ARL-TR-8115 ● AUG 2017

US Army Research Laboratory

Multiflash X-ray with Image Detanglement for 

Single Image Isolation

by Willard Casey Uhlig

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ARL-TR-8115 ● AUG 2017

US Army Research Laboratory

Multiflash X-ray with Image Detanglement for 

Single Image Isolation

by Willard Casey Uhlig

Weapons and Materials Research Directorate, ARL

Approved for public release; distribution unlimited.
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1. REPORT DATE (DD-MM-YYYY)

August 2017

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Technical Report

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January 2016–March 2017

4. TITLE AND SUBTITLE

Multiflash X-ray with Image Detanglement for Single Image Isolation

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Willard Casey Uhlig

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

US Army Research Laboratory 

Weapons and Materials Research Directorate

ATTN: RDRL-WMP-A

Aberdeen Proving Ground, MD 21005

8. PERFORMING ORGANIZATION REPORT NUMBER

ARL-TR-8115

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12. DISTRIBUTION/AVAILABILITY STATEMENT

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

A new capability was conceived and developed to capture multiple flash X-rays on a single film or phosphor screen and 

digitally detangle the resulting image into separate images for each individual flash. This is accomplished via the use of 

multiple flash X-ray heads geometrically placed with respect to a shadow mask in close proximity to the detection screen. The 

small holes in the shadow mask only allow X-rays from each head to pass through along a unique trajectory to specific pixels 

of the screen based on the placement of the head, mask, and screen. Thus, the location of the pixels associated with each flash 

are known and separated into individual images. A proof-of-principle study was performed using 4 X-ray flashes and copper

masks with sub-millimeter holes that allowed development of the required image-analysis algorithms. While using the method 

with 4 flashes decreased the image resolution by half and reduced the number of photons striking the phosphor screen to onequarter, 4 images were successfully detangled without any flash-to-flash overlap in the images.

15. SUBJECT TERMS

flash X-ray, ballistic diagnostics, shadow mask, radiographic diagnostics, multiple flash X-ray imaging

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19a. NAME OF RESPONSIBLE PERSON

Willard Casey Uhlig

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410-278-3997

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iii

Contents

List of Figures iv

Acknowledgments v

1. Introduction 1

2. Conceptual Outline 2

2.1 Geometric Configuration 2

2.2 Image-Analysis Algorithm 4

2.3 Calculation on a Test Image 6

3. Experimental Proof of Concept 7

3.1 Copper Mask One: 0.6 mm Thick, 1.0-mm Holes 8

3.2 Copper Mask Two: 1.8 mm Thick, 0.79-mm Holes 9

3.3 Future: Laser-Drilled Tungsten Mask 9

4. Conclusion 11

5. References 12

List of Symbols, Acronyms, and Abbreviations 13

Distribution List 14
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List of Figures

Fig. 1 Illustration of the electron beam lines for a cathode ray tube color 

television............................................................................................... 1

Fig. 2 Illustration of square grid shadow mask ............................................... 3

Fig. 3 Geometric configuration of the shadow mask concept, showing 2-D 

ray tracing of 2 of the 4 X-ray heads. The shadowed box at the bottom 

of the figure shows the front view of the placement of the 4 X-ray 

heads. .................................................................................................... 4

Fig. 4 Example of generating a new pixel from a region-of-interest from the 

original radiograph................................................................................ 5

Fig. 5 Test image to determine effectiveness of image-analysis algorithm.... 7

Fig. 6 a) Resulting images from the third “flash” using the algorithm on a test 

image with a multiple pixel object where the averaged sample area 

was too large and b) a reduced sample area.......................................... 7

Fig. 7 Radiographs from a thin copper shadow mask with 2.15-mm-hole 

spacing .................................................................................................. 8

Fig. 8 Four flash radiograph of a 30-cal. bullet in 4 different positions with a 

1.8-mm-copper shadow mask with 1.8-mm-hole spacing and the 

associated detangled images................................................................. 9

Fig. 9 X-ray photon attenuation of tungsten and tantalum sheets of 380- and 

245-μm thicknesses............................................................................. 10

Fig. 10 Proposed shadow mask design for 0.38-mm-thick tungsten sheet for 

X-ray image detanglement with 0.5-mm resolution ........................... 11
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Acknowledgments

The author would like to thank Dr Charles Hummer for the inspiration of this 

concept. Also, thanks is given to Mr Keith Mahan and Mr Steven Davis for their 

efforts in machining some of the shadow masks.
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vi

INTENTIONALLY LEFT BLANK.
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1

1. Introduction

In the 1940s and early 50s, RCA made the color television a reality by developing 

a 3-electron gun and shadow mask combination, which ingeniously enabled 

separate illumination of red, green, and blue pixels.1 An illustration of the electron 

beam lines and shadow mask concept, as well as an image of an original RCA color 

television tube, is shown in Fig. 1. A nice overview of the development and design 

of the scheme is presented in a 1963 Popular Science article.

2 For decades, that 

basic concept dominated the color television market. Those were the days when a 

large color television could fill up an entire room. 

Fig. 1 Illustration of the electron beam lines for a cathode ray tube color television

In a similar vein, a new capability was conceived and developed to capture multiple 

flash X-rays on a single film or phosphor screen and digitally detangle the resulting 

image into separate images for each individual flash. Traditionally, multiple flash 

X-rays onto a single film have been used in ballistic and other high-speed events,

with X-ray sources separated over a sufficient distance in an attempt to minimize 

overlap of the multiple flashes. However, in even relatively simple experiments the 

X-ray overlap can become difficult to decipher and obtain clear characterization of 

the event. The new capability presented here seeks to eliminate that complexity. 

This is accomplished via the use of multiple flash X-ray heads geometrically placed 
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2

with respect to a shadow mask, which is in close proximity to the detection screen. 

The small holes in the shadow mask only allow the X-rays from each head to pass 

along a unique trajectory to specific pixels of the film/phosphor screen based on 

the placement of the head, mask, and screen. Thus, the location of the pixels 

associated with each flash are known and separated into individual images. 

A proof-of-principle study was performed using 4 X-ray flashes and copper masks 

with sub-millimeter holes that allowed development of the required image-analysis 

algorithms. While using the method with 4 flashes decreased the image resolution 

by half and reduced the number of photons striking the phosphor screen to onequarter, 4 images were successfully detangled without any flash-to-flash overlap in 

the images.

2. Conceptual Outline

The use of 4 X-ray flashes results in a simple square grid shadow mask requirement. 

While the geometric location of the exposed screen is known, it must be correlated 

with the appropriate pixels of the resulting digital image as well as any pixels that 

may only be partially exposed and any uncertainties in the physical ability to 

precisely establish the experimental configuration.

2.1 Geometric Configuration

The most simple and direct way to align 4 X-ray tubes is to place the 4 tubes at the 

corners of a square, aligned with a square grid shadow mask. An illustration of such 

a shadow mask with square holes of width (w) and hole-to-hole spacing (b) is 

shown in Fig. 2. If the distance (d) from the X-ray source to the mask is much 

greater than the distance (s) between the shadow mask and the screen, then the 

width (P) of the exposed area on the screen is approximately equal to w. Thus, the 

hole-to-hole spacing can simply be set as twice the hole width. In Fig. 2, a box with 

dashed outlines shows the area of the screen (behind the mask) that would be 

exposed by the 4 flashes (labelled 1–4) through one of the holes as aligned directly 

with the first source for such a configuration. In reality w would be made slightly 

smaller than half b to avoid any overlap. (Section 2.2 will show how this can also 

be accomplished digitally by discarding boarder pixels if there is sufficient 

resolution in the original X-ray image.)
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Fig. 2 Illustration of square grid shadow mask

Using this mask geometry, the distance from the center line on the screen (i.e., the 

location of exposure for a given hole and X-ray source) will be

FF = (ss+dd)

dd (mm ± HH) ∓ HH , (1)

where m is the distance from the center line to the hole location on the mask and H

is the distance from the center line to the X-ray source as shown in Fig. 3, which 

gives a 2-D view of the shadow mask concept. The ± is determined by the position 

of the source with respect to the center line. The exposed area for a given 

hole/source pair (from which the intensity will be extracted for the new pixel of the 

processed image) will have a width of 

PP = 2HH

dd = ww (ss+dd)dd . (2)

In full dimension, there will be an Fx and Fy for each corresponding mx and my on 

the mask.
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Fig. 3 Geometric configuration of the shadow mask concept, showing 2-D ray tracing of 2

of the 4 X-ray heads. The shadowed box at the bottom of the figure shows the front view of 

the placement of the 4 X-ray heads.

2.2 Image-Analysis Algorithm 

There are 2 core objectives in developing the image-analysis algorithm: 1) the 

physical location of the exposed region, which will become a pixel in the new image 

(referred to as image1, image2, image3, or image4, corresponding to the X-ray flash 

of interest), needs to be determined in terms of pixel locations on the original digital 

radiograph (ORAD); and 2) an appropriate averaging of the value of the pixels 

within the exposed region-of-interest on the ORAD needs to be calculated. To find 

the location from the center line of the new pixel for the nth hole of the shadow 

mask use

mmxx = 1−NNxx

2

bb + nnxxbb and mmyy = 1−NNyy

2

bb + nnyybb , (3)

where N is the total number of pixels to be analyzed along an axis and is odd. This 

is relating to the (0,0) pixel’s location in the top-left corner of the image. Using 

these values of m, Fx, and Fy as calculated in Eq. 1 would yield the center of the 

new image pixel. In obtaining the value for the new pixel, the average of the ORAD 

pixels completely encompassed within the region-of-interest and a weighted 

average of the partially covered pixels will be included. 
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Two separate scenarios are possible for averaging over the region-of-interest. If P

is greater than twice the ORAD pixel width po, then it is guaranteed that at least one 

full pixel will be covered. In this case the average of the center pixels (i.e., fully 

covered) will be combined with the average of the pixels at each edge multiplied 

by the fraction of the pixel exposed by the region-of-interest correlating to the new 

image pixel. The pixels at the corners will be discarded for convenience. This can 

be observed in the example shown in Fig. 4, where the fully exposed pixels are 

shaded dark grey and the partially exposed pixels are shaded with slanted stripes. 

If overlap occurs from the other flash X-rays due to imprecise physical alignment 

of the mask and sources, the region-of-interest can be decreased digitally, 

encompassing fewer pixels. In the illusory example in Fig. 4, the center line is 

determined to be at the pixel position (242.77,195.85) on the ORAD. If Nx = 43, Ny

= 33, d = 2000 mm, s = 43.5 mm, H = 25 mm, b = 2.15 mm, P = 0.92 mm (~ 15% 

reduction from b/2), and po = 0.254 mm, then the new (0,0) pixel of image1 would 

be centered at (63.29, 55.33) on the ORAD with Fx = –45.59 mm and Fy = –35.69 

mm. The fraction at the top edge would be 0.49, the bottom edge 0.15, the left edge 

0.53, and the right edge 0.11, for weighting the pixels in row 53, row 57, column 

61, and column 65, respectively. This would be repeated for each new pixel location 

in both the x and y directions.

Fig. 4 Example of generating a new pixel from a region-of-interest from the original 

radiograph

While averaging is not necessary where multiple pixels are encompassed (i.e., just 

the center pixel could be chosen), there is no reason to discard data that could 

enhance the image. On the other hand, if P < 2po the situation arises where it is 

possible that no original pixels are completely encompassed by the region-ofinterest and averaging of the fractional pixels is necessary. Cazamias3 generated an 

algorithm to calculate on a square mesh the area of overlap for individual elements 

with a square box. This method was adopted for the P < 2po case. Again, each 

partially covered pixel (including corner pixels) was combined in a weighted 
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average to determine the value of the new image pixel. Because of the limited 

number of pixels within a region-of-interest, digital correction for overlap is not 

possible in this situation and requires physical compensation on the mask (i.e.,

reducing w to less than b/2). Ideally, configuring the X-ray set up with P > 2po

allows greater flexibility in eliminating overlap and possible influence of 

neighboring pixels than the P < 2po case.

2.3 Calculation on a Test Image

To test the image-manipulation algorithms, an image with 4 bright pixels 

(representing an object moving in time and flashed with 4 pulsed X-ray sources) 

and a region of gradual brightness increase in the bottom-right corner (representing 

a stationary object) was created. The single bright pixels were placed in locations 

that would correspond to 4 different alignments through a mask and should be 

isolated on individual images after running the algorithm, while each image should 

include the gradual brightness increase in the corner. The left side of Fig. 5 shows 

the original image with a blue box outlining the region of the image used in the 

algorithm. The right side of Fig. 5 shows the 4 newly generated images. Each new 

image shows the bright bottom-right corner, but only 1 bright spot correlating to 

the moving object. For the test, P = 3.23 po, thus the algorithm averaged over an 

approximate 3 × 3 block of pixels, and the 4 resulting images have a resolution 6.5 

times lower than the original image (from b ~ 2P). Because the averaging covered 

9 pixels and only 1 pixel was bright in the original area, the relative intensity 

between the moving object (bright pixel) and stationary object (corner region) has 

decreased. This is only a concern for real applications if the object to be 

radiographed is smaller than P. However, in practice, the object should be several 

times larger than the eventual pixel size of the new image, Fn – Fn–1 or ~2P. 
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Fig. 5 Test image to determine effectiveness of image-analysis algorithm

If multiple pixels for the object are used or if the bright pixel is placed at the edge 

of the averaged area, overlap of the images can be replicated. Reducing P in the 

algorithm can reverse the effect as shown in Fig. 6, where a multiple pixel object 

having the same layout as Fig. 5 had too large of a sample area P (Fig. 6a) and a 

reduced sample area (Fig. 6b). The images should only show the results of the third 

“flash”; however, the larger sample area used in Fig. 6a allowed bleed-through of 

the first “flash”. Narrowing the averaging area completely removes the overlap in 

Fig. 6b.

Fig. 6 a) Resulting images from the third “flash” using the algorithm on a test image with 

a multiple pixel object where the averaged sample area was too large and b) a reduced sample 

area

3. Experimental Proof of Concept

The factors effecting experimental implementation of this method include precise 

alignment of the system as well as the design and manufacture of the shadow mask. 

The mask must be opaque to the photons of the X-ray source. This is dependent on 

the photon energy, mask thickness, and mask material. While significant resolution 

is readily available for optical masks, thicker masks required to attenuate X-rays 

sufficiently is a much different requirement that is not readily available off-theshelf. There also exists a limit in the ratio of the hole-width, the mask thickness, 

and the coverage area of the X-ray film or screen. If the material thickness is too 
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8

large compared to other critical geometries within the setup, significant shadowing 

will occur at the edges of the image. (Additionally, manufacturing deep holes whilst 

maintaining precise hole-size and shape becomes untenable.) Thus, the method will 

be limited to lower energy X-rays such as 150-kV flash X-ray tubes. With all these 

considerations in mind, a couple of masks made of copper with hole-sizes on the 

order of 1 mm (final image resolution order of 2 mm) were made to test the method. 

Then, a third mask was designed out of tungsten with 220-μm holes (final image 

resolution 0.5 mm) to test a realistic or usable system.

3.1 Copper Mask One: 0.6 mm Thick, 1.0-mm Holes 

A 0.6-mm-thick copper shadow mask (mask1) with w = 1.0 mm and b = 2.15 mm 

was made as an initial test of the X-ray detanglement concept. A 150-kV flash 

X-ray was taken of a 30 cal. bullet and a 22-mm-wide washer with s = 44 mm, d = 

2000 mm, and H = 25 mm. The bullet was moved approximately 12 mm to the right 

and a second flash X-ray was pulsed from position 2 (as outlined in Fig. 3). The 

mask was not precisely machined and the holes drilled (round) as can be seen in 

Fig. 7. Thus, P was reduced to 0.93 mm for the algorithm calculations.

Fig. 7 Radiographs from a thin copper shadow mask with 2.15-mm-hole spacing

Shadows from exposure through the holes of the mask are visible in Fig. 7 and 

appear as horizontal lines of dots due to the overlap of the 2 X-rays. (If all 4 X-rays 

were flashed, dots would fill in the spacing between the lines as well.) The 

algorithm was applied to the region where the bullet appears in the radiograph 

highlighted in the blue box (Fig. 7). Image1 and image2 from the detanglement are 

labeled as 1 and 2. Obviously, 2.15 mm resolution on a 30 cal. bullet is not ideal, 

but the concept is clearly demonstrated. Additionally, shadowing from the alternate 
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X-ray flashes are visible in the detangled images (i.e., the bullet from flash 1 is 

lightly visible in image2 and vice-versa). The imprecise machine most likely 

contributes to the shadowing, but the largest contribution is the thickness of the 

copper. The copper does not attenuate the X-rays sufficiently.

3.2 Copper Mask Two: 1.8 mm Thick, 0.79-mm Holes

A second copper mask was machined with a smaller drill (0.79 mm) for slightly 

better resolution and an increased thickness (1.8 mm) for greater X-ray attenuation. 

While greater precision of hole location was implemented, unfortunately, multiple 

drill bits broke during construction causing missing pixels in the mask. However, 

excellent elimination of shadowing from flash-to-flash was achieved. Four 150-kVflash X-rays were taken of a 30 cal. bullet in 4 different positions with a 

configuration of b = 1.8 mm, s = 40 mm, d = 2000 mm, and H = 22.5 mm. For the 

calculations P was set to 0.7 mm. The results are shown in Fig. 8. The black outlined 

box shows the region of the copper that was machined with the holes and thus used 

in the detanglement calculations. Despite the missing pixels, 4 separate radiographs 

are obtained clearly showing the position of the bullet as a function of time with 

very minimal shadowing.

Fig. 8 Four flash radiograph of a 30-cal. bullet in 4 different positions with a 1.8-mmcopper shadow mask with 1.8-mm-hole spacing and the associated detangled images

3.3 Future: Laser-Drilled Tungsten Mask

Ideally a material would be used that is much better than copper at attenuating the 

X-rays. Gold has potential, but it is expensive and not very durable. Therefore, 

tungsten and tantalum were investigated. The data in Fig. 9 was generated using 

the X-ray attenuation coefficients available in the National Institute of Science and 

Technology (NIST) X-ray Mass Attenuation Coefficients Database.4 The intensity 
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ratio is the photon intensity that passes through the material versus the initial 

intensity

II

IIoo

= ee−μμμμ , (4)

where μ is the X-ray mass attenuation coefficient and t is the thickness of the 

material. Minimizing this ratio will minimize shadowing between the detangled 

images. The energy profile of a 150-kV-flash X-ray pulse peaks between 60 and

90 kV. Thus, it is most important to focus in that region. Ironically, the attenuation 

shown in Fig. 9 shows a peak right in the middle of that region. Tungsten is a 

slightly better performer than tantalum and it is less expensive, so it was determined 

to use a 0.38-mm-thick tungsten sheet as a mask. Because sub-millimeter resolution 

is desired for the X-rays, precision laser drilling is the best hope for a mask of this 

material. A mask was designed with 0.22-mm-square holes with 0.5-mm hole-tohole spacing and a total area of 76 × 76 mm2

, as shown in Fig. 10. Once this method 

of manufacture and appropriate attenuation is confirmed, a larger mask for use in 

dynamic experiments will be acquired. 

Fig. 9 X-ray photon attenuation of tungsten and tantalum sheets of 380- and 245-μm 

thicknesses
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Fig. 10 Proposed shadow mask design for 0.38-mm-thick tungsten sheet for X-ray image 

detanglement with 0.5-mm resolution

4. Conclusion

A new capability was conceived and developed to capture multiple flash X-rays on 

a single film or phosphor screen and digitally detangle the resulting image into 

separate images for each individual flash. The geometric concept, computer 

algorithm, and experimental proof of principle were successfully completed using 

4 flash X-rays with low-resolution copper masks. Extending the method for higher 

resolution masks has been outlined and a method of manufacture determined. This 

concept can be adapted to any experimental facility that currently has flash X-ray 

capability and will enable multiple flash X-rays during dynamic events without any 

flash-to-flash overlap in the images.
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5. References

1. Abramson A. The history of television, 1942 to 2000. Jefferson (NC): 

McFarland & Co.; 2003.

2. Gilmore CP. Color TV: Is it finally worth the money? Popular Science. 1963 

Aug;178:80–83.

3. Cazamias JU. Overlap areas of a square box on a square mesh. Aberdeen 

Proving Ground (MD): Army Research Laboratory (US); 2017 Apr. Report 

No.: ARL-TN-0818.

4. Hubbell JH, Seltzer SM. Tables of X-ray mass attenuation coefficients and 

mass energy-absorption coefficients from 1 keV to 20 MeV for elements Z = 

1 to 92 and 48 additional substances of dosimetric interest. Gaithersburg (MD): 

National Institute of Standards and Technology; 2004 July. NISTIR 5632.
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List of Symbols, Acronyms, and Abbreviations

2-D 2-dimensional

NIST National Institute of Science and Technology

ORAD original digital radiograph
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1 DEFENSE TECHNICAL

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