Optical pyrometers are devices which are used to measure temperature without contact. This is done through the process of incandescense color, which is based on the fact that all black bodies have the same incandescense color at a given temperature. This means of temperature measurement allows for any temperature from which a hot object emits light to be taken.
An optical pyrometer is created starting with a small magnifying device, such as a monocular. A miniscule incandescent bulb is placed within this device and produces a sharp image when viewed through the eyes piece of the optical devices, such as a lens. The background is the object which is to be gauged. The current that flows through the filaments in the bulb indicate their temperature, controlled by a potentiometer placed between the power source and the bulb.
In order to display the readings of an optical pyrometer, an ammeter is used. This displays the temperature that the pyrometer measures. It ranges from 500 degrees C to 1600 degrees C. For most applications, this is perfectly acceptable and will be able to fit the purpose of most temperatures necessary in semiconductor manufacturing.
Optical pyrometers work under the principle of sight. Using the human eye, they match the brightness of a hot object to the brightness of a calibrated lamp filament that is contained within the device. The optical system has filters which restrict the wavelength-sensitivity of the devices involve, decreasing it to a narrow wavelength band of approximately 0.65 microns.
Some filters reduce the intensity in order to allow a single instrument to have a wider capability of temperature range. By restricting the wavelength response of the device to the red region, the lower end of the temperature range can be limited to 700°C as opposed to 500°C. For that reason, this is not done with a lower temperature.