COL 9(9), 092901(2011) CHINESE OPTICS LETTERS September 10, 2011
Theoretical studies on bioaerosol particle size and shape
measurement from spatial scattering profiles
Chunxia Feng (
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1,2
, Lihua Huang (
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1
, Jianbo Wang (
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1,2
,
Yongkai Zhao (
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1
, and Huijie Huang (
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)
1∗
1
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China
2
Graduate University of Chinese Academy of Sciences, Beijing 100049, China
∗
Corresponding author: huanghuijie@siom.ac.cn
Received March 22, 2011; accepted April 4, 2011; posted online June 16, 2011
A method of clarifying bioaerosol particles is proposed based on T-matrix. Size and shape characterizations
are simultaneously acquired for individual bioaerosol particles by analyzing the spatial distribution of
scattered light. The particle size can be determined according to the scattering intensity, while shape
information can be obtained through asymmetry factor (AF). The azimuthal distribution of the scattered
light for spherical particles is symmetrical, whereas it is asymmetrical for non-spherical ones, and the
asymmetry becomes intense with increasing asphericity. The calculated results denote that the 5
◦
–10
◦
scattering angle is an effective range to classify the bioaerosol particles that we are concerned of. The
method is very useful in real-time environmental monitoring of particle sizes and shapes.
OCIS codes: 290.5850, 290.1090.
doi: 10.3788/COL201109.092901.
The field of biological agent detection has recently re-
ceived considerable interest due to the high risk it poses
to personnel even at extremely small amount of agent
dose
[1−3]
. As a consequence of the high degree of lethal-
ity, it is essential to develop a new method that is espe-
cially sensitive to biological agents. It must be capable
of detecting low levels or even single particle agents.
Among the many particle characteriza tion techniques,
optical methods offer genuine real-time non-destructive
particle analysis and are highly cost effective. Fluores-
cence emission by particles is mainly used to discrimi-
nate b etween biological and non-biological pa rticles
[2,3]
.
Simultaneously, the magnitude or the total scattering
intensity of the elastic light scattering c an be used to de-
termine the particle size
[4−6]
, while the rate o f generation
of the light pulses can be related to particle number and
particle concentration within the sampled atmosphere
[7]
.
However, a further problem arises from the fact that
biological agent particles have similar size a nd fluores-
cence characteristics, but differ ent morphology cannot be
differentiated. Therefore, particle shape proved to be a
more effective method to discrimina te between airborne
particles of different types. Since the spatial pattern of
the scattered light contains particle shape information
[8]
,
further analysis of the elastic light scattering may be
effective in cla ssifying ae rosol particles by their shapes.
The T-matrix method is one of the most versatile,
efficient, and widely used theoretical technique for com-
puting the light-scattering properties of nonspheric al
particles based on the solution of Maxwell’s equations.
The T-matrix can be considered as a transfer matrix,
which transforms incoming electromagnetic wave into
scattered electromagnetic wave
[9]
. The T-matrix can
be applied to the analysis of e lectromagnetic scatter-
ing by homogeneo us and composite particles, clusters of
particles, discrete random media, and particles in the
vicinity of an interface separa ting two half-spaces with
different refractive indices
[10]
. This approach has been
applied to simulations of, for example, the optical prop-
erties of Bacillus subtilis spores, biconcave red blood
cells, and living bacterial cells
[11−13]
, as well as soot clus-
tered agglomerates
[14]
. Several expe riments have also
been devoted to the field of single-particle spatial light
scattering
[15,16]
. However, only few researches have re-
ferred to the shap e analysis of biological agents, which is
the subject o f this letter.
One of the problems with the characterization of
bioaeros ol lies in determining their exact refr active in-
dex. For homogeneo us biological cells, the refractive in-
dex n = n
r
+ in
i
is treated as a constant within the c e lls,
where n
r
and n
i
are the real and imaginary parts of the
refractive indx, respectively. Since different bioaerosols
have simila r compositions, we estimate their refrac tive
indices to be the sa me. The typical refractive indices n
r
of Bacillus subtilis spore range from 1.51 to 1.54 at λ =
0.589 µm
[12]
, thus 1.52 and 0.017 are chosen as the real
and imaginary parts of the refractive index of bioaerosol,
respectively
[17]
.
The manner in which a particle spatially scatters in-
cident light is a complex function of the size, shape,
composition, and orientation of the particle, as well as of
the properties of the illuminating radiation (wavelength,
polarization)
[18]
. As depicted in Fig. 1, the particle sit-
uates with the semi-major a xis along the positive y axis
of the lab oratory coordinate frame, and it is illuminated
with the circularly polarized beam dir ected along the
z axis and perpendicula r to the y axis. This is an im-
portant attribute tha t the pa rticles display in actual
measurement
[15]
. We assume that the refractive index of
the particle in the sample is known and unifor m, which is
true in most cases. In this way, the scattered light inten-
sity is only a function of scattering angle, particle shape,
and particle size
[19,20]
. Then through analysis of the an-
gular scattering pattern, the particle size and shape can
be theoretica lly obtained
[21−24]
. In this letter, spherica l
and ellipsoidal models are used to investigate individual
1671-7694/2011/092901(4) 092901-1
c
2011 Chinese Optics Letters