Before you begin. This procedure is general reference information, not professional advice. Equipment varies by manufacturer, model, and year. Always follow the documentation supplied with your equipment — where it differs from this page, follow the manufacturer. Verify that you have the tools, the working conditions, and the ability to do this task safely. If anything is unclear, if the equipment is damaged, or if the task exceeds your experience, stop and use a qualified professional. You are responsible for your own safety and for the outcome. This site accepts no liability for any loss, damage, or injury arising from the use of this information.
Specifications marked unverified could not be traced to a manufacturer source. Do not rely on them — obtain the value from your equipment documentation.
Summary
An infrared thermometer is a non-contact tool that reads the heat a surface radiates. Two specifications decide whether that reading means anything and neither appears on the display: the emissivity setting — which is wrong by default on any shiny surface — and the distance-to-spot ratio, which decides how much of the wall or equipment the thermometer is really averaging. This page covers the one field check anybody can make, using an ice bath as a fixed reference point at 0 °C (32 °F). What this page does not do is calibrate the thermometer in the metrology sense — that is work for the manufacturer. What it does is show you whether the thermometer is in the right ballpark for your purposes, and explain two settings that trap readers most often: emissivity and distance-to-spot ratio. A reading taken this way is a check and never a certification.
Safety
THIS PAGE IS A FIELD CHECK, NOT A METROLOGY CALIBRATION.
A reading at the ice bath tells you whether your thermometer is approximately correct. It does not prove the thermometer to a standard. For a certified calibration, send the instrument to the manufacturer or to a calibration laboratory.
Necessary equipment
| Item | Note |
|---|---|
| Infrared thermometer | Read your own instrument manual before you use it. The emissivity adjustment, the distance-to-spot ratio, and the temperature range are specific to your model. |
| Crushed or shaved ice | Use fresh ice from the freezer or from snow. Do not use ice that started to refreeze. |
| Insulated cup or container | An old thermos, a foam cup or a small insulated cooler works. Metal containers conduct heat and are less suitable. |
| A contact thermometer (optional, for comparison) | A probe, dial or digital contact thermometer lets you check the ice bath temperature. You do not need one. If you use one, read its own manual for accuracy. |
| Water | Use distilled water or tap water. The ice bath works because the ice and water reach the same temperature: the ice point. |
Start
Estimated 6 min to 12 min
No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.
READ YOUR OWN THERMOMETER MANUAL BEFORE YOU USE IT.
The emissivity setting, the distance-to-spot ratio, the temperature range, and the display mode are different for each model.
- Read the manual for your own infrared thermometer.5mReport a problem with step 1, Start
- Find the emissivity adjustment on your thermometer.
- Find the distance-to-spot ratio on the manual or on the tool.
- Find the units switch if your thermometer has one (Celsius or Fahrenheit).
- Set the units of your thermometer to Celsius.<1mReport a problem with step 2, Start
- A Celsius setting removes one source of error.
- The ice point is 0 °C by definition.
- If your thermometer does not have a units switch, note whether it displays Celsius or Fahrenheit.
- Set the emissivity to 1.0 (or the maximum value on your thermometer).1mReport a problem with step 3, Start
- Ice has an emissivity close to 1.0 — very close to an ideal black body.
- Setting the emissivity to 1.0 lets you read the ice temperature without compensation.
- You will adjust the emissivity later for other surfaces.
Prepare an ice bath
Estimated 5 min
HANDLE THE THERMOMETER CAREFULLY AROUND WATER.
Do not freeze the thermometer sensor. Do not expose the lens to condensation. Keep the sensor dry. If the lens becomes wet, dry it at once with a soft, clean cloth.
- Fill an insulated cup or container with crushed or shaved ice.2mReport a problem with step 1, Prepare an ice bath
- Use ice that came fresh from the freezer.
- Fill the cup to the top with ice.
- Do not use ice that is starting to refreeze — it will be above the ice point.
- Add a small amount of distilled or tap water to the ice.1mReport a problem with step 2, Prepare an ice bath
- Add just enough water to form a slurry with the ice.
- The water and ice together create the ice point: 0 °C.
- The ice and water must both be at the same temperature.
- Let the bath sit for two minutes without disturbance.2mReport a problem with step 3, Prepare an ice bath
- This time allows the ice and water to reach thermal equilibrium.
- The bath is now at the ice point: 0 °C (32 °F).
Measure the thermometer with the ice bath
Estimated 5 min
No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.
No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.
- Point the infrared thermometer at the ice bath from directly above.<1mReport a problem with step 1, Measure the thermometer with the ice bath
- Aim the thermometer at the ice surface, not at the side of the cup.
- Do not let the thermometer touch the ice or the water.
- Keep the sensor lens dry.
- Hold the thermometer at the distance recommended in the manual.1mReport a problem with step 2, Measure the thermometer with the ice bath
- Distance matters. The distance-to-spot ratio tells you what area the thermometer covers at any distance.
- A typical thermometer at 10 cm covers an area about 1 cm across.
- At 30 cm the same thermometer covers an area about 3 cm across.
- Press the trigger and read the display.<1mReport a problem with step 3, Measure the thermometer with the ice bath
- Wait for the reading to stabilize.
- Take note of the temperature shown.
- Compare the reading to 0 °C (32 °F).1mReport a problem with step 4, Measure the thermometer with the ice bath
- A simple infrared thermometer reads within ±2 °C (±3.6 °F) of the true value.
- If your reading is close to 0 °C (between −2 and +2 °C), the thermometer works within normal limits.
- If the reading is far from 0 °C (more than five degrees off), the thermometer needs service.
- Repeat the reading to show repeatability.2mReport a problem with step 5, Measure the thermometer with the ice bath
- Repeat steps 7 through 10.
- Two readings close together show that the thermometer repeats well.
- Repeatability matters more than accuracy. A thermometer reading 2 °C high is better than one that reads 0 °C then 5 °C.
Learn about emissivity
Estimated 9 min to 14 min
No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.
No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.
No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.
- Note that the emissivity setting is almost certainly wrong for your target surface.3mReport a problem with step 1, Learn about emissivity
- Emissivity is a property of the surface, not of the thermometer.
- The factory default suits matte, non-reflective surfaces.
- Shiny, polished or reflective surfaces have lower emissivity.
- Use the ice bath to find the emissivity setting for shiny or reflective surfaces.5mReport a problem with step 2, Learn about emissivity
- Point the thermometer at the ice bath again.
- Start with the emissivity at the factory default (often 0.95).
- If the reading is much higher than 0 °C, reduce the emissivity value. Read again.
- Reduce the emissivity until the reading is close to 0 °C.
- Note this emissivity value. Use it for your shiny surfaces.
- Keep a record of the emissivity settings you discover for different surfaces.1mReport a problem with step 3, Learn about emissivity
- Paint, matte materials and skin have emissivity close to 1.0.
- Shiny metals, polished surfaces and reflective coatings have much lower emissivity — sometimes as low as 0.1.
- A thermometer is most accurate when the emissivity is set correctly for the target surface.
Learn about distance-to-spot ratio
Estimated 5 min
- Locate the distance-to-spot ratio on your thermometer or manual.1mReport a problem with step 1, Learn about distance-to-spot ratio
- The distance-to-spot ratio has two numbers separated by a colon, such as 10:1 or 12:1.
- The ratio tells you the size of the area the thermometer covers at any distance.
- Understand what your distance-to-spot ratio means.3mReport a problem with step 2, Learn about distance-to-spot ratio
- A 10:1 ratio means that at 10 cm distance the thermometer measures a spot about 1 cm across.
- At 20 cm distance the same thermometer measures a spot about 2 cm across.
- A 12:1 ratio is more precise: at 12 cm distance it measures a 1 cm spot.
- A higher ratio gives a narrower field of view and better precision at distance.
- Make sure the target is at least twice the size of the spot at your working distance.1mReport a problem with step 3, Learn about distance-to-spot ratio
- If the target is smaller than the spot, the thermometer includes heat from the background.
- This reading will be wrong.
- If your thermometer has a 1 cm spot at your working distance, the target must be at least 2 cm across.
Terminology
This page uses one term for each item and holds it throughout, as ASD-STE100 requires. Equipment sellers and other guides use the names below for the same parts.
- infrared thermometer
- Also called: non-contact thermometer, IR thermometer, thermal scanner, laser thermometer, pyrometer
- emissivity
- Also called: emissivity setting, emissivity adjustment, surface emissivity, thermal emittance
- ice bath
- Also called: ice water bath, melting ice, ice point, ice slurry
- distance-to-spot ratio
- Also called: D:S ratio, field of view, spot size, measurement area
- ice point
- Also called: melting point of ice, freezing point of water, 0 °C, 32 °F
- calibration
- Also called: accuracy check, calibration check, reference check, field verification
When to use a professional
Stop if the ice bath reading is more than five degrees from 0 °C. The ice bath must be fresh and at equilibrium. The thermometer may need service. Send it to the manufacturer or to a calibration laboratory.
Stop if you do not understand the distance-to-spot ratio or the emissivity adjustment. Ask the manufacturer or contact a qualified technician.
Stop if you want a metrology calibration to a standard or certified reference. That work belongs to a calibration laboratory. An ice bath check tells you if the thermometer is correct. It does not prove the thermometer to any standard.
Stop if your target is smaller than twice the spot size. The reading will include background heat and will be wrong. Move closer or increase your distance-to-spot ratio or find a larger target.
Common questions
Is an ice bath check the same as a proper calibration?
No. An ice bath check is a field verification using a known temperature reference. A proper calibration is a metrology procedure done by the manufacturer or by a certified calibration laboratory. The ice bath check tells you if your thermometer is approximately correct. It does not prove the thermometer to a standard or give you a calibration certificate.
Why is emissivity important, and why is the default setting usually wrong?
Emissivity is the fraction that shows how much heat a surface radiates. The scale goes from 0 (a perfect mirror) to 1 (a perfect black body). Factory defaults are set to 0.95, which is right for matte surfaces like painted walls or skin. But shiny or reflective surfaces have lower emissivity — sometimes as low as 0.1 for polished metal. A shiny surface measured with the default emissivity reads too high because the thermometer sees reflected heat from the room, not the surface itself.
How do I find the right emissivity for a shiny surface I want to measure?
Use the ice bath to find the right setting. Point the thermometer at the ice bath with default emissivity and take a reading. If it is much higher than 0 °C, reduce the emissivity value. Keep reducing until the reading is close to 0 °C. That setting works for your thermometer on shiny surfaces. Emissivity tables exist, but the ice bath method works without a table: you measure your target surface's actual emissivity against a known reference.
What does distance-to-spot ratio mean, and why does it matter?
A 10:1 ratio means at 10 cm distance the thermometer measures a 1 cm spot. At 12:1, at 12 cm the spot is 1 cm. The ratio tells you the field of view. If your target is smaller than the spot, the thermometer averages the background and reads too high. Make sure the target is twice the spot size.
What should I do if my ice bath reading is far from 0 °C?
First, check your ice bath. Make sure it is fresh ice from the freezer and the ice and water are mixed. If the bath is correct and the reading is still far from 0 °C (more than five degrees), the thermometer may need service. Check your manual first. Some thermometers have narrow ranges or are for higher temperatures. Then send it to the manufacturer or a calibration laboratory.
Can I use a contact thermometer instead of an ice bath for comparison?
Yes, but the ice bath is better. A contact thermometer lets you verify that your ice bath is at 0 °C. But if your contact thermometer is old or unmaintained, it may not be accurate either. The ice bath is a physical reference point: ice and water at equilibrium are always at 0 °C. That is more reliable than any old thermometer.
My thermometer has an emissivity range of 0.1 to 1.0. How do I know which value to use?
Use the ice bath method. Set emissivity to 1.0, read the ice bath, and note the temperature. If it is higher than 0 °C, reduce the emissivity and read again. That setting works for ice and shiny surfaces. For matte surfaces, the default emissivity (often 0.95) works well. Keep a note of the values that work for different materials.
Related procedures
- How do I know if my meat thermometer or fridge thermometer is accurate, and how do I calibrate it?
- How hot should my hot water be and how do I check it
Sources
Every specification on this page is traced to one of these sources. Tier 1 is a manufacturer document, a published standard, or a government safety authority. See Standards for the full tiering rule.
- Tier 2Emissivity
- Tier 2Infrared Thermometer
- Tier 2Triple Point of Water
Revision history
Current version 1 · updated 2026-08-03 · next review due 2027-08-03
- v1 · 2026-08-03First draft. The ice point is defined by NIST as the melting point of ice at standard atmospheric pressure: 0 °C (273.15 K; 32 °F). The emissivity and distance-to-spot ratio guidance is sourced from Wikipedia articles on emissivity and infrared thermometers, which cite the standard ranges and typical thermometer accuracy (±2 °C). The procedure describes the ice bath check as a field verification method, not as a metrology calibration, and routes the reader to the manufacturer or a calibration laboratory for certified calibration work. Terminology includes the common aliases that readers search for, such as 'laser thermometer' and 'pyrometer', alongside the canonical term 'infrared thermometer'. No torque, pressure, voltage, or other safety-critical numeric value is published without tier-1 sourcing. The page is Phase 1 compliant: no mention of mains voltage, pressurized systems, fuel, hot surfaces, ladders, jacks, or rotating equipment.
Corrections to this page are listed in the public corrections log. Reports that are credible and not yet verified are listed in the public errata register, and an open one renders above the step it concerns.
Comments and corrections
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Every step above carries its own report control, which tags the report with that step so a correction reaches the right place. For anything about the procedure as a whole — the order of the sections, a step that is missing, something that did not match your equipment — use this: report something about this whole procedure.