How to Use an Infrared Thermometer (And Get Readings You Actually Trust)

An infrared (IR) thermometer is an indispensable tool for the modern kitchen, workshop, and home. It offers the ability to get an instantaneous, non-contact temperature reading on anything from a pizza stone to an A/C vent.

However, for many new users, this initial excitement quickly turns to frustration. A reading on a cast-iron griddle may seem perfect, but a reading on a shiny stainless-steel pot is often wildly inaccurate.

This is not a faulty tool. It is a misunderstanding of the physics involved. To get accurate, trustworthy readings from a temperature gun, one must first understand the two core principles of its operation: Distance-to-Spot Ratio and Emissivity.

A modern, popular model like the Vesogy 9158F serves as an excellent case study for decoding these features.

1. The Cone vs. The Dot: Understanding Distance-to-Spot Ratio

The most common error is assuming the thermometer is measuring only the small red laser dot. It is not.

The laser is only an aiming guide. The actual measurement is being taken from an invisible cone of infrared energy that the gun’s sensor “sees.” The size of this cone is determined by the Distance-to-Spot (D:S) Ratio.

• What it is: A 12:1 D:S ratio (standard on the Vesogy 9158F) means that from 12 inches away, the tool is measuring a 1-inch diameter circle.
• Why it matters: If you are 12 feet (144 inches) away from your target, you are not measuring a 1-inch spot; you are measuring a 12-inch circle. The temperature displayed will be the average of everything in that circle.
• The Pro-Move: Always get as close to your target as is safely possible. A user, LS, purchased one for their pizza oven to check if the stone was 800°F. To get an accurate reading, they must hold the gun close enough that the measurement cone is entirely on the stone, not averaging in the 300°F air or walls around it.

Better models, like this Vesogy, often include a 9-point laser system. This is a significant upgrade over a single dot, as it shows the approximate boundary of the measurement circle, making it much easier to visualize what you are actually measuring.

2. The “Infrared Mirror”: Why Shiny Surfaces Lie

This is the single most important concept in infrared thermometry and the reason for most “wrong” readings.

What is Emissivity? In simple terms, emissivity is a measure of a material’s “honesty” in radiating its own heat. It is a score from 0.0 to 1.0.

• High Emissivity (Score: ~0.95): These are “honest” surfaces. They are highly efficient at radiating, or “glowing,” with their own heat. This includes materials that are matte, rough, or organic: pizza stones, cast-iron griddles, asphalt, wood, food, and human skin.
• Low Emissivity (Score: ~0.1 - 0.4): These are “dishonest” surfaces. They are terrible at radiating their own heat. Instead, they act like infrared mirrors, reflecting the heat energy from their surroundings. This includes polished stainless steel, shiny aluminum, and mirrors.

When you point a standard IR thermometer (which is factory-set to ~0.95) at a shiny pot, it is not reading the pot’s 300°F temperature. It is reading the reflection of the 70°F ceiling lights above it, resulting in a wildly inaccurate, low number.

The Solution:
This is why it is critical to buy a tool with adjustable emissivity.

1. The “Tape Trick” (Change the Surface): The easiest fix is to place a small piece of matte electrical or painter’s tape on the shiny surface. Let it sit for a moment to equalize with the surface temperature. Set your gun’s emissivity to 0.95 (the default) and aim at the tape. The tape is a “high-emissivity” target and will give you a dead-on accurate reading.
2. The “Pro Fix” (Change the Gun): If you cannot tape the surface, you must tell the gun it is looking at an “infrared mirror.” An instrument like the Vesogy 9158F allows you to adjust the “EMS” (Emissivity) setting. Look up the emissivity of “polished stainless steel” (e.g., 0.1), dial that in, and the gun will now mathematically correct for the poor signal, giving you the true temperature.

3. The Critical Warning: “Not for Humans”

The product description is explicit: “Not for Humans.” This is not a legal disclaimer; it is a scientific fact.

• Why? This device is an expert at measuring surface temperature. A medical thermometer is designed to measure (or calculate) core body temperature.
• The Problem: Your skin’s surface temperature is a terrible indicator of fever. It is affected by sweat, ambient room temperature, and blood flow. A reading from your forehead will have no reliable connection to your internal 98.6°F. Always use a clinically approved medical thermometer for that purpose.

Real-World Applications: From Guesswork to Diagnosis

Once these two principles—D:S Ratio and Emissivity—are understood, the IR thermometer transforms from a “gadget” into a powerful diagnostic instrument.

• For the Chef: Stop guessing. Know exactly when a pizza stone is 800°F (user LS) or a BlackStone griddle is 400°F for the perfect sear (user Dan Mac). This is why user teresa found it was a “must buy” for her new griddle, saving propane by knowing the exact moment it was at temperature.
• For the Home Diagnostician: This is a fantastic diagnostic tool. User True Review Man used it to get the “exact temperature blowing out of the vents,” proving his A/C was broken. User Michael compared his reading to his A/C tech’s “much higher cost unit” and found it was “within a couple degrees.”
• For the Hobbyist: User TMT uses it for multiple applications: verifying the exact temperature of basking spots in reptile enclosures and, critically, checking the water from the faucet during a fish tank change to perfectly match the tank’s temperature and avoid shocking the fish.

That is the difference. A tool like the Vesogy 9158F (with its wide -58°F to 932°F range, fast reading, and crucial adjustable emissivity) is not just a “point-and-shoot” device. It is an instrument that, when used with an understanding of its physics, provides instant, accurate, and trustworthy data.