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  Method for stabilizing the temperature dependency of light emission of an ledUSPTO Application #: 20070295912Title: Method for stabilizing the temperature dependency of light emission of an led Abstract: Method for correction of the temperature dependency of a light quantity L emitted by a light emitting diode (LED), being operated in pulsed mode with substantially constant pulse duration tP, and measured in a light detector, using a predetermined parameter X, correlated to the temperature T of the LED in a predetermined ratio, whereby a correction factor K is determined from the parameter X, preferably using a calibration table, especially preferred using an analytic predetermined function, whereby the measured emitted light quantity L is corrected for the temperature contingent fluctuations of the emitted light quantity, whereby the parameter X is determined from at least two output signals of the LED, which are related to each other in a predetermined manner. (end of abstract)
Agent: Ip Strategies - Asheville, NC, US Inventors: Stein Jurgen, Guntram Pausch, Karen Saucke, Guntram Pausch, Karen Saucke USPTO Applicaton #: 20070295912 - Class: 250363010 (USPTO) Related Patent Categories: Radiant Energy, Invisible Radiant Energy Responsive Electric Signalling, With Or Including A Luminophor, With Radiant Energy Source The Patent Description & Claims data below is from USPTO Patent Application 20070295912. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] The invention relates to a method for correcting the temperature dependency of a light quantity emitted by a light-emitting diode (LED), which is operated in pulsed mode with substantially constant pulse duration, and measured in a light detector. [0002] The emitted light quantity of an LED depends on its temperature. In laboratory applications according to prior art, according to which an LED is employed as reference light source, the LED and possibly the measuring apparatus associated therewith are tempered, resulting in the temperature and, thus, the emitted light quantity of the LED remaining constant. [0003] In applications outside the laboratory, in which such a climatisation is not possible at all or only at increased expenditure, it is therefore necessary to correct the measured values of the light quantity with respect to the temperature contingent influences, to thereby reduce the errors of the measured result. In case such an LED is used for example as light source for stabilization of a photo multiplier, which for example is employed as light detector in a scintillation detector, for example a mobile detector for identification of radio isotopes (hand held radio isotope identification device--RID), the LED is exposed to thermal fluctuations in the range of -20.degree. C. to +50.degree. C. Thereby, the system amplification of the light detector can fluctuate offhand for about 20% and more, such that a stabilization of the amplification of the light detector is necessary, to maintain the energy amplification and the energy resolution of the RID sufficiently good. For stabilization of Such a light detector with an LED, it is therefore necessary, to know the temperature dependency of the light quantity emitted by the LED. [0004] Methods for stabilization are known, according to which the temperature is measured at or in the detector and the temperature caused effects are adjusted by means of previously measured calibration tables. These methods, however, have the drawback that a temperature measurement with fast temperature changes is only hardly realizable, particularly for the reason that often no uniform temperature distribution can be expected in the detector. Besides, the amplification of, for example, a photo multiplier does not only depend on its temperature, but rather also on the effective counting rate and its previous history, i.e. its hysteresis and age. It has been found that sufficiently exact prediction of the amplification under consideration of all parameters is not possible. [0005] For stabilization, therefore, often active methods are employed during the actual measurement. Mostly, radio active calibration sources or natural background radiation are used, to achieve such an active stabilization. This, however, leads to optimization problems, because a compromise of sufficiently short but nevertheless sufficiently exact calibration measurements has to be found. Additionally, each additional radio active radiation leads to a reduction of the total sensitivity of the system. [0006] An alternative is the separated stabilization of light detector and scintillator--the latter is for example disclosed in PCT/EP2004/050754. It is known to use a pulsed light source, for example an LED, as measured standard for the stabilization of the light detector. It is also known to stabilize and to monitor the amplification of light detectors in this manner in laboratory applications. Disadvantageous with respect to this prior art is that the light emission of an LED depends on its temperature, more particular, on its junction temperature T.sub.LED. Thus, according to known methods, it is either necessary, to keep the temperature constant or to monitor it at least, or to monitor the light quantity emitted respectively by the LED with a separate measurement apparatus precisely. Such an assembly is not only technically complex and cost intensive, but rather requires also additional energy and additional space, complicating the use in battery operated mobile RIDs. [0007] From sensor techniques, a method is known, to measure the temperature of semiconductor elements by means of a current measurement at constant operating voltage or by means of a measurement of the flux voltage at constant current. [0008] Therefore, it is an object of the invention, to provide a method avoiding the drawbacks of prior art mentioned above, to reduce the expenditure for the stabilization of light detectors by means of pulsed LEDs. [0009] Further, it is an object of the invention, to provide a light detector, the signals of is which, including the pulse amplitude spectrum produced by the associated electronics, can be corrected and, thus, stabilized by means of pulsed LED with respect to temperature dependency and otherwise caused fluctuations. Moreover, it is an object of the invention, to provide a detector for measuring radiation, preferably ionized radiation, which can be stabilized by a pulsed LED. [0010] These objects are at first solved by the method and devices according to the claims. Thus, a method is provided, according to which the emitted light quantity L of a light emitting diode being temperature dependent is corrected, using a predetermined parameter X being in a predetermined relation to the temperature T of the LED. From the parameter X, a correction factor K is thereby determined, preferably using a calibration table, especially preferred using an analytic predetermined function, according to which the measured emitted light quantity L is corrected for the temperature-caused fluctuations of the emitted light quantity. Thereby, the diode is operated in pulsed mode with substantially constant pulse duration t.sub.P. The parameter X, thereby, is determined from at least two output signals of the LED itself, which are related with respect to each other in a predetermined manner. [0011] Thereby, it has been found to be advantageous, to determine at first the temperature T of the LED from the measured parameter X, whereby a calibration table can be used. Preferably, it is also possible to use an analytic predetermined function. Subsequently, the correction factor K is determined from the temperature T, whereby also preferably a calibration table or an analytic predetermined function is used. [0012] Moreover, a method for temperature stabilization of a light emitting diode (LED) is provided, whereby the LED is operated in pulsed mode with substantially constant pulse duration t.sub.P, whereby a predetermined parameter X is used as command variable, associated to the temperature T of the LED in a predetermined relation, whereby the parameter X is determined from at least two output signals of the LED, which are related to each other in a predetermined manner. [0013] It has been found to be advantageous, to operate the LED such that the pulse duration t.sub.P is substantially constant, the voltage applied to the LED, however, changing between at least one first voltage U.sub.P1 and at least a second voltage U.sub.P2, being different from the first voltage U.sub.P1. During the pulse, the respective voltage is substantially constant. Then, the average light quantities L(U.sub.P) of the pulses at different voltages U.sub.P are measured, thus, at least the average light quantity L(U.sub.P1) of the pulse at voltage U.sub.P1 and the average light quantity L(U.sub.P2) of the pulse at voltage U.sub.P2. The determination of the parameter X is then derived from the ratio of the light quantities L(U.sub.P) with respect to each other. The use of the ratio of at least two light quantities at constant pulse duration but at different voltages leads to the fact that amplification fluctuations of the light detector caused by temperature fluctuations or by other effects do not have any influence on the determination of the parameter X. [0014] The method can also be configured such that a current to the LED being in pulsed mode at also constant pulse duration t.sub.P, periodically alternating between at least a first value I.sub.P1 or at least a second value I.sub.P2, being different from the first one, is applied. During the pulse, the current, flowing through the LED, is respectively substantially constant. Then, the average light quantities L(I.sub.P) of the pulses with the different currents I.sub.P, thus, at least the average light quantity L(I.sub.P1) of the pulse with the current I.sub.P1 and the average light quantity L(I.sub.P2) of the pulse with the current I.sub.P2, are measured. The parameter X is then determined from the ratio of the light quantities L(I.sub.P) with respect to each other. [0015] To suppress the influence of turn on and turn off effects or similar influences to the light emission of the LED, it has been found to be especially advantageous, to determine the parameter X as follows: Operating the LED in pulsed mode such that the pulse duration t.sub.P takes substantially two different substantially constant values t.sub.PS and t.sub.PL and the voltage alternates between at least a first voltage U.sub.P1 and at least a second voltage U.sub.P2, being different from the first voltage U.sub.P1 periodically at the LED, measuring the average light quantities L(U.sub.P; t.sub.PS) and L(U.sub.P; t.sub.PL) of the pulses with at least the voltages U.sub.P1 and U.sub.P2 and the pulse durations t.sub.PS and t.sub.PL, determining the differences D.sub.P1 and D.sub.P2 of the light quantities L(U.sub.P1; t.sub.PL) and L(U.sub.P1; t.sub.PS) as well as L(UP.sub.2; t.sub.P1) and L(U.sub.P2; t.sub.PS), and determining the parameter X from the ratio of the differences of the light quantities. [0016] Just as well, it is possible to determine the parameter X as follows: Operating the LED in pulsed mode such that the pulse duration tp takes substantially two different substantially constant values t.sub.PS and t.sub.PL, and the current flowing through the LED alternates periodically between at least a first value I.sub.P1 and at least a second value I.sub.P2, being different from I.sub.P1, measuring the average light quantities L(I.sub.P; t.sub.PS) and L(I.sub.P; t.sub.PL) of the pulses with at least the currents I.sub.P1 and I.sub.P2 and the pulse durations t.sub.PS and t.sub.P1, determining the differences D.sub.P1, and D.sub.P2 of the light quantities L(I.sub.P1; t.sub.PL) and L(I.sub.P1; t.sub.PS) as well as L(I.sub.P2; t.sub.PL) and L(I.sub.P2; t.sub.PS), and determining the parameter X from the ratio of the differences of the light quantities. [0017] Further, it has been found to be advantageous, if the light quantities L(U.sub.P;) and L(I.sub.P), respectively, i.e. at least the light quantities L(U.sub.P1) and L(U.sub.P2) or L(I.sub.P1) and L(I.sub.P2), are determined with a light detector, preferably a photo multiplier, a hybrid photo multiplier, an Avalanche photo diode or a photo diode with amplifier. The light quantities measured with this light detector are preferably determined by application of one or more of the following method steps: Carrying out pulse amplitude spectrometry of the detector signals and/or measuring the average current flow in the light detector and/or measuring the charge quantity produced in the photo sensitive layer of the light detector by the LED pulse, preferably by means of spectrometry of the, already amplified, charge signals triggered by the LED pulses. [0018] It is further advantageous, if the LED comprises a series resistance, whereby the series resistance is selected particularly advantageous in that its resistance does not depend on temperature T in a linear manner, especially preferred in a manner that the dependency or at least the non-linearity of the dependency of the correction factor K from the temperature T is compensated approximately by the temperature dependency of the series resistance. [0019] Further, a method for stabilizing a light detector is claimed, preferably a photo multiplier, a hybrid photo multiplier, an Avalanche photo diode or a photo diode with amplifier, whereby the light detector is optically connected to at least an LED, whereby at least an LED is operated in pulsed mode and according to which the output signals of the light detector are stabilized with a stabilizing factor, whereby the stabilizing factor is generated by the signals of the at least one LED and according to which the temperature dependency of the light emission of at least one LED is corrected by means of one of the methods described above. [0020] Further, a method for stabilization of signals generated by a scintillating detector for measuring radiation is claimed, preferably ionized radiation, whereby the signals are generated by the radiation which is at least partly absorbed in the detector, and which depend on the operating temperature of the detector, whereby by scintillating detector has at least one light detector and at least one LED optically connected thereto, whereby the stabilizing factor for stabilizing the scintillation detector is generated from the signals emitted by at least one LED, and according to which the temperature dependency of the light emission of the LED is corrected according to one of the methods described above and claimed in claims 1 to 11. It can also be an advantage, if at least one, preferably the optical connection between the LED and the scintillating detector is designed in a heat conducting manner, because then the temperature of the LED being heat-conducingly connected to the scintillating detector substantially corresponds to the temperature of the scintillator. [0021] In all the methods described above, signal processing is preferably carried out digitally. [0022] Moreover, a light detector with a signal processing device is claimed, preferably a photo multiplier, a hybrid photo multiplier, an Avalanche photo diode or a photo diode with amplifier, whereby at least one LED is optically connected to the light detector, according to which at least an LED is operated in pulsed mode and the output signals of the light detector are stabilized by a stabilizing factor, whereby the stabilizing factor is generated from the signals generated by the at least one LED, and according to which the temperature dependency of the light emission of at least one LED is corrected with a method described above and claimed in claims 1 to 11. Here, the signal processing preferably is carried out digitally. [0023] Further, a scintillation detector for measuring of radiation is claimed, preferably ionized radiation, whereby the scintillation detector has at least one light detector described above, measuring the light generated by the scintillation detector at least partially. In a special embodiment, signals are measured which are generated by the radiation absorbed at least partially in the detector and being dependent on the operating temperature of the detector, and are stabilized by a stabilizing factor being in a predetermined relation to the temperature T of the scintillator, whereby at least an LED of the light detector is connected to the scintillation detector in a heat conducting manner, and whereby the temperature dependency stabilizing factor S for stabilizing the scintillation detector in a predetermined manner, preferably using a calibration table, in particular preferred using a predetermined functional dependency, is determined from parameter X of at least one LED being connected to the scintillation detector in a heat-conducting manner according to one of the process steps described above. [0024] The present invention provides a technically very simple and convenient method for temperature stabilization of LED reference light sources, which, for example, are used for stabilization of light and/or scintillation detectors, in that it analyses the pulse amplitude spectrum of LED signals, which have to be measured anyway for stabilization. Therefore, neither a radio active calibration source is necessary, nor the use of an additional light detector for monitoring the light quantity emitted by the LED in dependency from the temperature. The light detector, being present anyway, is sufficient, the stability of which does not matter anyway, as long as its amplification only alternates in periods of time, which are larger than the switching interval of the different LED modi. This switching interval can be kept very small (up to <1 ms), but is at least as large as the temporal distance between two LED pulses. Continue reading... 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