Sensors-8495-2013.R1 Accepted 1

Abstract-We have developed focal plan array (FPA) imagers based

on very inexpensive miniature neon indicator lamp detectors called

Glow Discharged Detectors (GDD's). The novelty of this paper is a

technique of calibration which permits reliable imaging with such

an inexpensive FPA. GDDs are commercial lamps which lack

uniformity as detectors. The technique developed in this study

cancels out differences in response between the FPA lamps caused

by both non-uniformity of the MMW collimated beam, and non-

uniformity of the GDDs and other components in the electronic

circuit. Utilizing this method in the new imaging system unifies

detector response. Results, shown with two types of sensor

configurations ( and scanner array), indeed show

better performances and improvement in visibility of details.

Index Terms— Millimeter waves, Imaging, Calibration

technique, Focal plane array, Systems embedded

I. INTRODUCTION

MMW/THz imaging systems in the electromagnetic spectrum

between and are required for applications in

homeland security, communications, medicine, and space

technology mainly because there is no known ionization

hazard for biological tissue, and atmospheric attenuation of

MMW/THz radiation at 100 GHz frequency relatively low for

practical imaging distances of even several kilometers (less

The paper was submitted for reviw in 23-Jun-2013. The authors are

grateful for the support of the Office of Naval Research and the US Army Night Vision and Electronic Sensors Directorate.

A. L. Author is a Ph.D. candidate in Electrical and Computer Engineering

Department, Ben-Gurion University of the Negev, Beer-sheva. (e-mail: assaflev@ee.bgu.ac.il)

N. S. Kopeika Author is a professor in Electrical and Computer

Engineering Department, Ben-Gurion University of the Negev, Beer-Sheva, Israel, (e-mail: kopeika@ee.bgu.ac.il)

Y. Y. Author is a Ph.D. Researcher and a member of Staff with the

Electro-optic Engineering Department, Ben-Gurion University of the Negev, Beer-Sheva, Israel. (e-mail: ytshak@bgu.ac.il )

D. R. Author is a Ph.D. candidate in the Electro-Optical Engineering

Department, Ben-Gurion University of the Negev, (e-mail: rozbandaniel@ee.bgu.ac.il)

A. A. Author is a Ph.D. Researcher and a member of Staff with the

Department of Electrical and Electronic Engineering, Ariel University Center of Samaria, Ariel, Israel (e-mail: amir007@ariel.ac.il)

than 1dB/km) except for heavy rain [1], let alone tens of

meters that are needed for many security applications. New

techniques of distinguishing between objects and absorbing

background in order to recognize concealed objects through an

interfering media are very popular.

We use the known and simple fundamental phenomena of

glow discharge plasma indicator lamps as THz detectors. The

lack of inexpensive room temperature high speed detectors

makes focal plane arrays (FPAs) very expensive.

Consequently, scanning is often employed instead of real time

FPAs.

Room temperature detectors such as pyroelectric, Golay

cell, and bolometer devices, feature relatively long response

times on the order of milliseconds or more which makes real

time imaging difficult. Direct detection GDD NEP at

is about ⁄ as measured using

modulation frequency. [2, 10]. For lower modulation

frequencies such as it is closer to

⁄ . The difference stems from circuit electronics.

Microbolometers coupled with antennas are much more

sensitive. However, GDDs are orders of magnitude both faster

and cheaper, with response time on the order of a microsecond

[3] as limited by circuit electronics. Recently, using faster

electronics, this has been reduced to less than 100 ns. Speed is

important for video frame rates, especially for imaging

moving targets. Cost is important for fast focal plane arrays

and possible wide spread implementation in crowded areas.

Ability of plasma to detect electromagnetic (EM) radiation

has been studied since the 1950's at least. Investigations

involved rare gas and rare gas mixtures enclosed in a glass

envelope containing at least two electrodes. The plasma is

formed by breaking down the gas via a DC voltage applied

between two electrodes. The plasma is then illuminated with

EM radiation of frequency greater than the plasma frequency

so that the incident radiation can be absorbed. At least two

mechanisms of detection have been identified, enhanced

ionization collision rate [4-9] and enhanced diffusion [5, 8, 10-

12]. The former increases the bias current, while the latter

decreases it. Change in current is proportional to incident EM

Calibration Method for MMW Imaging using

Inexpensive Miniature Neon Indicator Lamp

Detectors

Assaf Levanon1, Natan S. Kopeika

1,2, Yitzhak Yitzhaky

2, Daniel Rozban

2, Amir Abramovich

3

1Department of Electrical and Computer Engineering, Ben Gurion University of the Negev, Beer Sheva, Israel

2Department of Electro-Optical Engineering, Ben Gurion University of the Negev, Beer Sheva, Israel

3Department of Electrical and Electronic Engineering, Ariel University, Israel

This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.The final version of record is available at http://dx.doi.org/10.1109/JSEN.2014.2301933

Copyright (c) 2014 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.

Sensors-8495-2013.R1 Accepted 2

wave power. The dominant detection mechanism in such

devices was found to be the enhanced ionization leading to

increase of lamp current [8-12].

GDD plasma properties have been studied in three-

electrode lamps in which the third electrode is used as a

Langmuir probe, and such internal avalanche amplification

has been calculated to be on the order of six orders of

magnitude [9]. Response by such devices is linear with

incident electromagnetic wave power [9]. In order to separate

the detected signal from the large dc bias, it is desirable to

modulate the intensity of the radiation, in which case the GDD

acts as an envelope detector.

Imaging experiments with a single GDD in a scanning

system indicate essentially no difference in image quality from

that obtained using a Schottky diode detector instead [13].

Indeed, GDDs have been shown to have potentially higher

responsivity and greater speed than Schottky diodes [14].

However, success in developing less expensive real-time

millimeter wave and terahertz imaging systems depends

largely on the ability to develop inexpensive focal plane

arrays, especially using inexpensive detectors such as GDDs.

Indeed, small GDD arrays such as and scanner

have been developed and tested and found to yield very good

image quality despite the large size and dimensions of the

lamps [15-19]. Recent results include using oversampling sub-

pixel shifting to obtain high resolution images [17-19] on the

order of 0.9 milliradians. If diameter lamps are used,

0.5 milliradian resolution should be achievable. The

oversampling increases signal-to-noise ratio and allows

thresholding at the noise level. This removes larger angle

diffraction blur from the image since such blur is buried in the

noise. The blur size reduction improves resolution beyond the

diffraction limit. Such performance in real time requires

simultaneous exposure of all detectors to the incident image.

The oversampling would require large numbers of detectors.

Hence, low cost of detectors becomes a significant issue.

The detector operation, architecture, and direct detection

imaging have been described previously. It was shown that it

is possible to detect metallic materials, and interpolate over

the image with the assistance of DSP algorithms. A quasi

optical system for the detector FPA was also designed, and

various kinds of imaging were demonstrated [16-19]. Lab

VIEW controlling software permits graphical algorithm

display and image processing. Those capabilities permit all

post processing in real time. The GDDs used here are in

diameter. There also exist commercial GDDs of

diameter. Both types of lamps exhibit similar sensitivity, as

quantified above. The lamps have shown excellent imaging

results when used as image scanning single detectors [13, 14]..

II. IMAGE SETUP

Two separate systems were designed, based on two kind of

FPA which utilize miniature neon lamp GDD devices as

detectors with a small parabolic mirror behind each one to

focus the incoming radiation back into the sensor.

Fig.1 depicts the experimental MMW imaging system

featuring the FPA size of . Each line

turns on for reading operation. The software system images all

8 rows to obtain a 64 pixel image. The source is

based on GaAs multipliers of Virginia Diodes, Inc.

(Charlottesville, VA, USA) that multiply a low frequency

source of . Fig.2 shows the second

detector system, the scanner detector array. The new

scanner contains 18 GDDs in 2 columns, with a horizontal

movement motor. The two off-axis columns of GDDs are

displaced horizontally to produce interlaced images.

Compared to the FPA, the scanner detector array

has increased field of view (FOV) in both directions. The

image size could reach lengthwise (vertically) and

widthwise (horizontally) which is dramatically larger

compared to the previous FPA size of . This is made possible by the horizontal scan motion

extent [width] and the relatively long vertical scanner

dimension [length]. A user-defined scan step size determines

the number of pixels within field-of-view. Each vertically

scanned location causes the lamps rows to be turned on for

reading operation. The imaging system includes a MMW

source of based on GaAs multipliers from Virginia

Diodes, Inc. (Charlottesville, VA, USA).

Both systems sources are amplitude modulated sinusoidal at

. The transmitted signal was coupled to free space by

a rectangular horn antenna, producing an approximately

fundamental mode Gaussian beam with waist on the order of

[20]. Scene illumination is therefore non-uniform in

intensity. The imaging quasi optical set-ups in both system are

the same, and include an off- axis collimating parabolic mirror

900 (OPM) of focal length with aperture of .

An on-axis spherical mirror of focal length with

aperture of diameter was used to generate the image.

A diagonal mirror was used in order to lay objects in

horizontal positions on a table. Imaging is therefore performed

without the need to move the optical setup for different

objects. The system components are described in Figs.1, 2.

The quasi optic systems were designed to image according to

geometrical optics principles. ZEMAX optical development

software was used to optimize the system for minimum

geometrical aberrations. Simulation of an “F” shape object

was employed in order to predict diffraction affects.

The dimensions obey the Gaussian imaging equation , where and are object and image distances,

and is focal length [ ].

Fig.1: Setup design of the MMW FPA system. The MMW source [lower

left] transmits the beam to the target [lower right]. A parabolic imaging

mirror [upper right] reflects the returned radiation from the target onto

the FPA [upper right].

This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.The final version of record is available at http://dx.doi.org/10.1109/JSEN.2014.2301933

Copyright (c) 2014 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.

Sensors-8495-2013.R1 Accepted 3

Fig.2: Setup design of the MMW scanner detector array system. The

MMW source [lower left] transmits the beam to the target [lower right].

A parabolic imaging mirror [upper right] reflects the returned radiation

from the target onto the scanner detector array [upper-right].

In system 1, the set-up was designed to perform

to ) optical magnification of the object,

which was located from the imaging spherical

mirror . The GDD array was placed in the image plane

which was from the imaging spherical mirror .

In system 2 objects were placed distance from

imaging spherical mirror , so that the image plane appears

from there , yielding approximately unity

optical magnification.

III. CALIBRATION METHODS

It is worthwhile to develop methods for resolution and

identification improvement of details in the image. Such

operations open the possibility to perform successful imaging

by inexpensive commercial lamp FPAs as sensors

Developing a calibration technique in order to cancel out

differences in signal strength between FPA lamp signals is of

utmost importance. Differences and inconsistencies in the

image formation process need to be canceled. Lack of

uniformity is caused by non-uniformity of the collimated

beam, inconsistency in electrical components at the electrical

circuit, and GDD inconsistent power detection.

These characteristics are not unexpected, since GDD devices

are manufactured as inexpensive indicator lamps rather than

MMW radiation detectors.

In order to limit the non-uniformity across the GDD array, a

look-up-table (LUT) was created to yield effectively the same

response for each lamp. This LUT formation is based on the

linear response of each lamp when incident radiation is

detected [6]. The method procedure algorithm was

implemented in both and FPA systems through

novel system-based Lab-VIEW software (see Fig.3). This

system prevents the differences between lamps automatically,

by calculating the transform LUT. Fig.3 demonstrates the

graphical user interface (GUI) of the novel calibration system

of both FPA systems. The principle was similar, but each

system needs to implement the method in a different way

before data is transformed to the display unit.

Fig.3: are coordinates with respect to the source power

(source intensity levels), n and m are the line equation parameters

adjusted for each lamp sensor; (a) A calibration graphical interface of

FPA. (b) Calibration graphical interface of scanner detector

array

The procedure of the calibration is as follows:

In order to produce the LUT, the coordinates with respect to

source power of the system (intensity levels) and are

selected (see Fig. 4 – horizontal axis). These coordinates

reference to radiation illumination intensity irradiated from the

source. An MMW image is taken with intensity and another

with intensity. Each lamp (pixel) produces its own line

equation (where m and n are the slope and the

-intercept parameters of the line) using the volt root mean

square (VRMS) outputs and with respect to the intensity

of the GDD (Fig. 4 – vertical axis) in each detector test image.

These line equations are the function of intensity transform,

and parameters and are adjusted for each lamp so as to

yield a uniform response. New intensity value (new in

Fig.4) will transform to the appropriate by using the known

n and m, and this presents the calibrated value for

Pixel . Consequently, by choosing different gaps

between the values of and in the test image, we can

control the intensity resolution in the display unit.

As we present in the results, both systems perform such

calibration, so that the same absorbed radiation intensity will

effectively produce the same response for each lamp.

Fig 4: Calibration technique description: X’s are the coordinates which

refer to radiation illumination intensity irradiated from the source. Y’s

are the real VRMS intensity results for the detected sinusoidal modulated

wave.

This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.The final version of record is available at http://dx.doi.org/10.1109/JSEN.2014.2301933

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Sensors-8495-2013.R1 Accepted 4

The quality of the calibrated MMW images was improved

after the implementation of this technique and results are

presented in Figs.6-8. Each imaging result is presented over

the Lab View Graphical Interface User (GUI) and stored for

post-processing procedures. The resulting MMW images here

are presented in a spatially interpolated contour-view.

A contour line of a function of two variables is a curve along

which the function has a constant value (as show in

Fig.5). The space between each pair of adjacent elevation

lines is filled with respectively constant mapped gray level.

This technique produces a non-pixelated interpolated version

of the image, with a larger number of pixels (see MMW

photos of Figs.6-8). Here, the images are created with 8 bits

quantization, which means 256 gray levels.

Fig.5: Description of same-elevation (contour) lines.

Figs. 6 (b, c) and 7 (b, c) are product of an pixel

resolution while Figs. 8 (b, c) and Figs.8 (e, f) were created

with and pixels resolution, respectively.

Fig. 6(a) describes a resolution chart with

3 rectangular holes of size . As seen, in Fig.

6 (c) the resolution chart rectangular holes are much more

recognizable than in Fig.6 (b) where the holes almost cannot

be seen. Fig.7(c) also shows better results in comparison to

Fig.7 (b) after the lamps were calibrated over the image plane.

As mentioned above, the aim of this technique is to obtain a

reliable space-invariant linear-intensity response image.

Results indicate that it is indeed possible.

Fig 6: (a) A metal of resolution chart object, (b) 2D

contour-based interpolation MMW image before applying calibration, (c)

same image after applying calibration.

Fig 7: (a) A photograph of a metallic object with circular hole in plate, (b)

2D contour-based interpolation MMW image result before applying

calibration, (c) same image after applying calibration.

Fig.8 describes results with FPA scanner array

detector. As can be seen, the figures transformed by the

calibration technique are much better in terms of uniformity of

intensity.

In order to recombine the two off-axis column detectors of the

scanner and obtain their true spatial location, the

default step size is . Fig.8 (b, c) are made with this

default scan step in order for the interlace column to include

all 36 lamps. Images with that scan setup include

pixel resolution and are of size.

Fig.8 (b) includes high intensity in the middle of the image,

which disappeared by use of the calibration technique (as seen

in Fig.8 (c))

Figs.8 (e, f) are produced by only one column of 18 lamps, in

scan steps of 1mm. It was made in order to obtain only

pixel resolution, with image size of . The over sampling technique is used together with

the calibration technique to obtain excellent results. Fig.8 (e)

presents illumination with the un-collimated beam (higher

intensity in the middle of the image plane).Moreover there are

some GDD lamps with noticeable differences in responsivity.

The outputs of both imaging structures seem to be improved

with the new calibration technique.

Figure 8: Results with FPA scanner array detector (a) A

photographs of a metallic “F” object, (b) 2D contour-based interpolation

MMW image result before applying calibration, (c) the same image after

applying the calibration process, (d) an object with circular holes in a

metal plate, (e) 2D contour-based interpolation MMW image result

before applying calibration, (f) image after applying calibration.

IV. DISCUSSION AND CONCLUSIONS

We succeeded in obtaining MMW images of metallic and

dialectical objects at , with two different systems.

One is an FPA, and the other is a scanning FPA

imager. Both imagers use very low cost commercial GDD

lamps as detectors. Indicator lamps as detectors can greatly

reduce the cost of millimeter wave imaging systems, while

obtaining both speed and good image quality. However, non-

uniform illumination and non-uniform GDD response limit

image quality noticeably.

This research shows that methods such as calibration solve

both problems and thereby improve the image, permitting

This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.The final version of record is available at http://dx.doi.org/10.1109/JSEN.2014.2301933

Copyright (c) 2014 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.

Sensors-8495-2013.R1 Accepted 5

imaging with much better capability of identification and

accuracy especially when combined with oversampling...

In conclusion, the low cost of GDD FPAs can permit real

time relatively high image quality by illuminating all detectors

simultaneously, and using oversampling and calibration as

shown here.

ACKNOWLEDGMENT

The authors are grateful for the support of the Office of

Naval Research and the US Army Night Vision and Electronic

Sensors Directorate. We also appreciate the support of the

Institute for Future defense Technologies research named for

the Medvedi, Schwartzman, and Gensler Families.

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First A. Author, Assaf Levanon was

born in Israel in 1975. He received the

M.Sc. degree in Electrical and

Computer Engineering from Ben-

Gurion University of the Negev, Beer-

sheva, Israel, in 2011, where he is

currently pursuing a Ph.D. His current

research interests include millimeter

and sub-millimeter wave systems, expertise in image

processing algorithms for enhancing THz images, new

detection methods, beyond limit of diffraction resolutions

issues, and hardware design configuration for Ne lamp FPAs

for mm wave security, military, and civilians applications

Second B. Author, Natan S. Kopeika

(SM’79–LSM’10) was born in

Baltimore, MD, in 1944. He received

the B.Sc., M.Sc., and Ph.D. degrees in

Electrical Engineering from the

University of Pennsylvania,

Philadelphia, in 1966, 1968, and 1972,

respectively. He joined the Ben-Gurion

University of the Negev, Beer-Sheva, Israel, in 1973, where he

was the Chair of the Department of Electrical and Computer

Engineering for two terms from 1989 to 1993, Reuven and

Francis Feinberg Professor of Electrooptics since 1994, and

the first Chairman of the new Department of Electro-Optical

Engineering. He has authored or co-authored over 180 papers

in internationally reviewed journals and well over 100 papers

at various conferences. He has authored the textbook entitled

A System Engineering Approach to Imaging (SPIE Press,

1998) [first printing 1998, second printing 2000]. He is the

Topical Editor of Marcel Dekker for the topic “Atmospheric

Optics” in their Encyclopedia of Optical Engineering. He has

co-authored a book entitled Applied Aspects of Optical

communication and LIDAR (CRC Press, 2010). His current

research interests include interactions of electromagnetic

waves with plasmas, optogalvanic effects, environmental

effects on optoelectronic devices, imaging system theory,

propagation of light through atmosphere, imaging through

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Copyright (c) 2014 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.

Sensors-8495-2013.R1 Accepted 6

atmosphere, image processing and restoration from blur,

imaging in presence of motion and vibration, light detection

and ranging (LIDAR), target acquisition, and image quality in

general. Kopeika was a recipient of the J. J. Thomson Prize

from the IEE in 1999. He is a fellow of SPIE.

Third C. Author, Yitzhak Yitzhaky

received his BS, MS, and PhD degrees in

electrical and computer engineering from

Ben Gurion University, Israel, in 1993,

1995, and 2000, respectively. From 2000

to 2002, he was a postdoctoral research

fellow at the Schepens Eye Research

Institute, Harvard Medical School,

Boston, Massachusetts. He is currently with the Electro-Optics

Unit at Ben Gurion University. His research is mainly in the

fields of image restoration and enhancement, image analysis,

and imaging systems.

Forth C. Author, Daniel Rozban

received the B.Sc. degree in Electronic

engineering from Ariel University of

Samaria, Ariel, Israel, in 2006, and the

M.Sc. degree in Electro-Optical

Engineering from Ben-Gurion University

of the Negev, Beer-Sheva, Israel, in 2009,

where he is currently pursuing the Ph.D.

degree. He is a Researcher with the

Submillimeter Wave Laboratory, Department of Electrical and

Electronic Engineering, Ariel University of Samaria.

Fifth E. Author, Amir Abramovich

received the B.Sc. and M.Sc. degrees in

Electrical and Computer Engineering

from Ben-Gurion University of the

Negev, Beer-Sheva, Israel, in 1989 and

1991, respectively, and the Ph.D. degree

from Tel-Aviv University, Tel-Aviv,

Israel, in 2001. He was with the Counter

Measures and Spectroscopic

Characterization Department, TAAS Israel Industries. He is a

Researcher and a member of Staff with the Department of

Electrical and Electronic Engineering, Ariel University Center

of Samaria, Ariel, Israel, where he heads the Millimeter and

Submillimeter Wave Laboratory. Dr. Abramovich was a

recipient of research grants to develop and construct THz

imaging systems for homeland security purposes and THz

spectroscopy for material recognition and identification.

This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.The final version of record is available at http://dx.doi.org/10.1109/JSEN.2014.2301933

Copyright (c) 2014 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.