Scanning Direction Research Articles

High-speed 2D beam steering based on a thin-film lithium niobate optical phased array with a large field of view

An on-chip optical phased array (OPA) is considered as a promising solution for next generation solid-state beam steering. However, most of the reported OPAs suffer from low operating bandwidths, making them limited in many applications. We propose and demonstrate a high-speed 2D scanning OPA based on thin-film lithium niobate phase modulators with traveling-wave electrodes. The measured modulation bandwidth is up to 2.5 GHz. Moreover, an aperiodic array combined with a slab grating antenna is also used to suppress the grating lobes of far-field beams, which enables a large field of view (FOV) as well as small beam width. A 16-channel OPA demonstrates an FOV of 50°×8.6° and a beam width of 0.73°×2.8° in the phase tuning direction and the wavelength scanning direction, respectively.

Unveiling of a puzzling dual ionic migration in lead‐ and iodide‐deficient halide perovskites (d‐HPs) and its impact on solar cell J–V curve hysteresis

AbstractHalide perovskite solar cells are characterized by a hysteresis between current–voltage (J–V) curves recorded on the reverse and on the forward scan directions, and the suppression of this phenomenon has focused great attention. In the present work, it is shown that a special family of 3D perovskites, that are rendered lead ‐and iodide‐ deficient (d‐HPs) by incorporating large organic cations, are characterized by a large hysteresis. The strategy of passivating defects by K+, which has been successful in reducing the hysteresis of 3D perovskite perovskite solar cells, is inefficient with the d‐HPs. By glow discharge optical emission spectroscopy (GD‐OES), the existence of the classic iodide migration in these layers is unveiled, which is efficiently blocked by potassium cation insertion. However, it is also shown that it co‐exists with the migration of the large organic cations characteristics of d‐HPs which are not blocked by the alkali metal ion. The migration of those large cations is intrinsically linked to the special structure of the d‐HP. It is suggested that it takes place through channels, present throughout the whole perovskite layer after the substitution of PbI+ units by the large cations, making this phenomenon intrinsic to the original structure of d‐HPs.

Fiber laser cutting of steel plate by twin spot beam setting in scanning direction

This study investigated the effects of laser beam intensity distribution on the reduction of dross height in fiber laser cutting of a steel plate with 3.2 mm thickness. A twin-spot beam was produced by splitting a single Gaussian beam into two beams using a special axicon lens, and these beams were set in the scanning direction for cutting experiments. The power ratio of two beams (R:F = Rear power:Front power) was varied to discuss the intensity balance for the effective reduction of dross. After cutting experiments, ray tracing analysis was conducted using an optical analysis to calculate the absorbed power density distributions in the kerf. A smaller dross height of 18 μm can be achieved at a power ratio of R:F = 8:2, and its value is lower than that by a single Gaussian beam. At a power ratio of R:F = 8:2, the front beam of lower power is irradiated at the upper part of the workpiece, and the rear beam of higher power is absorbed at the lower part of the workpiece. Thus, effective heat input to the lower part of the workpiece can contribute to a reduction of the dross height. Variation of power ratio in the rear and the front beams is effective in controlling the cutting front shape, and the uniformity of absorbed power in the thickness direction can be improved by setting the rear beam of about four times higher power to the front beam of lower power to obtain a smaller dross height in the case of a 3.2 mm steel plate.

An improved Compton parameter map of thermal Sunyaev–Zeldovich effect from Planck PR4 data

ABSTRACT Taking advantage of the reduced levels of noise and systematics in the data of the latest Planck release (PR4, also known as NPIPE), we construct a new all-sky Compton-y parameter map (hereafter, y-map) of the thermal Sunyaev–Zeldovich (SZ) effect from the Planck PR4 data. A tailored Needlet Internal Linear Combination (NILC) pipeline, first validated on detailed sky simulations, is applied to the nine single-frequency Planck PR4 sky maps, ranging from 30 to 857 GHz, to produce the PR4 y-map over 98 per cent of the sky. Using map comparisons, angular power spectra, and one-point statistics, we show that the PR4 NILC y-map is of improved quality compared to that of the previous PR2 release. The new y-map shows reduced levels of large-scale striations associated with 1/f noise in the scan direction. Regions near the Galactic plane also show lower residual contamination by Galactic thermal dust emission. At small angular scales, the residual contamination by thermal noise and cosmic infrared background (CIB) emission is found to be reduced by around 7 and 34 per cent, respectively, in the PR4 y-map. The PR4 NILC y-map is made publicly available for astrophysical and cosmological analyses of the thermal SZ effect.

Residual stresses in circular steel tubular columns repaired by laser-cladding additive manufacturing

This paper presents a study that investigates the magnitude and distribution of residual stress (RS) in laser cladding (LC) repaired circular steel tubular columns. The study involves experimental tests and numerical simulation. The familiar blind-hole method is employed in the experimental investigation for two RS measurement tests, while the finite element method is used for numerical thermal-mechanical analysis to obtain the temperature field and RS field. The results indicate the presence of residual tensile stress in the LC sheet and its surrounding area. The RS in the LC sheet area and in the vicinity of the LC sheet-substrate boundary is much greater than that in the substrate area located far away from the LC sheet. Additionally, the RS in the LC sheet along the scanning direction is larger than that perpendicular to the scanning direction. Nevertheless, the RS values in both the LC sheet and substrate are less than their respective yield strength values. The temperature-time cooling curves obtained from the finite element analysis, which simulate the laser cladding process for repairing the circular steel tubular column, are used to develop a theoretical model for predicting the RS in the LC sheet. A simplified RS model is proposed based on the thermal-mechanical theory, utilising the linear-exponential two-stage cooling curve. Comparisons between the experimental results and the theoretical results that are predicted using the proposed simplified RS model show both results to be in good agreement. Therefore, the simplified RS model proposed in this paper is suitable for efficiently and accurately predicting the RS in LC sheet for circular steel tubular columns repaired by LC.

Full-arch restoration with the NEXUS IOS® system: A retrospective clinical evaluation of 37 restorations after a one year of follow-up

ObjectivesReport the results with a novel workflow of digital restoration for completely edentulous patients with implant supported full arch fixed dental prostheses (ISFDP). MethodsThis multicenter retrospective clinical study was based on the evaluation from a cohort of 29 patients restored with 37 ISFDP designed and manufactured from the data captured by a direct intraoral scan, using a novel full digital system (NEXUS IOS®, Osteon Medical, a Keystone Dental Group company, Melbourne, Australia). Data was collected over a 3-year period, in six different dental centers. This study reported on the clinical parameters including: precision of marginal fit, functional and aesthetic integration of Nexus ISFDP. All patients were followed for a period of one year post delivery. Implant survival, biologic and prosthetic complications were assessed, at one year. A statistical analysis was conducted. ResultsAll 37 ISFDP were deemed clinically acceptable on insertion. Implant survival at one year was 100 %. The biologic and prosthetic complications were minimal during the follow-up period. ConclusionsISFDP, designed and manufactured using the NEXUS IOS® system, are clinically acceptable, with a low incidence of complications at one year. Long-term clinical studies are needed. Statement of clinical relevanceWithin the limitations of this study (retrospective design, small patient sample, limited follow-up) the NEXUS IOS® system seems to represent a viable solution for the restoration of completely edentulous patients with ISFDP, in a full digital workflow.

Simulation Study on the Influence of a Laser Power Change on the Residual Stress of a Laser-Melting RuT300 Valve Seat

In order to effectively suppress the cracking induced by the excessive residual stress of a laser-melting RuT300 valve seat, the influence of a laser power change on the residual stress was studied by constructing a finite element simulation model of a new power valve seat. The absorption rate of the laser energy on the surface of the material and the change in thermophysical parameters with temperature were taken into account in the model. The results show that the melting and phase-change-hardening areas can be obtained by the laser-melting process. With the increase in laser power, the peak temperature of the molten pool increased almost linearly. The melting zone area and the phase-change-hardening zone depth increased. When the laser power was increased from 2000 to 2600 W, the peak temperature of the laser-melting RuT300 valve seat increased from 2005.09 to 2641.93 °C, the maximum depth of the melting area increased from 0.55 to 0.86 mm, the maximum width of the melting area increased from 3.42 to 4.21 mm, and the maximum depth of the phase-change-hardening area increased from 0.55 to 0.64 mm. The circumferential residual tensile stress in the melting area was much higher than in the radial and axial directions. Along the laser scanning direction, the residual stress in the melting area increased as a whole, and the residual stress in the laser-scanning finishing area greatly increased. With the increase in laser power, the circumferential residual stress at the previous scanning moment decreased, and at the closing moment of the scan, the circumferential residual stress increased with the increase in laser power.

Effects of laser power on microstructures and mechanical properties of CoCrFeNiMn high entropy alloy with the addition of Y2O3 by directed energy deposition

The bulk CoCrFeNiMn high entropy alloy with the addition of Y2O3 is fabricated by directed energy deposition, and its microstructures and mechanical properties were investigated in relation to laser power. The XRD results revealed that FCC and Cr-rich precipitates were constant in all samples, regardless of laser power. Moreover, the lattice constant is maximum at 800 W laser power due to the increased Y content. The EBSD results indicated that higher laser power can coarse grain, but it is refined at 800 W due to the increased concentration of new compounds and Y2O3 in the sample. The texture strength of the sample processed at 1000 W is approximately 1.6 times greater than that observed in the 700 W process. Besides, the sample shows the minimum texture strength at 800 W, owing to the action of small grain size. Additionally, the higher laser power is beneficial to form <001> texture along the scanning direction and the building direction. The laser power has a proportional relationship to the average KAM value and low-angle grain boundary. The results of the deformation behavior suggest that the sample has excellent mechanical properties at 800 W, which exhibits ductile fracture.

Effect of Process Parameter and Scanning Strategy on the Microstructure and Mechanical Properties of Inconel 625 Superalloy Manufactured by Laser Direct Energy Deposition

This study aimed to investigate the effect of process parameters on the microstructure and mechanical properties of Inconel 625 alloy manufactured by direct energy deposition process. The Inconel 625 samples were produced by varying the laser scanning speeds from 720 - 960 mm/min while maintaining the same the laser energy volume density. The microstructure and mechanical properties of the produced samples were evaluated, and their dimensional accuracy and mechanical properties were also analyzed in terms of the laser scanning strategy. Microstructural observations at the same energy density revealed a dendrite substructure near the laser melt pool boundaries, indicating that the dendritic microstructure was primarily formed at the beginning of the solidification of each laser bead. When the solidification further progressed into the melt pool, solidification cell substructures became dominant regardless of the laser scanning speed. The size of the solidification cell and the dendrite structure were nearly unchanged as laser scanning speed increased. This suggests that changing the laser scanning speed while maintaining the volumetric energy density does not significantly alter the solidification rates of the Inconel 625. As a consequence of the similar cell sizes, the samples produced with different laser scanning speed led to similar mechanical properties. When samples produced with two different scanning strategies, of unidirectional and 90o rotation, were compared, a better dimensional accuracy was obtained with the 90o rotation strategy, compared to that obtained with the unidirectional approach. Comparisons of mechanical properties obtained along different directions with the different laser scanning strategies revealed that the Inconel 625 produced by the laser direct energy deposition had pronounced anisotropic mechanical properties, and was the highest in strength but the lowest in maximum elongation along the laser scanning direction.

Laser textured Ti-6Al-7Nb alloy for biomedical applications: An investigation of texturing parameters on surface properties.

Surface texturing with a laser beam is an effective method for engraving on the surface of biomaterials. The four laser texturing parameters (scan speed, frequency, fill spacing, and pulse width) having five different values were associated with five different scanning strategies (scan direction), and a total of 25 texturing conditions were tested on the Ti-6Al-7Nb alloy surface. The surface roughness and wettability of the textures created with a 20 W nanosecond fiber laser with a wavelength of 1064 nm on the surface of Ti-6Al-7Nb biocompatible alloy were investigated. Laser texturing parameters were analyzed according to the lowest surface roughness and a hydrophilic surface by creating L25 orthogonal arrays. The surface roughness values ranged between 2 and 26 µm. The lowest surface roughness with a value of 2.21 µm was achieved when the texture was processed with a frequency of 150 kHz, a fill spacing of 0.02 mm, a scan speed of 800 mm/s, a pulse width of 250 ns, and a cross-hatch strategy of 0°/90°. Considering the wettability test results, it was revealed that most of the textured surfaces have super hydrophilic and hydrophilic characteristics except the surface with a contact angle of 92.93°. The relevant surface was textured with 75 kHz frequency, 1000 mm/s scan speed, 0.05 mm fill spacing, 200 ns pulse width, and 45°/-45° cross-hatch strategy.

Micromotion compensation of trapped ions by qubit transition and direct scanning of dc voltages.

Excess micromotion is detrimental to accurate qubit control of trapped ions, thus measuring and minimizing it is crucial. In this paper, we present a simple approach for measuring and suppressing excess micromotion of trapped ions by leveraging the existing laser-driven qubit transition scheme combined with direct scanning of dc voltages. The compensation voltage is deduced by analyzing the Bessel expansion of a scanned qubit transition rate. The method provides a fair level of sensitivity for practical quantum computing applications, while demanding minimal deviation of trap condition. By accomplishing compensation of excess micromotion in the qubit momentum-excitation direction, the scheme offers an additional avenue for excess micromotion compensation, complementing existing compensation schemes.

Low Power Compact 3D-Constructed AlScN Piezoelectric MEMS Mirrors for Various Scanning Strategies.

In this paper, the newly developed 3D-constructed AlScN piezoelectric MEMS mirror is presented. This paper describes the structure and driving mechanism of the proposed mirror device, covering its driving characteristics in both quasi-static and resonant scan modes. Particularly, this paper deals with various achievable scan patterns including 1D line scan and 2D area scan capabilities and driving methods to realize each scanning strategy. Bidirectional quasi-static actuation along horizontal, vertical, and diagonal scanning directions was experimentally characterized and even under a low voltage level of ±20 V, a total optical scan angle of 10.4° was achieved. In addition, 1D line scanning methods using both resonant and non-resonant frequencies were included and a total optical scan angle of 14° was obtained with 100 mVpp under out-of-phase actuation condition. Furthermore, 2D scan patterns including Lissajous, circular and spiral, and raster scans were realized. Diverse scan patterns were realized with the presented AlScN-based MEMS mirror device even under a low level of applied voltage. Further experiments using high voltage up to ±120 V to achieve an enhanced quasi-static scan angle of more than 20° are ongoing to ensure repeatability. This multi-functional MEMS mirror possesses the potential to implement multiple scanning strategies suitable for various application purposes.

Correlation between the scan strategy, residing spatter distribution, and parts quality in laser powder bed fusion

Spattering may cause defects in the Laser Powder Bed Fusion (LPBF) process, and the inert gas flow has been identified as an effective method for removing/mitigating the negative effects and preventing vapor interference in the melting process. Given the variability of scan strategies in LPBF, optimizing the scan strategy to minimize residing spatters on powder bed is challenging but important. This study focuses on the correlation between the distribution of residing spatters, parts quality, and scan strategies (particularly scan direction and scan sequence here) under the influence of inert gas flow. A single lens reflex camera and image processing are employed to detect the distribution of spatters residing on the powder bed, providng unprecedented insight and valuable guidance for the design of scan strategy. The results reveal that the scan sequence against the gas flow induces fewer spatters residing on the powder bed. The large size residing spatters, with the diameter ranging from 80 μm to 210 μm, are concentrated along the scan direction in close proximity to the melting area. Scan sequence along the gas flow leads to rougher surface of the part, and more defects are formed between layers caused by spatter inclusions, which deteriorates the tensile properties obviously. It is recommended to employ orthogonal S-scanning strategy (S-45° with 90° layerwise rotation, scan sequence against gas flow) during the LPBF process. The selection of scanning parallel or perpendicular to the gas flow is dependent on the distribution of the surrounding processing areas, to ensure consistency of the parts quality.

In-situ X-ray computed tomography tensile tests and analysis of damage mechanism and mechanical properties in laser powder bed fused Invar 36 alloy

Laser powder bed fusion (LPBF) is a potential additive manufacturing process to manufacture Invar 36 alloy components with complicated geometry. Whereas it inevitably introduces specific microstructures and pore defects, which will further influence the mechanical properties. Hence, aiming at exploring the LPBF process-related microstructures and pore defects, and especially their influences on the damage mechanism and mechanical properties, Invar 36 alloy was manufactured by LPBF under designed different laser scanning speeds. The microstructure observations reveal that higher scanning speeds lead to equiaxed and short columnar grains with higher dislocation density, while lower scanning speeds result in elongated columnar grains with lower dislocation density. The pore defects analyzed by X-ray computed tomography (XCT) suggest that the high laser scanning speed gives rise to numerous lamellar and large lack-of-fusion (LOF) pores, and the excessively low laser scanning speed produces relatively small keyhole pores with high sphericity. Moreover, the in-situ XCT tensile tests were originally performed to evaluate the damage evolution and failure mechanism. Specifically, high laser scanning speed causes brittle fracture due to the rapid growth and coalescence of initial lamellar LOF pores along the scanning direction. Low laser scanning speed induces ductile fracture originating from unstable depressions in the surfaces, while metallurgical and keyhole pores have little impact on damage evolution. Eventually, the process-structure-property correlation is established. The presence of high volume fraction of lamellar LOF pores, resulting from high scanning speed, leads to inferior yield strength and ductility. Besides, specimens without LOF pores exhibit larger grain sizes and lower dislocation density at decreased scanning speeds, slightly reducing yield strength while slightly enhancing ductility. This understanding lays the foundation for widespread applications of LPBF-processed Invar 36 alloy.

Issues of forage quality under industrial milk production in the south of Ukraine

The purpose of the article is to determine the average quality of traditional forage (maize silage and alfalfa haylage) and innovative dietary components – rye silage on the example of average milk production farms in Odesa Oblast with moderate and intensive levels of technological process and to determine the impact of the risky land use zone in the South of Ukraine against the background of global warming on the quality of these dietary ingredients in order to determine further directions of forage production in the southern region of Ukraine. Scientific and economic experiments were conducted in Odesa Oblast according to the methods generally accepted in dairy farming, and laboratory studies of corn silage, alfalfa haylage, rye silage samples in a specialized laboratory for forage research using the NIRS technique were used to assess the quality of forages. NIRS has been successfully used in the prediction of nutritional value through direct scanning of forage samples. The analysis of corn silage shows that the basic indicators of its quality, such as dry matter content, metabolizable energy concentration, pH, level of digestibility of organic matter as a percentage of total dry matter, and starch content are below the existing standards, because due to hot weather conditions, silage is often forced to be harvested during the suboptimal phase of its maturity. An assessment of the mineral composition of corn, rye and alfalfa silage shows that the indicators are typical, taking into account the specifics of each crop in the southern region of Ukraine, so the existing deficit of manganese, cobalt, zinc and copper can be covered with the use of specialized premixes. Due to the difficulties of agrotechnical cultivation of corn silage and alfalfa haylage, which has recently developed in the risky land use zone of southern Ukraine and against the background of global warming, fodder crops have to be grown in more favorable (wet) seasons of the year, such as winter rye or triticale, etc. or their combination with corn silage and alfalfa haylage.

MeltPoolGAN: Auxiliary Classifier Generative Adversarial Network for melt pool classification and generation of laser power, scan speed and scan direction in Laser Powder Bed Fusion

A reliable classification architecture for melt pool images is crucial for real-time monitoring and control in additive manufacturing. It enables for the identification of discrepancies between actual and desired process parameters, which could lead to defects in printed part. Moreover, adjusting process parameters according to classification results promotes consistent melt pool geometry and leads to enhanced part quality. In addition to the classification, controlled generation of melt pool images can be employed for data generation, training of classifiers, and offline process parameter optimization along the scan path. This study introduces MeltPoolGAN, a novel machine-learning architecture designed for handling both of these tasks. The architecture is able to classify up to 371 classes consisting of process parameters such as laser power, scan speed and scan direction. This is a significant increase in the total number of classes as well as process parameters compared to state-of-the-art that only classified a maximum of 6 classes, consisting of a single process parameter. With this input, the MeltPoolGAN reaches accuracies of around 97% for power and scan speed class classification, and scan direction estimation errors below 3 degrees making it a reliable solution. Furthermore, the MeltPoolGAN is able to generate plausible melt pool images in controlled and flexible manner based on disentangled input parameters. Finally, since the architecture was trained and tested on two distinct datasets, the publicly available NIST dataset and the in-house created ETH dataset, its robustness and capabilities of being a general-purpose AI model are also demonstrated.

FIT-Net: Feature Interaction Transformer Network for Pathologic Myopia Diagnosis.

Automatic and accurate classification of retinal optical coherence tomography (OCT) images is essential to assist physicians in diagnosing and grading pathological changes in pathologic myopia (PM). Clinically, due to the obvious differences in the position, shape, and size of the lesion structure in different scanning directions, ophthalmologists usually need to combine the lesion structure in the OCT images in the horizontal and vertical scanning directions to diagnose the type of pathological changes in PM. To address these challenges, we propose a novel feature interaction Transformer network (FIT-Net) to diagnose PM using OCT images, which consists of two dual-scale Transformer (DST) blocks and an interactive attention (IA) unit. Specifically, FIT-Net divides image features of different scales into a series of feature block sequences. In order to enrich the feature representation, we propose an IA unit to realize the interactive learning of class token in feature sequences of different scales. The interaction between feature sequences of different scales can effectively integrate different scale image features, and hence FIT-Net can focus on meaningful lesion regions to improve the PM classification performance. Finally, by fusing the dual-view image features in the horizontal and vertical scanning directions, we propose six dual-view feature fusion methods for PM diagnosis. The extensive experimental results based on the clinically obtained datasets and three publicly available datasets demonstrate the effectiveness and superiority of the proposed method. Our code is avaiable at: https://github.com/chenshaobin/FITNet.

Customized Design for Ergonomic Products via Additive Manufacturing Considering Joint Biomechanics

This paper presents a customized design method for ergonomic products via additive manufacturing (AM) considering joint biomechanics. An ergonomic customized design model can be built based on kinesiology involving human joint biomechanics. Manifolds of the human bone can be reconstructed from X-rays, computed tomography (CT), magnetic resonance imaging (MRI), and direct 3D scanning. The conceptual and detailed design of customized products were implemented on ergonomic shoes and insoles. A lightweight lattice structure with variable porosity was generated via structural topology optimization for an ergonomic customized design. Notably, the upper surface of the custom-made insole may adhere perfectly to the plantar surface of the patient, resulting in a lower peak plantar pressure. Finite element analysis (FEA) can be employed to simulate the static or dynamic biomechanical characteristics. The conceptual ergonomic products were forwarded to the machine and fabricated via AM, driven by visual digital twin techniques. The experiments proved that a customized design suitability method for wearable ergonomic products via 3D printing is specifically tailored to the rehabilitation needs of individual customers, while consuming the least cost, time, and materials.

Tensile strength and crack tip stress distribution of modified glass in the composite laser beam separation

The tensile strength of the modified cross-section (MCS) determines to a certain extent whether the composite laser can complete the separation of glass, and the quality and efficiency of the separation. In this work, the effect of the picosecond Bessel laser on the tensile strength of soda-lime glass was systematically investigated for the first time. The relationship between the tensile strength and residual stress was not negatively correlated absolutely. The tensile strength was also related to the homogeneity and micro-structure of MCS, including micro cavities, cracks, and voids. Meanwhile, it was pointed out that by selecting appropriate modified parameters, the plasma shielding effect can be suppressed and the micro-morphology of MCS can be controlled to obtain smaller tensile strength and better cutting quality. Secondly, the stress field distribution at the crack tip was simulated to analyze the mechanism when the continuous wave (CW) laser interacted with the modified glass. The simulation results showed that only the stress perpendicular to the laser incidence direction and the scanning direction exceeded the tensile strength of glass material, ensuring that the cracks propagated only along the MCS. Finally, one-time separation of glass was achieved by using a composite laser beam separation (CLBS) technology with a speed of 50 mm/s, which was comparable to the industrial level, and the roughness of separated sidewall was less than 0.5 μm.

Comparative analysis of the trueness of linear measurements using direct and indirect scanning protocols compared to actual clinical measurements.

To assess the trueness of digital measurements using direct and indirect scanning approaches compared to the actual clinical measurements. The crown length, width, and width/length ratio were measured in 36 anterior maxillary teeth using three different methods. The first was clinically using a digital caliper, the second was by scanning the teeth using a digital scanner and the third was by making an impression of the teeth, constructing a stone cast in the lab, and scanning it to obtain digital measurements. Bland-Altman test and intraclass correlation were used to assess the data and make comparisons. Measurements taken using both approaches were highly reliable, with intraclass correlations ranging from 0.934 to 0.977 (p ≤ 0.000). Bland-Altman plot reflected a minimal mean difference between measurements especially in crown width measurements. Crown width/crown length measurement displayed the highest mean difference. Both direct and indirect optical surface scans showed similar high trueness in linear measurements of teeth. A higher discrepancy was detected in the crown width/length ratio. Digital dentistry is the new era in patient management. The use of conventional impression techniques and physical dental casts is associated with several disadvantages. Scanning dental casts to convert physical records into digital ones has multiple advantages. Optical surface scans (digital models) of the dentition are currently being more broadly used and advocated in the different dental disciplines including the construction of surgical guides for esthetic crown lengthening procedures. The trueness and reliability of linear measurements are of paramount importance to allow for proper fit and predictable outcomes. In this study, the trueness of these linear measurements obtained using direct and indirect methods was compared to the actual clinical measurements.