WO2023039701A1

WO2023039701A1 - 3d (3-dimensional) printing with void filling

Info

Publication number: WO2023039701A1
Application number: PCT/CN2021/118132
Authority: WO
Inventors: Peiyan CAO
Original assignee: Shenzhen Xpectvision Technology Co., Ltd.
Priority date: 2021-09-14
Filing date: 2021-09-14
Publication date: 2023-03-23
Also published as: TW202311059A; CN117916039A

Abstract

Disclosed herein is a 3D (3-dimensional) printing method. The method comprises: printing a first layer; locating a first unintended void in the first layer based on an image of the first layer; filling the first unintended void; and printing a second layer on the first layer after said filling the first unintended void is performed.

Description

3D (3-DIMENSIONAL) PRINTING WITH VOID FILLING

Background

A radiation detector is a device that measures a property of a radiation. Examples of the property may include a spatial distribution of the intensity, phase, and polarization of the radiation. The radiation may be one that has interacted with an object. For example, the radiation measured by the radiation detector may be a radiation that has penetrated the object. The radiation may be an electromagnetic radiation such as infrared light, visible light, ultraviolet light, X-ray, or γ-ray. The radiation may be of other types such as α-rays and β-rays. An imaging system may include one or more image sensors each of which may have one or more radiation detectors.

Summary

Disclosed herein is a 3D (3-dimensional) printing method, comprising: printing a first layer; locating a first unintended void in the first layer based on an image of the first layer; filling the first unintended void; and printing a second layer on the first layer after said filling the first unintended void is performed.

In an aspect, the method further comprises: locating a second unintended void in the second layer based on an image of the second layer; filling the second unintended void; and printing a third layer on the second layer after said filling the second unintended void is performed.

In an aspect, the method further comprises generating the image of the first layer, wherein the image of the first layer is a 3D image of the first layer, and wherein said locating the first unintended void comprises comparing the image of the first layer with a design of the first layer.

In an aspect, said generating the image of the first layer comprises: capturing M 2D (2-dimensional) images of the first layer, M being an integer greater than 1; and generating the image of the first layer from the M 2D images using computed tomography.

In an aspect, said capturing the M 2D images of the first layer comprises rotating a radiation source and a radiation detector around the first layer and about an axis perpendicular to the first layer.

In an aspect, said capturing the M 2D images of the first layer further comprises, for each 2D image of the M 2D images: sending with the radiation source a cone beam of X-rays through the first layer; and capturing with the radiation detector the 2D image of the first layer using radiation of the cone beam that has passed through and interacted with the first layer.

In an aspect, the method further comprises generating the image of the first layer, wherein the image of the first layer is a 2D image of the first layer, and wherein said locating the first unintended void comprises comparing the image of the first layer with a design of the first layer.

In an aspect, said generating the image of the first layer comprises: capturing N 1D (1-dimensional) images of the first layer, N being an integer greater than 1; and generating the image of the first layer from the N 1D images using computed tomography.

In an aspect, said capturing the N 1D images of the first layer comprises rotating a radiation source and a radiation detector around the first layer and about an axis perpendicular to the first layer.

In an aspect, said capturing the N 1D images of the first layer further comprises, for each 1D image of the N 1D images: sending with the radiation source a fan beam of X-rays through the first layer, wherein the fan beam is parallel to the second layer; and capturing with the radiation detector the 1D image of the first layer using radiation of the fan beam that has passed through and interacted with the first layer.

Disclosed herein is a 3D printing apparatus, comprising: a 3D printer; and an imaging system, wherein the 3D printer is configured to print a first layer, wherein the imaging system is configured to locate a first unintended void in the first layer based on an image of the first layer, wherein the 3D printer is configured to fill the first unintended void, and wherein the 3D printer is configured to print a second layer on the first layer after the 3D printer fills the first unintended void.

In an aspect, the imaging system is configured to locate a second unintended void in the second layer based on an image of the second layer, wherein the 3D printer is configured to fill the second unintended void, and wherein the 3D printer is configured to print a third layer on the second layer after the 3D printer fills the second unintended void.

In an aspect, the imaging system is configured to generate the image of the first layer, wherein the image of the first layer is a 3D image of the first layer, and wherein the imaging system is configured to locate the first unintended void in the first layer by comparing the image of the first layer with a design of the first layer.

In an aspect, the imaging system is configured to generate the image of the first layer by: capturing M 2D images of the first layer, M being an integer greater than 1; and generating the image of the first layer from the M 2D images using computed tomography.

In an aspect, said capturing the M 2D images of the first layer comprises rotating a radiation source and a radiation detector of the imaging system around the first layer and about an axis perpendicular to the first layer.

In an aspect, the imaging system is configured to generate the image of the first layer, wherein the image of the first layer is a 2D image of the first layer, and wherein the imaging system is configured to locate the first unintended void in the first layer by comparing the image of the first layer with a design of the first layer.

In an aspect, the imaging system is configured to generate the image of the first layer by: capturing N 1D images of the first layer, N being an integer greater than 1; and generating the image of the first layer from the N 1D images using computed tomography.

In an aspect, said capturing the N 1D images of the first layer comprises rotating a radiation source and a radiation detector of the imaging system around the first layer and about an axis perpendicular to the first layer.

Brief Description of Figures

Fig. 1 schematically shows a radiation detector, according to an embodiment.

Fig. 2 schematically shows a simplified cross-sectional view of the radiation detector, according to an embodiment.

Fig. 3 schematically shows a detailed cross-sectional view of the radiation detector, according to an embodiment.

Fig. 4 schematically shows a detailed cross-sectional view of the radiation detector, according to an alternative embodiment.

Fig. 5A –Fig. 5B schematically show perspective views of a 3D printing apparatus in operation, according to an embodiment.

Fig. 6 shows a flowchart generalizing the operation of the 3D printing apparatus.

Detailed Description

RADIATION DETECTOR

Fig. 1 schematically shows a radiation detector 100, as an example. The radiation detector 100 may include an array of pixels 150 (also referred to as sensing elements 150) . The array may be a rectangular array (as shown in Fig. 1) , a honeycomb array, a hexagonal array, or any other suitable array. The array of pixels 150 in the example of Fig. 1 has 4 rows and 7 columns; however, in general, the array of pixels 150 may have any number of rows and any number of columns.

Each pixel 150 may be configured to detect radiation from a radiation source (not shown) incident thereon and may be configured to measure a characteristic (e.g., the energy of the particles, the wavelength, and the frequency) of the radiation. A radiation may include particles such as photons and subatomic particles. Each pixel 150 may be configured to count numbers of particles of radiation incident thereon whose energy falls in a plurality of bins of energy, within a period of time. All the pixels 150 may be configured to count the numbers of particles of radiation incident thereon within a plurality of bins of energy within the same period of time. When the incident particles of radiation have similar energy, the pixels 150 may be simply configured to count numbers of particles of radiation incident thereon within a period of time, without measuring the energy of the individual particles of radiation.

Each pixel 150 may have its own analog-to-digital converter (ADC) configured to digitize an analog signal representing the energy of an incident particle of radiation into a digital signal, or to digitize an analog signal representing the total energy of a plurality of incident particles of radiation into a digital signal. The pixels 150 may be configured to operate in parallel. For example, when one pixel 150 measures an incident particle of radiation, another pixel 150 may be waiting for a particle of radiation to arrive. The pixels 150 may not have to be individually addressable.

The radiation detector 100 described here may have applications such as in an X-ray telescope, X-ray mammography, industrial X-ray feature detection, X-ray microscopy or microradiography, X-ray casting inspection, X-ray non-destructive testing, X-ray weld inspection, X-ray digital subtraction angiography, etc. It may be suitable to use this radiation detector 100 in place of a photographic plate, a photographic film, a PSP plate, an X-ray image intensifier, a scintillator, or another semiconductor X-ray detector.

Fig. 2 schematically shows a simplified cross-sectional view of the radiation detector 100 of Fig. 1 along a line 2-2, according to an embodiment. Specifically, the radiation detector 100 may include a radiation absorption layer 110 and an electronics layer 120 (which may include one or more ASICs or application-specific integrated circuits) for processing or analyzing electrical signals which incident radiation generates in the radiation absorption layer 110. The radiation detector 100 may or may not include a scintillator (not shown) . The radiation absorption layer 110 may include a semiconductor material such as silicon, germanium, GaAs, CdTe, CdZnTe, or a combination thereof. The semiconductor material may have a high mass attenuation coefficient for the radiation of interest.

Fig. 3 schematically shows a detailed cross-sectional view of the radiation detector 100 of Fig. 1 along the line 2-2, as an example. Specifically, the radiation absorption layer 110 may include one or more diodes (e.g., p-i-n or p-n) formed by a first doped region 111, one or more discrete regions 114 of a second doped region 113. The second doped region 113 may be separated from the first doped region 111 by an optional intrinsic region 112. The discrete regions 114 may be separated from one another by the first doped region 111 or the intrinsic region 112. The first doped region 111 and the second doped region 113 may have opposite types of doping (e.g., region 111 is p-type and region 113 is n-type, or region 111 is n-type and region 113 is p-type) . In the example of Fig. 3, each of the discrete regions 114 of the second doped region 113 forms a diode with the first doped region 111 and the optional intrinsic region 112. Namely, in the example in Fig. 3, the radiation absorption layer 110 has a plurality of diodes (more specifically, 7 diodes corresponding to 7 pixels 150 of one row in the array of Fig. 1, of which only 2 pixels 150 are labeled in Fig. 3 for simplicity) . The plurality of diodes may have an electrical contact 119A as a shared (common) electrode. The first doped region 111 may also have discrete portions.

The electronics layer 120 may include an electronic system 121 suitable for processing or interpreting signals generated by the radiation incident on the radiation absorption layer 110. The electronic system 121 may include an analog circuitry such as a filter network, amplifiers, integrators, and comparators, or a digital circuitry such as a microprocessor, and memory. The electronic system 121 may include one or more ADCs (analog to digital converters) . The electronic system 121 may include components shared by the pixels 150 or components dedicated to a single pixel 150. For example, the electronic system 121 may include an amplifier dedicated to each pixel 150 and a microprocessor shared among all the pixels 150. The electronic system 121 may be electrically connected to the pixels 150 by vias 131. Space among the vias may be filled with a filler material 130, which may increase the mechanical stability of the connection of the electronics layer 120 to the radiation absorption layer 110. Other bonding techniques are possible to connect the electronic system 121 to the pixels 150 without using the vias 131.

When radiation from the radiation source (not shown) hits the radiation absorption layer 110 including diodes, particles of the radiation may be absorbed and generate one or more charge carriers (e.g., electrons, holes) by a number of mechanisms. The charge carriers may drift to the electrodes of one of the diodes under an electric field. The electric field may be an external electric field. The electrical contact 119B may include discrete portions each of which is in electrical contact with the discrete regions 114. The term “electrical contact” may be used interchangeably with the word “electrode. ” In an embodiment, the charge carriers may drift in directions such that the charge carriers generated by a single particle of the radiation are not substantially shared by two different discrete regions 114 ( “not substantially shared” here means less than 2%, less than 0.5%, less than 0.1%, or less than 0.01%of these charge carriers flow to a different one of the discrete regions 114 than the rest of the charge carriers) . Charge carriers generated by a particle of the radiation incident around the footprint of one of these discrete regions 114 are not substantially shared with another of these discrete regions 114. A pixel 150 associated with a discrete region 114 may be an area around the discrete region 114 in which substantially all (more than 98%, more than 99.5%, more than 99.9%, or more than 99.99%of) charge carriers generated by a particle of the radiation incident therein flow to the discrete region 114. Namely, less than 2%, less than 1%, less than 0.1%, or less than 0.01%of these charge carriers flow beyond the pixel 150.

Fig. 4 schematically shows a detailed cross-sectional view of the radiation detector 100 of Fig. 1 along the line 2-2, according to an alternative embodiment. More specifically, the radiation absorption layer 110 may include a resistor of a semiconductor material such as silicon, germanium, GaAs, CdTe, CdZnTe, or a combination thereof, but does not include a diode. The semiconductor material may have a high mass attenuation coefficient for the radiation of interest. In an embodiment, the electronics layer 120 of Fig. 4 is similar to the electronics layer 120 of Fig. 3 in terms of structure and function.

When the radiation hits the radiation absorption layer 110 including the resistor but not diodes, it may be absorbed and generate one or more charge carriers by a number of mechanisms. A particle of the radiation may generate 10 to 100,000 charge carriers. The charge carriers may drift to the

electrical contacts

119A and 119B under an electric field. The electric field may be an external electric field. The electrical contact 119B may include discrete portions. In an embodiment, the charge carriers may drift in directions such that the charge carriers generated by a single particle of the radiation are not substantially shared by two different discrete portions of the electrical contact 119B ( “not substantially shared” here means less than 2%, less than 0.5%, less than 0.1%, or less than 0.01%of these charge carriers flow to a different one of the discrete portions than the rest of the charge carriers) . Charge carriers generated by a particle of the radiation incident around the footprint of one of these discrete portions of the electrical contact 119B are not substantially shared with another of these discrete portions of the electrical contact 119B. A pixel 150 associated with a discrete portion of the electrical contact 119B may be an area around the discrete portion in which substantially all (more than 98%, more than 99.5%, more than 99.9%or more than 99.99%of) charge carriers generated by a particle of the radiation incident therein flow to the discrete portion of the electrical contact 119B. Namely, less than 2%, less than 0.5%, less than 0.1%, or less than 0.01%of these charge carriers flow beyond the pixel associated with the one discrete portion of the electrical contact 119B.

3D PRINTING APPARATUS

Fig. 5A schematically shows a perspective view of a 3D (3-dimensional) printing apparatus 500, according to an embodiment. Specifically, in an embodiment, the 3D printing apparatus 500 may include a 3D printer 510+520 and an imaging system 100+530.

In an embodiment, the 3D printer 510+520 may include a print bed 510 and a hotend 520. The print bed 510 may be used for supporting the object to be printed. The hotend 520 may be used for releasing printing material so as to form the object to be printed. In an embodiment, the 3D printer 510+520 may further include components such as extruder, filament, display unit, etc. However, these components are not shown for simplicity.

In an embodiment, the imaging system 100+530 may include the radiation detector 100 and a radiation source 530. The radiation source 530 may be configured to generate radiation used for imaging in the imaging system 100+530. In an embodiment, the radiation generated by the radiation source 530 may be X-rays.

OPERATION OF THE 3D PRINTING APPARATUS

PRINT A FIRST LAYER

Assume that an object 540 (e.g., a solid pyramid) is to be printed by the 3D printing apparatus 500. In an embodiment, the 3D printer 510+520 may print the object 540 layer by layer (i.e., one layer after another) . Assume that the 3D printer 510+520 has just finished printing a layer 542 of the object 540. For simplicity, the layers of the object 540 that were printed before the layer 542 is printed are shown as a single block (i.e., not shown individually) .

LOCATE UNINTENDED VOIDS IN THE FIRST LAYER

In an embodiment, after the 3D printer 510+520 prints the layer 542, the imaging system 100+530 may generate a 3D image of the layer 542 of the object 540.

Specifically, in an embodiment, while the 3D printing apparatus 500 is in a first arrangement as shown in Fig. 5A, the radiation source 530 may generate a radiation beam represented by an arrow 532a (hence, hereafter referred to as the radiation beam 532a) toward the layer 542 and the radiation detector 100. Using the radiation of the radiation beam 532a that has passed through and interacted with the layer 542, the radiation detector 100 may capture a first 2D (2-dimensional) image of the layer 542 of the object 540. In the example shown in Fig. 5A, the radiation beam 532a is parallel to the layer 542. The radiation beam 532a may have other suitable arrangements.

In an embodiment, after the radiation detector 100 captures the first 2D image of the layer 542, the radiation detector 100 and the radiation source 530 may rotate around the layer 542 and about an axis (not shown) perpendicular to the layer 542 so that the 3D printing apparatus 500 is in a second arrangement as shown in Fig. 5B.

In an embodiment, while the 3D printing apparatus 500 is in the second arrangement as shown in Fig. 5B, the radiation source 530 may generate a radiation beam represented by an arrow 532b (hence, hereafter referred to as the radiation beam 532b) toward the layer 542 and the radiation detector 100. Using the radiation of the radiation beam 532b that has passed through and interacted with the layer 542, the radiation detector 100 may capture a second 2D image of the layer 542 of the object 540.

In an embodiment, after the radiation detector 100 captures the second image of the layer 542, the radiation detector 100 may generate the 3D image of the layer 542 from the first and second 2D images of the layer 542. In an embodiment, the radiation detector 100 may generate the 3D image of the layer 542 from the first and second 2D images of the layer 542 using computed tomography.

In an embodiment, after the radiation detector 100 generates the 3D image of the layer 542, the 3D image of the layer 542 may be compared with a design of the layer 542 so as to locate any unintended voids in the layer 542. In an embodiment, the radiation detector 100 may perform this comparison.

FILL UNINTENDED VOIDS IN THE FIRST LAYER

With reference to Fig. 5A & Fig. 5B, assume that an unintended void 542v is located in the layer 542 by the comparison described above. In an embodiment, the 3D printer 510+520 may fill the unintended void 542v by moving the hotend 520 to the location of the unintended void 542v and then causing the hotend 520 to release an amount of printing material sufficient to fill the unintended void 542v.

PRINT A SECOND LAYER ON THE FIRST LAYER

After the unintended void 542v is filled, the 3D printer 510+520 may print a next layer 544 on the layer 542. In an embodiment, if multiple unintended voids (similar to the unintended void 542v) are located in the layer 542 by the comparison described above, the 3D printer 510+520 may fill these multiple unintended voids one by one and then print the next layer 544 on the layer 542. In an embodiment, if no unintended void is found in the layer 542 by the comparison described above, the 3D printer 510+520 may print the next layer 544 on the layer 542.

SUMMARY

In summary, right after a current layer (e.g., the layer 542) is printed, the step of locating unintended voids in the current layer is performed (by comparing an image of the current layer with a design of the current layer) . If unintended voids are located in the current layer, then the unintended voids are filled one by one and then the next layer (if any) is printed on the current layer. If no unintended void is found in the current layer, then the next layer (if any) is printed on the current layer.

FLOWCHART FOR GENERALIZATION

Fig. 6 shows a flowchart 600 generalizing the operation of the 3D printing apparatus 500 described above. Specifically, in step 610, a first layer is printed. For example, in the embodiments described above, with reference to Fig. 5A and Fig. 5B, the layer 542 is printed.

Next, in step 620, a first unintended void in the first layer is located based on an image of the first layer. For example, in the embodiments described above, the unintended void 542v in the layer 542 is located based on the 3D image of the layer 542.

Next, in step 630, the first unintended void is filled. For example, in the embodiments described above, the unintended void 542v is filled.

Next, in step 640, a second layer is printed on the first layer after said filling the first unintended void is performed. For example, in the embodiments described above, the layer 544 is printed on the layer 542 after the unintended void 542v is filled.

ADDITIONAL EMBODIMENTS

In the embodiments described above, with reference to Fig. 5A & Fig. 5B, the 3D image of the layer 542 is generated from two 2D images of the layer 542 (i.e., the first and second 2D images) . In general, multiple 2D images of the layer 542 (similar to the first and second 2D images) may be captured by the imaging system 100+530. Then, the 3D image of the layer 542 may be generated from these multiple 2D images using computed tomography.

In an embodiment, the radiation beams 532a and 532b may be cone beams. Alternatively, the radiation beams 532a and 532b may be fan beams that are parallel to the

layers

542 and 544. In an embodiment, the

radiation fan beams

532a and 532b may be sufficiently thin such that the resulting first and second 2D images are in essence 1D (one-dimensional) images (i.e., each image having 1 × P picture elements, with P being an integer greater than 1) . As a result, the generated 3D image of the layer 542 is in essence a 2D image of the layer 542 (i.e., having 1 × Q × R picture elements, with Q and R being integers greater than 1) .

While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.

Claims

A 3D (3-dimensional) printing method, comprising:

printing a first layer;

locating a first unintended void in the first layer based on an image of the first layer;

filling the first unintended void; and

printing a second layer on the first layer after said filling the first unintended void is performed.
The method of claim 1, further comprising:

locating a second unintended void in the second layer based on an image of the second layer;

filling the second unintended void; and

printing a third layer on the second layer after said filling the second unintended void is performed.
The method of claim 1, further comprising generating the image of the first layer,

wherein the image of the first layer is a 3D image of the first layer, and

wherein said locating the first unintended void comprises comparing the image of the first layer with a design of the first layer.
The method of claim 3, wherein said generating the image of the first layer comprises:

capturing M 2D (2-dimensional) images of the first layer, M being an integer greater than 1; and

generating the image of the first layer from the M 2D images using computed tomography.
The method of claim 4, wherein said capturing the M 2D images of the first layer comprises rotating a radiation source and a radiation detector around the first layer and about an axis perpendicular to the first layer.
The method of claim 5, wherein said capturing the M 2D images of the first layer further comprises, for each 2D image of the M 2D images:

sending with the radiation source a cone beam of X-rays through the first layer; and

capturing with the radiation detector the 2D image of the first layer using radiation of the cone beam that has passed through and interacted with the first layer.
The method of claim 1, further comprising generating the image of the first layer,

wherein the image of the first layer is a 2D image of the first layer, and

wherein said locating the first unintended void comprises comparing the image of the first layer with a design of the first layer.
The method of claim 7, wherein said generating the image of the first layer comprises:

capturing N 1D (1-dimensional) images of the first layer, N being an integer greater than 1; and

generating the image of the first layer from the N 1D images using computed tomography.
The method of claim 8, wherein said capturing the N 1D images of the first layer comprises rotating a radiation source and a radiation detector around the first layer and about an axis perpendicular to the first layer.
The method of claim 9, wherein said capturing the N 1D images of the first layer further comprises, for each 1D image of the N 1D images:

sending with the radiation source a fan beam of X-rays through the first layer, wherein the fan beam is parallel to the second layer; and

capturing with the radiation detector the 1D image of the first layer using radiation of the fan beam that has passed through and interacted with the first layer.
A 3D printing apparatus, comprising:

a 3D printer; and

an imaging system,

wherein the 3D printer is configured to print a first layer,

wherein the imaging system is configured to locate a first unintended void in the first layer based on an image of the first layer,

wherein the 3D printer is configured to fill the first unintended void, and

wherein the 3D printer is configured to print a second layer on the first layer after the 3D printer fills the first unintended void.
The 3D printing apparatus of claim 11,

wherein the imaging system is configured to locate a second unintended void in the second layer based on an image of the second layer,

wherein the 3D printer is configured to fill the second unintended void, and

wherein the 3D printer is configured to print a third layer on the second layer after the 3D printer fills the second unintended void.
The 3D printing apparatus of claim 11,

wherein the imaging system is configured to generate the image of the first layer,

wherein the image of the first layer is a 3D image of the first layer, and

wherein the imaging system is configured to locate the first unintended void in the first layer by comparing the image of the first layer with a design of the first layer.
The 3D printing apparatus of claim 13, wherein the imaging system is configured to generate the image of the first layer by:

capturing M 2D images of the first layer, M being an integer greater than 1; and

generating the image of the first layer from the M 2D images using computed tomography.
The 3D printing apparatus of claim 14, wherein said capturing the M 2D images of the first layer comprises rotating a radiation source and a radiation detector of the imaging system around the first layer and about an axis perpendicular to the first layer.
The 3D printing apparatus of claim 15, wherein said capturing the M 2D images of the first layer further comprises, for each 2D image of the M 2D images:

sending with the radiation source a cone beam of X-rays through the first layer; and

capturing with the radiation detector the 2D image of the first layer using radiation of the cone beam that has passed through and interacted with the first layer.
The 3D printing apparatus of claim 11,

wherein the imaging system is configured to generate the image of the first layer,

wherein the image of the first layer is a 2D image of the first layer, and

wherein the imaging system is configured to locate the first unintended void in the first layer by comparing the image of the first layer with a design of the first layer.
The 3D printing apparatus of claim 17, wherein the imaging system is configured to generate the image of the first layer by:

capturing N 1D images of the first layer, N being an integer greater than 1; and

generating the image of the first layer from the N 1D images using computed tomography.
The 3D printing apparatus of claim 18, wherein said capturing the N 1D images of the first layer comprises rotating a radiation source and a radiation detector of the imaging system around the first layer and about an axis perpendicular to the first layer.
The 3D printing apparatus of claim 19, wherein said capturing the N 1D images of the first layer further comprises, for each 1D image of the N 1D images:

sending with the radiation source a fan beam of X-rays through the first layer, wherein the fan beam is parallel to the second layer; and

capturing with the radiation detector the 1D image of the first layer using radiation of the fan beam that has passed through and interacted with the first layer.