Principles and Applications of Optical Temperature Measurement

Kaiyuan Zhan

School of National Defense and Nuclear Science and Technology, South University of Science and Technology, Mianyang,

Sichuan, 621010, China

Keywords: Fiber Optic Temperature Measurement Technology, Non-Contact Temperature Measurement, Current Status

of Fiber Optic Temperature Measurement, Bragg Grating.

Abstract: Temperature measurement is a key link in industry and scientific research. Traditional temperature

measurement methods have limitations in complex environments such as high temperature, high pressure,

and strong corrosion. Optical temperature measurement has gradually become a research hotspot due to its

advantages, such as non-contact measurement, fast response, high precision, and high sensitivity. With the

continuous progress of optical technology and material science, the development of new optical temperature

measurement methods has important research value. The existing optical temperature measurement methods

still face room for technical improvement and optimization in practical applications. This article focuses on

the principles and applications of optical temperature measurement methods, and focuses on the fiber optic

temperature measurement technology (TMT). By analyzing the basic principles and performance

characteristics of point TMT, quasi-distributed TMT, and distributed TMT, we study their principles, their

application advantages, and problems in different scenarios. The research results show that fiber optic TMT

has significant advantages such as high sensitivity, high precision, corrosion resistance, and distributed

measurement, and is especially suitable for complex environments, and can effectively overcome the

limitations of traditional TMT. The research conclusions of this paper provide a theoretical basis and practical

guidance for the further development and application of fiber optic TMT, and provide new ideas and methods

for temperature monitoring in related fields.

1 INTRODUCTION

Temperature is a basic physical quantity that

describes physical states. How to detect temperature

quickly and accurately has become an important part

of industrial development and scientific research.

Optical temperature measurement is used in many

fields, such as online detection of power cable line

operating temperature, tunnel fire warning and

forecasting system, and aircraft engine temperature

testing, which require high precision, high sensitivity,

high temperature resistance and corrosion resistance

(Zeng, Xu & Hou, 2007; Luo et al. 2007; Han et al.

2022). Zeng, Xu & Hou (2007) Based on the

temperature measurement principle of distributed

fiber, using the same fiber, can realize tunnel fire

monitoring and leakage detection. It has the

advantages of early warning, setting multiple alarms,

finding and positioning disasters (fires and leakages),

https://orcid.org/0009-0002-5930-014X

being accurate, easy to use and maintain, safe and

reliable, and long-distance monitoring. The system

operates normally and stably, and has strong anti-

interference ability. In addition, Yao et al. (2023)

applied quasi-distributed fiber grating sensing

technology to the measurement system, and after the

system design was completed, they verified the anti-

interference ability of the designed hydropower plant

temperature measurement system, proving that the

temperature measurement system can meet the actual

temperature measurement needs. In recent years,

fiber surface temperature testing technology has

made great progress. Hbisreuther et al. (2015)

produced a fiber grating high temperature sensor

based on multimode single crystal sapphire fiber, and

used this sensor to measure the high temperature of

1900°C, and its error is within the range of 2°C (Zhou,

2022).

This paper analyzes the principles and

applications of the light temperature measurement

578

Zhan, K.

Principles and Applications of Optical Temperature Measurement.

DOI: 10.5220/0013833500004708

Paper published under CC license (CC BY-NC-ND 4.0)

In Proceedings of the 2nd Inter national Conference on Innovations in Applied Mathematics, Physics, and Astronomy (IAMPA 2025), pages 578-583

ISBN: 978-989-758-774-0

method. As a new temperature measurement

technology (TMT), fiber optic TMT has high

sensitivity, accuracy, distributed measurement, and

corrosion resistance characteristics. Its application

fields and application prospects are broad. This article

will analyze the point TMT, quasi-distributed TMT,

and distributed TMT of fiber optic TMT, explore the

basic principles and performance characteristics of

the three fiber optic temperature measurement

technologies, and draw their application advantages

and problems faced.

2 THE BASIC PRINCIPLE OF

LIGHT TMT

The photometric TMT originates from the

relationship between the radiated power per unit area

and the absolute temperature to the fourth power

proposed by Joseph Sterfly (Sun, 1995). This method

is a non-contact TMT based on the relationship

between the radiation characteristics and temperature

of an object. Its principles mainly rely on the law of

bold radiation, the law of Wien's displacement, and

the law of Planck's bold radiation. In 1884, Austrian

physicist Ludwig Boltzmann deduced the same law

as the results of the Stefan experiment based on

thermodynamic theory and by assuming that light

(electromagnetic wave radiation) is used as the

working medium of the heat engine, thus providing a

theoretical basis for the Stefan law (Huang, 2023).

Therefore, this law is also called the Stefan-

Boltzmann law. Wien's law of displacement was

proposed by German physicist William Wien in

1893(Yang, Li & Ma, 2015). These two studies are

also important principles of the photometric

temperature method.

3 FIBER OPTIC TEMPERATURE

MEASUREMENT

3.1 Point TMT

Point TMT is a technology that measures temperature

at specific points and is widely used in industries,

power, medical care, environmental monitoring, and

other fields. It measures the temperature of the target

point directly or non-contactly through a sensor or

detector. The fiber temperature sensing device for

fluorescent radiation in the dot fiber temperature

measurement scheme and the fiber temperature

sensing device using semiconductor material crystals

are more engineering-friendly.

The measurement system is shown in Figure 1.

Figure 1: Scheme of the measurement system (Lu, Chen & Yin, 2024).

The system consists of three parts: an optical

system, a fiber probe, and a signal processing system.

The light source emits ultraviolet light and is tuned

into pulse excitation light by the dimmer. It is

transmitted to the probe through the optical fiber(TOF)

to generate fluorescence. Two silicon photoelectric

diodes detect the two fluorescence intensity signals

received by TOF. Then it converts the two

fluorescence signals into electrical signals. After

amplification, it takes the average value and sends it

to the divider to obtain the ratio signal, and then

converts it into a digital signal through A/D. The ratio

of the average value of the two signals is calculated

by a computer and used to obtain the measured

temperature. Fluorescent fiber temperature

measurement has a fast dynamic temperature

response, and can achieve accurate temperature

measurement under harsh environmental conditions

such as high voltage, large current, strong

electromagnetic fields, and flammable and explosive

environments. Traditional platinum resistors and

thermocouples based on thermoelectric effects are

prone to inaccuracy due to electromagnetic

interference in a strong electromagnetic field

environment, and the fluorescent fiber thermometer

makes up for this deficiency. Fluorescent fiber TMT

Principles and Applications of Optical Temperature Measurement

579

is currently mainly used in harsh environmental

conditions such as strong electromagnetic fields, high

voltages, and high currents.

Fluorescent fiber sensors have the advantages of

small size, anti-electromagnetic interference, and

high temperature resistance. They can be buried in the

windings, which can effectively monitor the internal

hot spot temperature of the power transformer and

accurately understand the location of the hot spot. The

application of fluorescent fiber thermometers

fundamentally increases the possibility of obtaining

the correct thermal model of the power transformer,

as well as the online monitoring of the excitation

system temperature. According to the survey, the

maximum allowable error of fluorescent fiber

thermometers currently used in the power industry is

± 1.0°C. Due to its good insulation, fluorescent

fiber thermometers can be directly attached to the

surface of the thyristor component or buried inside it

to achieve accurate temperature measurement. (Lu,

Chen & Yin. 2024).

3.2 Quasi-Distributed Optical Fiber

TMT

Quasi-distributed fiber optic TMT is a technology

that uses a fiber grating as a temperature sensor,

which can modulate and measure the temperature

along TOF. This technology has many advantages,

such as high sensitivity, small size, easy integration,

and corrosion resistance.

3.2.1 Principles of Quasi-Distributed Fiber

TMT

During the specific implementation process, the light

emitted by the light source enters TOF through the

circulator and is input into the TOF grating along

TOF. TOF grating reflects the optical signal 56 back

to the circulator, and the final optical signal is

analyzed by a spectrometer or a demodulator. The

corresponding temperature information is obtained

from the wavelength information of the optical signal.

The Fiber Bragg Grating(FBG) Temperature Sensor's

Operating Principle is to expose and etch the Bragg

fiber grating with different center wavelengths along

the longitudinal direction of TOF. Each Bragg fiber

grating is power-to-reflection for a specific optical

wavelength. In the TOF transmission direction,

multiple Bragg fiber gratings are connected in series

in sequence to form a spatially discrete quasi-spatial

distribution temperature measurement system. A

beam of wide-spectral light containing multiple

wavelengths is injected into TOF, and the beam

passes through a series of FBGs, each grating

reflecting back a monochromatic light signal

corresponding to its central wavelength. (Huang,

Xiao, & Li, 2022).

Figure 2: Fiber grating sensor equipment (Huang, Xiao, & Li, 2022).

The principle is shown in Figure 2. The FBG

sensor device has a built-in wide-spectrum light

source, which is coupled to the on-site FBG sensor

through a coupler. The sensing part of the FBG sensor

is distributed in different spatial positions in the

temperature measurement area.

3.3.2 Distributed Fiber TMT

While the light waves are conducted in TOF, the

specific wavelengths of light generated by different

physical processes are different in TOF, and the

signal light carrying temperature characteristics is

demodulated at one end of TOF to achieve fully

distributed temperature measurement. As shown in

Figure 3, when light waves are transmitted in optical

fibers, backscattered light is generated, including

Rayleigh scattering, Raman scattering, and Brillouin

scattering. When light irradiates matter, the photon

interacts with molecules or atoms in matter, causing

the energy of the photon to change, thereby producing

scattered light. Raman scattering is one of the

inelastic scattering phenomena, and the frequency of

the scattered light is different from the frequency of

the incident light.

IAMPA 2025 - The International Conference on Innovations in Applied Mathematics, Physics, and Astronomy

580

Detecting the backscattering generated by various

points along TOF, through the relationship between

these backscattered light and the measured (such as

temperature, stress, vibration, etc.), a distributed fiber

sensing distributed fiber TMT can be realized, which

is to use the temperature modulation characteristics of

a certain specific light propagating in TOF to

demodulate this temperature-carrying signal light at

one end of TOF, thereby realizing distributed TMT

(Liu & Sun, 2009).

Figure 3: Principle diagram of optical fiber temperature measurement (Bo, 2024)

Figure 4: Principle of Distributed Fiber Optic High

Temperature Sensor (Tong & Song, 2014).

During the transmission of a pulsed laser, the

backscattered light signal of the sensing optical fiber

will return to the optical coupler along the original

transmission path and be coupled into the optical

processing subsystem (Figure 4). Through the

spectrometer, Stokes light and anti-Stokes light of

different frequencies in the Raman scattered light are

separated and entered into different optical paths for

processing. The bandpass light filter filters out other

scattered light and interfering light, allowing only

Stokes and anti-Stokes Raman scattered light with

temperature information to pass through. These two

scattered lights are photoelectrically converted

through their respective avalanche photodiodes

(APDs), and the electrical signal is amplified, sent to

the data acquisition and processing module, and the

temperature is demodulated at different time

positions, and then the spatial distribution

information of the temperature is reconstructed

through the OTDR principle, and displayed in a table

or graphical format. Since the entire system has

certain noise and losses, multiple measurements are

required during operation. The data is then

accumulated and averaged to eliminate adverse

effects and obtain more accurate temperature

distribution data. Finally, the data is stored in a

computer database for the next step of application

analysis (Al-Harb, 1998).

4 CURRENT STATUS OF

OPTICAL FIBER TMT

TOF temperature measurement is the preferred

solution for temperature measurement in the power

industry due to its excellent characteristics, such as

moisture resistance, corrosion resistance,

electromagnetic interference resistance, insulation,

small size, and high sensitivity. In particular, point

optical fiber temperature measurement and fully

distributed optical fiber temperature measurement

have been widely used in different application

scenarios (Huang, Xiao, & Li, 2022)

4.1 Application of Fiber Point

Temperature Measurement in

Power Systems

For various reasons, the connecting parts of electrical

equipment, such as isolating switches, switch cabinet

dynamic and static contacts, cable heads, etc., when

the current passes, the temperature rises, causing the

equipment to age and the insulation to drop. In severe

Principles and Applications of Optical Temperature Measurement

581

cases, it will cause short circuits, damage the

equipment, and interrupt the power supply. During

the operation of the transformer, the overtemperature

of the winding will cause insulation aging, burning,

breakdown, and other accidents. Other physical and

chemical changes inside the transformer (such as

local discharge and local overheating, etc.) will also

cause changes in the transformer temperature, making

the transformer perform differently from the

temperature rise trajectory of normal operation.

Previously, the on-duty personnel regularly used

infrared cameras, thermometers, or temperature

indicators to monitor the equipment offline. This

method can only measure the temperature of the

contact point exposed outside, while there is no

effective monitoring method for the contact point

temperature in the metal-enclosed switch cabinet. The

fluorescent fiber temperature measurement solution

has been used in power systems due to its small size,

convenient integration, safe and reliable performance,

anti-electromagnetic interference, good insulation

performance, convenient installation, and flexible

networking. The fluorescent fiber probe has a small

size and can be measured deeply in the internal heat

generation point to achieve real and effective

monitoring of the heat generation point; it can be

easily integrated into the intelligent switch cabinet

and passed the type test; it can realize on-site display,

which is convenient to integrate into the control

system (Huang, Xiao, & Li, 2022).

4.2 Application of Fully Distributed

Fiber Temperature Measurement

in Power Systems

In recent years, distributed fiber temperature

measurement systems based on Raman scattering and

Brillouin scattering have been vigorously developed

and promoted. Taking advantage of the inherent

advantages of fiber sensing and combining the

characteristics of fiber transmission and sensing, an

optical fiber temperature measurement system with a

monitoring distance of more than 30 km, continuous

space monitoring, high monitoring sensitivity,

corrosion resistance, and anti-interference is realized.

It only requires the deployment of monitoring

equipment at one end of the optical cable, and no

additional sensing units are needed along the way.

While achieving long-distance monitoring, it reduces

the difficulty of deployment and maintenance. Raman

distributed fiber TMT based on single-mode

communication optical cables Because common

communication optical cables are used as temperature

measurement sensors, no additional equipment is

required, and the temperature measurement

sensitivity is high and the performance is stable, it is

very suitable for temperature monitoring applications

in business scenarios such as power pipeline corridors

and overhead lines (Huang, Xiao, & Li, 2022).

5 CONCLUSION

This article discusses the principles and applications

of the optical temperature measurement method in

depth, focusing on optical fiber TMT. As a non-

contact TMT, photothermal measurement has

gradually become an important means in industrial

and scientific research, with its advantages such as

high accuracy, rapid response, and suitability for

complex environments. Its principle is mainly based

on the law of bold radiation, Wien's displacement law,

and Planck's bold radiation law, and the temperature

is calculated by measuring the radiation

characteristics of an object. In terms of fiber optic

TMT, this paper analyzes point TMT, quasi-

distributed TMT, and distributed TMT in detail. Point

TMT uses the fluorescent radiation phenomenon to

determine temperature by measuring changes in

fluorescence intensity or lifetime. It has high

sensitivity and rapid response and is suitable for local

temperature monitoring. Quasi-distributed TMT is

based on the characteristics of TFG. It has the

advantages of high precision and corrosion resistance,

and is suitable for multi-point temperature monitoring.

Distributed TMT uses backscattered light (such as

Raman scattering) in optical fibers to achieve

continuous temperature monitoring, which can

provide temperature distribution information along

TOFs and is suitable for long-distance and large-area

temperature monitoring.

This article provides the theoretical basis and

practical guidance for the further development and

application of fiber optic TMT, and provides new

ideas and methods for temperature monitoring in

related fields. With the continuous advancement of

technology, fiber optic TMT is expected to be widely

used in more fields, providing more reliable

temperature monitoring solutions for industrial

production and scientific research.

REFERENCES

Al-Harbi, K.M.A.S., 1998. Sharing fractions in cost-plus-

incentive-fee contracts. International Journal of

Project Management, 16(2), 73–80.

IAMPA 2025 - The International Conference on Innovations in Applied Mathematics, Physics, and Astronomy

582

Bo, Z., 2024. Application of fiber optic TMT in engineering

practice. Soda Ash Industry, (05), 40–42.

Han, Q., Liu, X., Lei, X., Zhang, P., Zhou, F., 2022.

Application of fiber optic sensing technology in aircraft

engine temperature testing. Acta Instrument and

Metering, 43(01), 145–164.

Huang, C., Xiao, J., Li, X., 2022. Research and

development trends of current situation of fiber optic

TMT. Electric Power Information and Communication

Technology, 20(2), 102–108.

Huang, J., 2023. Development of quantum theory from the

perspective of statistical mechanics—from Boltzmann

to Planck. Physics Bulletin, (12), 154–158.

Liu, D., Sun, Q., 2009. Distributed fiber sensing technology

and its application. Advances in Laser and

Optoelectronics, 46(11), 29–33.

Lu, Z., Chen, Z., Yin, X., 2024. Fluorescent fiber TMT and

application. China Inspection and Testing, 32(01), 24–

27.

Luo, J., Zhou, Z., Li, H., Luo, M., 2007. Research on the

application of online detection technology for power

cable line operation temperature. High Voltage

Technology, (01), 169–172.

Sun, Y., 1995. Application of microcomputers in the

experiment of thermal radiation and temperature

relationship. Journal of Harbin University of Science

and Technology, (01), 68–70.

Tong, X., Song, L., 2014. Current status and application

prospects of optical fiber high temperature sensors.

Optical Communication Technology, 38(10), 8–10.

Xiang, M., Li, Z., Ma, X., 2015. Analysis of Wien's law of

displacement. Journal of Xinjiang Normal University

(Natural Science Edition), 34(03), 44–47+2.

Yao, M., Hu, D., Liang, Y., Zhou, W., Yang, X., 2023.

Application of quasi-distributed fiber grating sensing

technology in temperature measurement systems of

hydropower plants. Mechanical Design and

Manufacturing Engineering, 52(03), 126–130.

Zeng, T., Xu, W., Hou, J., 2007. Application of distributed

fiber TMT in tunnel fire and leakage detection. Journal

of Disaster Prevention and Mitigation Engineering,

(01), 52–56.

Zhou, F., 2022. Research on the application of aircraft

engine optical fiber surface temperature testing

technology. Electronic Components and Information

Technology, 6(06), 13–16.

Principles and Applications of Optical Temperature Measurement

583