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Most infrared thermometer failures are measurement errors, not instrument failures. Check three things first: the spot size at your working distance, the emissivity setting for the surface, and whether the target fills the spot. A reading that is wildly wrong on shiny metal but correct on matte black tape is almost always an emissivity problem. A reading that changes when you move closer or further is a spot-size problem. A meter that will not power on is a battery or contact problem. Only after those checks should you suspect the sensor itself. If you need to measure a surface you cannot see clearly, or a small component inside a live panel, a thermal camera or a contact probe is the right tool, not a better infrared gun.
Start with the three errors that account for most wrong readings
An infrared thermometer does not measure temperature. It measures the infrared energy a surface emits and converts that to a temperature using a model of how the surface behaves. Three inputs drive that conversion: how much energy arrives, how large an area it came from, and how emissive the surface is assumed to be. Get any one wrong and the number on the display is wrong even though the instrument is working exactly as designed.
The first check is spot size. Every infrared thermometer has a distance-to-spot ratio, and the spot grows with distance. A 12:1 instrument looking at a target 36 cm away reads a circle about 3 cm across. If the target is a 1 cm wire, the instrument is averaging the wire with whatever is behind it. Move closer and the reading changes; that is the signature of a spot-size error. The listing for the Etekcity 774 states a 12:1 ratio and recommends 36 cm for best results, which is the manufacturer telling you where the spot is a sensible size for typical targets.
The second check is emissivity. Shiny metal reflects infrared from its surroundings and emits little of its own, so a default emissivity setting of 0.95 will make it read far too low or wildly variable. Matte tape, a painted patch, or a thermocouple bonded to the surface gives a far better reading. The third check is the target itself: glass, plastic film and thin surfaces can be partly transparent to infrared, so the instrument reads what is behind them. A reading that is plausible on a painted panel and nonsense on bare aluminium is not a broken thermometer.
Emissivity, spot size and distance: the arithmetic that explains the symptom
Spot size is simple geometry. The diameter of the measurement circle is the distance divided by the ratio. At 1 m with a 12:1 instrument the spot is about 8.3 cm; with a 16:1 instrument it is about 6.3 cm. The target should fill at least the stated spot, and ideally more, because the edges of the spot are weighted less than the centre. A small target at long range is the single most common reason a reading looks wrong.
Emissivity is a number between 0 and 1 describing how efficiently a surface radiates compared with a perfect black body. Most organic and painted surfaces sit around 0.9 to 0.95, which is why instruments default there. Bare polished metals sit far lower, often below 0.2, and the exact value depends on the alloy and finish. The practical fix is not to guess a number but to change the surface: a piece of matte electrical tape, a dab of flat black paint, or a contact probe. The listing for the Etekcity dual laser model states adjustable emissivity from 0.1 to 1.0, which is the feature that lets you correct for a known surface rather than guess. The listing for the Fluke 62 Max+ does not publish an emissivity adjustment in its bullets, so on unknown shiny surfaces you are relying on the default and on surface preparation.
Distance also matters for a reason that is not obvious: the instrument has a fixed focus and a finite detector. Beyond the rated distance the spot grows faster than the ratio suggests and the reading drifts toward the background. If a reading improves dramatically when you step closer, the instrument was never reading the target you thought.
When the display is dead, erratic or stuck: power, lens and sensor checks
A thermometer that will not turn on is usually a battery problem. Check the battery type and orientation, and check the contacts for corrosion. Some instruments ship with a plastic tab over the battery that must be removed. If the display comes on but the reading is stuck at a fixed value, the trigger or the hold function may be latched; press and release the trigger and check for a hold icon. A reading that jumps wildly between samples usually means the spot is moving across a boundary between two surfaces with very different temperatures, or the target is reflective and picking up a moving reflection.
The lens is the other common culprit. Dust, fingerprints and condensation on the lens scatter infrared and bias the reading. Clean it with a soft lens cloth, not a solvent. A scratched or cloudy lens is a permanent error source and usually means the instrument needs replacing. The listing for the TempPro TempScan states a 16:1 ratio and a 0.5 second response, which is a specification you can use to sanity-check behaviour: if the display takes several seconds to settle, something is wrong with the trigger, the battery or the sensor.
If the instrument passes all of these checks and still reads implausibly, compare it with a contact thermometer on the same surface. A difference of a degree or two is normal; a difference of tens of degrees is a real fault or a real emissivity error. Do not assume the infrared instrument is the wrong one until you have checked the surface.
What each instrument in this category actually measures, and what it cannot
Every product referenced here is a non-contact surface thermometer. None of them measures human body temperature, and the listings say so plainly. The Fluke listing carries the phrase not for human temp, and the Etekcity dual laser listing states that readings for humans or animals will not be correct. That is not a limitation to work around; it is a statement about the calibration and the emissivity model. A surface instrument assumes a surface emissivity and a spot size suited to objects, not to the human forehead.
| Instrument | Stated range | Stated accuracy | Distance-to-spot | Emissivity adjustment |
|---|---|---|---|---|
| Fluke 62 Max+ | -20 to +1202 F (-30 to +650 C) | +/- 1 C or +/- 1.0 percent of reading from 0 to 650 C | Not stated in the listing bullets | Not stated in the listing bullets |
| Etekcity 774 | -58 to 842 F (-50 to 450 C) | Not stated in the listing bullets | 12:1 | Not stated in the listing bullets |
| Etekcity dual laser | -58 to 1022 F (-50 to 550 C) | Not stated in the listing bullets | 12:1 | 0.1 to 1.0 |
| TempPro TempScan | Not stated in the listing bullets | +/- 1.5 percent | 16:1 | 0.1 to 1.0 |
The table makes the honest position clear: no listing here publishes a full set of figures. Where a value is missing, do not assume it. The Fluke listing, for example, gives an accuracy figure and a range but not a distance-to-spot ratio in its bullets; the instrument may well have one, but it is not stated, so you cannot use it to calculate spot size from the listing alone. The Etekcity 774 gives a ratio but not an accuracy figure. If you need both to plan a measurement, you need a listing that publishes both.
A meter is not an alarm. None of these instruments will warn you that a surface is dangerously hot unless you are looking at the display. For safety-critical work, a contact probe or a fixed alarm is the right instrument, not a hand-held gun.
When the right answer is a different instrument
If the target is small, moving, or inside an enclosure you cannot see into, a hand-held infrared thermometer will keep giving you spot-size errors no matter how carefully you use it. A thermal camera images the whole scene and lets you see where the hot spot is before you measure it. The trade-off is resolution and cost, and the thermal camera versus infrared thermometer comparison sets out where each one wins.
If the surface is shiny, transparent or has an unknown emissivity, a contact probe gives a direct reading that does not depend on emissivity at all. If the surface is live electrical equipment, a non-contact instrument is the safe choice but you must accept the emissivity error and work with a known reference surface. If the question is about air temperature rather than surface temperature, an infrared thermometer is the wrong tool entirely; it reads surfaces, and the air between the instrument and the surface is largely transparent to it.
The most common mistake is buying a better infrared thermometer to fix a measurement problem that is not about the instrument. A higher ratio helps with small targets at distance, and adjustable emissivity helps with shiny surfaces, but neither fixes a target you cannot see or a surface you cannot characterise. For a structured approach to matching the instrument to the job, see how to choose an infrared thermometer.
The short version
An infrared thermometer that reads wrong is usually telling you the truth about a measurement you set up badly. Check spot size at your working distance, check emissivity for the surface, and check that the target fills the spot. Clean the lens and check the battery before you suspect the sensor. If the target is small, shiny, transparent or hidden, the instrument is not broken; it is the wrong instrument, and a camera or a contact probe is the honest answer. For a wider view of the category, the best infrared thermometers roundup and the individual Fluke, Etekcity and TempPro reviews give the stated figures product by product.
Frequently Asked Questions
Why does my infrared thermometer read much lower on metal than on plastic?
Bare metal has low emissivity, so it emits little infrared of its own and reflects its surroundings. The instrument assumes a high emissivity by default, so it interprets the weak signal as a low temperature. Put matte tape or flat black paint on the metal and measure that, or use a contact probe.
Why does the reading change when I move closer or further away?
The spot grows with distance. If the target is smaller than the spot, the instrument is averaging the target with the background. Move to a distance where the spot fits inside the target, or use an instrument with a higher distance-to-spot ratio.
My thermometer will not turn on. Is it broken?
Check the battery first, including the plastic shipping tab and the contact orientation. Check for corrosion on the contacts. If it still will not power on with a known-good battery, the fault is in the instrument.
Can I use these thermometers to check a fever?
No. The listings for the instruments here state that they are for inanimate objects and that human readings will not be correct. They are calibrated for surface measurement, not body temperature.
The listing does not give a distance-to-spot ratio. Can I still use it?
You can use it, but you cannot calculate spot size from the listing. Keep the target large relative to the instrument and test at a known distance against a contact probe. Where the ratio is not stated in the listing bullets, treat it as unknown rather than guessing.
Why does the reading drift slowly instead of settling instantly?
The detector and the electronics need to reach thermal equilibrium, and a cold instrument moved into a warm room will drift for a while. Let it acclimatise. A response that never settles suggests a battery or sensor fault, not normal behaviour.