Nevertheless, the Shulchan Aruch explicitly permits measuring for the ill or for the purpose of a mitzvah. Therefore, checking an ill person’s temperature with a thermometer will be permitted as long as the use of the thermometer is permissible.
Clearly the ideal form of thermometer for public domains would be a thermometer that can hygienically read temperatures from a distance without the need for it to be handled by one or more people. Infrared electronic thermometers can serve that purpose throughout the week, but can they be made in a manner that is permissible for Shabbat?
Since the thermometer is not actually being used to heal a sick individual, but rather to identify if someone has a fever, using a regular electronic or digital thermometer is not an option.
The Zomet Institute developed an infrared thermometer that operates automatically, ensuring that no electrical circutis will be closed on Shabbat and no device will be turned on or operated. The automated Shabbat thermometer is programmed to check the temperature every 4 seconds whether or not anyone is standing in front of it. Every 4 seconds it automatically sends out an infrared temperature reading, showing the temperature on its little screen.
Any change in electric output is treated as an act of shinui zerem chashmali, a modification of an existing current, which is permitted in these instances by poskim such as Rav Shlomo Zalman Auereabach and many others.
The characters on the electrophoretic display of the temperature reading are also not viewed as an act of writing.
For these reasons, the Zomet Institute devised the ModChom Shabbat, Shabbat Infrared Thermometer, to automatically operate on Shabbat to allow digital temperature readings in a permitted manner for health-related uses of a thermometer.
For a longer discussion of the halakhic rationale of the device, see below.