Infrared thermometers are widely used for non contact based temperature measurements in covid-19 situation.
The main working principle of infrared thermometer is the thermal radiations coming from a body, this radiation is received by a temperature measuring sensor focused by a lens, which converts heat to electrical signals. This signal is the displayed on a LCD.
For measuring temperature from a infrared thermometer, one need to align the sensor of the thermometer within 3-10 cm from center of person forehead.
Now click the trigger of the infrared thermometer & the measured temperature will be displayed.
The temperature measured are displayed in Celsius and Fahrenheit. Nearly all thermometers have options to switch between the two. You can go to settings or function & navigate to switch over between the two measurement units.