System and method for tomographic fluorescence imaging for material monitoring

Abstract

A computed axial lithography (CAL) system for producing a three-dimensional (3D) object includes a 3D object system comprising a vessel to hold a material for producing the 3D object; a source of patterned radiation arranged to project patterned radiation onto the material to produce the 3D object; and a tomographic fluorescence monitoring system arranged to receive fluorescence light emitted from the material during production of the 3D object. The tomographic fluorescence monitoring system includes a radiation source arranged to illuminate a region within the vessel in which the 3D object is produced with monitoring radiation to cause fluorescence of the material, a fluorescence imaging system arranged to receive the fluorescence light emitted from the material during production, and an image processing system arranged to determine at least one of three-dimensional temperature information or three-dimensional degree of conversion information based on the fluorescence light received.

Description

Att’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2 SYSTEM AND METHOD FOR TOMOGRAPHIC FLUORESCENCE IMAGING FOR MATERIAL MONITORING CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present patent application claims priority benefit to US Provisional Patent Application No.63 / 532,812, filed on August 15, 2023, the entire contents of which is incorporated herein by reference. FEDERALLY SPONSORED RESEARCH

[0002] This invention was made with government support under grant number 2036849 awarded by the National Science Foundation. The government has certain rights in the invention. BACKGROUND 1. Technical field

[0003] The currently claimed embodiments of the present invention relate to volumetric additive manufacturing (VAM), and more particularly to systems and methods that monitor at least one of three-dimensional temperature information or three-dimensional degree of conversion information during production of a three-dimensional object by VAM. 2. Discussion of related art

[0004] From point-scanning to plane-scanning and volume-at-once methods, advancements in parallelization of additive manufacturing (AM) have reduced the processing time by orders of magnitude. While parallelization has dramatically improved the practicality of these AM methods, the challenge of heat generation has become increasingly critical. Vat AM techniques such as continuous liquid interface production [1], [2], xolography [3], and computed axial lithography (CAL) [4] all exothermically solidify a large volume of photoresins in the order of minutes. For volumetric AM (VAM) such as CAL, the generated heat can auto-accelerate the polymerization reaction, trigger convection flows which lead to part floating [5]–[7], and cause optical aberrations. Due to the strong influence of temperature on reaction rates, rheologicalAtt’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2 behavior, and thermal lensing effects, temperature measurement alongside conversion state monitoring is crucial for devising mitigation strategies and implementing process control. Although heat generation has been acknowledged as one of the fabrication speed limiters in layer- based AM[2], the equivalent thermal challenges in VAM are not yet studied due to the lack of a volumetric thermal monitoring method. Infrared (IR) thermometry cannot be easily applied in volumetric measurements as it faces challenges such as limited transmission, material-dependent absorption peaks, and high background signals emitted by other objects. Thermal imaging using visible light from blackbody radiation, as used in reactive sintering [8], is not applicable to VAM due to much lower reaction temperature.

[0005] Currently, methods for in-situ monitoring in volumetric AM are primarily focused on degree of conversion (DOC) rather than temperature. Color schlieren tomography (CST) [9],

[0010] is designed to reconstruct local refractive index changes and quantify local material conversion through conversion–index relationships. On the other hand, optical scattering tomography (OST)

[0011] ,

[0012] infers material conversion from reconstructed scattering density. While both spatiotemporally resolved techniques have the potential to provide process control feedback from a conversion perspective, their current development has not addressed the exothermic nature of the process.

[0006] Fluorescence thermometry, also known as luminescence molecular thermometry, offers a viable alternative. This technique has been successfully employed to monitor temperatures in small-scale and biological environments

[0013] . It provides a variety of readouts including fluorescence intensity, fluorescence lifetime, and absorption / emission spectrum, offering resolution down to sub-degree Celsius. Yet, these measurements are commonly made in 2D. Although 3D measurements have been made with confocal microscopy, tomographic measurements, which enable larger (cm-scale) isotropic measurements, have not been performed. Therefore, there thus remains a need for improved systems and methods that monitor at least one of three-dimensional temperature information or three-dimensional degree of conversion information during production of a three-dimensional object by VAM.Att’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2 SUMMARY

[0007] A computed axial lithography (CAL) system for producing a three-dimensional (3D) object according to an embodiment of the current invention includes a 3D object system comprising a vessel to hold a material for producing the 3D object; a source of patterned radiation arranged to project patterned radiation onto the material to produce the 3D object; and a tomographic fluorescence monitoring system arranged to receive fluorescence light emitted from the material during production of the 3D object. The tomographic fluorescence monitoring system includes a radiation source arranged to illuminate a region within the vessel in which the 3D object is produced with monitoring radiation to cause fluorescence of the material, a fluorescence imaging system arranged to receive the fluorescence light emitted from the material during production, and an image processing system arranged to determine at least one of three- dimensional temperature information or three-dimensional degree of conversion information based on the fluorescence light received. At least one of the 3D object system or the tomographic fluorescence monitoring system includes an assembly to effect relative rotation of the tomographic fluorescence monitoring system around the 3D object system.

[0008] A tomographic fluorescence monitoring system for a volumetric additive manufacturing (VAM) system for producing a three-dimensional (3D) object according to an embodiment of the current invention includes a radiation source arranged to illuminate a region within a vessel in which the 3D object is produced with monitoring radiation to cause fluorescence of the material; a fluorescence imaging system arranged to receive the fluorescence light emitted from the material during production; and an image processing system arranged to determine at least one of three-dimensional temperature information or three-dimensional degree of conversion information during production of said 3D object. At least one of the vessel or the fluorescence imaging system comprises an assembly to effect relative rotation therebetween.

[0009] A method of tomographic fluorescence monitoring of a volumetric additive manufacturing (VAM) process for producing a three-dimensional (3D) object according to an embodiment of the current invention includes illuminating a region within a vessel in which the 3D object is being produced with monitoring radiation to cause fluorescence of the material;Att’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2 receiving the fluorescence light emitted from the material during production by a fluorescence imaging system to provide imaging signals; and processing the imaging signals to determine at least one of three-dimensional temperature information or three-dimensional degree of conversion information during production of the 3D object. At least one of the vessel or the fluorescence imaging system is rotated for tomographic imaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Embodiments of the present invention, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.

[0011] FIG. 1 is a schematic illustration of a tomographic fluorescence imaging system according to an embodiment of the present invention.

[0012] FIG. 2 shows normalized absorption (blue, left-hand peak) and normalized fluorescence intensity (orange, right-hand peak) of PEGDA-700 + 5 µM RhB upon excitation at 532 nm, and normalized absorption of 2 mM I369 in isopropanol (gray, far left). The 405 nm patterning light used in this study is denoted in dashed line.

[0013] FIG. 3 shows fluorescence intensity (right axis) and the relative fluorescence intensity compared to a baseline of 25°C and 0 mJ / cm3absorbed dose (left axis) of PEGDA-700 + 5 µM RhB + 0.3 mM BAPO, as a function of temperature and absorbed optical dose. The polymer precursor is excited at 532 nm.

[0014] FIG. 4 shows normalized absorption (blue, left-hand peak) and normalized fluorescence intensity (orange, right-hand peak) of PETRA + 5 µM RhB upon excitation at 532Att’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2 nm and normalized absorption of 2 mM I369 in isopropanol (gray, far left). The 405 nm patterning light used in this study is denoted by a dashed line.

[0015] FIG.5 shows fluorescence intensity (right axis) and relative fluorescence intensity compared to a baseline of 25°C and 0 mJ / cm3absorbed dose (left axis) of PETRA + 5 µM RhB + 0.1 wt.% I369, as a function of temperature and absorbed optical dose. The polymer precursor is excited at 532 nm.

[0016] FIG. 6 shows absorbance of PETRA + 5 µM RhB + 0.1 wt.% I369 of various absorbed optical dose measured on a well plate. Onset of scattering is indicated by increase in measured absorbance.

[0017] FIGS. 7A-7D show a) Geometry of trihelix target object, b) Raw fluorescence image captured at 166 s into printing of a trihelix, c) Geometry of pyramid truss target object, d) Raw image captured at 133 s into printing of a pyramid truss (black region at the bottom: bottom of the vial, grey region directly above: reduced fluorescence at the bottom of the pyramid truss).

[0018] FIGS.8A-8F show central vertical (a-c) and horizontal (d-f) reconstruction slices of relative fluorescence intensity during printing of a trihelix target. Scale bar: 3 mm.

[0019] FIGS.9A-9F show central vertical (a-c) and horizontal (d-f) reconstruction slices of relative fluorescence intensity during printing of a pyramid truss target. Scale bar: 3 mm.

[0020] FIGS. 10A-10B show (a) a vertical slice of the reconstructed trihelix print at 85.36s. (b) temporal temperature profile at a location near the top of the trihelix structure (red dot in (a)), where absorbed dose is assumed to be zero. At this location, a spatial average is taken over relative intensity in nine adjacent voxels in the horizontal plane of reconstruction. The average intensity is then mapped to temperature with the material’s response surface along the line of zero absorbed dose. The red star indicates the temperature value at the location at 85.36s.

[0021] FIG.11 is an illustration of obtaining material state information from RhB material response surface. Colored surface plot: response surface of PETRA + 5 µM RhB + 0.1 wt.% I369, in terms of fluorescence intensity relative to that at 25 °C and 0 mJ / cm3absorbed dose. IntersectionAtt’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2 of a specific reconstructed relative fluorescence intensity (gray plane) and the response surface gives a set of possible corresponding temperatures and absorbed dose (gray contour line). At spatial location where the resin is unexposed (absorbed dose = ^^ ^^ ^^. ^^ ^^ି ^^), the temperature at that location is uniquely determined on the response surface (red star).

[0022] FIG. 12 shows a printed trihelix object while capturing fluorescence intensity. Scale bar: 1 mm.

[0023] FIG. 13 shows real-time FTIR data of acrylate group conversion of PETRA + 5 µM RhB + 0.1 wt.% I369. DETAILED DESCRIPTION

[0024] Some embodiments of the current invention are discussed in detail below. In describing embodiments, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. A person skilled in the relevant art will recognize that other equivalent components can be employed, and other methods developed without departing from the broad concepts of the present invention. All references cited anywhere in this specification are incorporated by reference as if each had been individually incorporated.

[0025] The term “light”, and related terms such as optical, are intended to have a broad meaning to include both visible and non-visible portions of the electromagnetic spectrum. For example, “light” can include infrared light and ultraviolet light in addition to including visible light.

[0026] Tomographic volumetric additive manufacturing rapidly solidifies free-form objects via photopolymerization, which raises both local temperature and degree-of-conversion (DOC). In-situ monitoring of temperature and DOC during the printing process is crucial for metrology and process control. Accordingly, some embodiments of the current invention provide a tomographic fluorescence imaging technique to detect the spatiotemporal evolution ofAtt’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2 temperature and DOC during volumetric printing. An embodiment employs a fluorescent dye that is sensitive to both variations in temperature and DOC. DOC is considered as a function of, and is expressed in, the absorbed optical dose of the resin. By tomographically measuring changes in the dye’s fluorescence intensity and feeding such changes to a temperature–dose–intensity calibration, the local dose and temperature during the print can be confined to a set of possible states. At exemplar locations where a priori information is known for one quantity, the other quantity can be resolved. Some embodiments of the current invention are directed to two- wavelength spatiotemporal measurement systems that uniquely resolve both temperature and DOC.

[0027] Some embodiments of the current invention exploit the dependence of Rhodamine B (RhB) on the surrounding temperature and DOC to infer their possible values during a print. RhB is a cost-effective, widely available, and well-studied fluorophore. It is highly soluble in the resins used in the following examples at the concerned concentration (5 µM). RhB’s sensitivity to viscosity and temperature of the environment has been demonstrated previously

[0014] –

[0016] . Here, we employ tomographic fluorescence imaging to measure spatiotemporally the intensity of these luminescent molecular thermometers, providing a relationship between temperature and DOC (expressed in terms of absorbed dose) at various points in space and time. With a priori information on one of the quantities, this relationship allows us to determine the other unknown quantity. For materials undergoing significantly exothermic reactions, our method can discern the geometry of the printed object before material conversion develops to a stage where it begins to scatter light appreciably. The method can also quantify the temperature change in the surrounding uncured region. In some embodiments, the system can be used after production is finished to image DOC tomographically (even after the temperature had equilibrated). For example, this could be useful if we wanted to get a finer spatial resolution or determine the effects of any dark polymerization, so imaging offline after exposure could be useful. There could be reactions that continue longer than the patterning process. We have observed dark polymerization over 5-10 minutes after. Some applications can make measurement after reaction and thermal equilibrium as it can be more representative of the final material state. In some embodiments, an additive that is sensitive only to temperature could be used (e.g. an encapsulated fluorescent particle that is isolated from the polymer network and so not sensitive to the changing molecularAtt’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2 confinement provided by increasing DOC) and an additive that is sensitive to both DOC and temperature (e.g., the dye) could be used. This could make distinguishing the two effects more robust. Furthermore, other sensitive probes can be used, such as, but not limited to encapsulated dyes and / or other nanoparticles. Different dyes or probes could be used for their different temp / DOC sensitivities in different materials, for example. Probes can be engineered to be disproportionately / exclusively sensitive to either temperature or DOC, for example.

[0028] FIG. 1 is a schematic illustration of a computed axial lithography (CAL) system 100 for producing a three-dimensional (3D) object according to an embodiment of the current invention. The CAL system 100 includes a 3D object system 102 that includes a vessel 104 to hold a material for producing the 3D object, a source of patterned radiation 106 arranged to project patterned radiation 108 onto the material to produce the 3D object, and a tomographic fluorescence monitoring system 110 arranged to receive fluorescence light emitted from the material during production of the 3D object. The tomographic fluorescence monitoring system 110 includes a radiation source 112 arranged to illuminate a region within the vessel 104 in which the 3D object is produced with monitoring radiation to cause fluorescence of the material, a fluorescence imaging system 114 arranged to receive the fluorescence light emitted from the material during production, and an image processing system 116 arranged to determine at least one of three- dimensional temperature information or three-dimensional degree of conversion information based on the fluorescence light received. At least one of the 3D object system 102 or the tomographic fluorescence monitoring system 110 includes an assembly to effect relative rotation of the tomographic fluorescence monitoring system 110 around the 3D object system 102.

[0029] FIG.1 illustrates some details for the CAL system 100 that may be suitable in some cases; however, the general concepts of the current invention are not limited to those examples. For example, the source of patterned radiation 106 can include a 405nm projector with other optics as appropriate for the particular application. Various projector systems for projection pattern radiation for CAL systems can be used, for example. The general concepts of the current invention are not limited to these examples. Also, the radiation source 112 can include a 532nm laser diode but is not limited to that example. See discussion below for some general considerations regarding possible radiation sources. Furthermore, the radiation source 112 can include additional opticsAtt’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2 such as lens components and / or a multimode optical fiber as shown, but the general concepts of the current invention are not limited to only that example. The fluorescence imaging system 114 can include optics as well as a camera as illustrated in FIG. 1, but the general concepts of the current invention are not limited to only that example. The image processing system 116 can be programmed and / or hard wired according to algorithms, and variations thereof, as is described below with reference to some examples. In addition, the image processing system 116 can be implemented on one or more computers and the one or more computers can also be configured to control the 3D object system 102, the source of patterned radiation 106, and the tomographic fluorescence monitoring system 110 to coordinate them according to some embodiments of the current invention. See below for the description of some examples in more detail.

[0030] In some embodiments, the 3D object system 102 also includes a rotatable stage assembly 118 and the vessel 104 can be at least one of held on or attached to and rotated by the rotatable stage assembly 118.

[0031] The CAL system 100 can further include an illumination control system configured to communicate with the source of patterned radiation 106 and the tomographic fluorescence monitoring system 110 as mentioned above whether it is configured as a separate component and / or implemented on one or more computers along with the image processor 116. In some embodiments, the illumination control system interleaves actuation of the source of patterned radiation 106 and the tomographic fluorescence monitoring system 110 such that the fluorescence imaging system 114 receives the fluorescence light emitted from the material during time periods while the source of patterned radiation 105 is in an off state.

[0032] In some embodiments, the patterned radiation is optical radiation in a first wavelength region of the electromagnetic spectrum, the monitoring radiation is optical radiation in a second wavelength region of the electromagnetic spectrum, and the first wavelength region of the electromagnetic spectrum is different from the second wavelength region of the electromagnetic spectrum. In some embodiments, the material from which said 3D object is produced includes at least one additive that fluoresces with at least one fluorescence wavelength in response to the monitoring radiation. In some embodiments, the material from which the 3DAtt’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2 object is produced includes at least one additive that fluoresces with at least two fluorescence wavelengths in response to the monitoring radiation.

[0033] In some embodiments, the fluorescence light emitted from the material during production of the 3D object received by the tomographic fluorescence monitoring system includes at least two distinguishable fluorescence wavelengths and the image processing system 116 of the tomographic fluorescence monitoring system 110 is further configured to determine at least three- dimensional temperature information and three-dimensional degree of conversion information during production of the 3D object based on at least the at least two distinguishable fluorescence wavelengths.

[0034] Another embodiment of the current invention is directed to a method of tomographic fluorescence monitoring of a volumetric additive manufacturing (VAM) process for producing a three-dimensional (3D) object. The method includes illuminating a region within a vessel in which the 3D object is being produced with monitoring radiation to cause fluorescence of the material; receiving the fluorescence light emitted from the material during production by a fluorescence imaging system to provide imaging signals; and processing imaging signals to determine at least one of three-dimensional temperature information or three-dimensional degree of conversion information during production of the 3D object. At least one of the vessel or the fluorescence imaging system is rotated for tomographic imaging. In some embodiments, the processing of the imaging signals determines at least three-dimensional temperature information and three-dimensional degree of conversion information during production of the 3D object.

[0035] The following describes some further aspects of the current invention by way of specific examples. However, the broad concepts of the current invention are not limited to only those examples. Methodology Resin formulation

[0036] To demonstrate the system’s capability to detect thermal change in the print, a polymer precursor is chosen such that a significant amount of heat is generated over the course ofAtt’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2 tomographic VAM printing with a typical patterning light intensity. Pentaerythritol Tetraacrylate (CAS no.4986-89-4, TCI) (PETRA) with 0.1 wt% Irgacure-369 (CAS no.119313-12-1, Sigma- Aldrich) (I369) was found to release substantial heat under typical VAM patterning light intensity (8 mW / cm2) over the course of 1–2 minutes, making the resin a good candidate for rapid testing of our detection system. Poly (ethylene glycol) diacrylate (average molecular weight 700) (CAS no. 26570-48-9, Sigma Aldrich) (PEGDA-700) with 0.3 mM Phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide (CAS no.162881-26-7, Sigma Aldrich) (BAPO) was also prepared for separate material characterization. The concentration of RhB (CAS no.81-88-9, Sigma Aldrich) dissolved in the resin is chosen to be 5 µM to provide sufficient fluorescence signal without significant attenuation of excitation light or reabsorption of emitted light within the resin volume. Absorbance / emission measurement and fluorescence calibration

[0037] The absorbance and emission spectra of resins were collected by a Vernier SpectroVis Plus UV-VIS spectrophotometer with a custom excitation light source at 532 nm. The fluorescence of RhB in PETRA resin as a function of absorbed optical dose and temperature was collected with a Molecular Devices SpectraMax M5 multimode microplate reader. The absorbed optical dose refers to the optical dose absorbed by the photoinitiator (I369 or BAPO). This quantity has unit of mJ / cm3and it is calculated by multiplying the areal optical dose in mJ / cm2with the absorption coefficient of the photoinitiator at the patterning wavelength (405 nm). Resin samples across optical dose 0-93 mJ / cm3were first cured in groups of four replicates on a clear-bottom 96- well-plate (CellVis P96-1.5H-N). The plate was then read on the SpectraMax M5 microplate reader in fluorescence end-point mode with excitation wavelength = 532 nm and emission wavelength = 580 nm. A cutoff wavelength of 570 nm was set by the plate reader to filter unwanted signal from excitation light. To account for signal variation across the well area induced by uneven curing or excitation illumination, a 3 × 3 point scan was done across each well and the median of the nine point signals was chosen to represent the fluorescence intensity measured at that well.Att’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2 Tomographic fluorescence imaging setup

[0038] Hardware for the printing and fluorescence imaging experiments is shown in Fig. 1. The optical projections used for printing had a center wavelength of 405 nm and maximum intensity of 8 mW / cm2and were discretized at a spatial sampling rate of 32.8 pixel / mm at the sample volume. The vial containing the resin was immersed in a mineral oil index matching bath and was rotated at 30 ° / ^^ during the experiment. A multimode optical fiber (Thorlabs AFS105 / 125Y) carried the excitation light output from a 532 nm diode laser to the fiber port mounted on the top of the hollow shaft rotation stage. From the optical fiber, the 532 nm light illuminated the resin from above and through a diffuser.

[0039] The sample fluorescence was collected and imaged by a 4f system comprising two lenses with focal length of 300 mm and 50 mm (Thorlabs AC508-300-A-ML and LA1131-ML), through a long-pass filter (Thorlabs FELH0600) to a monochrome camera (Thorlabs 340M-USB). At the resulting 6× demagnification, the camera effectively sampled the fluorescent resin at 22.5 pixel / mm.

[0040] Image acquisition was electronically triggered to synchronize with the intermittent off-time of the patterning projection. Effectively, this synchronization prevented the camera from capturing the non-uniform fluorescence caused by the 405 nm patterning beam. Details of this electronic synchronization are described below in the supplementary materials (see FIG.2). Computational reconstruction

[0041] A multi-processing Python program was written to control the projector, diode laser and camera. The data acquisition and processing pipeline is detailed in supplementary materials (see FIG.3). During printing, a sequence of pre-computed patterning images was delivered by the projector to the resin. The set of images spanned a full vial rotation and contained one projected image per degree of rotation. These projections were optimized for the test object with the OSMO algorithm

[0017] .

[0042] To estimate the spatial distribution of the resulting fluorescence intensity within the resin, the collected camera images were first denoised with a Gaussian filter and then used forAtt’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2 reconstruction via filtered back projection. This digital reconstruction process is detailed below in supplementary section S.2. The resulting reconstruction is a volumetric representation of the local fluorescence intensity ratio relative to the local intensity at the baseline (25 °C and 0 mJ / cm3optical dose). Over the course of an experiment, a temporal sequence of reconstructions can be produced, where each reconstruction is computed from images taken over one rotation period. Similar to CST [9], the multi-processing program is written such that reconstruction can be computed either in real-time during the experiment (at slower reconstruction refresh rates) or afterwards (at higher refresh rates). The reconstruction demonstrated in this work was computed in real-time while analyses were performed after the experiment. Results Temperature- and curing-dependent optical properties of RhB

[0043] The absorption spectrum of RhB in PETRA did not significantly overlap with that of I369 or the patterning light used in this work, which had a center wavelength of 405 nm (FIG. 4). Nevertheless, the residual light absorption at approximately 405 nm by RhB necessitated the application of time-gated and synchronized projection and image acquisition (supplementary material S.1). This ensured that the camera did not capture the non-uniform fluorescence created by the patterning beam.

[0044] As shown in FIG. 5, the fluorescence of RhB excited by 532 nm decreases with temperature and absorbed optical dose of PETRA resin. In the case of unexposed resin, the fluorescence intensity drops 34% over temperature increases of 34 °C. This corresponds to an average thermal sensitivity of 1% per degree Celsius. Both the fluorescence intensity and its sensitivity with temperature decrease with absorbed dose. This effect can be attributed to the increased restriction of RhB molecules in the formed polymer network.

[0045] Notably, RhB’s fluorescence intensity in PETRA resin declines significantly as a function of the resin’s absorbed dose. A potential explanation for this could be the ionic nature of RhB versus the lower polarity of PETRA, given its tetrahedral acrylate moieties. This polarity difference could induce RhB aggregation and consequently result in fluorescence quenching

[0018] .Att’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2

[0046] On the other hand, we also characterized the response surface of RhB in PEGDA (at the same concentration of 5 µM), which exhibited opposite trend of fluorescence with absorbed dose (FIG.3). PEGDA has a higher polarity than PETRA, which may facilitate the dispersion of RhB and allow the other temperature / exposure-dependent mechanism to dominate, such as the sensitivity related to the rotation of diethylamine groups in the molecule upon excitation as summarized in

[0019] ,

[0020] . These reports suggest elevated local viscosity is associated with the non-radiative decay of excited electrons, resulting in decreased fluorescence, while an increase in temperature has an opposite effect. This is in line with our intensity characterization data in FIG. 3.

[0047] The formation of an amorphous polymer network induces spatial variation of refractive index and leads to scattering of both excitation light and emission from fluorophores within the material volume. As scattering lowers detected intensity in addition to temperature and exposure effects, the onset of scattering thereby established a cut-off for the absorbed dose range examined in the response surface characterization. In the above characterization, this scattering onset is indicated by an increase in measured absorbance on a well-plate following an absorbed dose of 74.6 mJ / cm3(FIG.6).

[0048] With the above material considerations in mind, we sought to detect two VAM- printed objects ― a trihelix and a pyramid truss ― from their changes in fluorescence intensity. The objects were printed in the PETRA resin and their fluorescence images were collected, processed, then used for tomographic reconstruction. From the raw fluorescence images (FIGS. 7A-7D), we can detect reduced intensity near the object. In the reconstruction (

[0049] FIGS.8A-8F, FIGS.9A-9F) we can recover the volumetric distribution of relative fluorescence intensity ( ^^^^^^௧^௩^) at each query time. From the resulting reconstructions, the reduction of ^^^^^^௧^௩^distributed in a similar shape as the printing object but extends further beyond the design part surface. The change of ^^^^^^௧^௩^is non-uniform, indicating potentially different rates of conversion and heat diffusion. From the reconstruction in the printing of the pyramid truss (Att’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2

[0050] FIGS. 9A-9F), the largest decrease of ^^^^^^௧^௩^happens in the five tips of the pyramid where the members of the truss meet. For an example location above the trihelix object, we extract the time series of ^^^^^^௧^௩^and resolve its local temperature under the assumption that this location has zero absorbed optical dose as it is located outside the part and above the height range under patterning exposure. As shown on

[0051] FIGS.10A-10B, the temporal temperature profile computed from locally averaged ^^^^^^௧^௩^shows a clear increase after the end of patterning at 60 s. The onset of scattering in the prints were quantified separately with dark-field imaging (supplementary section S.3) and only the results generated prior to that time point are presented in Error! Reference source not found.FIG. 5,

[0052] FIGS.8A-8F,

[0053] FIGS.9A-9F. A separate run of the experiment was performed without printing to detect photobleaching of RhB by 532 nm excitation light. The effect of photobleaching was negligible over the course of printing up to 450 s.

[0054] There is substantial room for improvement in the signal-to-noise ratio (SNR) on the acquired image and the computed reconstructions. The high noise level in the results can be attributed to limited light collection during the short exposure time (9 ms) and with the low numerical aperture (0.085) imaging system. In other embodiments, SNR can be improved by using a laser with higher power and more concentrated fluorophore in resin. Temporal averaging over consecutively collected images can also be used to reduce noise. Discussion

[0055] Our research has established a tomographic fluorescence imaging system within the VAM framework for temperature and DOC characterization. The fluorescent probe implemented has demonstrated reasonable sensitivity to both temperature changes and photopolymerization exposure. Tomographic reconstruction further facilitates online and offline metrology, allowing us to capture volumetrically the evolution of temperature-dose profile both inside and outside the print, prior to the onset of substantial scattering. In a broader context, ourAtt’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2 work serves as an inaugural demonstration of tomographic measurement of material-state properties via an environment-sensitive fluorescent probe. Existing confocal measurement of state-dependent fluorescence

[0013] have limited axial sectioning capability at low numerical aperture (NA) due to the diffraction limit. The tomographic nature of our reconstruction method offers isotropic voxels in a low NA regime and therefore enables volumetric measurement at cm- scale or larger. This spatiotemporally resolved material-state measurement technique also presents potential applications in other additive manufacturing modalities.

[0056] While RhB exhibited a thermal sensitivity of 1% per degree Celsius, we also found that it demonstrated varying degrees of absorbed-dose sensitivity in different polymer precursors. RhB’s thermal and absorbed-dose sensitivity is also subject to the polarity of the resin and the aggregation effect, necessitating characterization specific to each resin. In other embodiments, different fluorescent sensors can be examined for improving sensitivity. For example, BODIPY- derived molecular rotors have demonstrated tunable sensitivity to viscosity and / or temperature in both polar and non-polar solvents

[0019] .

[0057] Certain limitations were identified in the above example, particularly regarding the sensitivity of the optical system. The minimum detection level of voxel intensity change depends on the image pixel sampling rate, available bit-depth and noise level. A relation between spatial and fluorescence intensity resolution in tomographic imaging is derived in supplementary material S.4. Apart from hardware, there are various image processing decisions (such as denoising parameters) that represent tradeoffs between spatial resolution and intensity resolution. Furthermore, scattering posed a significant challenge to interpreting results generated after the onset of the phenomenon. Scattering arises from local changes in the refractive index and the self- lensing effect associated with directed patterning illumination

[0021] . For reconstructions after the onset of part scattering, we limited quantitative analysis to levels above the object height, as the tomographic light propagation model’s accuracy was compromised by unaccounted scattering within the same height level as the object. We characterized the scattering onset of the print using a dark-field imaging setup (supplementary section S.3) and only presented results generated prior to that time point in Error! Reference source not found.FIG.5,

[0058] FIGS.8A-8F andAtt’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2

[0059] FIGS. 9A-9F. To mitigate the impact of scattering, other embodiments could implement techniques such as tomosynthetic printing

[0022] ,

[0017] or latent image VAM

[0021] , or utilize a low-shrinkage resin with a smaller change in refractive index. Other embodiments could also explore dual-mode detection for simultaneous resolution of both temperature and DOC. For instance, multiple chemical probes at orthogonal wavelengths could be employed, as well as two- color ratiometric measurements on a single chemical probe. Conclusion

[0060] Our study established the use of RhB and tomographic fluorescence imaging for real-time monitoring of temperature and degree of conversion in volumetric additive manufacturing. The fluorescent properties of RhB demonstrated sensitivity to temperature and curing, though the exact effects varied across different polymer precursors. Scattering of light during printing limits the valid observation time and points to future investigations in conjunction with scattering-free printing methods. Other embodiments include two-wavelength spatiotemporal measurement systems that uniquely resolve both DOC and temperature.

[0061] References [1] J. R. Tumbleston et al., “Continuous liquid interface production of 3D objects,” Science, vol.347, no.6228, pp.1349–1352, Mar.2015, doi: 10.1126 / science.aaa2397. [2] D. A. Walker, J. L. Hedrick, and C. A. Mirkin, “Rapid, large-volume, thermally controlled 3D printing using a mobile liquid interface,” Science, vol.366, no.6463, pp.360–364, Oct.2019, doi: 10.1126 / science.aax1562. [3] M. Regehly et al., “Xolography for linear volumetric 3D printing,” Nature, vol.588, no. 7839, Art. no.7839, Dec.2020, doi: 10.1038 / s41586-020-3029-7. [4] B. E. Kelly, I. Bhattacharya, H. Heidari, M. Shusteff, C. M. Spadaccini, and H. K. Taylor, “Volumetric additive manufacturing via tomographic reconstruction,” Science, vol.363, no.6431, pp.1075–1079, Mar.2019, doi: 10.1126 / science.aau7114. [5] T. Waddell et al., “Use Of Volumetric Additive Manufacturing As An In-space Manufacturing Technology,” in Advancements in materials applications and rapid prototyping,Att’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2 72 st International Astronautical Congress (IAC), Sep.2022. [Online]. Available: https: / / calfdm.com / wp-content / uploads / 2022 / 09 / IAC-CAL-final-15Sep22-print1.pdf [6] R. Salajeghe, C. S. Kruse, D. H. Meile, D. Marla, and J. Spangenberg, “Investigating the influence of thermal and mechanical properties of resin on the sedimentation rate of the printed geometry in the volumetric additive manufacturing technique,” 2022. doi: 10.26153 / tsw / 44203. [7] R. Salajeghe, D. H. Meile, C. S. Kruse, D. Marla, and J. Spangenberg, “Numerical modeling of part sedimentation during volumetric additive manufacturing,” Addit. Manuf., vol. 66, p.103459, Mar.2023, doi: 10.1016 / j.addma.2023.103459. [8] H. Wang, D. J. Kline, and M. R. Zachariah, “In-operando high-speed microscopy and thermometry of reaction propagation and sintering in a nanocomposite,” Nat. Commun., vol.10, no.1, Art. no.1, Jul.2019, doi: 10.1038 / s41467-019-10843-4. [9] C. Chung Li, J. Toombs, and H. Taylor, “Tomographic color Schlieren refractive index mapping for computed axial lithography,” in Symposium on Computational Fabrication, Virtual Event USA: ACM, Nov.2020, pp.1–7. doi: 10.1145 / 3424630.3425421.

[0010] C. C. Li et al., “Computational optimization and the role of optical metrology in tomographic additive manufacturing,” in Advanced Fabrication Technologies for Micro / Nano Optics and Photonics XV, SPIE, Mar.2022, pp.38–44. doi: 10.1117 / 12.2610558.

[0011] A. Orth et al., “On-the-fly 3D metrology of volumetric additive manufacturing,” Addit. Manuf., vol.56, p.102869, Aug.2022, doi: 10.1016 / j.addma.2022.102869.

[0012] A. Orth et al., “Optical Scattering Tomography for Volumetric Additive Manufacturing,” in Conference on Lasers and Electro-Optics (2022), paper ATh1D.3, Optica Publishing Group, May 2022, p. ATh1D.3. doi: 10.1364 / CLEO_AT.2022.ATh1D.3.

[0013] J. Zhou, B. del Rosal, D. Jaque, S. Uchiyama, and D. Jin, “Advances and challenges for fluorescence nanothermometry,” Nat. Methods, vol.17, no.10, Art. no.10, Oct.2020, doi: 10.1038 / s41592-020-0957-y.

[0014] R. Mercadé-Prieto, L. Rodriguez-Rivera, and X. D. Chen, “Fluorescence lifetime of Rhodamine B in aqueous solutions of polysaccharides and proteins as a function of viscosity and temperature,” Photochem. Photobiol. Sci., vol.16, no.11, pp.1727–1734, Nov.2017, doi: 10.1039 / c7pp00330g.

[0015] M. A. Bennet, P. R. Richardson, J. Arlt, A. McCarthy, G. S. Buller, and A. C. Jones, “Optically trapped microsensors for microfluidic temperature measurement by fluorescenceAtt’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2 lifetime imaging microscopy,” Lab. Chip, vol.11, no.22, pp.3821–3828, Oct.2011, doi: 10.1039 / C1LC20391F.

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[0020] K. G. Casey and E. L. Quitevis, “Effect of solvent polarity on nonradiative processes in xanthene dyes: Rhodamine B in normal alcohols,” J. Phys. Chem., vol.92, no.23, pp.6590– 6594, Nov.1988, doi: 10.1021 / j100334a023.

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[0022] C. M. Rackson, “Fundamental Developments in Volumetric Additive Manufacturing,” Ph.D., University of Colorado at Boulder, United States -- Colorado, 2022. Accessed: Jun.24, 2023. [Online]. Available: https: / / www.proquest.com / docview / 2671944553 / abstract / 7D2531D42BFC444FPQ / 1

[0023] W. Van Aarle et al., “Fast and flexible X-ray tomography using the ASTRA toolbox,” Opt. Express, vol.24, no.22, p.25129, Oct.2016, doi: 10.1364 / OE.24.025129. Supplementary Materials S.1 Electronic synchronization of patterning projection and image acquisition

[0062] To prevent imaging the fluorescence excited by the patterning light, the patterning and image acquisition were electronically synchronized to take place alternately. The LED of theAtt’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2 projector was pulsed at 60 Hz at a duty cycle of 33%. The camera was triggered immediately when this LED transition to off state and starts a 9 ms of exposure. The exposure time was chosen such that the patterning and imaging do not overlap in time.

[0063] While this approach effectively rejects fluorescence excited by the patterning light, it poses a tradeoff between time spent in patterning and in image acquisition. The maximum intensity of the patterning projections was reduced to an average of 8 mW / cm2at 33% duty cycle from 24 mW / cm2at 100% duty cycle. The short exposure of the camera also reduced the SNR of the acquired images.

[0064] While the camera was triggered at 60 Hz, the frames were sampled at 30 Hz in experiments such that the camera capture exactly 360 images per vial rotation for reconstruction. In future experiments, a temporal averaging technique may be applied over images collected at 60 Hz to improve SNR. The SNR could also be improved by using a stronger excitation light source and higher concentration of RhB. S.2 Data acquisition, processing, and tomographic reconstruction pipeline

[0065] All hardware control, data acquisition and processing were handled by a custom Python program. Using the multi-processing module, the main Python process spawns a new process for each of the tasks which includes image acquisition, image pre-processing, tomographic reconstruction, and data saving. This architecture was designed to concurrently process a real-time stream of data.

[0066] The image pre-processing steps included denoising, background subtraction and Ram-Lak filtering. All collected images were first Gaussian-filtered with a kernel size of 11 pixels and a standard deviation of 7 pixels for denoising. Prior to printing, a set of denoised background images was collected without the presence of excitation or patterning light while the vial was rotating. All subsequent images used for reconstruction had the background image at the corresponding angle subtracted. For the subsequent filtered back-projection step in tomographicAtt’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2 reconstruction, the images were filtered in the horizontal direction (perpendicular to the rotation axis) by a Ram-Lak filter before they were sent to another process downstream.

[0067] The tomographic reconstruction process was performed by filtered back projection with the Astra toolbox

[0023] . There are multiple methods to group images together for tomographic reconstruction. These methods are defined as time segmentation schemes in CST [9],

[0010] . Under the same definition, the reconstructions in this work used images within a moving window spanning one full rotation. In the real-time computation of reconstruction in this work, the entire image set was collected prior to the time of reconstruction. The reconstruction timestamps were set to be equal to the time when the last image in the set was taken. This contrasts with an offline implementation where reconstruction can be centered in the moving window.

[0068] The native tomographic reconstructions were expressed in units of greyscale value of the camera. To obtain the reconstructions in relative intensity, all reconstructions are normalized by a reference reconstruction. This reconstruction was collected one rotation prior to the start of the printing at reference temperature and 0 ^^ ^^ / ^^ ^^ିଷabsorbed dose with excitation light turned on. Normalization was performed elementwise in each reconstruction voxel. After normalization, the reconstructions are expressed in ^^^^^^௧^௩^which physically represents the ratio of fluorescence intensity relative to the same voxel at the reference state. The local ^^^^^^௧^௩^can then be interpreted with the response surface of RhB. S.3 Scattering observation by dark field imaging

[0069] The scattering observation was done with a method similar to that of Orth et al.

[0011] . A diffused 635 nm laser source (Thorlabs CPS635S) illuminated top-down into the printing vial in the CAL system. The scattered red light entered a camera (Panasonic Lumix GH4) viewing orthogonally to the light source. A long-pass filter (FELH0600) was placed in front of the camera to remove the 405 nm patterning light from the projector. The onset of scattering was determined directly on the acquired image by the appearance of any local increase of brightness against theAtt’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2 background at the location of the printing object. This onset happened at 42s and 72s for the printing of trihelix and pyramid truss target respectively. S.4 Derivation of relation between spatial and fluorescence intensity resolution in tomographic imaging

[0070] In tomographic fluorescence imaging, the value of each pixel in an image can be expressed as the line integral of fluorescence power along the line of imaging (optical axis). This approximation is accurate at the paraxial limit (when system has low NA). From this relationship, it can be shown that the resolution of fluorescence power density and spatial resolution of the tomographic reconstruction shares the total bit-depth of the camera. A high-bit-depth camera is hence desirable for obtaining high spatial resolution and temperature / DOC resolution.

[0071] In a hypothetical scenario, let ^^ be the coordinate along the optical axis of a 4f imaging system with magnification ^^, and ^^ and ^^ be the coordinates in the horizontal and vertical directions respectively. The imaging target is a fluorescent object when substantial thickness along the optical axis. The camera sensor plane is conjugate to the center plane of the imaging target. We are interested in determining the minimum change of fluorescence power density change in one reconstruction voxel that the camera pixel can detect. In a parallel beam tomographic configuration, the reconstruction voxel width equals the width of one camera pixel.

[0072] Let ^^^ ^^ᇱ, ^^ᇱ^ be the intensity at a particular point on the sensor plane parametrized by ^^′ and ^^′, ^^^ ^^, ^^, ^^^ be the solid angle subtend by the objective lens from a particular point ^ ^^, ^^, ^^) in space, ^^^ ^^, ^^, ^^^ be the local isotropic fluorescence power density of a point ^ ^^, ^^, ^^) in space in units of mW / cmଷ, and ^^^^^, ^^^be ^^-thickness of the fluorescent object at ^ ^^, ^^^. Assuming all light received by the objective lens is relayed to the corresponding point on the sensor, the intensity at a point on the sensor plane can be expressed as ^^^^^ᇱൌ െ ^^ ^^ᇱൌ െ ^^ ^^^ൌ ^ ^^^^^^^^^^^^^^ ^^^1^Att’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2 under the imaging condition.

[0073] From this point onwards, we only focus on events at a particular ^^-line at ^ ^^, ^^^ and omit the functional dependence on ^ ^^, ^^^ for a concise derivation. Let the bit-depth of the camera be ^^, and ^^ be the saturation intensity at the given sensor setting. Each intensity level of the camera pixels is separated byௌଶ್. Let ^^^^^^ ^^^ be the fluorescence power density at the reference state.

[0074] of fluorescence power density to be detectable, ^^2^^ Δ ^^ ൌ ^ ^^^ ^^^ ^^^ ^^^ ^^ ^^ െ ^ ^^^ ^^^ ^^^^^^ ^^^ ^^ ^^ ^2^ ^^

[0075] In low-NA systems and for object with limited thickness such that ^^ can be approximated as a constant, ^^2^^ Δ ^^ ൌ ^^^ ^^^ ^^^ െ ^^^^^^ ^^^ ^^ ^^ ^3^

[0076] If the changewithin a range ^ ^^^െ^௭ଶ , ^^^^^௭ଶ ^ where Δ ^^ is the thickness of one reconstruction voxel orof one camera pixel size, then ^^^ ^^^^ and ^^^^^^ ^^^^ can be represented by their local spatial average ^^௭బand ^^^^^,௭బrespectively. We can write ^^ ௭ ା^௭^௭ బ௭బାଶ ^^^ ^^ ^ଶ^^^ ^^^ ^^ ^ ^^^ ^^ ^^ ^ ^^ ^^ ^^From equation (4), it is clear that the product of ൫ ^^௭బെ ^^^^^,௭బ൯ and Δ ^^ must be greater than a constantௌ. For a small change in fluorescence, Δ ^^ mustthe system has a poorresolution. On the other hand, if Δ ^^ is required to be small, the fluorescence change must be large to be detectable. For a fixed hardware specification, it represents a tradeoff between spatial resolution and resolution of fluorescence power. To the goalAtt’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2 of achieving both high spatial resolution and high temperature-DOC resolution, a high-bit-depth camera (large ^^) is desirable.

[0078] We can write this equation in non-dimensional form. Let ^^ be the ratio between the sensor saturation level and the reference state intensity at this ^ ^^, ^^^ position, then ^^ ൌ^^ ^^ ^^ ^^^^^^ ^^^ ^^ ^^. We can rewrite the detection criterion to be1൫ ^^௭ െ ^^^^^, ൯Δ ^^2^ ^ బ ௭బ^^ ^^ ^ ^ ^ . ^5^^^^ ^ ^^ ^^

[0079] With a uniformthe reference state is spatially constant. This leads to a form in which both the fluorescence powerdensity change and the voxel size are non-dimensionalized:1^^ െ ^^^ ^ 1^^ ^ ௭బ ^^^^^ ^ Δ ^^൬^^ ^ . ^6^

[0080] Obviously, thenoise-free environments. In practical situations, the minimum change in intensity change must also be significantly greater than the average noise level Δ ^^^^^^^. Following a similar argument as above, we can write the signal-to-noise requirement as Δ^^^^^^^ ≪ Δ ^^ ൌ ^^^ ^^^^^^െ ^^^^^^^^^^^ ^^ ^ ^^൫ ^^௭బ െ ^^^^^,௭బ൯Δ ^^^7^

[0081] SNR can be improved by using a larger NA (larger ^^), an actively cooled low-noise camera, or noise-reduction settings (longer exposure time and low gain, which lead to lower Δ ^^^^^^^). S.5 Resin characterizationAtt’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2

[0082] See FIGS.2, 3 and 6 and the related brief descriptions. S.6. Fourier transform infrared spectroscopy measurement

[0083] The DOC (acrylate C=C double bond conversion) as a function of absorbed dose was quantified ex situ using Fourier transform infrared (FTIR, Bruker Vertex80) spectroscopy. Samples of thickness ~1 mm between glass slides were prepared and the instrument backgrounded to blank slides. Samples were exposed to 405 nm light (Excelitas Omnicure S2000 with a Thorlabs FBH405-10405 nm bandpass filter) for the duration of the experiment. The intensity of the light was measured before each experiment using a handheld radiometer (Thorlabs PM100D with S121C detector). During exposure, RT-FTIR scans were done in the range of 4000 – 8000 cm-1with 4 cm-1at a frequency of 40 kHz and measurement averaging of 64 with the CaF2 beamsplitter and LN-MCT Mid IR detector from Bruker. The conversion was calculated from double bond concentrations determined by integrating the 6160 cm-1C=C stretch peak using tangent skim method.

[0084] The embodiments illustrated and discussed in this specification are intended only to teach those skilled in the art how to make and use the invention. In describing embodiments of the disclosure, specific terminology is employed for the sake of clarity. However, the disclosure is not intended to be limited to the specific terminology so selected. The above-described embodiments, and following examples, may be modified or varied, without departing from the invention, as appreciated by those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the claims and their equivalents, the invention may be practiced otherwise than as specifically described. For example, it is to be understood that the present disclosure contemplates that, to the extent possible, one or more features of any embodiment can be combined with one or more features of any other embodiment.

Claims

Att’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2 CLAIMS We Claim:

1. A computed axial lithography (CAL) system for producing a three-dimensional (3D) object, comprising: a 3D object system comprising a vessel to hold a material for producing said 3D object; a source of patterned radiation arranged to project patterned radiation onto said material to produce said 3D object; and a tomographic fluorescence monitoring system arranged to receive fluorescence light emitted from said material during production of said 3D object, wherein said tomographic fluorescence monitoring system comprises: a radiation source arranged to illuminate a region within said vessel in which said 3D object is produced with monitoring radiation to cause fluorescence of said material, a fluorescence imaging system arranged to receive said fluorescence light emitted from said material during production, and an image processing system arranged to determine at least one of three- dimensional temperature information or three-dimensional degree of conversion information based on said fluorescence light received, and wherein at least one of said 3D object system or said tomographic fluorescence monitoring system comprises an assembly to effect relative rotation of said tomographic fluorescence monitoring system around said 3D object system.

2. The CAL system according to claim 1, wherein said 3D object system comprises a rotatable stage assembly, said vessel being at least one of held on or attached to and rotated by said rotatable stage assembly.

3. The CAL system according to claim 2, further comprising an illumination control system configured to communicate with said source of patterned radiation and tomographic fluorescence monitoring system,Att’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2 wherein said illumination control system interleaves actuation of said source of patterned radiation and said tomographic fluorescence monitoring system such that said fluorescence imaging system receives said fluorescence light emitted from said material during time periods while said source of patterned radiation is in an off state.

4. The CAL system according to any one of claims 1, 2 or 3, wherein said patterned radiation is optical radiation in a first wavelength region of the electromagnetic spectrum, wherein said monitoring radiation is optical radiation in a second wavelength region of the electromagnetic spectrum, and wherein said first wavelength region of the electromagnetic spectrum is different from said second wavelength region of the electromagnetic spectrum.

5. The CAL system according to claim 4, wherein said material from which said 3D object is produced comprises at least one additive that fluoresces with at least one fluorescence wavelength in response to said monitoring radiation.

6. The CAL system according to claim 4, wherein said material from which said 3D object is produced comprises at least one additive that fluoresces with at least two fluorescence wavelengths in response to said monitoring radiation.

7. The CAL system according to any one of claims 1 to 4, wherein said fluorescence light emitted from said material during production of said 3D object received by said tomographic fluorescence monitoring system comprises at least two distinguishable fluorescence wavelengths, and wherein said image processing system of said tomographic fluorescence monitoring system is further configured to determine at least three-dimensional temperature information and three-dimensional degree of conversion information during production of said 3D object based on at least said at least two distinguishable fluorescence wavelengths.

8. A tomographic fluorescence monitoring system for a volumetric additive manufacturing (VAM) system for producing a three-dimensional (3D) object, comprising:Att’y Ref.: 130996-598965 Applicant Ref.: BK-2024-013-2 a radiation source arranged to illuminate a region within a vessel in which said 3D object is produced with monitoring radiation to cause fluorescence of said material; a fluorescence imaging system arranged to receive said fluorescence light emitted from said material during production; and an image processing system arranged to determine at least one of three-dimensional temperature information or three-dimensional degree of conversion information during production of said 3D object, wherein at least one of said vessel or said fluorescence imaging system comprises an assembly to effect relative rotation therebetween.

9. The tomographic fluorescence monitoring system according to claim 8, wherein said image processing system is further configured to determine at least three-dimensional temperature information and three-dimensional degree of conversion information during production of said 3D object based on at least two distinct fluorescent wavelengths of light received by said fluorescence imaging system.

10. A method of tomographic fluorescence monitoring of a volumetric additive manufacturing (VAM) process for producing a three-dimensional (3D) object, comprising: illuminating a region within a vessel in which said 3D object is being produced with monitoring radiation to cause fluorescence of said material; receiving said fluorescence light emitted from said material during production by a fluorescence imaging system to provide imaging signals; and processing imaging signals to determine at least one of three-dimensional temperature information or three-dimensional degree of conversion information during production of said 3D object, wherein at least one of said vessel or said fluorescence imaging system is rotated for tomographic imaging.

11. The method of tomographic fluorescence monitoring according to claim 10, wherein said processing said imaging signals determines at least three-dimensional temperature information and three-dimensional degree of conversion information during production of said 3D object.

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