PROCEDURE FOR CONTROLLING A RADIATION SOURCE AND CONTROLLABLE RADIATION SOURCE

Abstract: The present invention concerns a procedure for controlling a radiation source which has been implemented with the aid of light-emitting diodes, or LEDs, from the radiation produced by them being separated the desired wavelength range, of which the intensity is controlled or maintained constant. The radiation source is implemented by means of a LED row (2) formed of semiconductor chips, or LED elements, (21, 22, 23,..., 26), from the radiation of which is separated a wavelength range ( DELTA lambda 1, DELTA lambda 2,...) depending on the location of the LED element in said arrays with an optical means dispersing the radiation to a spectrum, and the intensity of the radiation in this wavelength range, or of the output radiation, is controlled or maintained constant by observing the intensity thereof and regulating with its aid the current passing through the respective LED element. The wavelength ranges of the output radiation are selected electrically by activating a suitable LED element (21, 22, 23,..., 26) in the LED row (2).

Description: PROCEDURE FOR CONTROLLING A RADIATION SOURCE AND CONTROLLABLE RADIATION SOURCE The present invention concerns a procedure for con- trolling a radiation source, said radiation source being implemented with the aid of light-emitting diodes, or LEDs, of the radiation produced by them the desired wavelength range being separated and the inten sity of this radiation range being controlled or kept constant. The present invention also concerns a controllable rad iation source which has been composed of light-emittin diodes, or LEDs, and which comprises optical means for separating the desired wavelength range from the radia tion produced by said LEDs and means for controlling the intensity of said wavelength range or keeping it constant. A radiation source of the kind described is used for instance in spectrometers and photometers. In frequent cases the radiation source is the most significant fac tor limiting the capacity of performance and usability of an instrument. In apparatus meant to be used in in- dustrial conditions, as radiation source have usually been employed thermal radiators, such as sources based on an incandescent filament, for instance. Their prob¬ lem is, however, poor optical efficiency and consequen high heat dissipation, as well as poor vibration toler ance, short .service life and difficulty of modulation. In recent years the development of semiconductor tech¬ nology has introduced on the market efficient lasers based on semiconductor junctions, and light-emitting diodes, or LEDs. They afford several advantages over traditional radiation sources: for instance, small size and low energy consumption, good reliability, long ser— vice life (even more than 10° hours), high operating speed, easy connection to optic fibres. Furthermore, they can be electrically modulated with ease. Semicon¬ ductor radiation sources are nowadays available for the wavelength range about 4O0 n to over 10 μm; admitted¬ ly, though, for operation at room temperature only up to about.3200 nm. The said range is usable in quantita¬ tive and qualitative analysis of most substances. Semiconductor lasers are nearly ideal radiation sources for spectrometers in view of their narrow spectrum. However, high' price and poor stability are their prob¬ lems. It is also a fact that the selection of standard wavelengths is scanty, particularly in the near IR range. LEDs enable, owing to their wider radiation spectrum, a considerably wider wavelength range to be covered', and they are also lower in price. The spectral radiance of LEDs is on the same order as that of most thermic radiation sources, or higher. The radiations spectrum of LEDs is mostly too wide to allow them to be used as such in spec roscαpic measure¬ ments. Moreover, the shape of the radiation spectrum, the peak wavelength and the radiant power change power— fully with changing temperature and driving current, and with time. In spectrometer designs of prior art based on the use of LED sources, the measuring band is separated from the LEDs, usually, with the aid of separate filters, or the LED is used without filtering, in which case the resolution will also be poor. The variation of radia¬ tion intensity has most often been compensated for, either by mere electric compensation or by maintaining constant temperature of the LED, which has lead to a demanding and expensive mechanical design.Owing to the high price and difficult manipulation of the filters (e.g. miniaturizing, cutting), the number of wavelengt bands in such pieces of apparatus is usually small (2 to 10) . The object of the present invention is to eliminate, among others, the drawbacks mentioned above and to pro vide a novel procedure for controlling a radiation source, and a controllable radiation source. This is realized with the aid of the characteristic features of the invention stated un the claims hereto attached. With the aid of the invention the following advantages are gained, among others. On the basis of the procedure of the invention a controllable radiation source can be realized which is simple as to its construction and contains no moving parts. Furthermore, the radiation source can be realized in a design with small external dimensions. Θood wavelength resolution can be achieved in spite of small size. The wavelength resolution is determined, in the first place, be the size of the LED elements (typically only 300 μm by 300 μ ) and by the angular dispersion and dimensions of the optics which disperse the radiation into a spectrum. The number of measuring channels in the spectrometer in which the procedure of the invention is applied may with ease be increased to be several dozen. The amount of required electronic control and driver apparatus is not neces¬ sarily dependent on the number of LEDs. According to the procedure, the intensity of the wavelength bands is also stabilized.* The creeping of the spectrum of the LEDs and of the total radiant power will then have no influence on the output intensity of the radiation source. According to the procedure, the wavelengths of the radiation source may be electrically selected. The modulation frequency can be made very high if required (e.g. less than 1 μs per LED). It is possible in connec¬ tion with the controllable radiation source of the in- vention to use for detector a single-channel radiome¬ ter, in which advantageously one single detector ele¬ ment is used. The procedure, and the radiation source applying it, can be used in the visible light and IR radiation ranges. The invention may be applied in the transmitter part of the spectrometer. In certain applications it is also possible to integrate the whole spectrometer to be one single component, which may be hermetically encapsulat ed. A reliable and stable spectrometer, and one which is usable in field work, is obtained with the aid of such integration. Significant advantages are gainable with the aid of the procedure of the-invention, and of the radiation sourc employing it, in reflection or transmission measure¬ ments on diffuse objects, compared with the multiple detector technique, which has become commonly used in recent years. In multiple element spectrometers the diffuse radiation has to be collected on small-sized detector elements through a narrow entrance slit, whereby high optical collection losses are incurred. In the controllable radiation source of the invention, th optical collection losses can be minimized by using fo detector the single-channel radiometer mentioned above, which has a large-sized detector element and, at the same time, also a wide collection angle. The invention is .described in detail in the following with the aid of the' attached drawings; wherein:- Fig. 1 presents schematically a spectrometer in which the invention is applied; Fig. 2 presents by way of example the radiation spec¬ trum of a LED and the radiation spectrum achievable with the aid of the invention; Fig. 3 presents in the form of a block diagram a LED row and its driver and control means; Fig. 4 presents a radiation source according to the invention in which an optic fibre is utilized; and Fig. 5 presents schematically a reflection spectromete in which the invention is applied, Fig. 6 presents a radiation source row, Fig. 6B, by which a LED row, Fig. 6A, may be replaced. In the procedure of the invention the radiation source is realized with a LED array composed of semiconductor chips, or LED elements, or equivalent. From the radia¬ tion of' the LED elements a wavelength range depending on the location of the LED element in said array is separated with an optical means dispersing radiation t a spectrum, and the intensity of this wavelength range or output radiation, is controlled or maintained con¬ stant by observing its intensity and with its aid regu lating the current passing through the respective LED element. The desired wavelength range may then be selected electrically by activating the respective ele ment in the LED array. In the spectrometer of Fig. 1 the procedure of the in¬ vention is applied. The radiation source 1 of the spec trometer has been composed of light-emitting, diodes, o LEDs. Specifically,' the radiation source 1 consists of a row of LEDs 2 which comprises a plurality of side-by side LED semiconductor chips, or LED elements 21, 2 5 source further comprises optical means for separating the desired wavelength range from the radiation pro¬ duced by the LEDs, and means for maintaining the inten sity of the radiation in the wavelength range constant or on desired level. Said optical means consist of op¬ tical pieces of equipment, such as lenses, mirrors, gratings, slits and beam dividers, the radiation pro- duced by the LEDs being collected and dispersed to a spectrum with the aid of said means, and the radiation of the desired wavelength range being directed on an output slit or equivalent. The optical means in the spectrometer of Fig. 1 include a radiation-collecting lens 3, a reflecting grating 5 dispersing the radiation to a spectrum, and a stop 6 presenting an output slit 6. It is equally possible to use for radiation-collect¬ ing component e.g. a concave mirror, and in place of the reflection grating one may use a transmission grat¬ ing or a prism. These components may also be combined by using, for instance, a focussing reflector- or trans¬ mission 'grating. The controllable radiation source 1 of the invention further comprises optical means and a detector 7 for observing and/or measuring the intensity of the outgo¬ ing radiation. The optical means employed in the spec¬ trometer of Fig. 1 consist of a beam divider 8. The radiation source also comprises a driver and control means 9, to which the detector 7 has been connected. The driver and control means 9 is connected to the LED row 2, to each of its elements 21, 22, ..., 26. With the aid of the driver and control means 9, the desired LED element is selected. Thus from the radiation pro— duced by each LED element the desired wavelength range is directed on the output slit, this wavelength range depending on the location of the LED element 21, 22, ..., 26. From the radiation going out from the radia¬ tion source, part is separated with the aid of the op- tical means to go to the detector 7, the current flow¬ ing through the respective LED element and producing the radiation in question being regulated in accordance with the intensity data supplied by the detector and in such manner that the radiation intensity of that wave¬ length range, and thus the intensity of the output radiation, is constant. Also other optical means may further be attached to the radiation source 1. In Fig. 1, the output radiation passing through the slot 6 is rendered parallel with a lens 10, whereafter it is directed on the object of measurement, 11. After the object of measurement, in the direction of propagation of the radiation, follows a receiver 12, in which capacity in measurements made on diffuse objects advantageously serves a wide-area radiometer. The electric signal from the receiver 12 is amplified in an amplifier 13 and fed to a calculation unit 14, and it is possibly displayed on a display pro¬ vided in conjunction therewith. With the calculating unit 14, or by controls on a panel provided in conjunc¬ tion therewith, the driver and control unit 9 of the radiation source 1 may possibly also be controlled. For reflection and/or transmission measurements, it is advantageous to devise at least the radiation source 1 to be an integral unit. It can be suitably shielded against ambience, for instance enclosed in a hermeti¬ cally sealed housing, for improved reliability. The circuitry of the driver and control means 9 is pre¬ sented in block diagram form in Fig. 3. The LED ele- ments 21, 22, ..*.,, 2(N-1), 2N (N = an integer 1, 2, 3, ... ) has been connected over a current control circuit 15 to a voltage source 30. The LED elements are con¬ nected to a selector means 16, such as a decoder, and this is connected to the calculating unit 14. The de- sired LED element, for instance the element 23, and the desired wavelength range of the output radiation from the radiation source are selected with the aid of the selector means 16. Part of the radiation obtained from the LED elements is picked up with the aid of the opti¬ cal means S and carried to the detector 7. The detector 7 is connected with the controller 9a. The current con- trol circuit 15 is governed by the controller 9a with the aid of the signal from the detector 7, in such man¬ ner that the output signal of the detector has con¬ stantly the desired magnitude, whereby the intensity of the output radiation also maintains the desired level. Fig. 2 illustrates the spectrum of the output radiation obtained by the procedure of the invention. In the rec¬ tangular coordinates, the ordinates represent the radi¬ ation intensity I and the abscissae, the wavelength The radiation spectrum of a standard LED has the shape of a broad bell curve, L, Fig. 2A. The spectra of the radiation coming from, a radiation source 1 according to the invention are narrow bands of desired height, or ranges, S, Figs 2B—2E, within the range delimited by the bell curve L. The spectrometer of Fig, 1 and the radiation source therein employed operate in principle as follows. The driver and control means 9 selects and activates in the LED row 2 the first LED element 21. The radiation of the LED element is collected with the lens 3 and sent as a parallel beam to the reflection grating 4. The lens 3 further produces of the radiation reflected by the grating a spectrum on the stop 5. That part of the radiation (wavelength range ΔX ) passes through the output slit 6 which is determined by the location of the LED element 21 in the row and by the locations and dimensioning of the other optical means 3,4,6. From the lens 10 a parallel output radiation is obtained, strik- ing the object of measurement 11. Through the optical means 8, part of the output radia- tion is directed to strike the detector 7, and the sig nal representing the intensity that h§s been obtained is recorded with the driver and control means 9. The driver and control means adjusts the current going to the LED element 21 to have a value such that the signal obtained from the detector 7 rises to desired level, whereby the intensity of the output radiation also as¬ sumes the desired value, e.g. I β . Hereafter the actual measurement takes place in the wavelength range Λ ^ x (Fig. 2B) and the result is recorded in the calculat¬ ing unit 14. Next, the driver and control means 9 ac¬ tivates the LED element 22 and, after the output inten¬ sity has similarly been adjusted to required level, measurement takes place in the wavelength range -4A a (Fig. 2C) . The driver and control means 9 activates all LED elements 21 ... 2N in succession and measurements are similarly carried out in the wavelength ranges Λλ 1 ..., AΛ (Fig, 2E) , whereafter the driver and control means 9 activates again the LED element 21, and so on. The calculating unit 14 is electrically synchronized with the driver and control means 9 so that all results of measurement that have been recorded can be coordi¬ nated with the correct wavelength ranges Δ i. , . . . M . In the foregoing in the spectrometer of Fig. 1 the op¬ tical means for collecting the radiation from the LED rows, for dispersing it to a spectrum and for directing the desired wavelength range on the output slit 6, com¬ prising a lens 3 and a reflection grating 4 or equival- ent. In the .radiation source of Fig. 4, said, optical means comprise a beam divider cube 17 'and on the sui— face thereof a focussing transmission grating 18. In this case the detector 7 for monitoring the intensity of the output radiation has been disposed in conjunc- tion with the beam divider cube 17. The dividing inter— face 17a of the beam divider cube reflects to the de¬ tector 7 part of the radiation going through the beam divider cube to the output slit. In the capacity of output slit serves an optic fibre connector 19 to which an optic fibre 31 has been connected. The radiation source 1 with its LED row 2 is enclosed in a suitable housing 32. Fig. 5 depicts a compact spectrometer intended for reflection measurements and comprising both the trans¬ mitter of the radiation source and the receiver. The radiation source 1 is equivalent in its design with the radiation source of Fig. 4, and the same reference numerals are employed to indicate equivalent compo¬ nents. In front of the radiation source 1, in the di¬ rection of propagation of the radiation, a receiver 33 has been disposed. This receiver consists of a beam divider 34 and a receiver detector 12. The output radi¬ ation is carried put from the device through an aper¬ ture 35. The object of measurement 36, which is a re¬ flecting surface, is placed in front of the radiation beam coming from the spectrometer. The radiation re¬ flected from the object of measurement 36 returns through the aperture 35 to the beam divider 34 of the receiver 33, from the dividing interface 34a of which the major part of the radiation is reflected to the receiver detector 12 and is detected. Both the radia¬ tion source 1 and the receiver 33 have in this case been integrated to constitute a single unit, which may be provided with a hermetically sealed housing 36 if required. In this way the reflection spectrometer can be made into a device usable in the field. The radia¬ tion from the spectrometer can be directed to strike the object of measurement 36 without any separate out¬ put optics, as has been set forth in the foregoing, or by using e.g. a standard lens optic system or a fibre- optic component. The LED row may be replaced with another type of radia- tion source row, as can be seen in Fig. 6. Fig. 6A pre sents, seen from the side, a LED row 2 comprising con¬ secutively placed LED elements 21, 22, ..., 2N. The alternative radiation source row depicted in Fig. 6B comprises a number of LED elements or separate, encap¬ sulated LEDs 21, 22, ..., 27, to each of them connecte an optic fibre 41, 42, ..., 47 by its first end 41a, 42a, ..., 47a. The other ends 41b, 42b, ..., 47b of th optic fibres are arranged in a configuration such as i desired. The ends of the optic fibres are then close together and are thus equivalent e.g. to the LED row presented in the foregoing. It should be noted that although the invention has bee presented in the foregoing in the first place only wit the aid of one measuring apparatus, it is obvious that the procedure for controlling a radiation source, and the controlled radiation source, can be employed in numerous other applications as well in which a stable, or easy to control , radiation source is needed which produces radiation within a given wavelength range in plurality of alternatingly active, and if need be na — row, wavelength bands. Moreover, in the embodiment ex¬ amples of the invention presented in the foregoing a LED row or equivalent is employed, but it is also pos¬ sible to use in its stead a LED matrix or any other equivalent LED array.

Claims: CLAIMS 1. A procedure for controlling a radiation source which has been implemented with the aid of light—emitt- ing diodes, or LEDs, from the radiation produced by them being separated the desired wavelength range, of which the intensity is controlled or maintained con¬ stant, characterized in that the radiation source is implemented by means of a LED array (2) or equivalent (2') formed of LED elements (21, 22, 23, ..., 26), from the radiation of which is separated a wavelength range (Δλ. A Δ s, ...) depending on the location of the LED element in said arrays with an optical means dispersing the radiation to a spectrum, and the intensity (I a ) of the radiation in this wavelength range, or of the out¬ put radiation, is controlled or maintained constant by observing the intensity thereof and regulating with its aid the current passing through the respective LED ele¬ ment. 2. Procedure according to claim 1, characterized in that the wavelength ranges ( Δ X , Δλ s t ...) of the out¬ put radiation are selected electrically by activating a suitable LED element (21, 22, 23, ..., 26) of the LED array (2,2'). 3. A controllable radiation source (1) which is com¬ posed of light-emitting diodes, or LEDs, and which com¬ prises optical means for separating the desired wave- length range from the radiation produced by the- LEDs and means for controlling or keeping constant the in¬ tensity of said wavelength range, characterized in that the radiation source consists of a LED array (2,2' ) which comprises a plurality of sidε-by-side LED semi- conductor chips, or LED elements (21, 22, 23, ..., 26) or equivalent; the optical means consist of optical apparatus (3,4,6) , with the aid of said apparatus the radiation produced by the LEDs being collected and the radiation dispersed to a spectrum and the desired wave length range { Λ , ΔA a , ...) of the spectrum being directed on an output slit (6) or equivalent; optical means (8) and a detector (7) for monitoring and/or measuring the intensity (I α ) of the output radiation; and a driver and control means (9) by the aid of which from the radiation produced by each LED element is selected to the output slit (6) the desired wavelength range, which depends on the location of the LED element (21, 22, 23, ..., 26) in the LED array (2,2' ) and from which output radiation part is with the aid of optical means (8) separated to go to a detector (7) according to the intensity values from which the driver and coπ- trol unit (9) regulates the current passing through the respective LED element so that the intensity (Ie») of the radiation in said wavelength range and of the out¬ put radiation is maintained at desired height. 4. Means according to claim 3, characterized in that the LED elements (21, 22, 23, ..., 26) of the LED array (2,2' ) are connected together and connected with a cur¬ rent control circuit (15) and a selector means (16) with the aid of which the desired LED element (e.g. 23) and the desired wavelength range (-^^a) of the output radiation of the radiation source are selected and which current control circuit (15) is governed by a controller (9a) with the aid of the signal from the detector (7) in such manner that the intensity (I α ) of the output radiatiqn=oπtinuously maintains the same height. 5. Means according to claim 3 or 4, characterized in that said optical means for collecting and dispersing to a spectrum the radiation produced by the LEDs and for directing the desired wavelength range on the out¬ put slit (6) comprise a focussing reflection or trans- mission grating. 6. Means according to claim 3 or 4, characterized in that said optical means for collecting and dispersing to a spectrum the radiation produced by the LEDs and for directing the desired wavelength range on the out¬ put slit (6) and on the detector (7) comprise a beam divider cube (17) and on then surface thereof a trans¬ mission grating (18). 7. Means according to claim 6, characterized in that the detector (7) for observing the intensity of the output radiation has been disposed in conjunction with the beam divider cube (17) so that the beam divider cube also serves as an optical means associated with the detector. 8. Means according to any one of the preceding claims 3—7 for performing reflection and/or transmission meas- urement, characterized in that at least the radiation source (1) has been formed to be an integral unit (32). 9. Means according to any one of the preceding claims 3—7 for performing reflection measurements, character- ized in that, the radiation source (1) and the receiver (33) have been integrated to constitute an integrated unit, which is advantageously provided with a hermeti¬ cally sealed housing (37). 10. Means according to any one of the preceding claims 3-7, characterized 'in that the LED array (2') consists of a plurality of LED elements or separate encapsulated LEDs (21, 22, ..., 27,Fig. 6B) to each of which is con¬ nected an optic fibre (41, 42, ..., 47) by its first end (41a, 42a, ..., 47a) and the second ends (41b, 42b, ..., 47b) have been arranged in a desired configura¬ tion.

Inventors: KERAENEN HEIMO, MALINEN JOUKO, KERAENEN, HEIMO, MALINEN, JOUKO
Applicants: VALTION TEKNILLINEN, VALTION TEKNILLINEN TUTKIMUSKESKUS
Application year: 1988
Main classification: G05D25
IPC codes: G01J1, G01J3, G01N21