The visual inspection and probing currently used in clinical practice can only detect cavities deep to the dentin or involving the pulp. Those non-cavity-forming enamel and dentin damages on the surface of the teeth are difficult to detect with the naked eye. Only with the use of new diagnostic techniques it is possible to detect those pre-clinical and very early lesions. The instruments used for early caries diagnosis are: 1. DIAGNOdent DIAGNOdent is a small portable instrument for early caries diagnosis with simple operation. It mainly consists of three parts: central processor, detector and transmission device (hose). The laser diode in the central processor can emit pulsed light of limited wavelength, which can excite different wavelengths of fluorescence when encountering teeth with different degrees of calcification. As the degree of tooth demineralization due to caries increases, the wavelength of fluorescence excited increases. The detector collects these fluorescence, and after processed by the electronic system in the central processor, it is expressed digitally on the screen of the instrument. Based on the magnitude of the value, the current mineralization status of the tooth can be determined against the diagnostic criteria and the depth of the caries can be determined. It has both conical and planar detectors and can be used to detect early demineralization on occlusal and smooth surfaces. The diagnostic result of this instrument is affected by some factors, such as tartar, plaque, etc.; filling materials such as glass ion and composite resin can also interfere with the measurement result, which is unfavorable to the diagnosis of secondary caries; in addition, there is a difference between the examination result in wet and dry state of teeth. Laser fluorescence diagnostic instrument 2.Electrical impedance instrument is mainly used for the study of early caries of occlusal surface, and its principle is: tooth enamel is a poor conductor of electricity, enamel demineralization forms tiny pores, and saliva penetrates into the tiny pores to form current conduction pathway. As the degree of tooth demineralization caused by caries increases, the resistance of enamel decreases greatly, and the degree of decrease of resistance value is proportional to the depth of caries damage. By measuring the resistance value from the tooth surface to the pulp chamber, the state of tooth mineralization can be judged. This instrument is one of the more ideal methods for early caries diagnosis at present. This technique is called resistance resistance method. The main products produced according to this principle are early and recent ECM. there are two methods: point specific method and surface specific method, the former is to select a number of points on the joint surface for detection, the common instrument is the latter is to determine the occurrence of caries by measuring the resistance of the whole sulcus system, the common instrument is ECM. When using the resistance resistance resistance method for caries diagnosis, because the current through the pulp cavity is very small, this method is considered It is a more sensitive non-invasive technique for early caries quantification and the operation is simple and easy to master. However, its shortcoming is that the negative diagnostic correct rate of electrical impedance method is still low (77%), and the application of this instrument for clinical diagnosis may lead to unnecessary treatment and destroy the normal structure of teeth. It has been proposed to increase the caries threshold (referring to the monitor reading) to improve the specificity, but some authors believe that this is at the expense of high sensitivity, and the best combination of sensitivity and specificity should be achieved. In addition, the thickness of the enamel has a great influence on the examination results, e.g. differences in enamel thickness due to enamel abrasion can affect the measurement results. Therefore, there is no unified diagnostic standard that can cover the above issues. Electrical impedance meter 3.QLF This instrument can take fluorescence photos of the caries area on the tooth surface and quantitatively analyze the amount of mineral loss and the size of the lesion in this caries area. It is suitable for the quantitative evaluation of the early enamel caries on the uncaved tooth surface, and also for the quantitative monitoring of the change of mineral content (demineralization and remineralization process) in the lesion area over a period of time. The principle is that when irradiated with high intensity blue light, the tooth will emit light in the green spectral band, and this fluorescence is directly related to the mineral content of the enamel; thus the difference in the induced fluorescence intensity between normal and early carious dentin tissue, which is highly correlated with the degree of demineralization of the carious dentin, can be correlated and analyzed by special software for the final purpose of quantitative diagnosis. The main components of QLF are composed of: argon ion laser generator (for blue-green light, λ=488nm), CCD camera (equipped with light filter), PC (for displaying, storing and analyzing data), etc. The light-emitting system consists of a 50-watt xenon arc lamp miniature light source, an optical filter and a liquid light guide. The arc length of the miniature light source is 4.2mm, and the emitted light is filtered through the optical filter (maximum filtering wavelength is 370nm) and then reaches the liquid light guide. The optical fiber core of the liquid light guide is 5 mm in diameter and is connected to a dental mouth mirror. Since the liquid light guide absorbs part of the light, the intensity of light reaching the tooth surface is about 10mW/cm2 (0.1mW/mm2), and the peak wavelength of light on the tooth surface is about 404nm. The demineralized area photographed by the camera is darker than normal and can be displayed on the computer screen, and then the outline is drawn manually on a transparent film paper. The fluorescence intensity of the normal tissue next to the caries is used to reconstruct the normal image of the caries, and the two images are compared to calculate the magnitude of the decrease in fluorescence intensity of the caries. This method can be used to measure the change of mineralization on the smooth surface of teeth, and it can also be used in clinical trials and evaluation of preventive methods. The test proved that the method can measure the reduction of demineralized area of the tissue around the brackets after one year of effective preventive treatment after orthodontics, and the increase of fluorescence intensity of the enamel, indicating the occurrence of remineralization. However, the method is more complicated to perform. QLF is also an effective method for the diagnosis of occlusal caries, but its clinical testing is still needed. Equipment composition Until 1996, a 488 nm blue light from an argon ion laser was used as the light source. The light reaches the sample or the patient’s tooth through the optical fiber, providing sufficient light for taking pictures. The blue light source is monochromatic and can be easily distinguished from the fluorescence emitted by the tooth surface using a filter. However, because the laser has a certain interference effect on the human body, when using the laser, the operator and the subject must wear a more bulky protective glasses, which is very inconvenient to use. Therefore, scientists developed another non-interfering visible light source-namely, the existing QLF-and compared it with the original laser fluorescence diagnostic system, yielding a correlation coefficient of r = 0.93 for both fluorescence systems [18]. Currently, the tooth-generated fluorescence of the newly developed QLF is collected by a CCD camera (lens focal length 12 mm, aperture F/2.0) with 795 × 596 pixels fixed on a dental mouth mirror (lens focal length 12 mm, aperture F/2.0) after passing through a high filtering lens with a conversion wavelength of 520 nm (to prevent interference from other external light) to obtain a fluorescence photograph of the tooth, which is displayed on a screen and saved. The photographs of the teeth were taken by Sony HyperHAD (CCD model: 1/4″, 440k pixels) using DSP (Digital Signal Processing) technology to achieve high resolution image quality with a signal-to-noise ratio of 44 dB. The final resolution of 760 x 570 tricolor photos can be obtained, each eight-bit color level pixel points are equivalent to 20 x 20um2 area on the dental surface. The display size of the photo is also an adjustable parameter. Third, the relationship between the operator and the equipment The accuracy of any measurement technique is influenced by both the precision of the equipment used and the operating skills of the user. The ideal situation is that the sophisticated design of the instrumentation can offset the errors caused by the user’s unskilled operation. The QLF equipment currently available on the market is well designed so that the operator only needs to adjust the position of the CCD camera to align it with the demineralized area on the tooth and obtain the exact focus. The liquid light guide is directly attached to the camera and does not require separate adjustment. The intensity of the light source can also be adjusted, but in order to ensure consistent light intensity during longitudinal observation, it is usually not necessary to do so. The camera’s lens automatically retracts back and forth, and when the operator feels that the image displayed on the screen is appropriate, the camera takes a picture and saves it in the computer by depressing the foot control. The operator can analyze the photos through a visual wizard. Up to six reference points can be selected for comparison with the baseline photo in subsequent photos, which are used to assist in correcting geometric changes in subsequent photos due to different light levels. The selected reference points must be points that remain constant in the series, such as cusps or gingival marginal points. After comparison and adjustment, the QLF analysis program can calculate and derive the values of ΔF, Area, and ΔQ. The results can be displayed simultaneously with the quantitative analysis and the fluoroscopy, which can be saved and displayed in 3D format or in different colors when needed. All photographs of the same tooth can be displayed with the reference point selected by the operator. The program analyzes the subsequent photographs according to the area of the selected lesion area in the baseline photograph. Normally, the “normal” border of the baseline photograph is copied and pasted into the subsequent photographs, or if the lesion area is expanded, a new lesion border can be redefined using mouse dragging. However, since the extent of the analysis must be consistent when monitoring the lesion longitudinally, the redefined extent of the lesion must be copied and pasted into the baseline photograph and reanalyzed. This analysis procedure does not allow direct comparison of the size of ΔFoldΔFnew.