Device and Method for Measuring Semiconductor-Based Light Sources

Abstract

The invention relates to a method for the sequential measurement of a plurality of semiconductor-based light sources such as LEDs, OLEDs or VCSELs, in particular comparatively low-luminosity light sources such as so-called micro-LEDs. The invention further relates to a device for carrying out the method. The object of the present invention is to provide a method that operates faster, more accurately and more sensitively than the known methods, which operate by scanning with a photodiode or with a spectrometer. The method according to the invention proposes for this that a current pulse is applied by means of a pulsed current source (1) to the low-luminosity light sources consecutively or simultaneously. The emitted light pulse of the LED (2) is converted into electric charge carriers by means of a photodiode (3), the electric charge carriers are added up by means of an integrator circuit (5), the added-together charge carriers are converted by means of an A/D converter (6) into a digital signal and the digital signal is forwarded to a measurement and control unit (7). The invention also relates to a method and a corresponding device for the sequential measurement of a plurality of optical pulses, wherein the pulsed light radiation enters an Ulbricht sphere (10) through an inlet opening (11), a first portion of the light radiation, which exits the Ulbricht sphere (10) following interaction with the same through a first outlet opening, is measured by means of a first detector (14, 18) and a second portion of the light radiation, which exits the Ulbricht sphere (10) without interaction with the same through a second outlet opening (19), is measured by means of a second detector (14′).

Claims

1. A method for the sequential measurement of a plurality of optical pulses, which are converted by means of a photodiode (3) into electric charge carriers, wherein the electric charge carriers are added up by means of an integrator circuit (5), the added-together charge carriers are converted by means of an A/D converter (6) into a digital signal and the digital signal is forwarded to a measurement and control unit (7).

2. Method of claim 1, wherein the optical pulses are generated in that a current pulse is applied respectively to an arrangement of light sources (2a) simultaneously or consecutively by means of a pulsed current source (1).

3. Method of claim 1, wherein triggering takes place at each light pulse by means of a comparator circuit (8) connected in parallel to the integrator circuit (5).

4. Method of claim 1, wherein the generation and measurement of the optical pulses is controlled in a synchronized manner.

5. Method of claim 1, wherein the light sources are semiconductor-based light sources such as LEDs, VCSELs or OLEDs.

6. Method of claim 1, wherein the light sources are subdivided into groups and the groups are sequentially measured.

7. Method of claim 1, wherein an Ulbricht sphere (10) is used to detect the optical pulses.

8. Method of claim 7, wherein the Ulbricht sphere (10) has an outlet opening (19) opposite the inlet opening (11) of the Ulbricht sphere (10) and a detector (14′) behind the outlet opening (19) is used to detect the optical pulses.

9. Method of claim 1, wherein imaging optics (15) are used to detect the optical pulses.

10. Method of claim 1, wherein a spectrometer (18), in particular a spectroradiometer, is used to measure the optical pulses.

11. Method of claim 1, wherein the conversion efficiency of each light source is determined from the ratio of the electric charge impressed by the current pulse and the electric charge of the added-up charge carriers.

12. Method of claim 1, wherein the frequency of the current pulses that are applied to the light sources consecutively is more than 100 kHz, preferably more than 1 MHz, and the duration of the impressed current pulses is less than 100 μs, preferably less than 1 μs.

13. A method for the sequential measurement of a plurality of optical pulses, wherein the pulsed light radiation enters an Ulbricht sphere (10) via an inlet opening (11), a first portion of the light radiation, which exits the Ulbricht sphere (10) following interaction with the same through a first outlet opening, is measured by means of at least one first detector (14, 18) and a second portion of the light radiation, which exits the Ulbricht sphere (10) without interaction with the same through a second outlet opening (19), is measured by means of at least one second detector (14′).

14. Method of claim 13, where the second outlet opening (19) is arranged opposite the inlet opening (11) with respect to the central point of the Ulbricht sphere (10).

15. Method of claim 13, wherein the first detector (14, 18) measures a radiometric parameter, such as the radiation output, and/or a photometric parameter, such as the luminous flux.

16. Method of claim 13, wherein the first detector (14, 18) is a spectrometer, preferably a spectroradiometer.

17. Method of claim 13, wherein the second detector (14′) measures the temporal progression of the individual optical pulses, wherein the second detector (14′) comprises a photodiode, and A/D converter connected thereto and a digital memory, which records the temporal progression of the voltage and/or the current of the photodiode, and wherein the second detector (14′) detects the temporal progression with a temporal resolution of 10 ns or less, preferably 1 ns or less.

18. Method of claim 13, wherein the optical pulses are generated by a current pulse being applied respectively to an arrangement of a plurality of light sources (2a) simultaneously or consecutively by means of a pulsed current source (1).

19. Method of claim 13, wherein the light sources are semiconductor-based light sources such as LEDs, VCSELs or OLEDs.

20. Method of claim 13, wherein the conversion efficiency of the light sources (2a) is determined from the ratio of the electric energy impressed by the current pulse and the energy of the individual light pulse measured by means of the second detector (14′).

21. Method of claim 13, wherein the frequency of the current pulses which are applied consecutively to the light sources is more than 100 kHz, preferably more than 1 MHz, and wherein the duration of the individual light pulse is less than 1 μs, preferably less than 100 ns.

22. A device for the sequential measurement of a plurality of optical pulses, with a pulsed current source (1), a photodiode (3), an integrator circuit (5), an A/D converter (6) and a measurement and control unit (7), which is configured to perform the method according to claim 1.

23. A device for the sequential measurement of a plurality of optical pulses, with an Ulbricht sphere (10), which receives pulsed light radiation via an inlet opening (11), wherein a first portion of the light radiation, which exits the Ulbricht sphere (10) following interaction with the same through a first outlet opening, encounters a first detector (14, 18) arranged behind the first outlet opening, wherein a second portion of the light radiation, which exits the Ulbricht sphere (10) without interaction with the same through a second outlet opening (19) arranged opposite the inlet opening (11), encounters a second detector (14′) arranged behind the second outlet opening (19), and with a measurement and control unit (7) connected to the first and the second detector.

24. A device of claim 23, the measurement and control unit (7) being configured to execute the method according to claim 13.

25. Device of claim 23, wherein the first and/or the second outlet opening (19) is connected via an optical fiber (17, 17′) to the first or second detector (14, 14′, 18), respectively.

26. Device of claim 25, wherein the optical fiber (17, 17′) comprises an input and at least two outputs, the light radiation exiting the outputs at different intensities, wherein each output of the optical fiber (17, 17′) is assigned a first or second detector (14, 14′, 18).

27.-33. (canceled)

Description

[0032] Exemplary embodiments of the invention are illustrated by means of the drawings and explained below. The drawings show in:

[0033] FIG. 1: an equivalent circuit diagram of a device according to the invention in schematic form;

[0034] FIG. 2: a schematic view of a device according to the invention in a first embodiment;

[0035] FIG. 3: a schematic view of a device according to the invention in a second embodiment;

[0036] FIG. 4: a schematic view of a device according to the invention in a third embodiment;

[0037] FIG. 5: a schematic view of a device according to the invention in a fourth embodiment;

[0038] FIG. 6: a schematic view of a device according to the invention in a fifth embodiment.

[0039] In FIG. 1, a pulsed current source is identified by the reference character 1. The pulsed current source 1 applies a current pulse to an LED 2. The LED 2 to which the current pulse is applied emits a light pulse. The light pulse is detected by a photodiode 3 and converted into an electric current proportional to the light pulse. The current emitted by the photodiode 3 is converted via a transimpedance amplifier 4 into an amplified voltage proportional to the current. This voltage serves as an input voltage of an integrator circuit 5. The integrator circuit 5 adds up the charge at its capacitor. The charge added up by the integrator circuit 5 is thus proportional in relation to the energy of the light pulse. The analog signal tapped at the output of the integrator circuit 5 is converted via an A/D converter 6 into a digital signal and forwarded to a measurement and control unit 7. A comparator circuit 8 is additionally provided parallel to the integrator circuit 5. The comparator circuit 8 triggers a signal for both a rising and a falling pulse slope in the event of a threshold value that can be set in each case. This signal is then forwarded to the measurement and control unit 7. Via the adjustable threshold value of the comparator circuit 8, the pulse duration can be measured, for example, or a trigger realized for synchronization with another measuring apparatus or the current source.

[0040] FIG. 2 schematically depicts the setup for the application of a method according to the invention in a first embodiment. A test object with a plurality of micro-LEDs 2a, here a micro-LED display 9 (or a sub-module of such a display with at least a few thousand micro-LEDs), for example, is to be tested. Each micro-LED 2a of the micro-LED display 9 must be measured individually here. The micro-LED display 9 has up to several million micro-LEDs 2a. The following measuring setup is provided for the measurement: the micro-LED display 9 is arranged in or at the inlet opening 11 of an Ulbricht sphere 10. Provided at the outlet opening 19 of the Ulbricht sphere 10 is a detector module 14.

[0041] The detector module 14 includes the photodiode 3 depicted in FIG. 1, the transimpedance amplifier 4, the integrator circuit 5, the A/D converter 6. The measurement and control unit 7 from FIG. 1 is represented here by a PC 7a.

[0042] The micro-LED display 9, the PC 7a and the detector module 14 are each communicatively connected to the pulsed current source 1. Moreover, each individual micro-LED 2a of the micro-LED display 9 can be addressed via the pulsed current source 1, that is, applied with a current pulse. The PC 7a can consequently activate each individual micro-LED 2a via the communication interfaces and allocate the related measuring result based on this. The sequential measurement of the individual micro-LEDs 2a is carried out in the MHz range. The precise synchronization of current source 1, detector module 14 and PC 7a is of great importance, therefore.

[0043] If an individual micro-LED 2a of the micro-LED display 9 is now activated by the pulsed current source 1, the light pulse emitted tends to be of low luminosity on the one hand and on the other is also distributed in various spatial directions. Due to the integrative effect of the Ulbricht sphere 10, the weak, originally unevenly distributed light pulse radiated by the individual micro-LED 2a is collected from all directions and is convertible into an easily measurable illuminance that is related simply to the light pulse sought. The integrative effect leads to the illuminance measured at the outlet by the detector module 14 being directly proportional to the total light pulse injected. The detector module 14 now executes the steps described above (see FIG. 1) and supplies the digital measuring signal described to the PC 7a.

[0044] The next micro-LED 2a then has a current applied to it by means of the pulsed current source 1 via the communications interfaces. The frequency of the sequential measurement extends here from the kHz into the MHz range, thus many thousands to a few million micro-LEDs 2a can be measured within a short period.

[0045] FIG. 3 schematically depicts the setup for the application of a method according to the invention in a second embodiment. The setup is very similar to the setup in FIG. 2. Instead of the Ulbricht sphere 10 used in FIG. 2, imaging optics 15 are used in the second embodiment depicted in FIG. 3. The imaging optics 15 fulfil a similar purpose to the Ulbricht sphere 10 in FIG. 2. In this case, the light pulse radiated by the individual micro-LED 2a in various spatial directions is bundled by means of the imaging optics 15, so that the illuminance measured by the detector module 14 is as directly proportional as possible to the total light pulse of the individual micro-LED 2a. the following method steps correspond to those for the first embodiment.

[0046] In FIG. 4, the setup for the application of a method according to the invention in a third embodiment is depicted schematically. A spectrometer 18 is added to the setup from FIG. 2, so that the light pulses can be measured additionally or alone using the spectrometer 18. The light pulses homogenized by the Ulbricht sphere 10 are first injected by fiber optics 16 into a light-conducting fiber 17 and fed by said light-conducting fiber 17 into the spectrometer 18, in which the light pulses, disassembled into their spectral components, are imaged on a line or array detector, it being possible to measure the spectra of the light pulses with this detector. The measuring and readout processes of the aforementioned detector can be executed via the PC 7a or by trigger signals from the current source 1, for example; ideally the measuring and readout processes are synchronized with the generation of the light pulses. With this setup the spectra of the light pulses generated by a single light source 2a or a group of light sources 2a excited in parallel can be measured. Furthermore, a sequence of light pulses generated with successive current pulses can be measured spectrally by integration or averaging of the signal in the spectrometer 18 over several light pulses. A spectroradiometer is understood to be a combination of spectrometer 18 and coupling optics (e.g. fiber optics 16), for which absolute calibration exists in respect of wavelength and spectral detector sensitivity. This calibration can be stored e.g. on the PC 7a.

[0047] FIG. 5 schematically depicts the setup for the application of a method according to the invention in a fourth embodiment. An outlet opening 19 is added to the setup from FIG. 2, so that a portion of the light pulses can be measured following multiple reflection 13 by the detector 14, and a portion of the light pulses can be measured following a single passage 20 through the Ulbricht sphere 10 without multiple reflection by another detector 14′. The detector 14′ can be a fast photodiode, e.g. based on gallium phosphide, silicon, indium gallium arsenide or germanium. The detector 14′ permits time-resolved measurement of the pulses. The detector 14′ is connected to the current source 1 and synchronized. The light pulses 20 are imaged on the detector 14′ by imaging optics 15.

[0048] In FIG. 6, a portion of the light pulses 13 homogenized by the Ulbricht sphere 10 are injected by means of fiber optics 16 into a light-conducting fiber 17 and fed by said light-conducting fiber 17 into the spectrometer 18, in which the light pulses, disassembled into their spectral components, are imaged on a line or array detector. The spectra of the light pulses are measured by this detector. The spectrometer 18 expediently has absolute calibration, so that relevant radiometric, photometric and colorimetric magnitudes can thereby be calculated by software from the spectral data. Another portion of the light pulses, which exits the Ulbricht sphere after passing directly through the Ulbricht sphere without interaction with the internal wall of the Ulbricht sphere, is coupled into further fiber optics 16′ and injected via a light-conducting fiber 17′ into a detector 14′. The detector 14′ can be a fast photodiode, e.g. based on gallium phosphide, silicon, indium gallium arsenide or germanium. The detector 14′ permits time-resolved measurement of the pulses, i.e. measurement of the temporal progression of the individual optical pulses (light intensity as a function of time). The detector 14′ is connected to the current source 1 and thus synchronized. The light-conducting fiber 17′ can be a fiber with one input and one output. The light-conducting fiber 17′ can also be a fiber with one input and several outputs, however, as depicted, which feeds the injected light in equal or different intensity ratios into several second detectors 14′.

REFERENCE CHARACTER LIST

[0049] Current source

[0050] 1 LED

[0051] 2a Micro-LED

[0052] 3 Photodiode

[0053] 4 Transimpedance amplifier

[0054] 5 Integrator circuit

[0055] 6 A/D converter

[0056] 7 Measurement and control unit

[0057] 7a PC

[0058] 8 Comparator circuit

[0059] 9 Micro-LED display

[0060] 10 Ulbricht sphere

[0061] 11 Inlet opening

[0062] 12 Irradiated light

[0063] 13 Multiply-reflected beam

[0064] 14 Detector module

[0065] 14′ Detector module

[0066] 15 Imaging optics

[0067] 16 Fiber optics

[0068] 16′ Fiber optics

[0069] 17 Fiber

[0070] 17′ Fiber

[0071] 18 Spectrometer

[0072] 19 Outlet opening

[0073] 20 Direct through-passage