Image Capturing and Processing

Night-vision camera combines
thermal and low-light-level images
Raf Vandersmissen, XenICs NV, Leuven, Belgium

Even on moonless but starlit nights, highly sensitive InGaAs cameras still
produce good images for short-wavelength infrared. By thinning the image
sensor chip and with illumination right through the substrate, the wavelength range of the detectors can be extended into the visible spectrum tothen span 400–1700 nm. Together with uncooled microbolometer arrays forlong-wavelength infrared, this allows the design of universal analysis toolsthat can take advantage of the fusion of optical and thermal images.
IR imaging is already used successfully inmany technical/scientific fields. Its development, like that of other imaging technologies, is heading in the direction of higher
resolution, increased sensitivity and higher
speed.
Added to this is the multispectral capture
across multiple IR spectral ranges (figure 1). These begin with short-wavelength
infrared (SWIR) which, with its wavelengthof 0.9–1.7 µm, follows on directly fromthe visible spectrum (VIS) and is therefore
also called near infrared (NIR). This is followed by mid-wavelength infrared (MWIR),
which ranges from 3–5 µm. And longwavelength infrared (LWIR), from approx.
8–14 µm, is also gaining in importance forvarious kinds of analysis.
As well as the improved performanceof IR image sensors, their manufacturewith common industry processes on standard production lines ensures steady pricereduction, which should in turn open up
further application areas to IR array-sensor
technology, including those with strong
market potential.

Turning night into day
In particular the high sensitivity anddynamics of SWIR image sensors in InGaAs
technology predestines them for use in
low-light-level cameras. These are used
wherever insufficient light renders the useof normal cameras impossible. This typeof camera is used for monitoring at dusk,
dawn or night time, e.g. airport, buildingor site surveillance (security services), forscientific applications in astronomy, meteorology, animal observation and behavioural research, as well as in various military
applications and undercover observations.
In addition, when there is insufficient daylight, the phenomenon of airglow can be2 Photonik international · 2008/2

Figure 1: New infrared camera types and their combinations are conquering
wider spectral ranges, which are classified according to the water-vapour absorption bands in the atmosphereutilized. Airglow occurs as weak luminescence in the upper region of the earth’s
atmosphere through selective absorptionof the UV and X-ray spectrum of thesunlight in air molecules and atoms. The
biggest share of airglow comes from airlayers at 70–300 km elevation. Of particular significance here is the photochemicaldissociation occurring in the ionosphere ofoxygen molecules into two oxygen atoms
in daylight, which then later – partly not
until night time – recombine with theemission of light into O2. Figure 2 shows
the glow, which is also visible from space
during the day. At night, the brightness of
this atmospheric residual light is 5–7 times
greater than starlight, spanning virtually
the whole SWIR range from 1.0–1.7 µm.
With their maximum sensitivity of 0.9–
1.7 µm, SWIR cameras in InGaAs technology are perfectly suited to exploiting theairglow phenomenon. They also “see”objects very clearly on moonless nights(figure 3). The fact that the airglow fromthe entire sky is emitted equally in all latitudes is an advantage here.
SWIR imaging occurs in a similar way toimaging in the visible wavelength range,
because it uses the light reflected by theobject. This makes it easier to interpretand analyze scenes compared with purely
thermal detection. As a result of the reflective principle, images from SWIR cameras
based on InGaAs also show shade andcontrast, and are comparable in resolution and detail with normal images undervisible light. Objects in airglow are thus
relatively easy to recognize.
A particular benefit of SWIR InGaAs cameras is the imaging through conventional
glass. Simple glass optics therefore replacelenses made from expensive special materi-

Originally published in German in Photonik 2/2008

Image Capturing and Processing

Figure 2: Airglow as a thin blue line above the earth’s horizon, photographed in pseudo-colour during the Clementine
mission 1994 [1]. The bright spots on the earth’s surface areilluminated urban areas(image: Naval Research Laboratory)
als such as germanium, silicon or zinc selenide. Moreover, these kinds of cameras,
unlike thermal IR cameras or SWIR cameras with InSb or HgCdTe detector arrays,
require neither a lens shutter nor expensive
cryo-cooling of the array. Without thesecomponents, the reliability increases, thecosts, overall size and weight fall, and thecameras are less sensitive to vibrations.
For night-vision applications, SWIR InGaAs
cameras can work with thermoelectricallycooled array sensors, or even manage
without any cooling at all [2]. Their growing and varied use in security applications
documents the rapid progress of SWIR
technology – with ever-increasing performance in ever-shrinking packages.

Thinner chips extend the spectrum
The structure of a current SWIR image sensor in InGaAs technology is shown in figure 4 on the left: the infrared photodiodes
are produced on an InP epi-substrate about
125 µm thick. Because this technology is
not especially well suited to implementing

Figure 3: Airglow illumination was enough for this shot on amoonless, starlit night with a sensitive SWIR camera

readout circuitry, the photodiode array isassembled with flip-chip technology on asilicon-based CMOS chip that carries thereadout integrated circuit (ROIC). The electrical contact is provided by indium bumps[3]. The illumination then occurs throughthe InP substrate, although this absorbs
all light in the visible spectrum and up to0.9 µm.
Now, however, there is a whole rangeof applications for which it is necessaryor desirable to capture not just the SWIR
but also the VIS spectrum with a single“VISWIR“ sensor. A typical example is coupled landing aids in air traffic operations,which consist of an LWIR sensor (in therange 8–14 µm) for thermal radiation anda detector in InGaAs technology for SWIR,
which in turn effectively captures the lightfrom the incandescent bulbs of the landing lights in the range between 0.9 and1.7 µm. However, to increase safety andreliability, these kinds of landing lights are
increasingly being replaced by energy-saving light sources with LEDs, which mainly
emit rays in the visible spectrum and hardly

Figure 4: Thinning of the substrate turns an InGaAs SWIR camera chip (left) intoa broadband VISWIR image sensor (right)

Originally published in German in Photonik 2/2008

in NIR. It is therefore expedient to extend
the sensitivity range of SWIR sensors into
the visible spectrum.
Because the strong absorption here of visible and NIR (up to 0.9 µm) light by thesubstrate has a detrimental effect, thesubstrate is made much thinner. To ensure
safe thinning of the substrate without
damaging the InGaAs detector, therefore,
additional layers of InGaAsP are introduced
just below the photodiodes, which in
the InP environment act as an etch stop.
Subsequent etching with hydrochloric acid(HCl) removes the InP epi-substrate selectively and precisely as far as the InGaAsP
etch-stop layer. This thins the sensor chipfrom 125 µm to just 5 µm (figure 4, right),
making it transparent for visible light [4], sothat the sensor can pick up the broad wavelength range of 0.4–1.7 µm (figure 5).
The InGaAs sensor for the VISWIR range isassembled on the CMOS readout circuitry
in the established way with a flip-chipbonder, and prior to etching.

Spectrally combined detection
For night-vision applications, SWIR imaging with InGaAs technology is enhancedwith thermal imaging cameras for MWIRand LWIR in the form of uncooled microbolometers or cooled infrared cameras.
Their thermal detectors only show thepresence of warm objects against a cooler
background (figure 6). In combination
with thermal images, SWIR cameras thus
simplify the identification of objects that in
the thermal image alone are more difficult
to recognise.
The sensor elements of microbolometer
cameras for LWIR are made up of IRPhotonik international · 2008/2 3

Image Capturing and Processing

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absorbing conductors orsemiconductors, whoseradiation-dependent resistance is measured. Because
polysilicon is also suitable as
an absorber, they can also
be made from polysilicon
as MEMS [5] and combinedwith evaluation circuits in
CMOS technology.
Real-time images from both
spectral bands can then bedigitally processed and –even more importantly –also overlaid. The systemic
fusion of SWIR and LWIR
thus allows selection of
the image best suited to a Figure 5: Already known from CCD technology – agiven usage situation. LWIR thinned, backlit sensor chip captures a wider spectrum
generally proves of value
for detecting people or vehicles based on
special reconnaissance missions and rescue
their thermal emission. SWIR images offer
operations. VISWIR InGaAs cameras also
additional advantages when visual recognioffer conceptual ideas for night-vision aidstion, for instance in undercover investigain cars. As a relatively new application, IR
tions, is required.
systems have great potential in security foroverland transport fleets, or on ferries andyachts. Applications for volume-dependent
Versatile applications
traffic guidance systems are in development. Industrial application areas lie inWith their extended optical characteristics,
research and development, in real-timesuch cameras thus open up a multitude ofmonitoring of high-temperature producindustrial and security-related application
tion processes or, for example, in thepossibilities, such as hyperspectral imagmaintenance of aircraft with regard toing, laser beam profilometry, night-vision
the performance of electrical, hydraulicenhancement in road and air traffic, chipand pneumatic systems, such as in turbine
inspection in semiconductor manufacturinspection.
ing, thermal imaging in a temperature
range of 200–800°C, online process control, and medical electronics.
Literature:
Special water-cooled versions reduce the[1]	www.cmf.nrl.navy.mil/clementine
effect of vibration from the cooling fan[2]	B. Grietens, R. Vandersmissen, J. Bentell,
Ungekühlte InGaAs-Arrays für kompakte Hochgefor microscope applications, and are also
schwindigkeits-Kameras im nahen Infrarot, Photeasier to use in dusty environments.
onik 5/2005, p. 76-78
Other typical fields of application include[3]	J. John, L. Zimmermann, P. Merken, G. Borghs,
EVS landing systems, building security,C.A. Van Hoof, S. Nemeth, Extended backsideilluminated InGaAs on GaAs IR detectors, Proc.
public places or authorities, monitoring
SPIE Vol. 4820, p. 453–459, 2003
industrial objects such as factories or[4]	D. Göhring, Digitalkameratechnologien, Technischepower stations, border controls, as well as
Informatik, Humboldt University Berlin, 2002[5]	V.N. Leonov, N.A. Perova, P. De Moor, B. Du Bois,C. Goessens, B. Grietens, A. Verbist, C.A. Van
Hoof, J.P. Vermeiren, Micromachined poly-SiGebolometer arrays for infrared imaging and spectroscopy, Proc. SPIE Vol. 4945, p. 54–63, 2003

Figure 6: Thermal imaging from an
uncooled microbolometer camera canenhance VISWIR images for self-radiating objects
4 Photonik international · 2008/2

Raf Vandersmissen
Technical Sales Engineer
XenICs NV
Ambachtenlaan 443001 Leuven
Belgium
Tel. +32/16/3899-00
Fax +32/16/3899-01
eMail: rvn@xenics.beInternet: www.xenics.be

Originally published in German in Photonik 2/2008(photographed with a XEVA640)

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