Optical Pyrometer 

An optical pyrometer will measure temperature from a distance.

Optical pyrometer is a filament that has its power supply and as you adjust the resistance the on it the color will change on the filament.Now the idea is when you are viewing through the view piece,it match the filament to the object surface temperature. Which will be a certain a certain amount of current.Now that current will be converted to a display showing the degree of temperature 

If you use the optical pyrometer to measure temperatures out un the open,your temperature reading will be consistently low. The error varies from a few degrees C for target objects with high levels of radiation to several hundred degrees C for target objects with low levels of radiation.So most outdoor measurements are made with infrared pyrometers.

Advantages of Optical Pyrometer

Flexibility
Portability
Monitor the temperature of moving object

Disadvantages of Optical Pyrometer 

Cannot measure the temperatures of clean burning gases because these gases do not radiate visible energy.
Manual optical pyrometer are unsuitable for measuring the temperature of target object with temperature below 800 degree C because at lower temperatures the radiation emitted is at too low a level to be recorded.