Abstract
The disclosure relates to a spectrometer, comprising: an illumination device for illuminating a spectrometric measurement region; a detection unit for detecting electromagnetic radiation coming from the spectrometric measurement region; and a spectral element, which is arranged in the beam path between the illumination device and the detection unit. The illumination device comprises: a light emitting diode having a first central wavelength, which is designed to emit first electromagnetic radiation having a first spectrum; and a luminescent element for converting a first component of the first electromagnetic radiation having the first spectrum into second electromagnetic radiation having a second spectrum. The first central wavelength is 550 nm or 3000 nm or has a value between 550 nm and 3000 nm. The first spectrum and the second spectrum have an overlap.
Claims
1. A spectrometer comprising: an illumination device configured to illuminate a spectrometric measurement region; a detection unit configured to detect electromagnetic radiation coming from the spectrometric measurement region; and a spectral element arranged in a beam path between the illumination device and the detection unit, wherein the illumination device comprises a light-emitting diode having a first central wavelength and adapted to emit first electromagnetic radiation having a first spectrum, and a luminescent element configured to convert a first fraction of the first electromagnetic radiation having the first spectrum into second electromagnetic radiation having a second spectrum and wherein the first central wavelength has a value in a range of 550 nm to 3000 nm, and the first spectrum and the second spectrum have an overlap.
2. The spectrometer as claimed in claim 1, wherein the luminescent element is arranged in the beam path.
3. The spectrometer as claimed in claim 1, wherein the luminescent element comprises: at least one further phosphor configured to convert the first fraction of the first electromagnetic radiation having the first spectrum into third electromagnetic radiation having a third spectrum; and a first phosphor configured to convert the third electromagnetic radiation having the third spectrum into the second electromagnetic radiation having the second spectrum.
4. The spectrometer as claimed in claim 1, wherein the luminescent element is applied as a coating on the light-emitting diode.
5. The spectrometer as claimed in claim 1, wherein the luminescent element is applied on one of a carrier and an optical element.
6. The spectrometer as claimed in claim 1, wherein the illumination device comprises a package in which the light-emitting diode is arranged.
7. The spectrometer as claimed in claim 1, wherein the illumination device comprises at least one optical element configured to adjust a propagation of electromagnetic radiation.
8. The spectrometer as claimed in claim 5, wherein the optical element comprises at least one of the following components: a diffuser; a directional diffuser; a reflector; a mirror; a micromirror; and an optical lens.
9. The spectrometer as claimed in claim 1, wherein the detection unit comprises: a computation unit configured to determine, using the electromagnetic radiation coming from the spectrometric measurement region, at least one of a spectrum and a spectral information of the spectrometric measurement region.
10. The spectrometer as claimed in claim 1, wherein the first central wavelength has a value in a range of 550 nm to and 1800 nm.
11. The spectrometer as claimed in claim 1, wherein the spectrometer is a miniature spectrometer.
12. A method for calibrating a spectrometer, comprising: providing a spectrometer including an illumination device configured to illuminate a spectrometric measurement region, the illumination device including a light-emitting diode having a first central wavelength and adapted to emit first electromagnetic radiation having a first spectrum with a value in a range of 550 nm to 3000 nm, and a luminescent element configured to convert a first fraction of the first electromagnetic radiation having the first spectrum into second electromagnetic radiation having a second spectrum, the first spectrum and the second spectrum having an overlap, a detection unit configured to detect electromagnetic radiation coming from the spectrometric measurement region, and a spectral element arranged in a beam path between the illumination device and the detection unit and at least one of using the first central wavelength of the light-emitting diode as a reference for wavelength calibration of the detection unit, and using an emitted intensity of the light-emitting diode as a power reference for the spectrometric measurement for the power calibration.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] Exemplary embodiments of the invention are represented in the drawings and will be explained in more detail in the following description. References which are the same in the figures denote elements which are the same or have the same effect.
[0034] FIG. 1 shows a cross section of an illumination device according to one exemplary embodiment,
[0035] FIG. 2 shows a cross section of an illumination device according to one exemplary embodiment,
[0036] FIG. 3 shows a first spectrum of a light-emitting diode according to one exemplary embodiment,
[0037] FIG. 4 shows an excitation spectrum of a light-emitting diode and an emission spectrum of a phosphor, the excitation spectrum and the emission spectrum not having an overlap,
[0038] FIG. 5 shows an outline of a first spectrum of a light-emitting diode and of a second spectrum of a luminescent element in a common coordinate system according to one exemplary embodiment, the first spectrum and the second spectrum having an overlap,
[0039] FIG. 6 shows an outline of an emission spectrum of an illumination device according to one exemplary embodiment, the first spectrum of the light-emitting diode and the second spectrum of the luminescent element corresponding to the first spectrum shown in FIG. 5 and the second spectrum shown in FIG. 5,
[0040] FIG. 7 shows a spectrometer in a reflection geometry according to one exemplary embodiment,
[0041] FIG. 8 shows a spectrometer in a transmission geometry according to one exemplary embodiment, and
[0042] FIG. 9 shows a flowchart of a method for calibrating the spectrometer according to one exemplary embodiment.
EXEMPLARY EMBODIMENTS OF THE INVENTION
[0043] A spectrometer 1000 comprises an illumination device 100 for illuminating a spectrometric measurement region 104, a detection unit 106 for detecting electromagnetic radiation 1004 coming from the spectrometric measurement region, and a spectral element 105 which is arranged in the beam path between the illumination device 100 and the detection unit 106.
[0044] FIG. 1 shows a cross section of the illumination device 100 according to one exemplary embodiment. A light-emitting diode 102 is arranged on a substrate 101. In FIG. 1, the substrate 101 comprises a recess in which the light-emitting diode 102 is arranged. The luminescent element 103 is arranged on the light-emitting diode 102 in the recess. The light-emitting diode 102 having a first central wavelength 1001″ is adapted to emit first electromagnetic radiation 1001 having a first spectrum 2001, the first spectrum having for example the Gaussian profile shown in FIG. 3. The first electromagnetic radiation 1001 passes through the luminescent element 103, the luminescent element 103 being adapted to convert a first component 1001′ of the first electromagnetic radiation 1001 into second electromagnetic radiation 1002 having a second spectrum 2002. That is to say, the luminescent element 103 comprises at least one phosphor which can be excited by the first electromagnetic radiation 1001 to emit the second electromagnetic radiation 1002. Not all energy states of charge carriers are allowed in the phosphor. Electronic bands or band structures that define which energies various charge carriers can have, and which they cannot, are therefore often referred to. In these band structures, energy bands or states may additionally be generated by deliberate introduction of extraneous atoms (also referred to as activators). The fundamental mode of action of the luminescent element 103 is based on the physical principle of luminescence. The light generation in this case takes place by excitation of an electron with the energy of the first electromagnetic radiation 1001 impinging on the luminescent element 103. The electron is thereby transported from a low energy state (valence band) into a higher energy state (generated by activators) or the so-called conduction band. A hole in the valence band is also created by this process. After a certain time, the electron releases its energy by emitting light and returns into the valence band. The second spectrum 2002 of the second electromagnetic radiation 1002 converted in this way is dependent on the band structure of the phosphor and on the activators. A second fraction 1001′″ of the first electromagnetic radiation 1001 passes through the luminescent element 103 without being converted. The emission spectrum 1003 of the illumination device is therefore given by a superposition of the spectrum of the unconverted second fraction 1001′″ of the first electromagnetic radiation 1001 and of the second spectrum 2002 of the converted first fraction 1001′, i.e. of the second electromagnetic radiation 1002. An exemplary profile of the emission spectrum 2004 of the illumination device 100 according to one exemplary embodiment is represented in FIG. 6.
[0045] As an alternative to the exemplary embodiment shown in FIG. 1, the light-emitting diode 102 may also be arranged on a substrate without a recess, and the luminescent element 103, which acts as a phosphor of the illumination device, may be applied onto the light-emitting diode 102, for example as a layer or coating.
[0046] One difference between the exemplary embodiment shown in FIG. 1 and the exemplary embodiment shown in FIG. 2 is that the luminescent element 103 is arranged directly on the light-emitting diode 102 in FIG. 1, while the luminescent element 103 is arranged as a so-called remote phosphor on a separate carrier 101′ in FIG. 2. In the exemplary embodiment shown in FIG. 2, the luminescent element 103 is kept separated from the light-emitting diode 102 by the carrier 101′. The carrier 101′ holds the luminescent element 103 at a distance above the substrate 101. The light-emitting diode 103 is arranged on the substrate 101, between the luminescent element 103 and the substrate 101.
[0047] For example, the light-emitting diode 103 may be arranged in a package, for example an SMD package. At least one optical element (for example a diffuser, directional diffuser, reflector, mirror, micromirror, optical lens), which influences and/or manipulates the light propagation, may likewise be fastened on the SMD package. The luminescent element 103 is conventionally applied on the light-emitting diode 103, as shown for example in FIG. 1, as a remote phosphor on a separate carrier 101′, as shown for example in FIG. 2, or it may for example also be arranged or applied on the optical element.
[0048] As an alternative or in addition, further optical elements may also be applied on the package or in the beam path between the LED package and the spectrometric measurement region. For example, the light propagation of the light source may be optimized for the spectrometry by a diffuser or directional diffuser or a (further) optical lens.
[0049] In FIG. 3, the first spectrum 2001, i.e. an emission spectrum of the light-emitting diode 102 before a fraction of the first electromagnetic radiation 1001 is converted by the luminescent element 103, is outlined according to one exemplary embodiment. The wavelength is plotted on the x axis 200 and the intensity or spectral ray density is plotted on the y axis 201. The first spectrum 2001 in this case has a profile similar to a Gaussian function. The spectrum 2001 of light-emitting diodes is usually expressed by a single wavelength, for example a central wavelength 1001″ of the light-emitting diode 102. The central wavelength 1001″ describes the wavelength which lies midway between two points (wavelengths) having a spectral density of 50% of the peak of the spectrum, i.e. 50% of the maximum of the spectrum. For a symmetrical spectrum such as the first spectrum 2001 shown in FIG. 3, the central wavelength 1001″ corresponds precisely to the wavelength at which the spectrum is maximal.
[0050] In FIG. 4, an excitation spectrum 20 of a light-emitting diode and an emission spectrum 2002 of a phosphor as described in the prior art are outlined, the excitation spectrum and the emission spectrum not having an overlap. The phosphor used in this case is excited with blue light (central wavelength 10′ of for example 460 nm, 490 nm or a value between 460 nm and 490 nm) and then emits electromagnetic radiation in the near infrared range, particularly in the range of from 700 nm to 1050 nm. A part of the blue light is not converted and therefore remains in the emission spectrum of an illumination device having a blue LED and the phosphor from the prior art described in this example, this light fraction lying outside the wavelength interval 2000 which is usually registered in a spectrometric measurement.
[0051] In FIG. 5, the first spectrum 2001, i.e. an emission spectrum of the light-emitting diode 102, which acts as an excitation spectrum for the luminescent element 103, and the second spectrum 2002, which describes the emission spectrum of the luminescent element 103 after excitation by the first electromagnetic radiation 1001′, are outlined by way of example according to one exemplary embodiment in a common coordinate system. The wavelength is plotted on the x axis 200 and the intensity or spectral ray density is plotted on the y axis 201. A wavelength range 2000 which is usable for spectrometry is plotted on the x axis. Typical wavelength intervals within which a significant photocurrent is generated are 400 nm to 1100 nm for photodetectors based on silicon, 600 nm or 900 nm to 1700 nm for photodetectors based on indium gallium arsenide (In.sub.0.53Ga.sub.0.47As), and 900 nm to at most 2600 nm for photodetectors based on indium gallium arsenide (In.sub.xGa.sub.1-xAs; with x>0.53). The first central wavelength 1001″ of the light-emitting diode 102 in this case lies in the wavelength interval 2000 which is usable for spectrometry. As represented in FIG. 5, the first spectrum 2001 and the second spectrum 2002 have an overlap 2000′. In this way, it is in particular possible, in addition to the emission spectrum 2002 of the luminescent element 103, also to use the spectrum of the light-emitting diode 102 for spectrometry. The curve profile of the second spectrum 2002 depends, in particular, on the chemical composition of the luminescent element 103.
[0052] The spectrometer 1000 comprises the illumination device 100, the light-emitting diode 102 according to one exemplary embodiment having the first central wavelength with a value of 630 nm, a near infrared phosphor, which emits the second electromagnetic radiation 1002 having the second spectrum 2002 with wavelengths in the range of from 700 nm to 1100 nm, being used as the luminescent element 103. Typical phosphors are based for example on garnets, silicates, oxynitrides or oxycarbonitrides, or nitrides or carbonitrides. The emission spectrum 2004 of the illumination device 100 in this exemplary embodiment comprises electromagnetic radiation having wavelengths in the interval of from 600 nm to 1100 nm. The entire emission spectrum 2004 therefore lies in the wavelength interval 2000 usable for spectrometry and has an approximately constant power in this wavelength range, and in particular electromagnetic radiation of all wavelengths in the usable wavelength interval 2000 is directed with a sufficient power onto an object which is intended to be spectrometric studied, so that the reliability of the measurement results of the detection unit 106 for the wavelengths of the wavelength interval 2000 can be increased.
[0053] The first central wavelength 1001″ of the light-emitting diode 103 may, for example, be 550 nanometers (nm) or 1000 nm or have a value between 550 nm and 1000 nm. As an alternative, the first central wavelength 1001″ of the light-emitting diode 102 may be 760 nm or 2500 nm or have a value between 780 nm and 2500 nm. As an alternative, the first central wavelength 1001″ of the light-emitting diode 102 may be 610 nm or 3000 nm or have a value between 610 nm and 3000 nm. As an alternative, the first central wavelength 1001″ of the light-emitting diode 102 may be 610 nm or 1000 nm or have a value between 610 nm and 1000 nm. As an alternative, the first central wavelength 1001″ may be 580 nm, 630 nm, 800 nm or 1200 nm.
[0054] In a further exemplary embodiment, the light-emitting diode 102 of the illumination device 100 of the spectrometer 1000 has the first central wavelength 1001″ with a value of 1200 nm, and the luminescent element 103 comprises a phosphor which emits the second electromagnetic radiation 1002 having the second spectrum 2002 with wavelengths in the range of from 1280 nm to 1800 nm. The emission spectrum 2004 of the illumination device 100 in this exemplary embodiment therefore comprises wavelengths of from 1150 nm to 1800 nm. The entire emission spectrum 2004 therefore lies in the wavelength interval 2000 usable for spectrometry and has an approximately constant power in this wavelength range, and in particular electromagnetic radiation of all wavelengths in the usable wavelength interval 2000 is directed with a sufficient power onto an object which is intended to be spectrometric studied, so that the reliability of the measurement results of the detection unit 106 for the wavelengths of the wavelength interval 2000 can be increased.
[0055] In a further version of the spectrometer 1000, a light-emitting diode 102 having 800 nm as the first central wavelength may be used. The luminescent element 103 may comprise a plurality of phosphors, which in total emits the second electromagnetic radiation 1002 having the second spectrum 2002 with wavelengths in the range of from 850 to 1700 nm.
[0056] In FIG. 6, an outline of the emission spectrum 2004 of the illumination device 100 according to one exemplary embodiment is shown, the first spectrum 2001 of the light-emitting diode 102 and the second spectrum 2002 of the luminescent element 103 corresponding to the first spectrum 2001 shown in FIG. 5 and the second spectrum 2002 shown in FIG. 5. The wavelength is plotted on the x axis 200 and the intensity or spectral ray density is plotted on the y axis 201. The curve profile generally depends on the chemical composition of the luminescent element 103 and on the light-emitting diode 102 used, in particular the first central wavelength 1001″ of the light-emitting diode 102. The emission spectrum 2004 of the illumination device 100 is given by a superposition of the spectra of the unconverted second fraction 1001′″ of the first electromagnetic radiation 1001 and of the second electromagnetic radiation 1002 emitted by the luminescent element 103.
[0057] FIG. 7 shows an exemplary embodiment in which the spectrometer 1000 is represented in cross section and is arranged in a reflection geometry. The illumination device 100, which for example has the same structure as the illumination device 100 shown in FIG. 1 or FIG. 2, and the detection unit 106 are arranged on a common side in relation to the spectrometric measurement region 104 in the reflection geometry, the detection unit 106 being arranged in such a way that, in particular, the electromagnetic radiation emitted 1003 by the illumination device 100 and reflected 1004 by the spectrometric measurement region 104 impinges on the detection unit 106 and can be registered by it. The detection unit 106 may for example comprise a detector element or a detector array, which comprises a plurality of detector elements. A radiation sensor, for example based on silicon (Si), germanium (Ge), germanium on silicon, indium gallium arsenide (InGaAs), lead selenite (PbSe) may be used as a detector element. For example, photodiodes or bolometers are also suitable as radiation sensors. Radiation sensors may, as a function of a property of the electromagnetic radiation impinging on the radiation sensor, output an electrical detection signal which is a measure of the radiation property. Radiation sensors may, for example, measure an intensity or an energy flux density of the electromagnetic radiation coming from the spectrometric measurement region. The spectral element 105 is arranged in FIG. 7 as a separate component in the beam path between the spectrometric measurement region 105 and the detection unit 106. In one exemplary embodiment, the detection unit 106 or the illumination device 100 may comprise the spectral element 105 or the spectral element 105 may be arranged in the beam path between the illumination device 100 and the measurement region 104.
[0058] FIG. 8 shows an exemplary embodiment in which the spectrometer 1000 is represented in cross section and is arranged in a transmission geometry. The illumination device 100, which for example has the same structure as the illumination device 100 shown in FIG. 1 or FIG. 2, and the detection unit 106 are arranged on mutually opposite sides of the spectrometric measurement region 104 in relation to the spectrometric measurement region 104. That is to say, the spectrometric measurement region 104 is arranged between the illumination device 100 and the detection unit 106. The spectral element 105 may, as described above in connection with FIG. 7, be configured as part of the illumination device 100 or as part of the detection unit 106, or it may be arranged as a separate component in the beam path between the illumination device 100 and the spectrometric measurement region 104.
[0059] The spectral element 105 may for example comprise a tunable Fabry-Perot interferometer (FPI), birefringent crystals and polarizers, or another wavelength-selective filter, as well as optionally optical lenses, optical apertures, microlenses, microlens arrays, beam splitters, mirrors, micromirrors, etc. The spectrometer 1000 may, for example, be configured as a static or mobile Fourier-transform spectrometer or as a Fabry-Perot spectrometer. The illumination device 100, the spectral element 105 and the detection unit 106 may be arranged in a common package. For example, the spectrometer 1000 may be configured as a portable instrument. For example, the spectrometer 1000 may be configured as a miniature spectrometer. In one exemplary embodiment, the spectrometer 1000 may be integrated into a mobile terminal, for example a smartphone.
[0060] In FIG. 9, a flowchart of a method 300 for calibrating the spectrometer 1000 is represented. The spectrometer comprises, for example, the illumination device 100 shown FIG. 1 or FIG. 2. The known emission spectrum of the light-emitting diode 102 may be used to calibrate the spectrometer 100, since because of the selection of the central wavelength the detection unit is sensitive to the first electromagnetic radiation 1001 which is emitted by the light-emitting diode. The method may comprise a wavelength calibration 301 and/or a power calibration 302.
[0061] In the method 300 represented in FIG. 9, both the wavelength calibration 301 and the power calibration 302 are represented in the flowchart. In the wavelength calibration 301, use is made of the fact that the first central wavelength 1001″ of the light-emitting diode 102 is known. During the wavelength calibration, the detection unit 106 registers the first electromagnetic radiation 1001′ having the first spectrum 2002, the value of the known first central wavelength 1001″ being assigned to the central wavelength of the spectrum registered. For example, a reference data set 301′, which may be applied to the measurement result of the spectrometric measurement, can therefore be generated. In the power calibration 302, the emitted intensity of the light-emitting diode 102 is used as a power reference 302′ for the spectrometric measurement. To this end, the measured spectrum is evaluated in terms of the LED intensity reflected by the object being studied. For example, the measured LED intensity may be compared with a 100% reflection stored in the electronics, so that an absolute value of the reflected intensity for this wavelength is obtained. In a further example, the LED intensity measured during a test exposure may be used in order to prevent saturation of the photodiodes during the subsequent measurement. In a further example, the spectrum is recorded repeatedly, so that the variation in the LED intensity allows conclusions about a modified measurement condition (for example change of the measurement distance, the measurement angle, of the object being studied, or the like).
