390
5 Laser Specklometer, Speckle-Optical Diagnostics and Laser …
the spectrum asymmetry coefficient K a in the band from f 1 to f 2 relative
to the intermediate frequency f m , which can be determined as
K a =
f 2
f m
A( f )d f
f m
f 1
A( f )d f
(5.6)
the band coefficient K b (ratio of the sum of high-frequency harmonics
in the frequency band from f 3 to f 4 to the sum of low-frequency ones
in the band from f 1 to f 2 ):
K b =
f 4
f 3
A( f )d f
f 2
f 1
A( f )d f
(5.7)
the coefficient K b of the overtones of frequency of muscle contractions,
which can be determined using (5.7) where f 1 = 21 Hz, f 2 = 26 Hz, f 3
= nf 1 , f 4 = nf 2 , and n changes in the range from 2 to 5.
The work of the LS is based on the principle of registration of time changes of the
speckle-field formed by dynamically active scattering surface and further spectral
analysis of the obtained data: f , S, K a , K b .
Optical-electronic part of the LS consists of the following units: a single-mode
gas laser LGN-208A; an adjustment mechanism; a light guide; PMT; and a power
indicator. Irradiation of the surface under study and receiving of the scattered radiation are provided by two light guides with a diameter of the active part of the fiber
of 4 μm. A single-mode gas laser (the wavelength is 0.63 μm) serves as a source of
coherent radiation. Distance between the face of the irradiating light guide and the
surface of the object under study is from 8 to 12 mm, and the size of the light spot on
the skin at a distance of 10 mm to the face of the irradiating light guide is not more
than 2 mm. Construction of the light guide unit is shown in Fig. 5.7.
In the preliminary processing unit (PPU), the registered PMT signal is intensified,
filtered and converted into a digital form and transmitted through the serial line into
the PC, which stores the necessary amount of the data in the memory. Then, the examination of the obtained time realization, the calculation of spectrum in the needed
frequency range with giving of spectral characteristics and the data documentation
can be organized. The structural scheme of the LS is in Fig. 5.11.
The main controlled parameter is the average frequency f of the main spectral
harmonic, calculated due to the regulated intensity fluctuations of the speckle-field,
which was formed during reflection of laser radiation from the diffuser, which is
situated on the reference vibrator, oscillation frequency f e of which is in the range
of 15–50 Hz, and the oscillation amplitude does not exceed 5 μm. The average
frequency f must coincide with f e with the accuracy of 10%. Control should be
conducted at three meanings f e in the range of 15–50 Hz.
During determination of the value of the main controlled parameter, such a value
of oscillation amplitude of the reference vibrator is set, at which the amplitude of
5 Laser Specklometer, Speckle-Optical Diagnostics and Laser …
the spectrum asymmetry coefficient K a in the band from f 1 to f 2 relative
to the intermediate frequency f m , which can be determined as
K a =
f 2
f m
A( f )d f
f m
f 1
A( f )d f
(5.6)
the band coefficient K b (ratio of the sum of high-frequency harmonics
in the frequency band from f 3 to f 4 to the sum of low-frequency ones
in the band from f 1 to f 2 ):
K b =
f 4
f 3
A( f )d f
f 2
f 1
A( f )d f
(5.7)
the coefficient K b of the overtones of frequency of muscle contractions,
which can be determined using (5.7) where f 1 = 21 Hz, f 2 = 26 Hz, f 3
= nf 1 , f 4 = nf 2 , and n changes in the range from 2 to 5.
The work of the LS is based on the principle of registration of time changes of the
speckle-field formed by dynamically active scattering surface and further spectral
analysis of the obtained data: f , S, K a , K b .
Optical-electronic part of the LS consists of the following units: a single-mode
gas laser LGN-208A; an adjustment mechanism; a light guide; PMT; and a power
indicator. Irradiation of the surface under study and receiving of the scattered radiation are provided by two light guides with a diameter of the active part of the fiber
of 4 μm. A single-mode gas laser (the wavelength is 0.63 μm) serves as a source of
coherent radiation. Distance between the face of the irradiating light guide and the
surface of the object under study is from 8 to 12 mm, and the size of the light spot on
the skin at a distance of 10 mm to the face of the irradiating light guide is not more
than 2 mm. Construction of the light guide unit is shown in Fig. 5.7.
In the preliminary processing unit (PPU), the registered PMT signal is intensified,
filtered and converted into a digital form and transmitted through the serial line into
the PC, which stores the necessary amount of the data in the memory. Then, the examination of the obtained time realization, the calculation of spectrum in the needed
frequency range with giving of spectral characteristics and the data documentation
can be organized. The structural scheme of the LS is in Fig. 5.11.
The main controlled parameter is the average frequency f of the main spectral
harmonic, calculated due to the regulated intensity fluctuations of the speckle-field,
which was formed during reflection of laser radiation from the diffuser, which is
situated on the reference vibrator, oscillation frequency f e of which is in the range
of 15–50 Hz, and the oscillation amplitude does not exceed 5 μm. The average
frequency f must coincide with f e with the accuracy of 10%. Control should be
conducted at three meanings f e in the range of 15–50 Hz.
During determination of the value of the main controlled parameter, such a value
of oscillation amplitude of the reference vibrator is set, at which the amplitude of