Forehead Thermometer Accuracy

Forehead Thermometer Accuracy: A Complete Guide

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If you’re new to using forehead thermometers, you may be wondering if they’re really as accurate or more accurate than traditional thermometers. This is understandable since with traditional thermometers you know that they’re taking the temperature reading inside the body rather than at the surface.

To help you understand how forehead thermometers can be accurate, we have to explain how they work.

How do Thermometers Work

In order to understand how to properly use a thermometer, it’s helpful to know how thermometers work, first. All thermometers are tools for measuring heat. All of them, no matter the type, rely on a small amount of a substance that reacts to heat in specific ways to measure temperature.

Old-school mercury thermometers, for instance, rely on the expansion of mercury when exposed to heat. Because mercury responds to heat in a predictable way, it’s easy to use it as a way to measure temperature.

The reason why this works is that heat is actually a form of energy. At the atomic level, heat is movement. If you were to take a hot object and put it under a microscope next to a cold object, you’d see that the atoms in the hot object are moving around a lot more than the ones in the cold object.

Heat also works like air- it wants to move from areas of high pressure to low pressure. To understand this, you need to remember that “cold” is really just the absence of heat- you can’t actually measure how cold something is, you can only measure how much heat something has. When something is cold, it has very little heat.

Thermometers Work

If you put a cold object next to a hot one, the heat wants to even out by warming up the cold object. The rate at which this happens depends on what those objects are made of, because some materials conduct heat better than others. Metal is more conductive than wood, and silicon and rubber materials conduct heat so poorly that they can be used as insulators.

For a long time, most thermometers used metal to measure heat. Mercury is, in fact, a metal. It’s simply metal that is liquid at normal temperatures, which makes it very responsive to heat. Other thermometers, including the ones that come built in to a lot of grills, cheap oven thermometers, and the ones that come in your Thanksgiving turkey, are bimetal thermometers.

These thermometers use two different metal strips- often steel and copper- that are attached to each other and conduct heat at different rates. As they’re heated, the combined metal strips will bend in one direction, and they’ll bend in the other direction when they’re cooled. These are popular because they’re cheap, but they’re also very inaccurate.

Thermometers that rely on metals responding to heat to measure a temperature are imprecise and bimetal thermometers especially can be off by 5-10 degrees. They also take a relatively long time to give you a temperature reading. This why professionals in fields that require regular temperature readings- chefs and doctors- almost always prefer digital thermometers.

Most digital thermometers use a sensor called a thermoresistor. These are usually small pieces of glass with two wires attached. As the glass heats up, its resistance to the flow of electricity changes. A small computer in the thermometer can detect that change, and uses it to calculate a much more accurate temperature reading. What’s more, these are often instantaneous readings.

How Forehead Thermometers Work

Infrared thermometers have been used by doctors for a long time- if you can remember the doctor taking your temperature through your ear, that was an infrared thermometer like this one. Since children and infants often find the sensation of having the thermometer in their ear very unpleasant, these thermometers didn’t catch on at home.

The development of forehead thermometers changed that. These thermometers are also infrared thermometers, but they can measure body temperature on the forehead, or more accurately in the temporal artery. This is a completely comfortable process, which makes it very easy to take a child’s temperature. Because it’s such a kid-friendly technique, it’s becoming popular as an in-home thermometer.

How Infrared Thermometers Work

To understand infrared thermometers, you need to understand black body radiation. Everything warmer than absolute zero (read: pretty much everything) has molecules vibrating. Heat is the result of these vibrating molecules. They also generate infrared radiation, which is a spectrum of light that isn’t visible to the human eye.

Infrared light is perhaps most famous from technology like night-vision goggles, which sense infrared radiation and allow the wearer to see at night. Everything generates infrared radiation, and the hotter an object is, the more infrared radiation it will emit.

An infrared thermometer works by measuring the amount of infrared radiation being generated. Infrared thermometers use a lens to concentrate the infrared light onto a sensor called a thermopile, which converts the light into heat. That heat is turned into electricity, and the amount of electricity generated is used to measure the temperature of the object.

Eardrum thermometers work because the eardrum is about the same temperature as the inside of the body, but the eardrum itself is very sensitive, and touching it would be painful. The infrared thermometer can instead measure the temperature of the eardrum from a short distance (less than an inch) away. Other infrared thermometers measure the infrared radiation coming off your body- specifically the temporal artery- to determine body temperature.

In the case of industrial or culinary thermometers, infrared thermometers measure the infrared light generated by a cooking surface or a fire. It’s a common misconception that the laser on these thermometers is what’s measuring the temperature, but actually, these are only used to help aim the thermometer accurately.

Advantages of Infrared Thermometers

While they still aren’t as widespread as digital thermometers, infrared thermometers are very popular and becoming more common. Here are the biggest advantages of infrared thermometers:

  • No physical contact needed; very comfortable and very safe
  • Temperature readings are instant
  • Can be used with newborns, infants, children, and adults
  • Very accurate readings

Disadvantages of Infrared Thermometers

Despite their impressive benefits, infrared thermometers are not perfect. Here are the biggest drawbacks of infrared thermometers:

  • They tend to be very expensive
  • Ear thermometers can be uncomfortable

Infrared thermometers are, generally, just as accurate as their digital counterparts and even easier to use, but the cost can make them impractical.

Why Infrared Thermometers Are Accurate

We’ve explained how forehead thermometers work, but we haven’t yet explained why they are accurate. In reality, infrared thermometers are not necessarily more accurate than digital thermometers- the main advantage they have over digital thermometers is their ease of use and the instant temperature readings they give.

That said, they aren’t any less accurate than digital thermometers either. That means that they have a lot of advantages over digital thermometers (faster temperature readings, more comfortable to use) without any real disadvantages.

The accuracy of forehead thermometers is due to science. A forehead thermometer is measuring the infrared radiation being generated by your body. Since there is a well-understood relationship between how hot an object is and how much infrared radiation it generates, using infrared radiation to measure heat is a very accurate method.

Infrared Thermometers Are Accurate

Forehead thermometers go a step further, though. Instead of just calculating heat from infrared radiation, they actually convert the infrared radiation back into heat, allowing them to measure the heat directly, even without touching your body. This is how they can be so accurate even without being in direct contact with the body.

Many people wonder how a forehead thermometer can accurately measure your internal body temperature when they aren’t being stuck in the mouth like a traditional thermometer. The answer is that forehead thermometers aren’t actually measuring the temperature on the skin of your forehead.

They’re measuring the temperature of the blood in your temporal artery. Since they’re using infrared radiation to measure temperature, they can actually use measure the temperature of something below the surface.

The temporal artery supplies blood to the brain, and since it’s a major artery the blood flow is quick. That, in turn, means the blood doesn’t have time to cool down after leaving the heart, and so it’s still at the same temperature as your internal body heat.

Since the forehead thermometer is measuring the temperature of this blood, it’s able to accurately measure your internal body heat.

Why Your Forehead Thermometer is Inaccurate

If you’ve found your forehead thermometer to be inaccurate, there are several possible causes.

First, it’s possible your expectations might be too high. There are countless variables that can affect every individual’s body temperature, and no thermometer is going to be 100% accurate. Thermometers will measure slightly different temperatures all over your body- an oral thermometer will always give a slightly different reading than a forehead thermometer because the temperature in your mouth is slightly different from the temperature in your temporal artery.

Different brands of thermometers will also give slightly different readings. This is not unique to forehead thermometers. If you buy two different brands of digital thermometers and take your temperature with both of them, in the same spot at the same time, there’s a good chance that each thermometer will give a slightly different temperature reading. The same is true for your forehead thermometers.

Forehead Thermometer is Inaccurate

There are several reasons for these discrepancies. Different brands may use different manufacturers for their thermopile sensors, or different infrared lenses, different processors, etc.

Even old-school analog thermometers with mercury will have these issues. If you’re worried that your forehead thermometer is inaccurate because it gives you a different temperature reading from other thermometers, you have nothing to worry about. This is perfectly normal and does not indicate that your thermometer is defective or inaccurate.

The most common reason for inaccurate readings with forehead thermometers is improper use. This means that if you get a truly inaccurate reading from your forehead thermometer, it’s almost certainly because you did not properly use it.

How to Use a Forehead Thermometer

First, make sure the forehead is dry. Running a fever often leads to sweating, and the sweat will interfere with the ability of the thermometer to take an accurate reading of the temperature in the temporal artery.

Once the forehead is dry. place the thermometer directly in the center of the forehead. Slowly, slide the thermometer across the forehead towards the top of the ear, keeping it in contact with the skin at all times. The temporal artery runs horizontally across the forehead, so you’re essentially following the path of the artery with the thermometer.

How to Use a Forehead Thermometer

About two seconds after your start, you should have your temperature reading. As long as you have followed this procedure correctly, you should have an accurate temperature reading.

Always be sure to read the instructions that came with the thermometer, as some models do not require you to slide the thermometer and others may require a longer amount of time to generate the temperature reading.

As long as you’re following the instructions, you should not experience inaccurate readings with a forehead thermometer.


A forehead thermometer is every bit as accurate as a digital thermometer. With a forehead thermometer, you are able to quickly, accurately, and painlessly measure temperature. One of the chief disadvantages of oral thermometers is that many people, especially children, find oral thermometers to be very uncomfortable. This can make it difficult to take a person’s temperature orally since they often won’t sit still long enough for the thermometer to take their temperature.

In fact, the speed and comfort of the forehead thermometer might be its biggest selling point. It’s widely accepted that rectal thermometers are the most consistently accurate, but as with oral thermometers, these are very uncomfortable.

Forehead thermometers don’t cause any discomfort and, while they may not be quite as accurate as a rectal thermometer, they come pretty close. Since they’re much easier to use and don’t sacrifice much accuracy, they’re likely the best choice for a thermometer.




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