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Photo Acoustic Effect
And its usage for spectroscopy
Photo-Acoustic effect principle
Modulated Light
Absorption: Ia = αI0
T 
C p V
Local Heating
V  V0 (  T )
Thermal Expansion
Pressure  (v ac
)( T )
Pressure Wave =
Sound wave
Discovery: A.G.Bell (1880)
Rotating disc
Modulated light
Thin Disk
Sun Light
Example applications of PA effect
Characterization of solid materials
Separation between different gases and
measuring gas concentration
Glucose level monitoring in blood
Imaging of blood vessels
Concentration of textile dyes
Concentration of soot particles in diesel engines
PA effect – wave equation
Volumetric Thermal Expansion =-1
[cm ]:
in volume with
Amount of light energy absorbed
1  V 
in thesample.
 
Also affects the light
beam itself
 1 2
 
   P 
(I ( ))
C p t
 vac t
Acoustic velocity
=> temporal delay
Specific heat =
Heat energy required to
increase temperature by T
Light beam considerations for
PA experiment
PA amplitude depends on light intensity
derivative -> light should be modulated\pulsed
Pulsed light is preferred:
 Low
average intensity (can use very short pulses)
 High derivative of CW beam requires high
frequency – attenuated by most liquids
Pulse requirement:
 Short
rise time -> higher derivative
 Short pulses – lower average energy (eye-safety)
 Narrow spatial beam
Light Beam
Spatial –
high absorption
Spatial –
low absorption
Semi-cylindrical wave
Pressure wave propagation
FDM simulation results
High absorption: α=11.6 [cm-1]
No reflections, no dispersion or medium attenuation
Pressure wave propagation
FDM simulation results
Low absorption: α=0.9 [cm-1]
No reflections, no dispersion or medium attenuation
Effect of absorption coefficient
Pressure wave at fixed location (FDM simulation)
Effect of spatial beam diameter
Additional considerations
For FDM simulation
Reflections from cuevette and pressure
wave generation in cuevette
 Dispersion of acoustic wave
 Medium MTF
 PZT’s spatial response
 Scattering effects
Pros & Cons for PA usage for
spectroscopy in the eye
 Doesn’t
depend on transmission or reflection of the light
beam – can work with opaque materials, higher
immunity to scattering effects
 May work in various wavelengths
 Signal depends on various characteristics of medium in
addition to absorption (heat capacity, acoustic velocity)
that may be used to improve detection
 Depends on light intensity derivative – may be used with
short pulses that have low average intensity
 Option to work in wavelength where good optical broadband detectors are hard to find
Pros & Cons for PA usage for
spectroscopy in the eye
 Requires
to find specific wavelength in which there’s
a good separation between signal of target proteins
and other proteins\solvent
 Requires a high-quality light source: pulses with very
short rise time