WO2013068627A1 - High-sensitivity multifunctional automatic spectrometer and method for using same - Google Patents

Abstract

The invention relates to a novel automatic spectrometer model having high sensitivity, precision and a broad range of wavelengths. Said spectrometer has a design made up of exchange carousels which house a plurality of optical elements such as Carpenter prisms (diffraction grating prism), filters and collimation and focus systems controlled by a data-capture and control card. The automatic nature of the entire spectral analysis system lies in said electronic card, which is assisted by the solid-state detector. The spectrometer is mainly characterised by eliminating the non-linearity which can be found in said optical diffraction systems.

Description

 High sensitivity multifunctional automatic spectrometer and its method of use

SECTOR OF THE TECHNIQUE

The invention presented in this Patent Report belongs to the sector of equipment and methods used in chemical analysis of gases and fluorescent compounds, solid or liquid, with applications both in fundamental research (plasma physics, astrophysics, materials) and in industry ( food compounds, pharmacy, emulsions, lacquers and varnishes, etc.).

STATE OF THE TECHNIQUE

Spectrometry is a chemical, structural and quantitative analysis technique, whose principles date back to the beginning of the 19th century. Its foundation is very simple, it is about decomposing the light emitted by an incandescent body in its fundamental colors knowing that this information unambiguously identifies its chemical composition. More specifically, a spectrograph is an apparatus that determines the intensity distribution of each of the wavelengths of the emitted light.

Technology has always tried to improve the sensitivity and spectral range of manufactured equipment. Basic information on this technique and its industrial evolution can be found in books such as: "The design of optical spectrometers", John F. James and RS Stemberg, editor, Capman and Hall Ltd. 1969. Similarly, the methods, the equipment, as well as the problems focused by physicists and engineers when manufacturing new systems, are described in: "A medium resolution 0.8-5.5 microns spectrograh and imager for the NASA infrared telescope facility", Astronomical Soc. of the Pacific. March 2000, and, "A spatial spectral scanning technique for a Fabry-Perot spectrometer" by G. Shepherd, Applied Optics vol.4, pp 267, March 1965. Other chemical analysis techniques have appeared in recent decades to compete with classical spectroscopy. Procedures such as gas chromatography allow a saving of time both during the preparation of the sample to be analyzed and in the operation and all this without diminishing the precision of the results. Seeking an improvement, the French patent of E. Froigneux, publication number 284685, with a filing date at the national office on November 25, 2002, introduces the use in a spectroscope of the optical device called "grisma". This is a new term of the scientific language introduced by Spanish researchers and engineers who translate, with excessive speed, the English "difraction grating prism" as "grisma". This is a familiar, understandable language license, which official dictionaries will take some time to accept. However, historically, the union of a glass prism with a diffraction net was well known in the technical literature by "Carpenter's prism", and in this specification the invention will be hereinafter referred to in both forms. For the rest, experts know the high sensitivity and separating power of this diffractor device, but they are also aware of two disadvantages, or defects, when used in spectroscopy. Well, on the one hand the non-linearity of the signal obtained in the detector is observed when this type of diffractor elements is used, and on the other, also, a lack of versatility that consists in that the spectral range of observation is fixed in advance . The technology in this metrological domain must evolve until it finds the competitive procedures that the instrumentation markets require, above all, taking advantage of the latest techniques and devices provided by the science of electronics and computer science.

BRIEF DESCRIPTION OF THE INVENTION

This invention deals with a new concept of high sensitivity automatic spectrograph, wide range of wavelengths, compact, multifunctional, lightweight and easily portable. Its design allows a large number of applications, spectral determinations and chemical analysis by having a series of optical elements arranged in three element exchangers that allow the User choose between several grays, also known as Carpenter prisms, filters and polarizers. Similarly, two identical refractive focal subsystems with 4 lenses each and a CCD camera thermalized by a thermoelectric cooling system allows optimum sensitivity to be achieved. Spectral calibration systems of the instrument and a command and data collection unit complete the system.

Due to the design and manufacturing characteristics of this device, it is ensured, at any time, the automatic control and change of the different modes of application of the instrument, the main characteristic of a multifunctional and versatile instrument.

DETAILED DESCRIPTION OF THE INVENTION.

 Figure 1 of this invention report presents the prototype of the spectrograph. Figure 2 is also a schematic representation of the previous figure that describes the path of the light beam emitted by the sample or object to be analyzed, through the different fundamental optical parts or elements of the spectrometric system. The light radiation affects, first, the slit (Figure 3) placed in the first exchange carousel (2) that houses a set, or set, of slits, of different length and width, to allow the user to obtain different spectral resolutions and / or adapt to the experimental conditions of the instrument in those cases in which the luminous flux of the source to be analyzed turns out to be reduced. The exchange carousel is articulated by a stepper motor (9). The selection of the slit is carried out, before the programmed spectral determination, by means of the electronic circuits present in the control card and data collection (8).

Next, the beam passes to a four-lens optical collimator system (3) that provides a parallel beam of light which strikes the gray or Carpenter prism shown in detail in Figure 4, that is, on an optical element formed by a diffraction net (10) attached to the hypotenuse face of a prism (eleven). The diffraction net (10) is a glass or a transparent resin in which a carving is carried out, usually of saw teeth giving rise to a density of several lines per millimeter. The aforementioned Grisma is one of those mounted on a second exchange carousel (4), mechanically identical to the previous one. The choice of the gray, or Carpenter's prism, to be used in the measurement is a function of the wavelength range to investigate. The second exchange carousel (4) is designed to house different types of grays and photometric filters which will allow you to obtain different spectral resolutions or perform photometry. It is also articulated by a stepper motor (9), commanded by the aforementioned control card (8) through the electrical contacts (12) (see Figure 5).

Figure 5 presents an overhead view of one of the three interchangeable carousels used that are mechanically and of identical dimensions with the primary purpose of making manufacturing cheaper; the only thing that changes in each of them is the optical element to choose which is housed in the holes of the rotating wheel (13). Radiation diffracted by the gray, or Carpenter prism, in the second exchanger, passes through an optical cutting filter, or a polarizing, or photometric filter, according to the convenience of the determination, mounted on a third exchange carousel (5) . Specifically, the order cut filter is an optical medium that only allows the passage of a certain range of wavelengths, also known as high-pass filter. In turn, the polarizing filter only lets light radiation oriented in a given plane pass through and the photometric filter is an optical medium that only allows the passage of a certain narrow range of wavelengths, also known as a pass-band filter. The convenient choice of the gray, or Carpenter prism, of the photometric, polarizing and "order cut" filters allows to avoid overlapping wavelengths between the first and second diffraction orders.

After passing through the third exchange carousel, the light beam, with its different colors, or wavelengths, well separated spatially, then passes through a system of optical lenses (6) that serve to focus the image of the slit (2) on an electronic CCD detector (7) (CCD is the acronym for the English term, "Charge Coupled Devices", that is, charge coupled detector). The image of the slit (2) (Figure 3), for each of the wavelengths, is formed in the CCD detector occupying a matrix of M x N pixels that is stored in the digital memory of the control card (8 ). The CCD detector is cooled by a thermoelectric cooling system to a working temperature of -95 ° C. Indeed, to obtain a higher sensitivity, it is necessary to reduce the temperature of said solid-state detector in order to obtain a high "optical signal / background noise" ratio. The temperature of the detector is controlled at all times by means of a thermal sensor housed inside the body that houses the CCD (7) and this information is provided through the control card (8). The system is completed with the emission lamp units for calibrating the device (1) housed before the slit. The calibration system (1) is driven by an automatic electric motor that allows the passage of light from the lamps or from the sample to be analyzed according to the measurement stage. The spectral range of work is between 380 nanometers and 1050 nanometers. All the elements, mechanical and optical, that make up the described apparatus are mounted on an indeformable metal structure, as seen in Figure 1.

The intended fields of application of the spectrograph presented in this invention report are the following:

 1. Physics and analytical chemistry; for the identification of substances by the spectrum emitted and / or absorbed by them or characterization of optically active substances by polarimetry.

 2. Industry; of lighting, for spectral, photometric and radiometric characterization of lamps used in lighting, or for quality control and process control in the pharmaceutical, food and chemical industry by polarimetry.

3. Astronomy and remote detection; in which it will be possible to measure the chemical composition, brightness and physical properties of astronomical objects, such as for example their speeds from the Doppler effect of their spectral lines.

As described, in order to carry out the different measurements and determinations, the prototype presented in this invention report has a wide range of resources. By construction, the spectrograph provides the user with three types of measures of matter; spectroscopy, photometry and polarimetry, each of which is governed by software, introduced in an electronic device of the control card and data collection (8), which automatically allows the rapid change of such working modes. The aforementioned control and data / results card (8) that governs the instrument consists of a board with a microcontroller and input / output ports. It consists of 14 digital input and output pins that operate at 5 Volts and another 6 analog input pins. Each digital input and output pin can provide or receive a maximum of 40 mA. The memory is 512 bytes EEPROM type and the clock frequency is 16 MHz. The board has a USB connector plus a serial converter to directly connect a computer where the software that controls the instrument is. The control card (8) also serves to eliminate the overlap that may exist between successive dispersion orders in the optical prism and in the diffraction network, that is, the problem of non-linearity already mentioned in previous lines. Indeed, a digital image processing computer program subtracts the spectra obtained with and without the corresponding low-pass filter. Also, the coating applied on the lenses of the optical system (14, 15, Figure 8) that forms the image in the detector has been chosen to absorb; or so that it is not transparent at certain short wavelengths that are known to result from overlap.

In fact, for each work mode, the invention contains opto-mechanical elements characteristic of the type of measurement to be performed; spectroscopic mode (slits, gray, cross-dispersers and order cut filters), photometric mode (photometric filters) and polarimetric mode (polarizers). All processes and Working methods have been recorded, previously, by means of the appropriate source code, in the electronics of the control card (8) to direct the determination operation by commanding the calibration sources (1), the slits, the grays and the filters (Figure 2). Other opto-mechanical and opto-electronic elements are common in all three modes of work, these are; lens collimator (3), lens focusing system (6), element exchangers (Figure 5) and CCD detector (7).

The operation of the instrument for each work mode is described below: a) Spectroscopy

 The light radiation enters the instrument from the left side of the diagram presented in Figure 2 and goes through, first, the calibration system (1) that contains two lamps whose spectral distribution is well known. The calibration consists in comparing the position and intensity of the spectral lines of the light coming from the sample with that of the reference spectral flow lamps. Secondly, both the light of the lamps and the one that comes from the sample to be analyzed will illuminate the slit (2) made of aluminum whose opening (see Figure 3) will define the resolution element of the light that passes through it.

The reason for manufacturing the aluminum slits is its low coefficient of thermal expansion, this means that the dimensions of its opening do not vary practically over a long time of continuous use in situations where temperature variations are not negligible, as could happen if the instrument is used in astronomical observations which are carried out throughout the night.

The light that crosses the slit through its opening is collimated with the first lens barrel (3) making the light beams coming from each point of the slit parallel to each other. The collimated light affects the gray (Figure 4), this being a disperser formed by a prism (11) and a transmission diffraction network (10), both joined with optical glue and located in the second exchanger (4). The use of grismas allows the optical path to be linear, which facilitates that in addition to performing spectrometry, a direct image of the object or sample to be analyzed can be obtained.

The next optical element that passes through the light, depending on the needs of the measurements, are the order cut filters located in the third exchanger (5). The order cut filters are, in this particular case, high-pass or low-pass optical filters that prevent the overlapping of wavelengths of contiguous diffraction orders, thus avoiding confusion in the measurements.

Finally, the light is focused through a set of four lenses that make up the last kite (6). This subsystem is responsible for creating the image of the slit projected on the CCD detector (7).

donde ω' es la anchura de la rendija proyectada en el detector, ω es la anchura física de la rendija, _cam es la distancia focal efectiva de la cámara y f_coi es la distancia focal efectiva del colimador. Both the collimator (3) and the focusing system (6) are twins, so the focal length of both is the same, so that the width of the resolution element projected on the detector is equal to the physical width of the slit used at the time of observation according to the following expression:

where ω 'is the width of the slit projected onto the detector, ω is the physical width of the slit _cam is the effective focal length f of the camera _co i is the effective focal length of the collimator.

Therefore, the image of each slit projected on the detector (7) will define the spectral resolution element Δλ, the spectral resolution being conceptually equal to λ / Δλ, so that through the first carousel (2) slits can be replaced of different opening width and therefore obtain different spectral resolutions, depending on the requirements of the measurement or observation.

If the instrument is used for astronomical observation, the slit change can be used to avoid losing light from celestial objects affected by a worsening of atmospheric Seeing as can be seen in Figure 6, however, in this specific case, work with slits of greater openness it would imply losing spectral resolution, so to maintain the same resolution, the disperser would be changed to a gray with greater angular dispersion by virtue of the following expression;

206 264.81 - - 7¾

 7 φ where R is the spectral resolution, d is the diameter of the pupil, DQ is the angular dispersion of the gray, a is the anamorphism factor of the disperser, D is the effective opening of the telescope and is the opening of the slit in seconds of Arc.

If atmospheric conditions were invariable throughout the observation or measurement time, the spectral resolution could be increased by changing only the gray with a greater angular dispersion but maintaining the same slit without the system losing efficiency. b) Photometry

For photometric mode, the first exchange carousel (2) will allow light to pass through. Figure 5 shows the characteristics of the exchanger. Inside it there is a wheel that rotates thanks to a stepper motor (9) that allows the positioning of 8 elements, which in the case of the present invention would be seven slits for spectroscopy and a gap to let the direct light and Be able to perform photometry directly. The second exchanger (4) of similar characteristics to the first would also house seven optical elements (gray and / or photometric filters) and a gap to let in direct light, and, also, the last exchange carousel (5) would house seven optical elements ( order cut filters, photometric and / or polarizers) and a hollow for direct image. c) Polarimetry Polarimetry would be performed by leaving a free space in the first exchange carousel (2), a free space or photometric filter in the second exchanger (4) and a polarizer in the last exchanger (5). In astrophysics, interest in the determination of stellar magnetic fields of high intensity has appeared very recently. The assembly presented in this patent carries out this assessment by applying the different polarimetric filters, determining the relative intensity of the polarized component of the emitted radiation. A simple application of the Faraday effect leads to the module value of said magnetic field vector. EXAMPLE OF THE APPLICATION OF THE INVENTION

 In order to test the application of the invention presented in this Report, a prototype spectrograph was manufactured and several analyzes were made in the laboratory. The photograph shown in Figure 1 is a sample of the finished device.

For the calibration of the prototype (1) and before the determinations to be made, two discharge lamps were used; mercury-argon and krypton, whose spectral distribution is represented in Figure 7. The first exchange carousel (2) already described in the Detailed Description houses four aluminum slits similar to the one shown in Figure 3 with openings, respectively, of 25, 50, 75 and 100 microns wide by 15 millimeters long each and a total diameter of 28 millimeters. For subsequent applications there is space in the exchanger to place another three slits. The design of the lens systems (Figure 8), one for collimation (14) and one identical (15) required to form the image in the detector (7) were manufactured with optical quality glasses, N-SF57, CaF ₂ , N-BK7 and N-SF8. The effective focal length of the two systems was 52 millimeters, radial field of view of 10.3 ° and the outlet diameter 6.5 millimeters. The window separating the second optical system from the CCD detector is transparent fused silica and all the lenses, eight, have an anti-reflective coating on both sides, for useful wavelengths, which allows them to have high efficiency , almost total absence of optical absorption, in the spectral range of work between 380 nanometers to 1050 nanometers. The second exchanger contained a single gray or Carpenter prism, manufactured from a rectangular prism and a transmission diffraction network. Its dimensions allow the light beam to pass through the entire field of vision of the system. The vitreous material of the prism was LZ-CTK19 and its upper angle is 17.5 degrees (Figure 4). The diffraction net was manufactured on a B270 neutral substrate, with a scratch density of 300 lines / mm. The third exchanger (5) contained four "sloan" photometric filters, centered on wavelengths, 475 nanometers (g ', in the usual notation of spectroscopy), 622 nanometers (r'), 763 nanometers (i ') and 905 nanometers (ζ '). It also contained two other "Johnson" filters centered on 658 nanometers (R) and 806 nanometers (I). The total number of filters that can be accommodated in the carousel for operational reasons is seven, to be distributed between photometric filters, cut-off and polarimetric filters.

The spectra of the compound contained in a fluorescent luminaire were determined. The compound is basically formed by mercury, argon and neon gases at very low pressure. At the top of Figure 9 is the real image of the spectrum recorded in the detector in which the characteristic spectral lines of these compounds can be observed. In the lower image of Figure 9, the intensity levels are clearly seen, clearly distinguishing the effect of the hot tungsten filament that fluorescent luminaires contain inside, since it emits continuously on which the lines corresponding to the mentioned gases stand out. It is easy to verify with the help of the spectral libraries, the full and unambiguous identification of the prepared gaseous mixture.

In the chemical industry, it is often necessary to know, immediately, the amount in which a given compound enters a mixture. This automatic spectrometer enables the user, by choosing the cut-off filters, to limit, or shorten, the range of observed wavelengths, with the consequent saving of operating time, can be up to the wavelength of a given and known emission peak. The multifunctional and tuning character of the device proposed in this report is always ensured by design. Thus, if a previous calibration has been saved on the electronic card, it can write the result of the determination on a screen or monitor.

To carry out the photometry test, the photometric filters were sequentially placed in the optical path by actuating the third exchanger. These measurement modes are automatically changed by programming a work routine in the software that governs the instrument. In Figure 10 the intensity levels can be seen as a result of such an operation in which an incandescent lamp has been used as a light source and the images obtained after each sequential change of each filter have been superimposed.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1. Photographic assembly of the elements, main components to be used in operations with the spectrograph. Figure 2. Detailed block diagram with description of the elements that make up the spectrograph.

 Figure 3. Drawing of an aluminum slit with the geometry of its opening.

Figure 4. Drawing of a gray, composed of a transmission diffraction network (10) attached to the hypotenuse face of a rectangular prism (11).

 Figure 5. Aerial view of one of the exchangers of optical elements of the system, in which the stepper motor (9), the electrical connection points (12) to command the motor and the rotating wheel where the housings are housed are distinguished elements (13).

Figure 6. Effect of atmospheric seeing on a celestial body (for example a star) on the opening of a slit. It is evident that as the worsening sees the amount of light that does not penetrate through the slit and therefore the instrument is greater.

 Figure 7. Graphical representation of the spectral lines of the mercury-argon lamps (above) and krypton (below) used in the prototype for calibration. Figure 8. Optical design of the collimating system (14) and the refractive focus system (15) of the prototype.

 Figure 9. Image obtained with the application example working in spectroscopic mode. The image belongs to the light emitted by a fluorescent lamp. Figure 10. Image obtained with the application example working in photometric mode. The image belongs to the light emitted by an incandescent lamp in which the photometric bands g ', r', i ', z', R and Y are identified.

Claims

1. High sensitivity multifunctional automatic spectrometer, characterized in that it comprises the following elements:  a) a first exchange carousel containing a set of slits, of different dimensions, to be chosen depending on the determination and / or analysis to be carried out,  b) a four-lens objective optical system, which collimates and projects the image of the slit chosen in a Grisma or Carpenter prism, c) a second exchange carousel that contains a set of Grismas or Carpenter prisms, to be chosen according to the determination and / or analysis to be carried out,  d) a third exchange carousel containing optical cutting filters, polarizing filters and photometric filters, to be chosen depending on the determination and / or analysis to be carried out,  e) a four-lens optical system, which projects light radiation dispersed in wavelengths, dispersed by the gray or Carpenter prism, in the input plane of a CCD solid state detector, f) a CCD solid state detector element , capable of working at room temperature or refrigerated to a temperature of -95 ° Celsius scale,  g) a system for calibrating the apparatus by means of regulated light emission lamps that emit according to known spectra,  h) an electronic command and data collection card that, in series with an external computer, controls the operations and movement of the three mobile carousels (a, c and d), controls the status and operation of the CCD detector, and stores the obtained spectra . 2. High sensitivity multifunctional automatic spectrometer according to claim 1, characterized in that, by means of the selection of the elements contained in the three carousels, claimed in 1, a, c and d, the measurement spectral of any sample, in the range of wavelengths chosen at the user's convenience, between the wavelength limit values of 380 to 1050 nanometers. 3. High sensitivity multifunctional automatic spectrometer, according to claims 1 and 2, characterized in that the exchange carousels (claim 1 a, c and d), are driven in their rotational motion, by a stepper motor which is governed by the contained software on the electronic command and data collection card. 4. High sensitivity multifunctional automatic spectrometer according to claims 1 and 2, characterized in that the electronic command and data collection card, together with the correct choice of the cutting filters (claim 1 d) eliminates the superposition of the different and successive orders of dispersion in, respectively, prism and diffraction network (claim 1 c), with the objective of having the linearity condition (sum and product) necessary in every metrological instrument. 5. High sensitivity multifunctional automatic spectrometer according to claims 1 and 2, characterized in that it contains two optical lens systems (claim 1 bye) of four lenses each, carved to give a convenient focal length and free of chromatic aberrations and shape. , whose surfaces have been coated with an anti-reflective layer to ensure transparency. 6. High sensitivity multifunctional automatic spectrometer according to claims 1, 2, 3, 4 and 5, characterized in that it allows spectral measurements and determinations to be performed in a range of wavelengths (claim 2), in the spectroscopic mode of operation, obtaining the spectral resolution by means of the choice of the entry slit (claim 1 a). 7. High sensitivity multifunctional automatic spectrometer according to claims 1, 2, 3, 4 and 5, characterized in that it allows measurements and spectral determinations to be performed in a range of wavelengths (claim 2) according to the polarimetric method. 8. High sensitivity multifunctional automatic spectrometer according to claims 1, 2, 3, 4 and 5, characterized in that it allows photometric measurements and determinations to be made by changing the different optical elements contained in the exchange carousels (claims 1 a, c and d). 9. High sensitivity multifunctional automatic spectrometer according to claims 1, 2, 3, 4 and 5, characterized in that it allows the determination of the value of stellar magnetic fields by applying the Faraday effect to the spectral determinations obtained. 10. High sensitivity multifunctional automatic spectrometer according to claims 1, 2, 3, 4, 5, 6, 7, 8 and 9, characterized in that its design and manufacturing ensure automaticity, high sensitivity, precision and easy handling, as has been tested in different measurements carried out in laboratories and astronomical observatories.