WO2010060915A2

WO2010060915A2 - Measuring apparatus and method for spectroscopic measurements using led light

Info

Publication number: WO2010060915A2
Application number: PCT/EP2009/065795
Authority: WO
Inventors: Maximilian Fleischer; Paul Herrmann; Remigiusz Pastusiak; Rainer Speh; Rainer Strzoda; Kerstin Wiesner
Original assignee: Siemens Aktiengesellschaft
Priority date: 2008-11-28
Filing date: 2009-11-25
Publication date: 2010-06-03
Also published as: WO2010060915A3

Abstract

Disclosed is a spectroscopic measuring apparatus comprising at least two light emitting diode units (6), each of which includes at least one light emitting diode/LED (5), the emission wavelength of which can be adjusted to illuminate an absorption path. The at least two LED units are designed such that an adjustable spectral width relative to the emission wavelength of an LED unit (6) overlaps with an adjustable spectral width of the other of the at least two LED units (6) in order to represent an extended adjustable spectral width. The spectroscopic measuring apparatus further comprises at least one receiver to absorb reflected or transmitted light. Also disclosed is a spectroscopic measuring head comprising a housing (10) with LED units (5), lenses (4) for directing beams to the absorption path, and a central front sensing probe (1) for receiving reflected light. The LED units (5) are annularly arranged around a sensing probe. At least two light emitting diode units (6) are provided, each of which includes at least one light emitting diode (5), the emission wavelength of which can be adjusted to illuminate an absorption path. The at least two LED units (6) are designed such that an adjustable spectral width relative to the emission wavelength of an LED unit (6) overlaps with an adjustable spectral width of the other of the at least two LED units (6) in order to represent an extended adjustable spectral width.

Description

description

Measuring device and method for spectroscopic measurements with LED illumination

Spectroscopy is a contactless method for material analysis, which works mostly with infrared (IR) light, but generally with light with a wavelength between 1 nm and 500,000 nm. The spectroscopy is used primarily for the quantitative determination of known substances whose

Identification, used for process control and process monitoring and quality assurance. A spectroscopic measurement setup includes a spectrometer that can operate in different wavelength ranges, such as: UV - ultraviolet light / VIS - visible light; NIR - near infrared light, MIR - middle infrared light and FIR - far infrared light, for the separation and measurement of the different light components, as well as a light for optical coupling to a sample.

Nowadays, VIS and NIR spectroscopy typically uses halogen lamps or mercury vapor lamps or deuterium lamps as the light source. These have a spectral distribution of the power density according to the Planck radiation law, which is illustrated in Figure 1. The spectral emissivity and right spectral radiance are left ^¬ give is on the ordinate. That is, the available spectral energy density I varies as a function of wavelength λ and temperature T, which must be taken into account in the evaluation of the spectrum. Depending on differing Lichtmtensitat ^¬ surface light sources for transmission measurements and / or reflection measurements can be used. For reflection measurements, it is advantageous if the light source has a very high light intensity. Especially with highly absorbent samples, the power of a halogen light source is particularly important. High power light sources usually need to be cooled by large heatsinks or fans. Despite the cooling ent ^¬ stand during operation very high temperatures, which considerably shorten the life of the light source of today, for example, between 50 and 5000 hours. The effort to replace a halogen lamp, especially in industrial applications such. As in Onlme production monitoring systems, cause difficulties.

Spectral ranges sensitive to spectroscopy have powerful and efficient light-emitting diodes (= LEDs) at their disposal. An LED generally has a width with respect to the emission wavelength of on average 100 nm, which is usually not broadband enough for a spectroscopy measurement. For example, a halogen lamp serving as a light source in the IR region emits light in the wavelength range of 300 nm to 2200 nm.

The object underlying the invention is to provide a measuring device and a method for spectroscopy with a long-lasting illumination and increased spectral bandwidth with respect to the emission wavelength of the illumination.

The solution of this task is done by the respective feature combination of independently formulated claims. Advantageous embodiments can be taken from the dependent claims ent ^¬.

In the considered here spectroscopy method, or when the measuring apparatus for reflection or Transmissionsmes ^¬ solution, the measuring apparatus consists of illumination that emits light in the desired wavelength range, and a measuring probe for receiving the reflected or Transmit oriented light.

In the present invention, a construction of a measuring head for spectroscopy measurements, in particular with a Measuring probe and own, active lighting with LEDs presented.

According to the invention, several, at least two, with regard to their performance, such as spectral radiation distribution / spectral width, coordinated LED units with at least one LED used to realize a light source or lighting over a spectral range ben needed at the same time. An LED unit preferably consists of several, up to a few hundred individual diodes (diode array system).

As shown in FIG. 2, by means of a suitable combination of the LEDs not only an extended wavelength range but also an almost constant spectral power density for the radiation can be achieved.

A very advantageous combination of the LEDs is obtained if the distances between the center frequencies of the individual LEDs result from the sum of the wavelength deviations of adjacent LEDs from the center frequency, at which the power density has fallen to 50%, as is also indicated in FIG.

To cover the entire wavelength range of, for example, 900 to 1500 nm, high current / power LEDs instead of a halogen lamp are used in the illustration of a measuring head, wherein a series of six so-called power LEDs, i. LED units with particularly high light output, with coordinated wavelength ranges, is available. Each individual LED unit emits the light in a different wavelength range, as shown in FIG. The spectrally matched tuning ranges of the LED units are selected in such a way that overlapping the light beam results in an extended tuning width of, for example, 900 nm to 1500 nm.

Depending on requirements and application, the individual LED units consisting of at least one LED can be used in one measuring head. hen, be installed. If the measurement is to be carried out in a smaller wavelength range, the number of LED units can be reduced. Due to the flexibility in the optical design, probes can be built that are active in several wavelength ranges and whose measuring range simultaneously covers zones in the UV, VIS to NIR. It Kings ^¬ nen So two, three, or four, 2, five, or six, or more as shown in FIG three LED units are used as in.

It is advantageous to assign the spectral positions of the different spectral widths of the LED units in each case in accordance with the number and the distribution of spectral lines of a target substance. With regard to the emission frequency, gaps may be present in the extended tunable spectral width of a plurality of adjacent LED units arranged in a row. Expressed differently, the measuring device is used several times, each with intermediate free frequency ranges.

The cooler a LED is during operation, the longer its life and the higher the output power. Therefore, the LEDs are mounted on a heat sink to better dissipate the heat and thereby increase the life.

Another characteristic of LEDs in contrast to ther ^¬ mischen emitters are their very fast on and off times, which are in the range of less than 1 ms.

The use of LEDs for illumination allows two operating methods for a spectrometer:

Energy Saving Operation: The LEDs are off most of the time and are only turned on for a very short time - preferably 2 s - 100 s to take a measurement. In addition to the already efficient lighting, the timing will saved energy. This is particularly advantageous in portable spectrometer arrangements, as well as wireless montier ^¬ ten stations with battery power and wireless connectivity.

Influences such as scattered light, dark current and background radiation:

Highly sensitive detectors in spectrometers have a non-negligible dark current. Due to the fast switchability of LEDs, the lighting can be switched off shortly in time for the actual measurement and the

Dark current are measured. This results in a more stable signal with less error. The dark current measurement can also be performed cyclically to improve the result.

Of course, both operating modes can also be combined.

In the following, exemplary embodiments accompanying the invention, but these non-limiting figures will be described:

FIG. 1 shows a comparison of the spectral radiance of a body of Gluh according to Plank's law of radiation,

2 shows a combination of LED units, wherein the total radiation density is approximately equal ^¬ remaining area indicated by the dotted line, Figure 3 shows a LED-transmitter with single power LEDs, so-called high-current LEDs; a power LED is a diode array system, shown here as a solution with individual LED units 6, FIGS. 4 to 8 show components of the measuring device, such as a measuring probe with the lens carrier, LED carrier,

Reflection probe and protective glass, FIG. 9 shows an exemplary measuring principle with converging lenses

Collimators, Figure 10 shows an exemplary measuring principle with glass conductors, such as glass rod.

FIG. 2 shows the extended spectral width of at least two, here four, LED units located in the emission wavelength.

Advantages of the erfmdungsgemaßen structure or method are in detail:

Power LEDs are usually powered by constant current sources, which keeps the current flowing through the LED constant and thus independent of line fluctuations. For example, power LEDs can achieve a much longer service life, such as 20,000 - 50,000 hours, unlike conventional LEDs and halogen bulbs that can reach several thousand hours. This is an essential factor in continuous systems.

Through the use of LEDs, the change in the spectral radiation density, which is unavoidable in the aging of the classic halogen lamps, avoided or reduced ^¬ who. As a result, a recalibration of the spectrometers with a standard of reflection is less frequent, or can be completely avoided.

LED lighting has a much smaller form factor than a light bulb. Thus, the lighting can be di- rectly mounted in a small measuring head, where previously a aufwandige and sensitive Lichtzufuhrung over glass fiber reinforced ^¬ ser had experience dictate.

LEDs are much more energy efficient than thermal radiators. They have a greater light output and require correspondingly less input power. LEDs can be switched on and off in times of << Is. As a result, for example, parallel measuring spectrometers with long Ru ^¬ hey and very short operating times are conceivable. On the other hand, a chopper-like modulation of the luminous intensity for improving the signal-to-noise ratio is possible.

FIGS. 4 to 8 show a diagram for a further exemplary embodiment of a new measuring head.

Figures 9-10 show two illumination pπnzipien. The exemplary measuring device or the measuring head consists of six components: housing 10, protective glass 15, Lmsen-carrier 11, LED carrier 12, reflection probe / probe 1 and a cover 14th

Housing 10:

The housing 10 is made of stainless VA steel. Depending on the application, other raw materials can be used.

Protective glass 15:

As a protective glass 15 for the entire optics, a cylindrical window made of quartz glass (sapphire glass), on both sides po ^¬ profiled, with chamfers 0.2-0.5 mm used. The protective glass 15 assumes a protective function against dust, mechanical damage, etc., for the optical components of the probe. It should be noted that the protective glass must be sufficiently permeable to the desired wavelength range.

Lmsen-Trager 11: The Lmsen-Trager 11 is also made of stainless steel VA / stainless steel and is used for attachment of collimators. The carrier 11 is fastened by means of three threaded pins with the housing. The function of the lenses 4 is to focus the light emitted by LEDs or LED clusters and to transmit them bundled to the sample 13 or absorption path. The lenses 4 and the protective glass 15 must be transparent in the desired wavelength range. The size, thickness and physical-optical specification of the LM sen 4 is to be determined for each application. The general optical laws apply here.

LED carrier 12: The LED carrier 12 is made of aluminum or copper because of the better heat conduction compared to a VA steel and serves to accommodate and attach the LEDs. The carrier 12 is connected by two long screws, adjusting screws with the lid. With the visible adjustment screws, the distance between the lens carrier 11 and the LED carrier 12 can be adjusted. This feature allows additional adjustment possibilities for the entire optical design, e.g. based on the distance between probe and sample 13.

Reflectance probe:

The measuring probe 1 used is a fiber-coupled process measuring head with a variable measuring field or spot diameter. The optical components are advantageously achroma ^¬ table corrected and antireflection-coated.

Cover 14:

To cover the probe, a ring made of stainless steel VA is used with a hole for the power and glass conductor connections.

FIGS. 4-8 show, in detail, measuring probe 1 with the lens carrier 11, LED carrier 12, and protective glass 15.

FIG. 9 shows a measuring principle with converging lenses and collimators.

FIG. 10 shows a measuring principle with glass conductors, such as glass rods.

The measuring principle with collimators, according to FIG. 9, is based on the overlapping beam path of all LEDs. In this measurement principle, the emitted light is collimated by the collimators and radiates specifically to the absorption route. The light spot of each LED should be calculated and adjusted so that all the beams overlap, creating a spot where all desired wavelength ranges coincide. The reflected light is received by the reflection probe and sent via fiber to the detector of the spectrometer.

In the second measuring principle according to FIG. 10, a glass rod is set at a specific angle to each individual LED or LED unit 6, the angle resulting from the distance between the sample and the measuring probe, which focuses the focused light in the focus the reflection probe should go. As with the first measuring principle, reflected light is received by the reflection probe and transmitted to a spectrometer.

The probe with the collimators and the adjustable LED carrier 12 brings another advantage. The possibility of a change in distance between the LED carrier 12 and Lmsen-Trager 11, the focus of the collimated light and the size of the measuring spot can be changed, which makes the probe more versatile, since the distance between the sample and the probe can be changed arbitrarily.

Claims

claims

A measuring apparatus for spectroscopy, comprising:

at least two LED units (6) for illuminating an absorption path,

- wherein the at least two LED units (6) are so ausgest ^¬ taltet that the emitted spectral width relative to the emission wavelength of an LED unit (6) with the emitted spectral width of another at least two LED units (6) overlaps to represent an extended emitted spectral width, and

- At least one receiver on the absorption path for receiving light reflected or transmitted in the absorption path.

2. Measuring device according to claim 1, wherein a plurality of LED units (6) are positioned overlapping with respect to the emitted spectral width in order to form an extended emitting ^¬ te spectral width with substantially continuous spectral power density.

3. Measuring device according to claim 1 or 2, in which the wavelength ranges of the LED units (6) overlap one another such that the power density (I) of the total emitted radiation is between two maxima for the at least two LED units ( 6) fluctuates less than 25%.

4. Measuring device according to one of the preceding claims, wherein the distances of center frequencies of LEDs of adjacent wavelength ranges are selected so that at a lying between the center frequencies wavelength spectral power density of both LED units (6) at least 50% of the emitted in the center frequency spectral Power density amounts.

5. Measuring device according to one of the preceding claims, in which the tunable spectral widths of different LED units (6) are active in a plurality of spectral ranges, then that they are simultaneously located in several of the areas UV, VIS or NIR.

6. Measuring device according to one of the preceding claims, wherein the LED units (6) based on the Spektralbrei- th, depending on the number and the spectral position of the bands of a target substance, formed multiple times and each associated with a band.

7. Measuring device according to one of the preceding claims, wherein the LED units (6) are connected for cooling with a heat sink.

8. Measuring head for spectroscopy, comprising - a housing (10) with LED units (6), lenses (4) for

Beam guide on the absorption path and a frontally centrally positioned measuring probe (1) for receiving reflected light, wherein

the LED units (6) are arranged annularly around a measuring probe,

- At least two LED units (6) are designed such that overlap their emitted spectral widths to represent an extended emitted spectral ^width over ^¬ .

9. Measuring head according to claim 8, wherein the LED units (6) and lenses (4) are mutually adjustable for adjusting the optimal illumination of an absorption path.

10. Measuring head according to claim 8 or 9, wherein as a measuring probe (1) a fiber-coupled measuring head with variable Messfeldbzw. Luminous spot diameter is present.

11. A method of operating a measuring device or a measuring head for spectroscopy, comprising the following steps:

an absorption section is illuminated by means of at least two light-emitting diode units (6), - wherein the at least two LED units (6) are so ausgest ^¬ taltet that the emitted spectral width of the emission shaft length of an LED unit (6), with the emitted spectral width of one of the other of the at least two LED units (6 ) overlaps,

- The LEDs or LED units are turned on only for a short time, in a range of Is to 100s, to perform a measurement.

12. A method for operating a measuring device or a measuring head for spectroscopy, comprising the following steps:

an absorption section is illuminated by means of at least two light-emitting diode units (6),

- wherein the at least two LED units (6) are designed such that the emitted spectral width of the emission wavelength of an LED unit (6), with the emitted spectral width of one of the further of the at least two LED units (6 ) overlaps,

- The LEDs or LED units are switched off shortly after the actual measurement briefly and dark current, scattered light and background radiation are measured and subtracted.

13. The method of claim 11 or 12, wherein a modulation of Lichtmtensitat to improve the signal-to-noise ratio

Ratio is used.

14. The method according to any one of claims 11 to 13, wherein of all the LED units (6) the emitted light is collimated via Kollimato- ren and radiates targeted as an overlapping beam path on the absorption path, for representing ei ^¬ nes a measuring spot in which all of desired wavelengths or frequency ranges coincide.

15. The method according to any one of claims 11 to 13, wherein each LED unit (6) is associated with a glass rod, which is arranged in dependence on the distance between the sample and the probe at a certain angle, so as the bundled Specifically to lead light into the focus of the probe and to forward to ei ^¬ nem spectrometer.