In contemporary scientific research and industrial applications, optical diagnostic techniques are playing an increasingly important role due to their non-invasive nature and high sensitivity. Among them, Laser-Induced Fluorescence (LIF) and various luminescence phenomena form the core foundation of optical detection technologies.

1. Basic Concepts and Principles of Fluorescence and Luminescence

Luminescence refers to all light emission phenomena that do not depend on the object's temperature and do not generate significant heat. Unlike the incandescent light of a light bulb, during luminescence, electrons in a substance are excited to higher energy levels by an external energy source, and subsequently release energy in the form of light radiation, while the material as a whole remains close to ambient temperature.

Fluorescence is a specific type of luminescence. Its physical essence involves a substance absorbing high-energy photons, causing electrons to transition from the ground state to an excited state. Upon returning to the ground state, these electrons emit photons with longer wavelengths (lower energy) than the excitation light. A key characteristic of fluorescence is its instantaneity; the excited state lifetime is extremely short, typically on the order of 10⁻⁹ to 10⁻⁸ seconds. Emission ceases almost immediately after the excitation stops. Another crucial feature is the Stokes shift, meaning the emitted light always has a longer wavelength than the excitation light. This occurs because some energy is lost as heat through processes like vibrational relaxation during the excited state .
 

Laser-Induced Fluorescence (LIF) technology skillfully combines the monochromaticity and high brightness of lasers with the high specificity of fluorescence analysis. When the photon energy of the laser matches the energy difference between two specific energy levels of the molecule under inspection, the molecule absorbs the photon and transitions to an excited state. It then returns to the ground state via spontaneous radiation, emitting fluorescence. Due to the precisely controllable wavelength and extremely high intensity of laser light, LIF can achieve very high detection sensitivity, even down to the single-molecule level. Compared to fluorescence excited by traditional broadband light sources, laser-induced fluorescence offers higher spatial resolution and signal-to-noise ratio, making it an indispensable tool in fields such as chemical analysis, bioimaging, and flow diagnostics.

Principle of Laser-Induced Fluorescence

2. Classification System of Fluorescence and Luminescence

2.1 Based on Excitation Mechanism, luminescence can be classified into the following major types:photoluminescence, chemiluminescence, bioluminescence, electroluminescence ,radioluminescence, mechanoluminescence and thermoluminescence.

2.2 Based on Luminescence Duration, it can be divided into:
Fluorescence: Characterized by an extremely short excited-state lifetime (nanosecond scale). Emission stops almost immediately after the excitation ceases. This instantaneity makes fluorescence particularly suitable for real-time dynamic monitoring, such as rapid physiological processes within cells.
Phosphorescence: Involves a longer excited-state lifetime (milliseconds to hours), originating from a spin-forbidden triplet-to-singlet transition. This long-lived characteristic is widely used in applications like glow-in-the-dark materials and time-resolved fluorescence detection, where delayed measurement can effectively eliminate interference from short-lived background fluorescence.

2.3 From the Perspective of Energy Conversion Direction, there exists a special category known as Upconversion Luminescence. This refers to a nonlinear optical process where a substance absorbs multiple low-energy photons and subsequently emits one higher-energy photon (with a shorter wavelength than the excitation light). Upconversion nanoparticles (UCNPs) are typical representatives of this phenomenon. They offer significant advantages in bioimaging: near-infrared excitation light provides greater tissue penetration depth and induces less autofluorescence, while the emitted visible light is easy to detect.
Laser-induced fluorescence technology itself has also developed several branches, tailored to different detection needs:
Planar Laser-Induced Fluorescence (PLIF): Used for 2D flow field imaging, widely applied in combustion diagnostics, microfluidic chip detection, etc. 
Micro Laser-Induced Fluorescence (μ-LIF): Aimed at measuring phenomena in micro-scale flows, achieving spatial resolution down to the micrometer level.
Time-Resolved Laser-Induced Fluorescence: Utilizes fluorescence lifetime information for probing the molecular environment or component analysis.
Multi-Photon Excitation Fluorescence: Employs near-infrared femtosecond lasers to achieve 3D imaging in deep tissues, reducing photodamage and light scattering.
 

Comparison of Characteristics between Fluorescence and Major Luminescence Types

3. Technical Principles and Key Characteristics of Laser-Induced Fluorescence

As an advanced form of traditional fluorescence spectroscopy, Laser-Induced Fluorescence leverages the unique high monochromaticity, high brightness, and high directionality of lasers to push the sensitivity and spatial resolution of fluorescence analysis to new levels.

A typical LIF system consists of four core components: the laser source, the optical transmission and collection system, the spectral analysis device, and the signal detection and processing unit. The choice of laser source is crucial, commonly used wavelengths include 355 nm, 405 nm, 488 nm, 532 nm, etc. . The optical system is responsible for directing the laser beam to the inspection area and efficiently collecting the emitted fluorescence, often utilizing lens assemblies, optical fibers, and mirrors. Spectral analysis typically employs grating monochromators or interference filters to resolve the fluorescence spectrum. For detectors, photomultiplier tubes (PMTs), charge-coupled devices (CCDs), and avalanche photodiodes (APDs) are common choices, each with different advantages in sensitivity, response speed, and dynamic range .

Typical Setup for Laser-Induced Fluorescence (LIF) 

LIF technology possesses a series of key characteristics that make it unique for precision measurements:
Ultra-High Sensitivity: The high power density of the laser ensures the generation of a large number of fluorescence photons, while low-noise amplification in photodetectors enables detection at the single-photon level. This often allows LIF to achieve detection limits in the nanomolar or even picomolar range.
Excellent Selectivity: Selectivity can be achieved both by choosing the laser wavelength to excite specific substances and by confirming the substance type based on its characteristic fluorescence "fingerprint" spectrum.
Good Spatial Resolution: The laser beam can be focused to a micrometer scale. Combined with confocal microscopy techniques, three-dimensional high-resolution imaging can be achieved.
Rapid Dynamic Response: Thanks to the nanosecond-scale lifetime of fluorescence, LIF can capture rapidly changing dynamic processes.
Non-Invasiveness: As an optical method, LIF does not disturb the physico-chemical state of the system under study, making it particularly suitable for in situmonitoring of fragile biological samples or sensitive chemical reaction processes.

Comparison of Laser-Induced Fluorescence and Traditional Fluorescence Spectroscopy

4.Application Fields of Fluorescence and Luminescence Technologies

Leveraging their non-invasiveness, high sensitivity, and rich information dimension, fluorescence and luminescence technologies demonstrate extremely high application value across numerous fields including scientific research, industrial inspection, and medical diagnostics. From microscopic single-molecule detection to macroscopic environmental monitoring, and from basic scientific research to routine medical testing, these optical technologies are profoundly changing how we understand the world and solve problems. 

Preparation of Phosphors and Upconversion Luminescence Properties	Fluorescence Lifetime Imaging Study
Digital PCR	Gene sequencing
Flow cytometry	Optogenetics research

For more information on sensory interests such as flow cytometry, gene sequencing, and optogenetics, please visit the Applications and Performance Catalog. 
5. Typical Lasers for Fluorescence and Luminescence from CNI​

Typical laser wavelengths used for fluorescence and luminescence applications include ​355 nm, 405 nm, 488 nm, 514 nm, 532 nm, 561 nm, 640 nm, 808 nm, 980 nm, 1064 nm, and 1532 nm, among others. Depending on the requirements, either continuous-wave or pulsed lasers can be selected. With advancements in laser technology, spectrometers, and detectors, Laser-Induced Fluorescence (LIF) technology continues to push the limits of its performance. New technologies such as Quantum Cascade Lasers (QCLs) and Optical Parametric Oscillators (OPOs)​​ have significantly expanded the range of tunable lasers. Time-Correlated Single Photon Counting (TCSPC)has improved the precision of fluorescence lifetime measurements to the picosecond level. Furthermore, confocal microscopy and ​two-photon excitation techniques have enabled high-resolution three-dimensional fluorescence imaging.These technological advancements continuously expand the application boundaries of LIF, making it an indispensable tool across a wide spectrum of fields, from basic scientific research to industrial inspection.

Beyond these typical single-wavelength lasers, other laser products might be used, such as:

Multi-wavelength flow cytometry lasers	Multi-wavelength gene sequencing light source	Integrated optogenetic system
Quantum-cascade laser	Tunable laser	Femtosecond laser

These applications require the selection of an appropriate spectrometer based on the specific requirements for wavelength measurement range and resolution. The spectrometers produced by CNI cover wavelength ranges including 200-1100nm, 185-240nm, and 950-1200nm,with a resolution of <0.1nm. For measuring single-frequency or extremely narrow spectral linewidths, more expensive linewidth analyzers are necessary, and CNI can provide related testing services.