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Bull. Soc. Photogr. Imag. Japan. (2016) Vol. 26 No. 1: 10–13

Original Paper

Digital Silica Photography with Future Readability and Fire ResistanceTakao Watanabe, Ryo Imai, Shigeki Mori, Toshiyuki Mine, Toshimichi Shintani, and Koichi Watanabe

Abstract:

 e proposed a concept of ‘digital silica photography’ for long-term preservation. It stores digital photographic data with decode-freeWanalog images inside a fused-silica plate using a femtosecond pulse laser. The digital data are written so as to be able to read out using
a normal camera in order to guarantee future readability. The decode-free analog images will explain digital contents and reading
method in the distant future. As a feasibility demonstration, photographic data of Ukiyo-e ( Japanese woodblock prints) were recorded at 290 μm under the surface of a fused-silica plate with a density of 1,693 dpi. The decode-free analog image was made by a 1-bit
error-diffusion dithering method. The digital data were successfully read out from a single-shot picture taken by a 24-M pixel camera
with a macro lens. We also showed the potential of fire resistance through an annealing test at 1,000˚C.

Key words:

Fused silica, Digital photography, Preservation, Future readability, Fire resistance

1. Introduction
In recent years, almost all photographic data are born-digital andinherently preserved in digital-storage media. Migration of photographic data from traditional media to digital ones is also underway.
Digital photography has an advantage that cannot be provided bytraditional analog formats. For example, it enables worldwide concurrent access via internet. However, curators and archivists feel anxiety about the digital photography due to following three concerns
in long-term preservation. The first concern is about archival lifetime of digital media. Traditional photographic films with a base ofpolyester having silver-gelatin are expected to have a lifetime over
500 years when they are processed appropriately and preserved at
low temperature and humidity1). Data retention of digital media has
been improved, but they seem not to have exceeded that of photographic films. For example, recent magnetic tapes whose thermal
stability has been improved 2) are expected to have retention time ofover 30 years 3), and optical discs(type ʻR’) can retain data for 100200 years 4). The second concern is about data migration. Executionof data migration in short period of time is fairly costly 5) and in-creases the risk of data loss. Even if digital media have a long lifetime, another risk in data migration still exists. For example, if reading equipment is not be reconstructed in the future then data
migration cannot be performed. The third concern is lack of durability against change of storage condition. For the long period of preservation, it is difficult to preserve storage media under precise control of temperature and humidity. In addition, storage media
probably suffer from conflagration in the long period of time. All theexisting recording media as well as newly proposed ‘bit on film’ 6)7)
have no resistance against fire.
Recording digital data inside fused silica utilizing non-linear effects induced by a femtosecond laser 8-17) is one of the promisingsolutions to solve concerns described above. Fused silica is expected
to have long lifetime without precise control of archiving environment since it is chemically stable. In addition, it has high resistanceagainst fire since its softening temperature is approximately 1,700˚Cand coefficient of thermal expansion is as small as 5.8 × 10−7 /K 18).
In this work, we proposed digital silica photography that records
photographic data both with future readability and fire resistance.
Table 1 shows comparison of this work with our previous work 17). In

Table 1. Comparison of this work with our previous work.
FeatureHigh density

MediumFused silica

Test data
Pitch of dots

Our previous work

Text data

2.3 μm (11,043 dpi)

Number of recording layers 100
Reading system

Laser used for writing

Microscope with transmitted illumination

Photographic data
Fused silica

15 μm (1,693 dpi)1

Macro lens with reflected illumination

Femtosecond laser with regenerative amplifier Femtosecond laser without regenerative amplifier

Received: 18th, February 2016; Accepted: 7th, June 2016

Center for Technology Innovation - Electronics, Research & Development Group, Hitachi, Ltd.
Kokubunji, Tokyo 185-8601, Japane-mail: takao.watanabe.yh@hitachi.com

This work

Simple reading system

Takao Watanabe, Ryo Imai, Shigeki Mori, Toshiyuki Mine, Toshimichi Shintani, and Koichi Watanabe

2. Setup of systems and making of digital data for writing
Figure 1 shows the concept of proposed ‘digital silica photography’. A decode-free analog image is recorded with digital photographic data in fused silica. The decode-free analog image is made by
a 1-bit error-diffusion dithering method * in order to show the contents of digital photography. Other information such as how to decode the digital data can be recorded in the same way by using illustrations and fonts. In this feasibility demonstration, we recorded
decode-free analog images and full-color digital data in different
fused-silica plates, but both data can be recorded inside the same
fused-silica plate as shown in the figure.
Figure 2 shows schematic diagram of our writing system to generate dots inside a fused silica plate. Writing was performed by afemtosecond laser whose center wavelength, pulse width, and repetition rate of the output beam were 780 nm, 190 fs, and 76 MHz,
respectively. The objective lens with the numerical aperture of 0.85
tightly focuses the laser beam inside a fused-silica plate. As a result,nonlinear absorption creates a micro void-like structure inside a
fused-silica plate. The micro void-like structure is used for both adecode-free analog image and digital data. For digital data, the micro void-like structure corresponds to bit one. And the area without
a micro void-like structure corresponds to bit zero.
Figure 3 is the reading system for digital data. The 24-Mpixel
camera with a 60-mm macro lens was used to take a picture of therecording area where a binary-bitmap pattern of multi-blocks of
Fused-silica plate

Decode-free
analog image

11

Fused-silica plate
Femtosecond laser

Shutter

Objective lens

NA 0.85

780 nm, 76 MHzPower

our previous work, we used text data and succeeded making the
one-hundred recording layer sample with a dot pitch of 2.3 μm. In
order to achieve the high density we used a microscope for reading
and a high energy femtosecond laser for recording. In this work, weused single layer and a dot pitch of 15 μm to simplify the reading
system. We verified that the photographic data can be read out using
a commercially available macro lens.

Digital Silica Photography with Future Readability and Fire Resistance

190 fsTime

Fig. 2. Schematic diagram of the writing system.

24-Mpixel camera

60-mm
macro lensSample(Fused silica)

LED lamp

Notebook PC

Fig. 3. Reading system for digital data.

two-dimensional barcode was recorded. The LED lamp was used forreflected illumination. The picture taken by the camera was transferred to a notebook PC for decoding.
Figure 4 explains the sequence of making data for recording afull-color photograph with description sentences. In the first step, abinary file of a full-color image was transformed into characters expressing hexadecimal numbers (00 to FF). In the second step, thelong character line was divided into set of shorter character lines andheader characters are attached for each divided line. The header
characters were used to connect the divided character lines and also
Transform photographic data into characters expressing
hexadecimal numbers (00 to FF).
Example of data
'3345AC980D・・・'
Divide the characters into short character lines and attachheader.

Digital data

Transform the character lines and the description sentences
into multi-blocks of two-dimensional barcode.
Two-dimensional barcode

Fig. 1. Concept of digital silica photography.
*1-bit error-diffusion dithering method: An image processing methodthat converts a n-bit grayscale image into a 1 bit(black and white)image

so that the density of black dots in the converted image approximates theoriginal gray level.

5 x 5 pixel
Bitmap of two-dimensional barcode blocks
Fig. 4. Sequence of making data for writing.

12Bull. Soc. Photogr. Imag. Japan. Vol. 26 No. 1 (2016)

to reconstruct the full-color image. In the final step, the set of character lines and the description sentences were transformed into
multi-blocks of two-dimensional barcode. Then, the blocks were arranged into a rectangular area to make a bitmap image for recording.
In this experiment, we used QR Code that is the one of the mostpopular two-dimensional barcode (QR Code is registered trademark of DENSO WAVE INCORPORATED).

Dot

1.4 μm15 μm

3. Experiments for feasibility demonstration

Table 2. Specification of samples used in experiments.
Material

Synthetic fused silica

Recording area

12 × 8 mm

Size

Depth of recording area
Pitch of dots

Recording density

20 × 20 × 2 mm
290 μm15 μm

1,693 dpi

Fig. 5. A microscopic image of recorded dots.

0.04← Higher resolution
Larger depth of field, higher periphery quality →

0.03

bER

Table 2 summarizes specification of samples used in experiments.
The size of a sample was 20 × 20 × 2 mm. An area of 12 × 8 mm thatwas located 290 μm under the surface was used for recording. Recording pitch of dots (the micro void-like structures) was set to be
15 μm for the use of a macro lens in the reading procedure. The resultant recording density was 1,693 dpi.
A microscopic image of recorded dots is shown in Fig. 5. Total
reflection at the surface of the micro void-like structure may contribute the bright area of each dot. Diameter of the bright area in a
dot is about 1.4 μm. At the beginning of reading experiments, we
determined the best F number of the macro lens for decoding digital
data. Figure 6 shows the relationship between the F number andbit-error rate (bER). Smaller F number provides higher resolution
since smaller F number provides larger numerical aperture. On theother hand, large F number provides higher periphery quality. The
tradeoff determines the best F number. The figure shows the F number of 8 was the best value for our experiments.
Figure 7 shows decode results of full-color images and their description sentences. An optical magnification of ×1.6 was used fortaking a picture of a sample. For each sample, 23 blocks and one
block of two-dimensional barcode were used for encoding the photographic image and the sentences. All data were successfully decoded. The bERs of the sample 1, 2, and 3 were 0.04%, 0.05%, and
0.18% which were calculated using all data in the recording area. The
bit errors were corrected by ECC (Error-Correcting Code) in decoding procedure of the two-dimensional barcode. The recording
time was 7,458 seconds (about 2 hours) for the sample 2. The value
was restricted not by the time for making a dot, but by the scanningspeed of the XY stage, which was not optimized in this experiment.
In order to check whether or not recorded data have fire resistance,
we annealed the sample 1 at 1,000˚C for 2 hours in an annealingoven with air of one atmosphere. The photographic data and thedescription sentences were read out correctly. The bER calculatedusing all of the data in the recording area showed no degradation
after the annealing.
Figure 8 shows the result of a decode-free analog image. Black-

0.02

Average

0.01

0.00
2

3

4

5

6

7

8

9

10

11

F numberFig. 6. Relationship between the F number and bit-error rate (bER)

and-white image for recording was made by the 1-bit error-diffusion dithering method from an 8-bit, monochrome-photographic
image. The size of recording area was about 12 × 8 mm, and otherrecording conditions were also set to be the same as the ones used
for digital-data recording. Gradation of the original image was expressed by the error-diffusion dithering method as shown in Fig. 8(a) without using gray-level pixels. The image shown in Fig. 8 (a)was correctly recorded as shown in Fig. 8 (b), though vertical stripes
due to uneven size of dots were observed in the background.

4. Conclusion
We propose digital silica photography that records photographic
data with sentences in digital format inside a fused-silica plate using
a femtosecond laser. It can also record decode-free analog image thatwill be indispensable to know the contents and method for decoding
in distant future. As a feasibility demonstration, decode-free analog
image and full-color digital data of Ukiyo-e were written inside a
fused-silica plates. The digital data were read out using a commercially available camera with a macro lens just like making a photographic copy of a 35-mm slide. The simple reading procedure ofdigital data enables reconstruction of reading tools in the future. We
confirmed the recorded data were read out after 1,000˚C annealing
without degradation. It implies that the data have fire resistance andstability for change of storage condition.

Takao Watanabe, Ryo Imai, Shigeki Mori, Toshiyuki Mine, Toshimichi Shintani, and Koichi Watanabe

SamplenumberCamera imageof a sample

Read out image *1

Read out sentencesbER *20.04%

330 x 232 pixelThis is an ukiyo-e (Japanese
woodblock print) titled“Kanagawa-Oki Nami-Ura” by
Katsushika Hokusai, a collectionof the Hiraki Ukiyo-e Foundation.
…

0.05%

340 x 224 pixelThis is an ukiyo-e (Japanese
woodblock print) titled “Yui: Satta
Peak, from the series of Fiftythree Stations of the Tokaido
Road” by Utagawa Hiroshige, acollection of the Hiraki Ukiyo-e …

0.18%

235 x 350 pixelThis is an ukiyo-e (Japanese
woodblock print) titled "Suruga
Province: Miho Pine Grove, fromthe series of Famous Places in
the Sixty-odd Provinces" by
Utagawa Hiroshige, a collectionof the Hiraki Ukiyo-e Foundation.
…

1

2

3

two-dimensional barcode

Digital Silica Photography with Future Readability and Fire Resistance13*1 The use of image data is allowed by the Hiraki Ukiyo-e Foundation.
*2 Error bits are corrected in decoding procedure.
Fig. 7. Decode results of full-color images and their description sentences.

(a) Bitmap data used for recoding(b) Picture of the image recorded
inside fused silica

Fig. 8. The result of decode-free analog image.

Acknowledgements
The authors wish to express their thanks to Hiraki Ukiyo-e Foundation for the use of the data of Ukiyo-e collections. They also wish
to express the thanks to T. Azuma, T. Morioka, and Dr. J. Tanaka of
Hitachi, Ltd. for their helpful support, and to Prof. K. Miura, Associate Prof. Y. Shimotsuma of Kyoto University for the tutorials on
their related works 10)12).

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