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The Multispectral Imaging System for the
Humanities and Archives (MISHA)
Kevin Sacca
Chester F. Carlson Center for Imaging Science
Rochester Institute of Technology
Abstract—Multispectral imaging systems are costly and
are usually designed with a particular application in mind,
like astronomical imaging or airborne remote sensing. For
document imaging applications, a system that can image
from ultraviolet to infrared light is needed, but not readily
available for reasons of cost. Document imaging is a field
of value to many museums, libraries, and archives, but
has not received adequate attention because of the cost
of necessary imaging systems. I propose an example of a
low-cost, portable multispectral imaging system for use
in the humanities and archives (MISHA). Using many
narrow-spectrum LEDs ranging from 360 to 940nm and an
inexpensive digital camera with a monochromatic sensor,
this system is able to image a target with a controlled
illumination of selected wavelengths of light. The concepts behind MISHA’s imaging techniques were validated
through an experiment on a homemade document having
different ink signatures. In the future this system could
be used by students and scholars to perform in-house
sampling of documents, perform materials analysis, and
characterize documents for further imaging.
factor motivating the spectral imaging of documents is
the understanding that while the pigments and inks used
to write have faded in the visible spectrum of light,
chemical traces or other effects due to the ink may
not have faded in other parts of the spectrum, such as
infrared or ultraviolet light.
I. I NTRODUCTION
Multispectral imaging is the process of collecting
images at several narrow, discrete spectral bands. This
is a useful technique because there may be more information to collect from a scene in other areas of the
electromagnetic (EM) spectrum. In addition to visible
light, multispectral cameras may have the capability to
image in the infrared (IR) and ultraviolet (UV) spectra
by using a panchromatic sensor, which is sensitive over
wavelengths from 360nm to 1100nm, and can sample
smaller, individual spectral bands if the incoming light
is filtered.
Multispectral imaging is often utilized to take advantage of the various spectral properties of materials.
By imaging materials with different spectral responses,
the materials can be visually separated at wavelengths
where the spectral reflectance properties of the materials
differ. This concept is crucial to the idea of using
multispectral imagery of old manuscripts to reveal and
digitally preserve text that has faded over time. The key
Fig. 1. (left) Visual appearance of a cartouche on the Martellus
Map [1] showing few legible characters; (right) luminance channel
of image produced by combining fluorescence bands obtained under
blue illumination and imaged through red, green, and blue filters,
showing completely legible text.
There are many documents that are good candidates
for multispectral imaging; the problem is that these may
be hidden among many others that are not in need of
imaging. Good candidates for spectral imaging show
traces of hidden text under infrared or ultraviolet light. It
takes time and computer power to test the candidacy of
a document for spectral imaging, as well as to perform
more thorough spectral imaging of the document. Time
itself poses a problem as the condition of documents
can deteriorate beyond preservation if too much time
passes before they are imaged, so there is a sense
of urgency among the document imaging community.
The problem is that multispectral imaging systems for
applications like this cost tens of thousands of dollars. If
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a cheaper, more accessible multispectral imaging system
were readily available for scholars to use, the number of
documents in queue for imaging, processing, and digital
preservation would permanently decrease.
II. BACKGROUND
There is a present and ever-growing need for the
spectral imaging of historic manuscripts, paintings, and
palimpsests. Over time, such delicate artifacts may fade,
decay, or even accumulate mold if kept in less than
optimum conditions. Many other documents have been
damaged intentionally due to war and are now unreadable. Additionally, there are palimpsests, which are
manuscripts which have been erased and written over,
often by persons of religious order who could not acquire
new substrate to copy religious text. Despite these types
of damages, employing spectral imaging and processing
is a method for revealing and preserving the information
that was originally there.
The Early Manuscripts Electronic Library (EMEL)
is an organization that strives to provide libraries and
archives with processed digital copies of the originals
that they possess [3]. This group not only utilizes highquality imaging systems to take superb images, they
extensively image documents with a multispectral system
if the document is a good candidate. Dr. Roger Easton
is a colleague of EMEL and has worked on various
projects with EMEL, such as the Mount Sinai Project, the
Archimedes Palimpsest, the St. Catherine’s Palimpsests,
and the David Livingstone’s diary pages [2]. These
documents are very important to scholars and he spent
a lot of time developing imaging techniques to reveal,
enhance, and preserve the texts that were thought to
be lost. There is still much work to be done, and it’s
estimated that over 60,000 documents are waiting to be
imaged.
The Palimpsests Project is an international coalition
of scholars, graduate researchers, and undergraduate
research students working to make the imaging technologies, processing, and analyzing of multispectral images of palimpsests more widely available [2]. There
are numerous libraries that have a definite need for
affordable, accessible, and open-source technologies to
aid them in preserving their catalog of manuscripts. If
a low-cost, portable, and effective multispectral imaging
system were widely available, smaller groups around the
world could pick up the catalog of palimpsests and other
manuscripts that need to be imaged.
III. R EQUIREMENTS
The key components to this system are the camera
(mainly the sensor), the illumination source(s), and the
(a) Visual Appearance
(b) Processed Pseudocolor
Fig. 2. Example leaf from the Archimedes Palimpsest [2]. Note the
deterioration present in the visual appearance as well as the abundant
damage shown in the processed image. It is very common for there
to be degradation of documents in the IR domain, where humans
cannot perceive light. This means that even if there are traces of ink
that reflect light under IR illumination, mold and fading may prevent
any attempt at finding, let alone reading, the text.
lens. The system must image the field under multiple narrow-band illuminations. Researchers have found
that the combination of white light sources and different narrow-bandpass filters over the sensor is not the
best way to collect multispectral imagery. The broadspectrum white light source has been found to actually
damage the document due to the high-energy output of
light that is not imaged. A solution to this problem is
the use of narrow-spectrum LEDs to illuminate the document with only the energy to be imaged. This method
can also be supplemented with color filters, which can
result in spectral bands with a bandpass around 10-30nm.
The reduced spectral energy output is small, which is
a well-received aspect of using LED illumination by
scholars and curators. In order to sample the scene with
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sufficient spectral resolution, the number of bands, (or
the number of different-wavelength LEDs that will be
used) will be a factor that will be determined through
this research. Using past research and the multispectral
images resulting from them, the illumination sources will
be chosen based on their effectiveness and usefulness.
The sensor is important for many reasons, as it addresses the spatial, spectral, radiometric, and temporal
resolution of the imaging system. It will be crucial to
choose a camera with a sensor that is a well-balanced
option between each type of resolution. The pixel size
and number of pixels will determine the spatial resolution, and higher resolution tends to be better. However, to
design a low-cost system that is capable of multispectral capture, spatial resolution will almost certainly be
sacrificed to some degree to allow the sensor chosen to
accommodate the other resolution requirements, such as
the spectral response and radiometric properties of the
electronics. The sensor also must be sensitive to a wider
portion of the electromagnetic spectrum than a typical
digital camera, which is often only effective from 400 to
700nm.
Most digital cameras include infrared and ultraviolet
filters installed, which limit the spectrum imaged to
visible light. Since the spectral information is produced
by the lights, a monochromatic sensor may be used. The
most desirable sensor in terms of its spectral capabilities
is one that is monochromatic (e.g. color filter array),
contains no extra filters, is sensitive from 300-1000nm,
and has low noise under low light level conditions. To
simplify design, the sensor and camera will be considered a single component when researching options.
TABLE I
E STIMATED S YSTEM B UDGET
Component
Estimated Total Price
Camera
Illumination Sources
Lens
Structure
Other Electronic Components
$1500
$1000
$600
$300
$100
Total
$3500
IV. D ESIGN
The camera chosen for MISHA was the Point Grey
Blackfly [4]. The camera offered a 5.0MP monochromatic sensor with USB3.0 connectivity and speeds. According to the documentation, the spectral response at
ultraviolet and near-infrared wavelengths was adequate.
It was fairly expensive for the specs but it was an easy
off-the-shelf solution to begin testing with. The lens
mount for the Blackfly is a C-mount, and a recommended
lens for the camera was a Fujinon 12.5mm lens speciallymade for 5.0MP cameras, meaning that my optics would
not be the leading contributor to low-spatial resolution
of my system.
The other key component to this system is the lens.
When an optical system has to image such a wide range
of wavelengths, its design must be more complex than
a single, static lens. Chromatic aberration is an effect of
variation in refractive index and will play a huge role in
the resolution of this system. No matter how perfect the
sensor and illumination, a low quality lens will reduce
the overall quality of the images. Therefore, lenses that
are designed for use in broad-spectrum applications will
be thoroughly researched.
Other than these components, some kind of rigid
structure that holds each component in place will be
needed. For any imaging system that captures long
exposures of a static target, the structure needs to be
very stable, yet adjustable, strong, yet light, and it needs
to be easy to maintain and assemble/disassemble. If this
system is made to be highly portable, a more expensive,
lightweight material may be chosen with a collapsible
design in mind.
Fig. 3. The MISHA system. (1) Raspberry Pi, (2) Point Grey
Blackfly, (3) 3D Printed LED Lamps.
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(a) PG Blackfly Camera
(b) LED Lamp
Fig. 4. 4a Point Grey Blackfly 5.0MP USB3.0 with a monochromatic
Sharp RJ32S4AA0DT sensor [4]. 4b Image showing the interior of
one of MISHA’s LED lamps with the diffuser removed. The colors
represent the colors of light each wavelength emits, with the black
circles representing UV or IR.
The illumination source for MISHA was decided after
purchasing some LEDs to test with, and realizing the
simplicity of using cheap LEDs. The 16 unique LEDs
purchased have central wavelengths of: 361nm, 400nm,
420nm, 440nm, 470nm, 505nm, 525nm, 575nm, 590nm,
610nm, 630nm, 660nm, 780nm, 850nm, and 940nm.
A broad-spectrum white LED type is also used. The
LEDs all have varying angular spread, intensity, and
bandwidths, but they were extremely inexpensive and
these properties are not bad. The varying angular spread
of the different LEDs was effectively normalized with the
use of soft light diffusers. The diffusers create a flat-field
illumination of the target to minimize specular effects
from the reflection off the target.
In order to simplify the design of the system, a
convenient method for organizing the lights was needed.
The solution chosen was to design and 3D print lamp
casings for the 16 LEDs. Two lamps were created to be
placed 45 degrees from nadir to maximize reflected light
in the vertical direction. [2] The diffusers fit between
two plates constructed into the LED lamps making for
a simple packaged light source.
Fig. 5. Spectral Quantum Efficiency of the Point Grey Blackfly
integrated in MISHA. [4]
Fig. 6. An approximate spectral output plot for each of MISHA’s
16 unique LEDS. Each curve is a normalized gaussian with a fixed
FWHM value of 50nm, which was the wider range of bandwidths
among the LEDs purchased.
The camera and the LEDs are controlled via an
onboard Raspberry Pi microcomputer. The Pi is an
incredibly capable device, and well-suited to systems
requiring few computing tasks. In MISHA’s case, the
Pi is really only responsible for providing some method
for turning on a specified LED at a time, capturing
an image, turning off the LED, and repeating. Other
solutions to this workflow are easily possible with other
technology, but the Pi also has a great amount of support
and multiple platforms to work with. The Pi allows
for server control via WiFi, which means that MISHA
can be operated remotely from any other computer. The
software to control MISHA was written exclusively in
open-source tools, namely Python and C.
Another challenge for MISHA was aiming to develop
a strong but portable and adjustable structure. A very
simple design is proposed, but it allows for easy adjustment and even folds down flat to fit inside a laptop bag.
80/20 aluminum was used for the structure because it
is easy to assemble, cut, and add/remove components.
The entire system was painted black to try to reduce the
possibility of reflections occurring during image capture,
which would result in imagery with undesirable specular
effects. Fig 3 shows the final system ready for imaging.
The anticipated budget for MISHA was $3500 but
only about $1700 total was spent building the system.
Many of the prices shown in Table II are very generous
estimates, and the cost of everything purchased for
testing is included as well. Therefore, building a system
of known components and streamlining the construction
would result in a much cheaper system. Also, the camera/lens are clearly the largest purchases, and a different
choice of camera could drastically bring down the price
of MISHA while not sacrificing quality. Even at a price
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TABLE II
T OTAL C OST OF MISHA
Component
Price
Point Grey Blackfly Camera 5.0 MP Mono
Fujinon 12.5mm C-mount Lens
Bulk LEDs, Resistors, and Cables
Raspberry Pi 2 Starter Kit
74” of 1”x1” 80/20 Aluminum
3D Printed Lamps and Light Diffusers
$1,000
$300
$250
$90
$30
$30
Total Cost
$1,700
point of $1700, MISHA is incredibly affordable, and
could be a solution for many researchers and scholars
to determine if a document is a good candidate for highresolution multispectral imaging.
V. R ESULTS
The evaluation of MISHA’s performance should be in
three main aspects of cost effectiveness: Spatial Resolution, Spectral Resolution/Sensitivity, and Accessibility.
MISHA is first and foremost an affordable alternative
to an expensive high-resolution multispectral imaging
system, so it’s performance should be rated by it’s
potential to be used as a preliminary step in document
imaging applications. If MISHA is shown to have the
capability to capture multispectral image cubes that show
the spectral properties of a target, then the concept will
have been proved successful.
Fig. 7. Example of a 16-band image cube as captured by MISHA.
The front band shown is the image exposed by the broad-spectrum
white LED. Note the varying intensity of each band along the spectral
axis, this is due to dim LEDs and poor QE at extreme wavelengths.
In terms of spatial resolution, MISHA needs to be
able to distinguish fine details in small documents and
text, and an acceptable resolution is about 300 pixels
per inch (ppi). With this high of resolution, a multispectral imaging system should be able to distinguish
between manuscript writing and stains/mold. MISHA
accomplishes 200ppi at the expense of field of view
(FOV). With the camera positioned 14 inches above
the target, the field of view of the camera/lens used is
approximately 6 inches by 7 inches, yielding the 200ppi
resolution captured by 1.25MP images. The spatial and
radiometric resolution of MISHA was limited mostly
by the Raspberry Pi’s data transfer capabilities. The
Raspberry Pi 2 has only USB2.0 ports, while the Point
Grey Blackfly camera is USB3.0. Image data sent from
the Blackfly is too large for the Pi to process and store
when the resolution of the image is above 1.25MP 8-bit
images. 1.25MP (1024x1224) was achieved using 2x2
binning on the 5MP (2048x2248) sensor. If the Raspberry Pi could achieve the full resolution of the camera,
then the resolution of the imagery would be doubled
to be 400ppi at the same capture height. The FOV of
the images is still at maximum with this technique. The
Blackfly is specified to be able to handle 10-bit and 12bit imagery, allowing for more ADU steps to saturation,
however the Raspberry Pi can’t handle the increased data
size of each image, so 8-bit is the only option for this
system. Higher bit-depth in imagery for document image
processing is actually very important, but again, for the
purpose MISHA serves, 8-bit imagery still works.
As for the spectral resolution and spectral range detectable by MISHA, the performance can be assessed by
the quality of each individual band MISHA has, which
is determined by the combination of the sensor’s spectral
quantum efficiency (SQE) and the spectral output of
each LED. Ideally, a multispectral imaging system will
have high QE for all wavelengths sampled and many
narrow bandpass LEDs. MISHA has 16 bands, with each
band exposed by two identical LEDs that have average
bandwidths between 40-50nm at FWHM [6]. To saturate
the sensor as much as possible, the longest exposure that
the camera hardware would allow is used for each band.
Increasing the exposure time also decreases the effects
of random photon noise that is usually a problem in lowlight conditions. Luckily, for document imaging applications, the scene is static and so motion blur is not an
issue to worry about. Three seconds was the maximum
exposure time programmable into the hardware using
the Raspberry Pi as the computer. Perhaps with another
system, exposures could be lengthened. The SQE of the
Blackfly is decent at wavelengths longer than 360nm but
for the 361nm centered UV LED, the QE and signal to
noise ratio are poor, resulting in an image appearing to
be random noise. The result of using an 1100nm LED
for a 17th band would be of similar quality.
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(a) 360nm - UV
(b) 400nm - UV
(c) 420nm
(d) 440nm
(e) 470nm
(f) 505nm
(g) 525nm
(h) 575nm
(i) 590nm
(j) 610nm
(k) 630nm
(l) 660nm
(m) 780nm -NIR
(n) 850nm - NIR
(o) 940nm - NIR
(p) Broad-Spectrum White
Fig. 8. All 16 monochromatic bands from a MISHA capture of a handmade document. The document used was a simple sticky note with
various inks and writing tools used, to show the varying spectral properties that are regularly encountered in document imaging. Note that
some inks are completely transmissive in IR light, revealing pencil text underneath.
Fig. 9. The truecolor image obtained by loading Fig 8k into the Red
channel, Fig 8g into the Green channel, and Fig 8e into the Blue
channel into an RGB render.
As you can see in Fig 8, the individual bands are
showing the unique spectral reflective and transmissive
properties of the different inks. This is the working
concept behind multispectral imaging systems and how
the spectral properties of materials can be sampled. In
document imaging, the multispectral sampling serves to
separate inks into different spectral layers so that one
type of ink can be isolated and read at a time. In Fig
8b, the fluorescent nature of some inks is shown, where
high frequency UV light is absorbed and light at lower
energies is emitted, which is why the ’Hello World!’ text
appears so bright. In Fig 8n and the other NIR bands,
the transmissive nature of some inks is shown, where the
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light simply passes right through the ink and the detected
light from the image sensor is the light reflected from
underneath the ink, showing the ’IMAGINE RIT’ text.
VI. C ONCLUSION
From the results shown in Fig 8, it is clear that
MISHA is proving the legitimacy of using inexpensive
components to build a multispectral imaging system for
document imaging applications. The spectral nature of
different inks is shown in 16 bands, each producing different effects such as absorption, reflection, transmission,
and fluorescence. From this result, it is safe to say that
this system and others like it could be used by scholars to
image their own documents to test for good candidacy
for high-resolution multispectral imaging. Radiometric
calibration, spectral characterization of the LEDs, and
MTF calculation would have been nice additions to
system characterization results, but none of these items
are absolutely necessary for multispectral imaging of
documents. Time permitting, these would have been
additional steps in the scope of the project.
Stretch goals for MISHA were to make it portable,
easy to use, and cheaper than $2500. All of these were
accomplished through careful design and simple components. It is important to those involved in document
imaging research to have access to cheaper equipment, so
lowering the total cost of the system will be the focus
of future work on this project. It would be very easy
to cut a significant portion of the total cost by using a
much cheaper camera and lens combination. Simplifying
the electronics in the LED lamp would also decrease the
amount of material needed to construct them, thereby
lowering the cost. Once the system costs around $500
total, many scholars and researchers around the world
could easily afford to own a MISHA, rather than rent
one or another very expensive system.
Hopefully this project leads to an advancement in
document imaging technologies and brings attention to
the global movement to image, process, and preserve old
documents of historical importance. Only with widely
available technology, motivated individuals, and quick
action will we begin to make strides in the effort to
preserve the humanities from centuries ago.
ACKNOWLEDGEMENTS
I’d like to thank R. Easton for advising me and giving
me the opportunity to work on such amazing projects. I’d
also like to thank R. Kremens for his insight into system
design and electronics. Thanks to J. Pow and the Chester
F. Carlson Center for Imaging Science for funding the
first MISHA system. A special thank you to D. Kelbe, K.
Boydston, M. Phelps, and C. van Duzer for inspiring me
to build this system for my senior project. Lastly, thank
you to S. Chan, M. Helguera, J. van Aardt, A. Vodacek,
J. Qiao, E. Lockwood, L. Cohen, Z. Mulhollan, H. Grant,
and all the other students, faculty, and staff who are a part
of the Imaging Science community and support students
and their great projects.
R EFERENCES
[1] R. E. et al., “Rediscovering text in the yale martellus map:
Spectral imaging and the new cartography,” in WIFS Rome,
August 2015.
[2] R. Easton. Imaging of historical manuscripts. [Online]. Available: http://www.cis.rit.edu/people/faculty/easton/manuscriptsshort.html
[3] M. P. et al. Emel: Early electronic manuscripts library. [Online].
Available: http://www.emel-library.org
[4] PointGrey. Blackfly 5.0 mp mono usb3 vision sharp rj32s4aa0dt.
[Online]. Available: http://www.pointgrey.com
[5] NEDCC.
Protection
from
light
damage.
[Online].
Available:
https://www.nedcc.org/free-resources/preservationleaflets/2.-the- environment/2.4-protection-from-light-damage
[6] LedSupplyCo. Led specifications. [Online]. Available:
http://www.ledsupply.com