating body. The materials should have good transmissivity. All optical materials allow only particular wavelength to pass through it with sufficient intensity. For other wavelengths they are opaque.
Material for windows
Transmissivity
Glasses like quartz, Pyrex, ruby etc.
Good in ultraviolet and visible region of wavelength but are opaque to infrared. Glass windows are useful for wavelengths lower than . Beyond wavelength of , transmissivity decreases drastically.
Barium fluoride and zinc sulphide
They have transmissivity in the infrared visible region.
Calcium fluoride
It has a very good transmissivity in visible and infrared region.
and lead sulphide around 1 to 
.
and dust should not obstruct the path of radiation. The dust particles scatter the radiation, whereas 
and water vapor selectivity absorbs radiation.
. Beyond wavelength of 
transmissivity in the infrared visible region.
