A Portable, Low-cost System for Optical Explosive Detection based

on a CMOS Camera

Ross N. Gillanders

, Iain A. Campbell

, Fei Chen

, Paulina O. Morawska

, Ifor D. W. Samuel

and Graham A. Turnbull

Organic Semiconductor Centre, SUPA, School of Physics & Astronomy, University of St Andrews,

Fife KY16 9SS, Scotland

Key Laboratory for Flexible Electronics, Institute of Advanced Materials (IAM), Nanjing Tech University,

30 South PuZhu Road, NanJing, 211816, P. R. China

Keywords: Explosives, Conjugated Polymer, Organic Semiconductors, Raspberry Pi, Imaging.

Abstract: Humanitarian demining requires a variety of methods and instrumentation for effective mine clearance,

since a wide range of materials are used in mine manufacturing. However, landmines release vapours over

time that can be detected, for example, by sniffer dogs. Optical sensor systems are especially suited to this

application due to the potential for lightweight, portable, low-cost systems that nevertheless have fast

response times and ppb-level sensitivity to explosive vapours. In this paper we present a system for

detection based on a low-cost Raspberry Pi platform with an integrated CMOS camera. The conjugated

polymers Super Yellow and Polyfluorene are excited by an LED, and the quenching effect by DNB vapour

is monitored by the camera to indicate the presence of explosives. The system shows potential as a user-

friendly, lightweight platform for explosive vapour sensing.

1 INTRODUCTION

The tools required for humanitarian demining must

be able to operate across a wide range of

environments, since legacy landmines remain in

countries with extremely disparate climates. These

climates can range from Mediterranean, such as that

in Croatia; to dry desert climates in Africa; to

monsoon-prone countries, for instance those in

South East Asia. Different tools for demining have

different degrees of success in the various

environments. The most common methods for mine

detection include metal detectors and sniffer dogs;

the combination of both is often used in the field.

However, these methods can have disadvantages:

metal detectors can identify harmless fragments of

metal, or, with potentially fatal consequences, be

unable to detect a mine made from another material

such as plastic. Canines have strict working

practices with the time the dogs can spend sniffing

for mines restricted by both guidelines and the

willingness of the dog at any particular moment

(Porritt et al., 2015). Other common methods are

inherently dangerous, such as prodders; thus there is

a need in humanitarian demining for advanced,

sensitive, robust instrumentation (Newnham and

Daniels, 2001). In this paper we present a portable

optoelectronic sensor based on explosive vapour

detection.

Mines can release small amounts of vapour for a

long time after being buried – for example,

trinitrotoluene (TNT) degrades over time to produce

dinitrotoluene (DNT), which then is released at

ground level. There has been extensive work done

on explosive sensing with conjugated polymers

(Wang et al., 2011a; Wang et al., 2011b; Wang et

al., 2012; Narayanan et al., 2008; Thomas et al.,

2007), which are electron-rich materials used in

organic semiconductor research. If nitroaromatic

explosive vapours adsorb onto the polymer surface

while these polymers are photoexcited by incident

light, electrons are transferred to the deficient

nitroaromatic vapour, resulting in loss of fluorescent

emission, as shown in Figure 1. The luminescence

can be monitored in real-time to provide a

quenching profile, thus indicating the presence of

explosive vapours.

One of the chief advantages to optical sensing is

that it can be achieved through relatively low-cost,

portable and fast-responding instrumentation. These

132

Gillanders, R., Campbell, I., Chen, F., Morawska, P., Samuel, I. and Turnbull, G.

A Portable, Low-cost System for Optical Explosive Detection based on a CMOS Camera.

DOI: 10.5220/0005739801300134

In Proceedings of the 4th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2016), pages 132-136

ISBN: 978-989-758-174-8

factors are crucial in humanitarian demining, since

the work is often conducted in challenging

environments. Additionally, the operators of such

devices will not necessarily be trained scientists or

technicians, so ultimately the operation of the

instrument should be user-friendly, with clear and

distinctive data visualisation.



Figure 1: Quenching mechanism for conjugated polymer

by explosive vapour.

While many fluorescence-based sensing

platforms successfully utilise low-cost electronics,

such as photodiodes, recent years have seen the

introduction of off-the-shelf consumer computing

platforms including the Arduino family of

microprocessors and the Raspberry Pi. Use of these

platforms in analytical instrumentation has gradually

begun to appear in literature (Bitella et al., 2014;

Tonacci et al., 2015; Leeuw et al., 2013), since they

offer advantages in cost, open-source hardware and

software, user-friendliness, small footprint, and

flexibility in deployment. Smartphones have

recently become tools for chemical sensors (Comina

et al., 2015), and since the CMOS camera used in

conjunction with the Raspberry Pi processor is

similar to those found in typical smartphones, the

technology presented here may eventually be

integrated with them, broadening and making more

accessible the system for communities in need of

easy-to-use, low-cost, portable optical sensor

systems for mine detection. While the change in

luminescence intensity from the sensor films can be

monitored using a standard silicon photodiode, this

generally allows only the measurement of a single

wavelength from a single film. The ability to

measure multiple luminescent sensor spots with a

single detector would present advantages in both

simultaneous sensing of multiple analytes, and in

selectivity to identify the, in this case, various

nitroaromatic groups. The use of a camera system

allows for this multi-analyte detection in the future

since the principle of the image processing, reported

here, can be applied to more than one emission spot

in a single image. This system could prove useful in

a “demining toolbox” in conjunction with other

technologies to allow a deminer to make a choice of

complementary systems appropriate for the

environment. For instance, a metal detector could

identify suspect sites, and the air sampled above

those sites for trace explosive vapours. Since the

system would also be suitable for sensing other

analytes, it may be useful for environmental, food

quality or other applications in the future.

In this paper we present an optical sensor system

using the Raspberry Pi platform in conjunction with

a CMOS camera, excitation LED, Polyfluorene

(PFO) and Super Yellow (SY) conjugated polymers

to detect dinitrobenzene (DNB) vapour. Early results

indicate that the system could ultimately be applied

for the detection of buried landmines.

2 EXPERIMENTAL

2.1 Film Fabrication &

Characterisation

Films based on Merck Super Yellow and PFO were

prepared by spin-coating a 6.5mg/ml solution of the

polymer in toluene at 2000rpm onto a glass slide.

The film thicknesses were measured with a Veeco

Dektak 150 surface profiler and found to be 100nm

thick on average. Absorption and

photoluminescence spectra were measured,

respectively, using a Cary 300 Bio UV-Vis

spectrometer and an Edinburgh Instruments FLS980

Fluorescence spectrometer. The peak absorption

wavelengths for PFO and SY were 384nm and

440nm respectively. Peak emission wavelengths

were 420nm for PFO and 590nm for SY.

Photoluminescent Quantum Yield (PLQY) was

measured with a Hamamatsu Photonics C9920-02

integrating sphere with excitation wavelengths of

384nm for PFO and 440nm for SY. The PLQYs of

the polymers were measured to be 46% for PFO, and

40% for SY.

2.2 Hardware

The excitation LED for each polymer was chosen to

match the absorption peak as closely as possible,

with a Royal Blue LUXEON LED from Philips

selected with a central wavelength of 447.5nm for

SY and a 370nm-centred LED from Thorlabs for the

PFO excitation. The appropriate LED was aligned

with the sample and detector as shown in Figure 2:

the excitation light was filtered using a high-pass

A Portable, Low-cost System for Optical Explosive Detection based on a CMOS Camera

133

filter, and a neutral density filter of optical density 1

or 2 was inserted to reduce any saturation effects on

the CMOS camera if necessary. The CMOS camera

module from Raspberry Pi has 5 megapixels and a

fixed focus.



Figure 2: Hardware set-up schematic. A Royal Blue

LUXEON LED excites the Super Yellow film, and the

High-Pass filter removes the source light. The emitted

light is collected by the camera and processed on-board

the Raspberry Pi platform.

2.3 Experimental Set-up

The polymer-coated slide was inserted into an in-

house designed airtight chamber with gas flow inlet

and outlet, and quartz windows on opposing sides

for light excitation and emission. Powdered

dinitrobenzene (Sigma Aldrich) was placed in a

glass tube with a nitrogen line flowing through the

powder, then via control valves leading to the

chamber inlet. To make a measurement, the nitrogen

flow was turned on and the luminescent emission

monitored in real time with the camera. The images

were taken at 20 second intervals. Measurements

were conducted at room temperature.

2.4 Image Processing

The image processing was performed in Python

using open source libraries from Raspberry Pi.

Briefly, the system works by taking the original

image, prior to binarization, i.e assigning “light” and

“dark” regions to the image. Subsequently automatic

circle detection defined the emission area, and

finally automatic Region-of-Interest selection is used

to define the monitored signal. The process is

illustrated in Figure 3, with the decision-making

flow-chart shown in Figure 4.

Figure 4: Flowchart for image processing.

Figure 3: Image Processing with an illuminated Super Yellow film.

PHOTOPTICS 2016 - 4th International Conference on Photonics, Optics and Laser Technology

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3 RESULTS

Figure 5 shows the quenching profile of the Super

Yellow film to DNB vapour at a vapour pressure of

approximately 30 ppb(Ewing et al., 2013). A clear

and measurable decrease in luminescent intensity is

shown, with the quenching effect apparent after the

first 20 seconds. The intensity decreases by

approximately 6% in 200 seconds.

Figure 5: Quenching profile of Super Yellow exposed to

DNB vapour in nitrogen.

The response of PFO to DNB is shown in Figure

6. The quenching response is slightly lower than that

of SY at around 4.5%.

Figure 6: Quenching profile of PFO exposed to DNB

vapour in nitrogen.

These results indicate that the CMOS camera

system is a suitable method for optical detection of

ppb-level explosive vapours. The inherent difference

between the two polymers’ response to the DNB

vapour suggests that both could be monitored

simultaneously, leading to selective measurements

for identification of explosive vapours. The system

reported here is lightweight, inexpensive and low-

cost in addition to its sensitivity, so further work is

ongoing to ruggedize the system for field trials,

build a library of polymer-vapour response curves,

and investigate simultaneous multi-analyte sensing.

4 CONCLUSIONS

An optical sensing system was developed using an

off-the-shelf Raspberry Pi processing platform in

conjunction with a CMOS camera, using the

conjugated polymers Super Yellow and Polyfluorene

as the explosive vapour-sensitive sensor films. The

quenching effect of the vapour on the polymers’

luminescence was successfully monitored by the

camera and profiles generated over around 200

seconds of vapour adsorption to the polymer surface.

The system shows promise to be used as a portable,

user-friendly explosive vapour sensor in

humanitarian demining, and in this be useful

complementing other methods such as metal

detectors to more thoroughly detect and identify

buried landmines. Improvements and further work to

the system have been identified towards

development of a robust, highly-sensitive

instrument.

ACKNOWLEDGEMENTS

This project has received funding from the European

Union’s Seventh Framework Programme for

research, technological development and

demonstration under agreement no 284747.

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