Note: Descriptions are shown in the official language in which they were submitted.
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Method OF LASER EMISSION SPECTROSCOPICALLY ANALYSIS
AND APPARATUS THEREFORE
BACKGROUND OF THE INVENTION
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1. Field of the Invention
This invention relates to a method of laser emission
spectroscopically analysis and an apparatus therefore and more
particularly to improvements in a method of laser emission
spectroscopically analysis and an apparatus therefore wherein a
light emitted from the surface of a sample when a laser beam
irradiates the surface of the sample is spectroscopically
analyzed, and which are suitable for use in direct analysis of
hot metal, molten steel, slag and the like.
2. Description of the Prior Art
With the progress of the laser technique, in various fields,
there have been seen such an attempt that the laser may be
utilized as a source of excitation to conduct an emission
spectroscopically analysis. More specifically, when a powerful
laser beam adapted to take the focus on the surface of a sample
by a focusing lens having a suitable focal length irradiates the
surface of the sample, a surface layer is rapidly heated.
Particularly, if the laser beam is formed into pulse shapes of
several ten nanosecond, then such conditions occur that energy
is locally poured into the sample before heat is diffused in the
sample, whereby melting and evaporation occur. Vapor is further
excited by the laser beam to be formed into plasma which emits
a light. According to the method of laser emission
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spectroscopically analysis, this light is transmitted to a
spectroscopes by means of a suitable light introducing system,
spectrally separated by a diffraction grating and the like and
formed into spectra, and thereafter, detected by a photographic
film, a ph~tomultiplier tube; a photo-diode and the like,
whereby contents of aimed elements are determined. This method
has such outstanding characteristic features that this method is
also applicable to non-electrically conductive materials, can
make analysis in atmosphere and so on. However, this method has
been disadvantageous in that variations in data are high and the
accuracy of analysis is unsatisfactory. The major cause is
generally considered to reside in the variations in the output
intensity of the laser, with the result that such a method has
been normally adopted that the intensity of laser is constantly
monitored by a suitable method and data are normally collected
only within a predetermined range. However, this method has
been disadvantageous in that it is impossible to cope up with
changes in the distribution of the intensities of laser beams
and the adverse influence of the variations in the evaporation
and excitation processes due to the contour of the surface of
the sample and also due to the presence of contaminations, an
oxide layer and the like cannot be removed.
SUMMARY OF TOE INVENTION
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The present invention has been developed to obviate the
above-described disadvantages of the prior art and has as its
object the provision of a method of laser emission
spectroscopically analysis and an apparatus therefore wherein an
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analysis can be conducted with high accuracy irrespective
of variations in the output intensity of laser and the like.
According to the present invention, there is pro-
voided a method of laser emission spectroscopically analysis,
wherein light emitted from a surface of a sample when
a pulsed laser beam irradiates the surface of the sample is spectra-
scopically analyzed, characterized in that said method in-
eludes the steps of:
(i) fixing the emission mode of the laser beam to
lo a gauss distribution type Tempo mode; and
(ii) measuring the spectral line intensity
of a sample element only when the intensity ratio
between a pair of preset spectral lines in the light
emitted from the sample is within a preset
range.
The present invention has been achieved on the
basis of the results of survey made by the present inventors
on variations in intensity of spectral line, which variations
are regarded as a drawback in the laser emission spectra-
scopical analysis. More specifically, the followings were
clearly known by the survey made by the present inventors.
(1) If an output from a laser oscillator is increased,
then the coefficient of variation (one obtained by dividing
a standard deviation by a mean value) of the intensity of
a spectral line decreases. However, if an amplifier is
positioned in a stage posterior to the laser oscillator and
the output is further increased, then the coefficient of
variation of the intensity of the spectral line increases,
to the contrary.
(2) When the amplifier is not used and a laser beam is
obtained only by a laser oscillator, the variations of the
intensity of spectral line is higher than the variations of
the output of laser.
According to the present invention, there is also
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provided an apparatus for 'maser emission spectroscopically
analysis, comprising:
a laser oscillating means for oscillating a
pulsed laser beam, wherein the emission mode of said laser
oscillating means is fixed to a gauss distribution type
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an amplifying means for amplifying the laser
beam oscillated from said laser oscillating means;
a focusing lens for focusing the laser beam
emitted from said amplifying means onto a surface of a
sample;
a spectral separating means for spectrally sepal
rating light emitted from the surface of said sample;
a monitoring light detecting means for measuring
the intensity of a preset spectral line in the light emit-
ted from the sample;
a measuring light detecting means for measuring
the intensity of a spectral line of a sample element and
converting the spectral line into an electric signal;
: 20 a spectral line intensity monitor for determine
in whether the intensity of the preset spectral line
in the light emitted from said sample is within a preset
range in accordance with an output from said monitoring
light detecting means;
a gate means to be opened by said spectral line
intensity monitor when the intensity of the preset spectral
line in the light from said sample is within the preset
range; and
an indicating means for obtaining and indicating
a measurement of the output of said measuring light detect
tying means, when said gate means is opened.
According to the invention, there is also pro-
voided an apparatus for laser emission spectroscopically
analysis, comprising:
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a laser oscillating means for oscillating
a pulsed laser beam, wherein the emission mode of said
laser oscillating means is fixed to a gauss distribution
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an amplifying means for amplifying the laser
beam oscillated from said laser oscillating means;
a focusing lens for focusing the laser beam
emitted from said amplifying means onto the surface of
a sample;
a spectral separating means for spectrally
separating light emitted from the surface of said sample;
dual monitoring light detecting means for
measuring the intensities of a pair of preset spectral
lines in the light emitted from the sample
a measuring light detecting means for measuring
the intensity of a spectral line of a sample element and
converting the spectral line into an electric signal;
a spectral line intensity monitor for determine
in whether the intensity ratio between the preset spectral
lines in the light emitted from said sample is within a
preset range in accordance with the output of the monitoring
light detecting means;
a gate means to be opened by said spectral
line intensity monitor when the intensity ratio between
. the pair of preset spectral lines in the light from said
sample is within the preset range; and
an indicating means for measuring and indicating
the output of said measuring light detecting means, when
said gate means is opened.
BRIEF DESCRIPTION OF THE DRAWINGS
The exact nature of this invention, as well
as other objects and advantages thereof, will be readily
apparent from consideration of the following specification
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relating to the accompanying drawings, in which like reference
characters designate the same or similar parts throughout
the figures thereof and wherein:
Fig. 1 is a chart showing the relationship between
the laser output and 271.4 nanometer wavelength Fe spectral
line intensity;
Fig. 2 is a chart showing a comparison between the
laser output, the coefficient of variation of the laser out-
put and the coefficient of variation of 271.4 nanometer
wavelength Fe spectral line intensity;
Fig. 3 is a block diagram showing the arrangement
of one embodiment of the laser emission spectroscopically
analysis, in which is adopted the method of laser emission
spectroscopically analysis according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Figs. 1 and 2 show the results of experiments.
The sample used in the experiments was an Fe alloy. As the
laser beam, an infrared laser having a wavelength of 1.06
micrometer and a pulse width of 15 nanosecond was used. As
apparent from Fig. 1, when the output of laser exceeds an
output 2 joule obtained through the utilization of an amply-
lien, the coefficient of variation suddenly increases. The
cause is considered to reside in the intensity distribution
of laser pulses. More specifically, in general, in the
intensity distribution (mode) of a laser beam emitted from
a laser oscillator in the direction of vertical section,
there are various symmetries. When such a high output as
used in the emission spectroscopically analysis is required,
there is adopted a multi-mode oscillation in which modes
are changed over from one to another per pulse. However, if
an amplifier is actuated when peaks of several intensity
distributions are present in a beam as described above, then
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a specified peak becomes amplified preferentially. In
consequence, if the laser beam having the above-described
intensity distributions is converged onto the surface of the
sample, then, in the spot caliber thereon, a zone of a high
irradiation density occurs locally. Since such wide flue-
tuitions in the irradiation density are varied from a pulse
to another, the intensities of emission spectra are varied.
Then, the present inventors thought of also fixing the laser
beam used in the emission spectroscopically analysis to the
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mode in which the gauss distribution type output is obtainable
through the utilization of a mode lock method used in the
holography and the like. Although Timely mode and the like in
which an annular distribution is obtainable may be utilized as
such a mode described above, it is considered that the gauss
distribution is most suitable for the emission spectroscopically
analysis. Additionally, if the mode lock is adopted, the laser
output decreases. However, an output of the laser oscillator is
increased and another amplifier is added for use, so that this
decrease in the laser output can be made up for. Needless to
say, when the laser beam is fixed to Tempo mode, even if the
amplifier is utilized, the mode is not changed.
On the other hand, Fig. 2 shows a comparison between the
coefficient of variation (solid line A) of 271.4 nanometer
wavelength Fe spectral line intensity and the coefficient of
variation (solid line B) of the laser output when a laser beam
identical with that shown in Fig. l irradiates an Fe alloy. In
spite of non-use of the amplifier, the variations in intensity
of spectral line are larger than the width of variation of the
laser output. The above-described variation in intensity of
emission spectra can ye improved by the aforesaid mode lock. In
addition to this, the influences of wide fluctuations in an
evaporation and an excitation processes occurring on the surface
of the sample during the irradiation of laser cannot be
disregarded. Since the latter cannot be removed only by
controlling the laser output and the mode, it has been thought
of that, in the case of a predetermined spectral line such as an
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Fe alloy which is obtainable during the irradiation of laser,
one or two of Fe spectral lines each having a suitable
wavelength are selected, the intensity or the intensity ratio
thereof is constantly monitored, and the spectral line intensity
of an aimed element to be measured is read only within the
preset variation range. Particularly, the letter method of
regulating the intensity ratio can restrict the temperature of
the plasma produced by the irradiation of laser, thus being
effective in improving the accuracy of analysis.
The present invention has been attained on the basis of the
above-described knowledge.
According to the present invention, an analysis with high
accuracy can be conducted irrespective of the variations in the
laser output intensity and the like.
While 13.0 % was obtained when the coefficient of variation
of the spectral line intensity measured according to the
conventional method in measuring So spectral line intensity of
288.2 nanometer wavelength in an Fe alloy, the coefficient of
variation was improved to 9.0 % when the laser output was fixed
to Tempo mode. Further, according to the present invention, the
laser output was fixed to Tempo mode, and only when the 271.4
nanometer wavelength Fe spectral line intensity being monitored
was within the range of plus-minus 5 % of a predetermined value,
288.2 nanometer wavelength So spectral line intensity was read,
then 7.3 % was obtained. Furthermore, also, according to the
present invention, the laser output was fixed to Tempo mode, two
Fe spectral lines of 271.4 nanometer wavelength and 273.1
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nanometer wavelength were monitored, and So spectral line
intensity was read when the intensity ratio thereof was
within the range of plus-minus 5 of a predetermined value,
then 5.9 was obtained. It was ascertained that, in the
cases of the both methods as described above, the coefficient
of variation in the spectral line intensity became small as
compared with the case of the conventional method or the
case of only fixing the laser output to Tempo mode.
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Detailed description will hereunder be given of one
embodiment of the apparatus of laser emission spectroscopically
analysis, in which is adopted the method of laser emission
spectroscopically analysis according to the present invention with
reference to the drawings.
As shown in Fig 3, this embodiment comprises:
a laser oscillator 10 assembled whereinto with a mechanism
for the mode lock, not shown, for oscillating a laser beam;
an oscillator power source 11 for driving the laser
oscillator 10;
a power source 12 of a high speed switching element, for
actuating a high speed switching element not shown, assembled
into the laser oscillator 10;
an up collimator 13 for expanding a beam caliber of a laser
beam oscillated from the laser oscillator 10;
an amplifier 14 for amplifying the laser beam emitted from
the up collimator 13;
an amplifier power source 15 for driving the amplifier 14;
a rectangular prism 16 for changing a direction of
irradiation of the laser beam emitted from the amplifier 14;
a focusing lens 18 for converging the laser beam onto the
surface of a sample 20;
a spectroscopes optical system 24 formed of a concave mirror
for example, for forming an image of a light emitted by the
irradiation of laser from the surface of the sample 20 directed
in a predetermined direction relative to the laser beam at an
inlet slit 26 of a spectroscopes 22;
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the electroscope 22 for separating the light emitted from
the sample 20 whose image is formed Nat the inlet slit 26 into
spectral lines by a diffraction grating and the like;
one or two monitoring light detectors 28 for detecting the
intensity of a spectral line having a wavelength for monitoring
out of the spectral lines separated by the spectroscopes 22 to
convert the same into an electric signal;
a measuring light detector 30 for detecting the intensity of
a spectral line of an element to be measured to convert the same
into an electric signal;
a spectral line intensity monitor 32 for detecting whether
an intensity of a preset spectral line or an intensity ratio
: between a pair of preset spectral lines in the light emitted
from the sample is within a preset range in accordance with an
; 15 output from the light detector 28 for monitoring;
a gate circuit 34 to be opened by the spectral line
intensity monitor 32 when the intensity of the preset spectral
line or the intensity ratio between the pair of preset spectral
lines in the light emitted from the sample is within the preset
range; and
an indication section 36 for obtaining a measured value from
an output of the light detector 30 for measuring, when the gate
circuit 34 is opened, to indicate the same.
As the laser oscillator 10, a ruby laser having a wavelength
of 0.69 micrometer or an infrared laser having a wavelength of
1.05 to 1.06 micrometer, for example.
To control the density of laser so as to less than a
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predetermined value, a laser substance used in the amplifier 14
is larger in caliber than that of the laser oscillator 10. The
up collimator 13 is used to offset the differences in caliber of
these laser substances.
The rectangular prism 16 is used to cause the laser to
irradiate the surface of the sample 20 at an angle of a
predetermined value, and, when a laser emission section
including a laser oscillator 10 is provided in a predetermined
direction from the beginning, the rectangular prism 16 can be
dispensed with.
Description will now be given of action.
Firstly, electric energy accumulated in the power source 11
for the oscillator and a power source 15 for the amplifier is
transmitted to the laser substances in the laser oscillator 10
and the amplifier 14 to excite the both laser substances. When
the energy of a predetermined value is accumulated in the laser
substance of the laser oscillator 10, a signal is delivered to
the power source 12 for the high speed switching element to
actuate the same, and the energy accumulated in the laser
oscillator 10 is released at once. The mode lock mechanism is
assembled into the laser oscillator 10, and the released
pulse-shaped laser beam is formed into the gauss distribution
type Tempo mode. The laser beam oscillated from the laser
oscillator 10 falls into the amplifier 14 through the up
collimator 13. Then, the energy accumulated in this amplifier
14 is also released in a moment, whereby the laser beam is
further intensified. The powerful laser pulses thus obtained
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are converged onto the surface of the sample 20 through the
rectangular prism 16 and the focusing lens I Then, the
surface of the sample 20 is locally heated for a short period of
time to thereby be formed into a plasma. At this time, the
light emitted from the plasma is led to the spectroscopes 22 by
the spectroscopes optical system 24, and dispersed by the
diffraction grating and the like of the spectroscopes 22. Out of
dispersed lights, a spectral line having a specified wavelength
for monitoring is detected by the monitoring light detector 28,
and in the spectra] line intensity monitor 32, it is detected
whether the intensity of the preset spectral line or the
intensity ratio of the pair of preset spectral lines is within
the preset range or not. Only when the intensity of the
spectral line or the intensity ratio is within the preset range,
the gate circuit 34 is opened, and an output from the measuring
light detector 30 which has received the spectral line of the
; element to be measured is delivered to the indication section
36, where the result of measurement is indicated. The intensity
of the spectral line of the element to be measured thus obtained
is data processed by an ordinary method.
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