An optical pyrometer measures temperature by comparing the brightness of thermal radiation from an object to a reference lamp, allowing for non-contact temperature assessment. It features components like an optical lens, eyepiece, reference lamp, and rheostat, providing high accuracy and enabling remote sensing. The device is effective for temperatures above 700°C but not suitable for continuous temperature readings at small intervals.

1. Optical Pyrometer
In an optical pyrometer, a brightness comparison is made to measure the temperature. As a
measure of the reference temperature, a color change with the growth in temperature is taken.
The device compares the brightness produced by the radiation of the object whose temperature
is to be measured, with that of a reference temperature. The reference temperature is
produced by a lamp whose brightness can be adjusted till its intensity becomes equal to the
brightness of the source object. For an object, its light intensity always depends on the
temperature of the object, whatever may be its wavelength. After adjusting the temperature,
the current passing through it is measured using a multimeter, as its value will be proportional
to the temperature of the source when calibrated. The working of an optical pyrometer is
shown in the figure below.
Optical Pyrometer - Working
Optical Pyrometer - Working

2. As shown in the figure above, an optical pyrometer has the following components.
An eye piece at the left side and an optical lens on the right.
A reference lamp, which is powered with the help of a battery.
A rheostat to change the current and hence the brightness intensity.
So as to increase the temperature range which is to be measured, an absorption screen is
fitted between the optical lens and the reference bulb.
A red filter placed between the eye piece and the reference bulb helps in narrowing the band
of wavelength.
Working
The radiation from the source is emitted and the optical objective lens captures it. The lens
helps in focusing the thermal radiation on to the reference bulb. The observer watches the
process through the eye piece and corrects it in such a manner that the reference lamp filament
has a sharp focus and the filament is super-imposed on the temperature source image. The
observer starts changing the rheostat values and the current in the reference lamp changes. This
in turn, changes its intensity. This change in current can be observed in three different ways.
1. The filament is dark. That is, cooler than the temperature source.
2. Filamnet is bright. That is, hotter than the temperature source.
3. Filament disappears. Thus, there is equal brightness between the filament and temperature
source. At this time, the current that flows in the reference lamp is measured, as its value is a
measure of the temperature of the radiated light in the temperature source, when calibrated.
Optical Pyrometer-Temperature Measurement

3. Optical Pyrometer-Temperature Measurement
Advantages
Simple assembling of the device enables easy use of it.
Provides a very high accuracy with +/-5 degree Celsius.
There is no need of any direct body contact between the optical pyrometer and the object.
Thus, it can be used in a wide variety of applications.
As long as the size of the object, whose temperature is to measured fits with the size of the
optical pyrometer, the distance between both of them is not at all a problem. Thus, the device
can be used for remote sensing.
This device can not only be used to measure the temperature, but can also be used to see the
heat produced by the object/source. Thus, optical pyrometers can be used to measure and view
wavelengths less than or equal to 0.65 microns. But, a Radiation Pyrometer can be used for high
heat applications and can measure wavelengths between 0.70 microns to 20 microns.
Disadvantages
As the measurement is based on the light intensity, the device can be used only in
applications with a minimum temperature of 700 degree Celsius.
The device is not useful for obtaining continuous values of temperatures at small intervals.
Applications
Used to measure temperatures of liquid metals or highly heated materials.
Can be used to measure furnace temperatures.