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What is “Distance-to-Spot” (D:S) Ratio of Infrared Thermometer?

Infrared thermometer measures temperature through non-contact method, which allows users to measure high temperatures at a safe distance. It plays an important role in production quality control, equipment maintenance and troubleshooting.

This aricle will introduce an important technical index in the purchase of infrared thermometer, “distance-to-spot” (D:S) ration.

First we need to know the function of the infrared thermometer laser. It serves as an indicator to help the thermometer aim at the measuring area. Taking UNI-T’s thermometers as an example, there are three common laser types:

Single laser: It provides a simple way to aim at the measuring area. However, it can only indicate the general area.

Dual laser: The distance between two laser points on the surface of the measured object is approximately the diameter of the circular measuring area.

Ring laser: The ring approximates the measuring area.

Dual laser and ring laser can indicate the measuring area more directly and accurately.

laser type

Because an infrared thermometer uses an optical system with a lens to focus infrared energy, the area from which that energy is focused is sometimes called the thermometer’s field of view. The field of view is also sometimes called the spot. To get the best results from an infrared thermometer, you need to understand its field of view.

When measuring, make sure that the measured target is larger than the light spot diameter . The smaller the target, the closer the test distance should be. It is recommended that the measured target be larger than twice the light spot diameter of the thermometer.

Field of view

The field of view widens as the distance between the Infrared thermometer and the surface it’s measuring increases. The orientation of the device to the surface being measured also affects the field of view. To get the tightest field of view, hold the device at a perpendicular or 90° angle to the surface you’re measuring.

To use an IR thermometer correctly, it’s important to keep in mind that an IR thermometer measures the average temperature over its entire field of view. When a surface does not fill the instrument’s field of view, the instrument will be unable to measure its temperature well, because its readout will also include the temperature of things that are next to or behind the surface.

To determine the size of an IR thermometer’s field of view at a given distance, you can use the device’s distance-to-spot (D:S) ratio, also sometimes called distance-to-target ratio (DTR). The D:S ratio is related to the sophistication of the device’s optical system and can vary from 10:1 on economical infrared thermometers to 50:1 or higher on professional instruments.

Taking D: S= 10: 1 and 50:1 as an example, the following figures show the diameters of the field of view with different D: S ratios at different measured distances.


Infrared thermometers with high D:S ratios, such as 50:1, can measure smaller areas from a greater distance. When selecting an infrared thermometer, think about how far away you’ll need to be from the targets you’re measuring, and how small targets things are. The smaller and farther away the target, the higher D:S ratio you’ll need.

Application example:

A practitioner in the manufacturing industry is heat treating a workpiece. If the practitioner wants to measure the object temperature at a far and safe distance to know the temperature change of the object and to control the quality, the practitioner can choose an infrared thermometer with D: S= 30: 1 or higher.

UNI-T’s thermometers have various optional D: S according to different application scenarios and requirements. The D: S of economical thermometer UT300A+ is 10: 1. For UT302+ series, it is 20: 1. And for professional thermometer UT305+ series, it is 55: 1.

I’m sure by now you all know how to buy an infrared thermometer based on D: S.

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