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Publication numberUS3697180 A
Publication typeGrant
Publication dateOct 10, 1972
Filing dateDec 7, 1970
Priority dateDec 8, 1969
Also published asDE2060409A1
Publication numberUS 3697180 A, US 3697180A, US-A-3697180, US3697180 A, US3697180A
InventorsIrizuki Mamoru, Mori Hajime
Original AssigneeJeol Ltd
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Apparatus for automatically recording the hedepolarization ratios of raman bands
US 3697180 A
Abstract
Two rays of light polarized parallel and perpendicular with respect to a plane are produced from a single ray of light generated by a laser. By alternately illuminating a sample with the said two rays of light, two Raman scattering light rays having different intensities are produced and in turn applied to a monochromator where they are dispersed according to wave length. After dispersion, the rays are detected by a photomultiplier and the resultant signals applied to a division circuit where the depolarization ratio is automatically calculated. This process is repeated for subsequent sets of signals, thereby effecting a continuous operational sequence.
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United States Patent Moriet al.

[ APPARATUS FOR AUTOMATICALLY RECORDING THE HEDEPOLARIZATION RATIOS OF RAMAN BANDS [72] Inventors: Hajime Mori; Mamoru Irizuki, both of Tokyo, Japan 73 Assignee: Nihon Denshi Kabushiki Kaisha,

.Tokyo, Japan [22] Filed: Dec. 7, 1970 [21] Appl. No.: 95,700

30 Foreign Application Priority Data Dec. 8, 1969 Japan ..44/98527 Feb. 28, 1970 Japan ..45/17383 [52] US. Cl. ..356/75, 356/114, 356/116 [51] Int. Cl ..G0lj 3/44, GOln 21/40 [58] Field of Search ..356/75, 103, 104, 114-119 Oct. 10,1972

OTHER PUBLICATIONS Murphy et al., The Journal of Chemical Physic, Vol. 47, No. 5, Sept. 1, 1967, pp. 1836- 1839.

l-lawes et al., Analytical Chemistry, Vol. 38, No. 13, Dec. 1966 pages 1842- 1847.

Claasen et 211. Applied Spectroscopy, Vol. 23, No. 1 Jan.- Feb. 1969, pages 8- 12.

Primary Examiner-Ronald L. Wibert Assistant Examiner-F. L. Evans Attorney-Webb, Burden, Robinson & Webb [5 7] ABSTRACT Two rays of light polarized parallel and perpendicular with respect to a plane are produced from a single ray of light generated by a laser. By alternately illuminating a sample with the said two rays of light, two Raman scattering light rays having different intensities are produced and in turn applied to a monochromator where they are dispersed according to wave length. After dispersion, the rays are detected by a photomultiplier and the resultant signals applied to a division 56 References Cited circuit where the depolarization ratio is automatically 1 calculated. This process is repeated for subsequent UNITED STATES PATENTS sets of signals, thereby effecting a continuous operat' al 3,572,938 3/1971 Bradford ..356/116 Sequence 3,414,354 12/1968 Siegler, Jr ..356/75 4 Claims, 11 Drawing Figures HALFWNI; PLATE.-

SAMPLE mi; 1',

B sncron ,4 SECTOR A CHOPPER 22 j NONOCHQOMATOR REOORDSR Division T c ii ir m 4 CJRCUIT j i J J v FILTER ggg l'r v PHOTOMULTIPUER 2 24 23 J0 PATENTEDBBT 10 1912 SHEET 1 BF 5 dune 5:50.510 w\ NEE??? Huh-E uwh 8555 uwDmOQmd Nwmmdj p (l) where Ru (X) represents the intensity of the rays of Raman light scattered from a sample as a result of illuminating the sample with a ray of incident lightpolarized perpendicular to a plane and Rh (X) represents the intensity of the rays of Raman light scattered from the sample by illuminating the sample with a ray of incident light polarized parallel to the said plane.

In the present mode of depolarization measurements, a Raman spectrum is first observed by illuminating the sample with an incident ray of light polarized parallel to aplane and then again observed by illuminating the sample with an incident ray of light polarized perpendicular to the said plane. The depolarization ratios are then calculated by comparing two intensities of the same line in the two spectra. This method, to say the least, is quite troublesome, not to mention the margin of error involved due to the fluctuation of the incident light intensity, plus, of course, the human element.

According to this invention, there is provided a method and apparatus for automatically recording the depolarization ratios of the Raman bands. It is an advantage of this invention to provide a method and apparatus for measuring the depolarization ratios rapidly and yet with precision.

In one embodiment of this invention, the sample is illuminated alternately by two rays of incident light respectively polarized parallel and perpendicular to, a plane. As a result, two rays of Raman light based on the .two rays of incident light are scattered from the sample.

The two rays of Raman light are then alternately applied to a monochromator where they are dispersed according to wavelength and detected by a detecting means such as a photomultiplier. That is to say, two electrical signals based on the two different rays of incident light are alternately detected by the detecting means. The signals are then amplified and-their ratio is automatically calculated by a division circuit and recorded by a recorder. This procedure is continuously repeated so that the ratio of each set of signals is continuously and automatically recorded by the recorder.

In this invention, the depolarization ratios are obtained by a single scanning of the monochromator, thereby shortening the ratio measuring time. Further, since the sample is illuminated alternately, the two Raman spectra are observed under identical condi- "tions, thus ensuring precise measurement of the depolarization ratios.

Various other objects and advantages of this invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings in which:

FIG. 1 shows one embodiment of the invention in which the light path between the light source and the sample is bifurcated.

FIG. 2 shows a sector used in the embodiment shown in FIG. 1.

FIG. 3 shows a chopper used in the embodiment shown in FIG. 1.

FIG. 4 shows another embodiment of this invention.

FIG. 5 shows a sector used in the embodiment shown in FIG. 4.

FIG. 6 shows the wave forms produced by the various electrical units constituting the embodiment shown in FIG. 4.

FIG. 7 shows again a further embodiment for obtaining the two incident rays of light.

FIG. 8 shows the sector used in the embodiment shown in FIG. 7.

FIG. 9 shows yet another embodiment of this invention in which the sector is inserted in the optical path between the sample and the monochromator.

FIG. 10 shows the sector used in the embodiment shown in FIG. 9.

FIG. 11 shows one more embodiment of this invention.

Now referring to- FIG. 1, a coherent ray of light polarized parallel to a reference plane, preferably the horizontal plane, is produced by a laser 1. The optical path of the coherent ray of light is changed by a sector 2 which takes the form of a fan-shaped reflector as shown in FIG. 2. The said sector is mounted on a shaft 3 rotated by a motor 4. Also mounted on the said shaft is a chopper 5 and a gear 6, the said gear 6 being meshed to gear 7, on a shaft 8 of which a second sector 9, identical in form to the sector 2, is mounted. Accordingly, the rotation of the sector 9 is synchronized with the rotation of the sector 2.

The light deflected by the sector 2 is reflected by a mirror 10 so as to follow Path B, the said light being polarized perpendicular to the plane by a half-wave plate 11 and directed to Path C by a mirror 12 and the sector 9.

On the other hand, the light emanating from the laser 1 which is not reflected by the sector 2, that is to say, the light passing through the non-bladed section of the sector 2, forms a Path A which joins up with Path C since the sector 9 is synchronized with the sector 2. As a result, two light rays, one polarized parallel to the plane and the other polarized perpendicular to the plane illuminate the sample 13 alternately, and in so doing, excite the said sample causing a scattering of Raman light rays. The light rays scattered from the sample are then directed to a scanning monochromator 14 where they are dispersed according to wave length. Once dispersed, the rays are converted into electrical signals by a photomultiplier l5 and amplified by an amplifier 16. The amplified signals are then fed into gate circuits l7 and 18. At the same time, light emanating from a light source 19 is detected by a phototransistor 20 and applied to the respective gate circuits via a waveforming circuit 21 as gate signals.

Further, the chopper 5, shown in FIG. 3, located between the light source 19 and the phototransistor 20, is synchronized so as to pass light when the sample 13 is illuminated by incident light polarized parallel to the plane and to intercept light when the sample 13 is illuminated by incident light polarized perpendicular to the plane. As a result, the gate circuit 17 is opened when the sample is illuminated by incident light polarized parallel to the plane, enabling signals based on Raman light rays generated by parallel incident light to pass through the said gate circuit and into filter 22 where they are smoothed.

Similarly, the gate circuit 18 is opened when the sample is illuminated by incident light polarized perpendicular to the plane, this time enabling signals based on Raman light rays generated by perpendicular incident light to pass through the said gate circuit and into filter 23 where they are smoothed. The output signals from filters 22 and 23 are then fed into a division circuit 24 where the intensities of the output signals from filter 23 are divided by the intensities of the output signals from filter 22. The resultant quotients, viz, the depolarizarion ratios, are finally recorded by the recorder 25.

FIG. 4 shows another embodiment of this invention in which the same optical system as described in the aforedescribed embodiment is used for obtaining the two incident light rays polarized parallel and perpendicular to the plane. In this case, however, the sectors 30 and 31 are different as shown in FIG. 5. Sections 32 of the sectors, for example, take the form of fan-shaped mirrors and sections 33 serve to absorb the light emanating from the laser 1. FIG. 6A shows the train of light signals illuminated onto the sample 13. Here, the period th represents the period in which the incident light polarized parallel to the plane illuminated the sample 13, period tu represents the period in which the incident light polarized perpendicular to the plane illuminates the said sample and period tc represents the period in which the sample is not illuminated at all. As in the former embodiment, Raman light rays scattered from the sample are dispersed according to wavelength by a monochromator 34.

According to the embodiment set forth in FIG. 4, one circuit produces and records the Raman spectrum. Another circuit (interconnected) produces and records the depolarization ratio.

Once dispersed, the rays are converted into pulse signals, having frequencies proportional to the intensities of the said rays, by a photomultiplier 35. The pulse signals are then amplified by pre-and main-amplifiers 36 and 37. Low amplitude noise, such as dark current, in the said pulse signals is removed by discriminator 38, the output of which is fed into a gate circuit 39 into which gate signals are also fed. The gate signals are based on the light generated by lamp 119. During the period th and the subsequent period tc, the light generated by the said lamp is detected by a phototransistor and converted into an electrical signal. Again, during the period tu and the subsequent period to, the light generated by lamp 19 is intercepted by a chopper 5. The signals detected by the phototransistor 20 are amplified by an AC amplifier 40 and wave formed by a Schmitt circuit 41. The output of the Schmitt circuit 41 is then fed into a gate signal forming circuit 42 in which the phases and the level of the signals are regulated. Thence, the pulsed signals enter the gate circuit 39 which is opened during the period th and the subsequent period to. The intensity of the Raman signals passed through the said gate circuit is counted by a photoncounter 43 and the Raman spectrum is recorded by 2-pen recorder 44.

The discriminator 38, in addition to feeding output pulses into the gate circuit 39, also feeds pulses intoa monomultivibrator 45. Once in the monomultivibrator the input pulses are wave formed to similar pulses as shown in FIG. 6M, where W is the pulse width and E is the pulse height. The monomultivibrator output signal waveform is shown in FIG. 6B. The intensities R'h (A) and Ru (A), namely, the output pulse frequencies of the monomultivibrator during the periods th and tu are expressed as follows:

where Rn is the intensity of the signal attributable to dark current or thermal noise.

The monomultivibrator pulses output is fed into a pulse controller 46 where the pulse widths and heights are controlled. The controlled pulses are then fed into a filter 47 and smoothed. The voltage E of the smoothed signal is expressed as follows:

where f is a pulse frequency.

Referring to formulas (2), (3) and (4), the voltages E E and E of the smoothed signals at th, tu and tc respectively are expressed as follows:

E Rn W E 7 The wave form of the filter output signals is shown in FIG. 6C.

The said output signals are amplified by an AC amplifier 48 where the DC component Edc is removed by a coupling condenser, so as to produce voltages E, E and E,, as shown in FIG. 6D. The said voltages are expressed algebraically as follows:

E =A (E,Edc)=A-Rn-W-EA-Edc 10) where A is the amplification degree of the AC amplifier 48.

With the DC component removed, the amplifier output signals are fed into gate circuits 50 and 51 to which gate signals are applied from the wave forming circuit 42. The th and following to signals pass through the gate circuit 50. FIG. 6E shows the output signal waveform of the gate circuit 50. The output of the said gate circuit is fed into a synchronizing rectifier 52 to which a synchronizing signal, which is double the output signal frequency of the Schmitt circuit 41, is applied from circuit 53 (refer to FIGS. 6K and 6L).

Once in the synchronizing rectifier 52, the th and tc input signals are rectified to form positive and negative signals respectively in accordance with the incoming synchronizing signals from circuit 53. FIG. 6F shows the output signal of the synchronizing rectifier 52 and FIG. 6G shows the rectified output after being smoothed by filter 54. The output voltage E of the said filter is expressed as follows:

On the other hand, the tu and to components in the output signal of the AC amplifier 48 pass through the gate circuit 51, the output of which'is shown in FIG. 6H, and arethen fed into a synchronizing rectifier 55.

The synchronizing signals as shown in FIG. 6L are also fed into the said synchronizing rectifier from the circuit 53. As a result, the tu and tc input signals are rectified By maintainii'ig E, at a constant voltage.K,p (p) can be expressed as follows:

),(h) E IK 17 Thus, E is linearly proportional to the depolarization ratio )\(h).

A comparison circuit 57 is used for maintaining th voltage E constant. This is to say the signal of the filter 54 is fed into the comparison circuit 57 and compared with a reference voltage. The resultant differential voltage is then fed backto a pulse controller 46 so thatthe pulse width and'height of the pulse passed through the pulse controller are controlled in order to maintain a constant filter output voltage. As a result, the output voltage of the filter 56 is also controlled at the same rate as the output voltage of the filter 54 by the pulse controller 46 and a voltage proportional to the depolarization ratio is obtained automatically from the filter 56. The output signal of the filter 56 is fed into the two-pen recorder 44 via a DC amplifier 58 where the depolarization ratios are recorded in accordance with the wavelength scanned by the monochromator.

In the above embodiment, the differential voltage is fed back to the pulse controller 46 in order to control the filter output voltages. It is also possible, however, to feed back the differential voltage to the AC amplifier 48 and to control the amplification degree A of the said amplifier in order to keep the output voltage of the filter 54 constant.

FIGS. 7 and 8 show another embodiment for obtaining the two incident rays of light polarized parallel and perpendicular to the plane. In this embodiment, a secinserted between the laser 1 and the sample 13. Section 63 forming part of sector 60- comprises a half-wave plate and sections64serve as light intercepters. As a result, the light polarized parallel to the plane passes through the sector 60 and the light passing through section 63 is polarized perpendicular to the plane by the half-waveplate. in this way, the two incident rays of light polarized parallel and perpendicular to the plane illuminate the sample 13 alternately.

lt-is possible to measure the depolarization ratios by means of two Raman light rays, one polarized parallel to the plane and the other polarized perpendicular to the plane, by illuminating the sample with circularly polarized light. Referring to FIG. 9, laser light produced by a laser 1 passes through a quarter wave plate and is circularly polarized. The said polarized light is then made to illuminate sample 13, resulting in Raman light rays being scattered from the sample. The said rays are directed to a monochromator (not shown) via a sector 71, (see FIG. 10), which is mounted on a shaft 72 of a motor 73. In FIG. 10, sections 74 and 75 represent polarizers in which the- Raman light rays polarized parallel and perpendicular to the plane respectively pass. Sections 76 serve to intercept the passageof light. Thus, the two Raman light rays enter the monochromator alternately. The depolarization ratios are measured by the electrical units used in the prior described embodiments. The gate signal is produced by chopper 77, mounted fixedly on the periphery of the sector and thereby rotating in unison with the said sector, a light source 78 and a phototransistor 79.

FIG 11 shows one more embodiment of this invention. In the figure; two light rays polarized parallel and perpendicular to the plane are produced by the optical system used in the embodiment shown in FIG. 1 and illuminate the sample 13 alternately. The light rays scattered from the sample are then directed to the monochromator 14 where they are dispersed according to wavelength. Once dispersed, the rays are converted into electrical signals by the photomultiplier 15 and amplified by the amplifier 16. The amplified signals are then fed into a synchronizing rectifier 81, to which a synchronizing signal is applied from a synchronizing signal generating circuit 82, so that a differential signal of the intensities of the two Raman light rays resulting from the two differently polarized incident rays of light is produced. The differential signal is fed into a motor 83 via anamplifier 84. The motor moves an attenuator 85 into the optical path between the two mirrors l0 and 12 so as to attenuate the intensity of the light polarized parallel to the plane. When the differential signal is zero, the relationship between the two Raman light rays is expressed as follows:

where K is the attenuation coefficient of the incident light polarized parallel to the plane. Therefore, when the differential signal is zero, the attenuation coefficient K is equal to the depolarization ratio p(h). The said coefficient, viz, the depolarization ratio, is recorded by a recorder 86 connected with the said motor 83 so that the movement of the attenuator 85 is recorded.

Having thus described the invention with the detail and particularity as required by the Patent Laws, what is desired protected by Letters Patent is set forth in the following claims.

We claim:

1. An apparatus for automatically recording the Raman Spectrum and the depolarization ratios of Raman bands of a sample comprising:

A. a monochromatic light source;

B. means for obtaining two incident rays of light polarized respectively parallel and perpendicular to a reference plane, said means being located in the optical path between said light source and said sample;

C. means for alternately illuminating said sample with said two incident rays of light;

D. a monochromator for dispersing the resultant scattered Raman light rays;

E. means for creating electrical signals indicative of the intensity of the dispersed Raman light rays;

F. detecting means for detecting the intensity of either one of the electrical signals resulting from the two differently polarized incident rays of light;

G. an analyzing circuit for providing a signal indicative of the ratio of the intermittent electrical signals resulting from the two differently polarized rays of light; and,

H. means for recording the output signals of said detecting means and said analyzing circuit simultaneously.

2. An apparatus according to claim 1 wherein said light source comprises a laser providing a plane polarized monochromatic output.

3. An apparatus for automatically recording the Raman Spectrum and the depolarization ratios of Raman bands of a sample comprising:

A. a monochromatic light source;

B. means for obtaining two incident rays of light polarized respectively parallel and perpendicular to a reference plane, said means being located in the optical path between said light source and said sample;

C. means for alternately illuminating said sample with said two incident rays of light;

D. a monochromator for dispersing the resultant scattered Raman light rays;

E. means for creating an electrical signal indicative of the intensity of the dispersed Raman light rays;

F. detecting means for detecting the intensity of either one of the electrical signals resulting from the two differently polarized incident rays of light;

G. an analyzing circuit for providing a signal indicative of the ratio of the intermittent electrical signals supplied thereto comprising:

i. circuit means for comparing the signal voltage of the Raman light resulting from the parallel polarized incident rays of light to a reference voltage;

. a control circuit and a circuit means for feeding back the differential voltage observed in the comparing circuit to control the intensity of both electrical signals maintaining the signal based on the parallel polarized incident rays of light constant; i circuit means for producing a signal being the difference between the signal voltage of the Raman light resulting from the perpendicular polarized incident rays of light, said signal being indicative of the depolarization ratios; and,

H. means for recording the depolarization ratios and the output signal of the detecting means simultaneously.

4. An apparatus for automatically recording the Raman Spectrum and the depolarization ratios of Raman bands of a sample comprising A. a monochromatic light source;

B. means for obtaining two incident rays of light polarized respectively parallel and perpendicular to a reference plane, said means being located in the optical path between said light source and said sample,

C. means for alternately illuminating said sample with said two incident rays of light;

D. a monochromator for dispersing the resultant scattered Raman light rays;

E. means for creating an electrical signal indicative of the intensity of the dispersed Raman light rays; F. detecting means for detecting the intensity of either one of the electrical signals resulting from the two differently polarized incident rays of light;

G. means for producing a differential signal of said electrical signals;

H. means for attenuating the intensity of the incident light polarized parallel to the plane according to said differential signal so as to equalize the intensities of the two Raman light rays; and,

1. means for recording the attenuation coefficient of the incident light polarized parallel to the plane and the output signal of said detecting means simultaneously.

t i k i i UNITED STATES PATENT OFFICE CERTIFICATE OF CECTEUN Patent 3. 697. 180 I Dated October 10. 1972 n fl Haiime Mori and Mamoru Irizuki It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown -below:

In'the Title, "HEDEPOLARIZATION" should read --DEPOLARIZATION-- Col. 4, l. 11, formula (3) should read --R'u (X) Ru (X) R C01. 4, l. 42, formula (8) should read "E5 =A (E2 -Edc )='A'R'h yu w-E 7 A-Edo col. 4. 1. 43, formula 9 should read- "E6 :A(E3 Edc A'R'h (jM-W- E -A-Edo col. 5, l. 2, the formula should read ...:A {AnR h (1)W0E AnEd ARnVVE Col. 5, 1. 3, the formula should read C01. 5, l. 4, formula (11) should read 1 A WE'Rh 4 F ORM PO-105O (10-69) USCOMM-DC 6037'7-069 UYS, GOVERNMENT PRINYING OFFICE I969 0-366-134 UNITED STATES PATENT QFFICE CERTIFICATE OF ECTTQN Patent No. 3, 697, 180 Dated October 1 O, 1972 Inventor( s) Hajime Mori and Mamo'rulrizuki (conll nued) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 5, l. 24, the formula should read --:1A'W E{Ru (1)- R 3-- A'W' E0 Ru 2 Col. 5, 1. 26, "p (d)" should read ,0 7L

Col. 5, l. 29, the formula should read P Rh 7 Col. 5. l. 32, formula (16) should read A'W'E'E8 8 Col. 5, 1. 36, "p /0 should read --/0 FORM po'mso USCOMM-DC scam-peg l5, GOVERNMENT PRINTING OFFICE! 1959 Q-TwG-JJA UNITED STATES PATENT OFFICE CERTIFICATE OF COEQ'HN Patent No. 3, 697, 180 Dated October 10, 1972 Inventor(s) Hajime Mori and Mamoru- Irizfiki (contimded) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 5, 1. 39, formu1a(17)shou1d'read --/0 7 E9 Col. 5, 1. 41, "in; should read own-- Col. 6, 1. 58, the formula should read --Rh (7L) K Re (24-- C01. 6, 1. 61, theformula should read K u 1-) R (74 C01. 6, 1. 66, p,( h Should read O(7-)-- Signed and sealed this 6th day of March 1973.

(SEAL) Att'est:

EUWARDM.FLETCHERJR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM PO-IUSO (10-69)

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US3414354 *Oct 28, 1964Dec 3, 1968Perkin Elmer CorpRaman spectrometers
US3572938 *Feb 18, 1969Mar 30, 1971Us NavyPolarimeter
Non-Patent Citations
Reference
1 *Claasen et al. Applied Spectroscopy, Vol. 23, No. 1 Jan. Feb. 1969, pages 8 12.
2 *Hawes et al., Analytical Chemistry, Vol. 38, No. 13, Dec. 1966 pages 1842 1847.
3 *Murphy et al., The Journal of Chemical Physic, Vol. 47, No. 5, Sept. 1, 1967, pp. 1836 1839.
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US3807862 *Dec 18, 1972Apr 30, 1974Sybron CorpRaman spectroscopy in the presence of fluorescence
US3817634 *Feb 29, 1972Jun 18, 1974Nat Res DevTesting of optically active substances by polarized radiation
US4672196 *Feb 2, 1984Jun 9, 1987Canino Lawrence SMethod and apparatus for measuring properties of thin materials using polarized light
US5139334 *Sep 17, 1990Aug 18, 1992Boston Advanced Technologies, Inc.Hydrocarbon analysis based on low resolution raman spectral analysis
US6124928 *Mar 6, 1998Sep 26, 2000Kaiser Optical Systems, Inc.Methods and apparatus for obtaining a selectively polarized induced emission
DE2650994A1 *Nov 8, 1976May 18, 1977Stefan RidderstedtVorrichtung zur abschliessbaren halterung eines gegenstands an einer stationaeren aufbewahrungsstelle
Classifications
U.S. Classification356/301, 356/367
International ClassificationG01N21/21, G01J3/44, G01J4/00
Cooperative ClassificationG01J3/44, G01N21/21, G01J4/00
European ClassificationG01J3/44, G01N21/21, G01J4/00