How an Infrared Thermometer Reads a Pizza Oven Floor

An infrared thermometer measures the heat a surface gives off. It doesn't touch the floor, and it doesn't read the air or the inside of the stone. That one fact explains most of the odd readings people see.

What's inside the gun

An infrared thermometer infers temperature from a portion of the thermal radiation sent out by the object being measured¹. The design is simple. A lens focuses the infrared radiation onto a detector, which turns it into an electrical signal that the display shows in units of temperature¹.

A modern pyrometer does the same job. It works out the temperature of a surface from the amount of thermal radiation that surface emits². Most other thermometers have to be placed in thermal contact with the object and left to reach thermal equilibrium². A gun skips that wait. That's useful on an oven floor you can't safely touch.

What the reading is

The reading is a surface temperature for the patch the gun can see. Three things follow from that.

• It says little about how hot the stone is a short way below the surface.
• It says nothing about the air above the floor.
• It depends on the surface giving off radiation the way the gun expects. Emissivity covers that.

The gun also sees the whole patch at once and gives one figure. If the patch has hot and cold areas, the indicated temperature may be higher than the actual average temperature¹. A flame-licked streak next to a pale, cooler area can pull the number up.

The laser dot

Most handheld guns project a red dot. The dot only helps you aim. It plays no part in the measurement¹. The area being measured varies among instruments, and it isn't limited to the visible spot¹. So the dot shows the middle of the patch, not its edge.

The size of the patch grows with distance. Where to Aim explains the distance-to-spot ratio and how to use it.

Where a gun can mislead you

Sometimes, especially near ambient temperatures, readings may be subject to error from radiation reflected off a hotter body, or from an incorrectly assumed emissivity¹. An oven is a place full of hotter bodies. A fire, a flame and glowing walls can all bounce radiation off a shiny spot.

Other limits to keep in mind:

• The gun reads one surface at one moment. A floor that has just had a pizza slid off it will show a cooler spot that recovers.
• Soot, ash or flour on the floor can change what the gun sees.
• The gun's own housing has limits. Check your thermometer's manual for its operating range, and don't hold it in the mouth of a hot oven.

How it compares with a probe

A contact probe, such as a thermocouple, touches the thing it measures. Thermocouples are widely used, yet their main limitation is accuracy³. A gun and a probe can both be right and still disagree, because one reads a surface from a distance and the other reads where it touches.

Many ovens also carry a gauge in the dome or door, and that one reads differently. It has its own page: what dome gauges show.

A short checklist before you trust a number

1. Know which surface the gun is aimed at.
2. Check that the spot is clean and not shiny.
3. Take more than one reading at the same place.
4. Read the manual for the gun's range and its emissivity setting, if it has one.