Achromatic and absorption reducing light collecting system, particularly adapted to optical spectrometric analysis

This system collects light emitted by at least one light source (52) and focuses it onto at least one light detection device (54). Preferably, it comprises a first mirror (58) that collects light emitted by the source and focuses it on a second mirror (60) that focuses it in turn onto the device. The system is provided with a chamber that is opaque to all light, particularly ultraviolet radiation, and in which the light source, the light detection device and the mirrors are placed, and means of creating a vacuum in this chamber and filling it with a gas that is transparent to ultraviolet radiation.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority based on International Patent Application No. PCT/FR2003/002947, entitled “Achromatic and Absorption Reducing Light Collecting System, Particularly Adapted to Optical Spectrometric Analysis” by Jean-Charles HUBINOIS, Vincent LAVOlNE, and Herve CHOLLET, which claims priority of French Application No. 02 12467,filed on Oct. 8, 2002, and which was not published in English.

DESCRIPTION

1. Technical Domain

The present invention relates to a light collection system. It is particularly applicable to optical spectrometric analysis.

More particularly, this invention relates to a combination of mirrors with different technical characteristics, in the optical paths domain.

These mirrors are associated with each other in a particular system that forms an optical system to collect light from a light source and to send it to a light detection device that can be used at least in the field of optical spectrometric analysis, and possibly in other optical applications.

FIG. 1diagrammatically shows a light collection system2placed between a light source4and a light detection system6through which there is a light entry slit8. The light path is marked with reference10.

2. State of Prior Art

At the present time, optical collection systems used depend on:the nature of the incident light, in other words the wavelengths of the light radiation making up this incident light,the distance separating the light source from the detection device, andthe dimensions and shape of the light source and the detection device.

There are various optical systems adapted to a polychromatic light source for which the size varies from a few millimeters to a few tens of millimeters and that is located at a distance from the detection device varying from a few millimeters to several tens of centimeters.

For example, for a detection device in which light can only penetrate through a small slit called an “entry slit” a few millimeters long and a few micrometers wide, existing light transmission and collection systems are composed either of a plate with parallel faces, or a plane-convex or biconvex focusing lens, or a set of two plane-convex focusing lenses.

FIG. 2shows the path12of light in the case of a light transmission system composed of a plate with parallel faces14. References16,18,20,22and23respectively show the light source, the detection device, the entry slit of the latter, the path of light and the light beam that enters the detection device.

FIG. 3shows the path24of light in the case of a light collection system composed of a biconvex focussing lens26.

FIG. 4shows the path28of light in the case of a light collection system composed of a set of two plane-convex focussing lenses30and32.

The system inFIG. 2transmits light without focussing it, in other words without amplifying the light flux. The systems inFIGS. 3 and 4collect a maximum amount of light from source16before focussing it, in other words concentrating this light on the entry slot20of the detection device18by amplifying the light flux. If the light collection system is further from the detection device than the light source, the system that uses a set of lenses (FIG. 4) makes it possible to transmit light according to a substantially parallel beam between the two lenses30and32and therefore to minimize risks of poor focussing on the entry slit20.

Although the light collection systems inFIGS. 3 and 4amplify the light fluxes, these systems have the following disadvantages.

1) They do not enable optimum transmission of light. The optical elements (plate with parallel faces or lenses) absorb light radiation to a variable extent depending on the wavelength of the radiation.

This absorption is sometimes negligible, particularly in the case of visible light for example passing through a magnesium fluoride lens. This absorption is often greater for radiation in the far ultraviolet (corresponding to wavelengths of less than 200 nm).

For example, in the case of a 120 nm wavelength radiation, about 80% of the incident light flux is absorbed by a 1.4 mm thick magnesium fluoride lens. Similarly, absorption may be high above 800 nm (infrared range).

2) They are incapable of focussing all radiation with different wavelengths making up polychromatic light at a single point due to the presence of chromatic aberrations, particularly longitudinal chromatic aberrations. The consequence of these chromatic aberrations is dispersion of focussing points along the optical axis, as a function of the wavelength of the radiation.

This phenomenon is due to variations in the refraction index of the material from which the light collection system is made as a function of the wavelength of the incident light. The formation of longitudinal chromatic aberrations for polychromatic light passing through a lens34made of magnesium fluoride is shown for example inFIG. 5.

InFIG. 5, the reference36represents polychromatic incident light, reference38represents the focal point of light with the shortest wavelength, reference40represents the focal point of light with the longest wavelength, reference42represents the detection device, reference44represents the entry slit of this detection device, reference46represents the image spot for the shortest wavelength and reference48represents the image spot for the longest wavelength.

FIG. 5shows the partial closing that occurs as a result at the entry slit.

This problem of a different focal point depending on the wavelength is particularly severe when the range of observed wavelengths is wide and induces a difference in the sensitivity of the detection device as a function of the wavelengths.

As an example, for two light radiations with different wavelengths, the light flux at a given position on the optical axis is different for each wavelength. It may be maximum if the entry slit is placed on the focal point of one of the two wavelengths, but it is necessarily lower for the second wavelength.

In summary, although known light collection systems comprising focussing lenses partly satisfy light flux amplification needs, they do not make it possible to maximise this amplification simultaneously for all wavelengths of polychromatic light.

This is firstly due to the sometimes severe absorption of light induced by the material from which the lens is made, and secondly to longitudinal chromatic aberrations (differences between positions of light flux maxima on the optical axis).

It may also be necessary to study one or more polychromatic light sources, particularly one or more sources for which the spectrum contains one or more ultraviolet components. There is then the need to detect light emitted by such sources after this light has been collected and focussed on a detection system.

However, known light collection systems do not include any means of minimising the absorption of ultraviolet radiation on its path from the light source(s) as far as the detection device, while achromatically amplifying the light flux at a point and preventing the detection device from receiving ultraviolet radiation from sources other than the source(s) being studied.

PRESENTATION OF THE INVENTION

The purpose of this invention is to correct the disadvantages mentioned above.

Its purpose is an optical system that can solve light absorption problems and chromatic aberration problems while satisfying needs for amplification of light flux (of all natures and wavelengths) between one or more light sources and one or more detection devices.

Specifically, the purpose of this invention is a light collection system, this system being intended to collect light emitted by at least one light source and to focus the collected light onto at least one light detection device, this system being characterised in that it comprises at least two mirrors, namely a first mirror and a second mirror, the first mirror being capable of collecting light emitted by the light source and focusing the collected light on the second mirror, this second mirror being capable of focusing the light that it receives from the first mirror on the light detection device, this system being amplifying and achromatic and having a low absorption, particularly in the ultraviolet, and in that the system is provided with:a chamber that is opaque to all light, particularly ultraviolet radiation, and in which the light source, the light detection device and the mirrors are placed, andmeans of creating a vacuum in this chamber or filling it with a gas that is transparent to ultraviolet radiation.

The light detection device may or may not comprise an entry slit.

According to a first particular embodiment of the system according to the invention, the first and second mirrors have the same axis, this axis forming the optical axis of the system, and the respective focal points of the first and second mirrors are located on this optical axis.

These respective focal points for the first and second mirrors may be coincident, or they may be distinct.

In the case of this first particular embodiment, the first mirror may comprise a central drilling that is capable of allowing light focussed by the second mirror to pass towards the light detection device.

According to a second particular embodiment, the first and second mirrors are offset from each other, at least one of the first and second mirrors being off axis.

Each of the first and second mirrors may be chosen from among spherical mirrors, parabolic mirrors and ellipsoidal mirrors.

Each of the first and second mirrors may be covered by a metallic or chemical deposit.

The light detection device may comprise an entry slit and the second mirror is then designed to focus the light that it receives from the first mirror on this entry slit.

The light detection device may be an optical spectrometric analysis device comprising an entry slit and the second mirror is then designed to focus light that it receives from the first mirror on this entry slit.

The light source may be a polychromatic source.

Light emitted by this light source may contain one or more ultraviolet components.

This light source may be a luminescent discharge lamp.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

An optical system according to the invention preferably uses two mirrors called the “first mirror” and “second mirror” respectively. The shapes and characteristics of these two mirrors are predefined and a metallic or chemical deposit may or may not be formed on these mirrors.

This metallic or chemical deposit is intended to protect the mirror on which it is formed, against possible mechanical or chemical aggression and to minimize absorption of light radiation.

The first mirror is designed to collect the maximum amount of light from the light source, after which the optical system is placed, and to focus light thus collected on the second mirror. This second mirror then focuses the light that it receives onto the light detection device that follows the optical system.

This device usually comprises an entry slit and the second mirror then focuses the light that it receives on this slit. In a preferred application of the invention, this device is an optical emission spectrometer that actually comprises such a slit.

The size of the mirrors depends on the power and size of the light source, the distance between this light source and the mirrors and the distance between these mirrors and the detection device, or more precisely the slit in this device.

The first and second mirrors are focusing, which makes it possible to amplify light fluxes.

Furthermore, the use of the first and second mirrors instead of lenses solves the light absorption problems mentioned above.

Chromatic aberration problems are solved by the use of mirrors that are inherently free of chromatic effects.

The first mirror used is preferably a spherical, parabolic or ellipsoidal mirror. The same is true for the second mirror.

When the two mirrors have the same axis and their respective focal points, or focussing points, are placed on this same axis that forms the optical axis of the system, there may be a hole in the first mirror to allow light to pass from the second mirror to the light detection device (case of examples inFIGS. 6,7and10).

There is no need for a hole to be drilled in the first mirror in the case in which the two mirrors are offset from each other to form an off axis setup (case of the example inFIG. 8).

We will now reconsider the examples inFIGS. 6 to 8.

The optical system50according to the invention that is diagrammatically shown inFIG. 6, is placed between a light source52and a light detection device54for which the entry slit is marked with reference56.

The first mirror58of the system50is concave while the second mirror60of this system is convex. Light62emitted by the source52is picked up by the mirror58and focused by the latter to the mirror60that in turn focuses it on the slit56.

In the example shown inFIG. 6, the size of the light source52is comparable to the size of the mirrors58and60. However, it could be larger.

The optical axis of the system50is marked as reference X1. It can be seen that the mirror58is much larger than the mirror60and is located between this mirror and the device54, and comprises a drilling64through which passes light that the mirror60focuses on the slit56.

Furthermore, the mirrors58and60may for example be of the spherical type, and have the same axis that is coincident with the X1axis and their respective focal points F1and F2are on this X1axis. The focal distances of the mirrors58and60are denoted d1and d2respectively, where d1is greater than d2. The focal points F1and F2are distinct in the example shown inFIG. 6, but they could be coincident in other examples.

The optical system66conform with the invention that is diagrammatically shown inFIG. 7, is placed between a light source68and a light detection device70, for which the entry slit is marked as reference72.

The first mirror74of the system66is concave while the second mirror76of this system is convex. Light78emitted by the source68is picked up by the mirror74and is focussed by the latter towards mirror76that in turn focuses it on the slit72.

In the example shown inFIG. 7, the size of the light source68is small compared with the size of the mirrors74and76. For example, it may be 16 times smaller.

The optical axis of the system66is marked with reference X2. It can be seen that the mirror74is much larger than the mirror76, is located between the latter and the device70and it comprises a drilling80through which light passes that the mirror76focuses on the slit72.

Furthermore, the mirrors74and76are for example of the spherical type, have the same axis that is coincident with the X2axis and their respective focal points F3and F4are on this same X2axis. The focal distances of the mirrors74and76are denoted d3and d4respectively, where d3is greater than d4. The focal points F3and F4are distinct in the example inFIG. 7, but they could be coincident in other examples.

The optical system80according to the invention that is diagrammatically shown inFIG. 8, is located between a light source82and a light detection device84, for which the entry slit is marked as reference86.

The first mirror88of the system80is concave while the second mirror90of this system is convex. Light92emitted by the source82is picked up by the mirror88and is focused by the latter towards the mirror90that in turn focuses it onto the slit86.

It can be seen that the mirror88is much larger than the mirror90. The two mirrors88and90are offset from each other and are off axis with respect to the optical axis. Furthermore, the mirrors74and76are for example of the spherical type and their respective focal points are coincident at the same point F. The focal distances of the mirrors74and76are denoted d5and d6respectively, where d5is greater than d6.

Thus, any polychromatic light emitted by any of the sources52,68and82is focussed on the entry slit of the corresponding light detection device.

We will now describe an example application of the invention, purely for information purposes and in no way restrictively; we will consider the case of optical emission spectrometry with luminescent discharge applied to the spectrometric analysis of emission lines, for example carbon, hydrogen, oxygen and nitrogen emission lines that are between 120 nm and 160 nm.

The examples given above (FIGS. 6 to 8) may be applied to the case in which the optical system is used to optimise collection of light output from a luminescent discharge cell or lamp (forming the light source) towards an optical wavelength-dispersive spectrometer (forming the detection system).

This type of light source emits polychromatic light for which the rays, after penetrating into the detection system, are dispersed as a function of their wavelengths.

Refer toFIG. 9which shows a light discharge lamp94, an optical wavelength-dispersive emission spectrometer96, and a light collection system98with mirrors according to the invention. The path followed by light in the assembly94-96-98inFIG. 9is marked with reference100.

The use of mirrors makes it possible to amplify the light fluxes and in particular to solve the absorption and chromatic aberration problems mentioned above. The assembly94-96-98inFIG. 9may be used for light with wavelengths of 121.567 nm, 130.217 nm, 149.262 nm and 156.144 nm respectively emitted by hydrogen, oxygen, nitrogen and carbon elements during radiative deexcitation within the luminescent discharge cell.

FIG. 9diagrammatically illustrates variant embodiments of a system according to the invention: in addition to light output from the source94, the optical system98can process light which is output from another light source102and which is forced to follow the same path100due to a semi-transparent mirror104adapted to the lights considered.

Light(s) output by the optical system98can also be treated through a spectrometer106, in addition to the spectrometer96.

An appropriate semi-transparent mirror108is then provided to transfer light(s) originating from the system98onto the slit110of the spectrometer106.

The use of a light collection system according to the invention enables:maximising the light flux transmitted from the light source to the detection system by this light collection system (amplification),minimising absorption of light rays by the optical elements, andfocusing all rays with different wavelengths to the same point (achromatism).

The system according to the invention can provide considerable gains in terms of transmitted and collected light flux and in terms of simultaneously observable spectral ranges.

It may be used with any known light detection device.

It is not limited to use in the ultraviolet range of light radiation.

Furthermore, it is not limited to use with a luminescent discharge lamp, but can be used with any light source.

This system is not limited to two mirrors (see the description ofFIG. 10).

Furthermore, it is not limited to the use of mirrors with a spherical, parabolic or ellipsoidal shape.

Nor is it limited to spectrometric analysis of the C, H, O and N elements; it is also applicable to the spectrometric analysis of any chemical element.

FIG. 10shows a variant embodiment ofFIG. 6in which another mirror112is used in addition to the mirrors58and60, to reflect light output from the system50towards the slit56in the device54.

For example, such an arrangement could be used when the device cannot be placed in line with the source52.

We will now reconsider the examples inFIGS. 6 to 10.

With detection devices54,70and84, we may want to study a polychromatic light, particularly a polychromatic light source for which the spectrum contains one or several ultraviolet components. This possibility has already been considered above, particularly in the case in which the source is a luminescent discharge lamp or cell.

In accordance with the invention, a chamber is provided, that is opaque to all light, and particularly ultraviolet radiation, and inside which the source, the detection device and the mirrors are placed, so that the measurements are not disturbed. Means are also provided for creating a vacuum in this chamber, or filling it with a gas transparent to ultraviolet radiation.

This is illustrated diagrammatically inFIG. 6which shows a chamber114that is sealed and is opaque to all light and in which the source52, the mirrors58and60and the device54are located. This chamber may for example be made of a metal such as stainless steel.

Pumping means116are provided to create a vacuum in this chamber, so as to eliminate all gas such as water vapour or dioxygen that could absorb ultraviolet radiation.

The chamber114and the pumping means116are also shown diagrammatically inFIGS. 9 and 10.

In the example inFIG. 7, these pumping means are replaced by means of filling the chamber114with a gas transparent to ultraviolet radiation and for example that does not contain water or dioxygen. For example, pure dinitrogen or a rare gas such as argon could be used.

These means of filling the chamber114with gas comprise means118of injecting this gas into the chamber. A hole119, at a distance from the gas inlet location in the chamber, is provided in the wall of this chamber to allow the gas to escape (after which this gas may be pumped by means not shown). The result is then circulation of the gas in the chamber.

In the example shown inFIG. 6, the chamber is rigid. However, a “flexible” chamber could also be used.

This is shown diagrammatically byFIG. 8in which the chamber is made of several parts; a main chamber120is used that contains the mirrors, with an auxiliary chamber122that contains the source82and that is connected to the chamber120in a sealed manner through a metal bellows124. Furthermore, the detection device84is located in a sealed chamber126and this chamber is connected in a sealed manner to the chamber120through another metal bellows128.

The device, the mirrors and the source are thus located in a “flexible” chamber due to the bellows. In particular, this makes it possible to move the mirrors to refine focusing settings.

Advantageously, such a “flexible” chamber could also be used in the examples inFIGS. 6,7,9and10.

In one example of the invention not shown, a rigid chamber is used, for example in the form of a tube containing the source and the mirrors, and this chamber is connected in a sealed manner through a rigid or flexible duct (bellows) to another sealed chamber containing the detection device.

All connections between chambers are obviously made so as to not hinder propagation of light from the source as far as the detection device.