METHOD AND SYSTEM ARRANGEMENT FOR SETTING A CONSTANT WAVELENGTH

Description

[0001] The present invention is directed to a method that allows, with minimal technical effort, to set a constant wavelength in a light-emitting diode such that a human observer, using the unaided eye, perceives a consistent color of the light-emitting diode. Furthermore, the present invention is directed to a correspondingly configured system arrangement and to a computer program product with control commands that execute the method or operate the system arrangement.

[0002] WO 2014 / 067830 A1 discloses a method and arrangement for temperature-corrected control of LEDs using look-up tables. It proposes a circuit for controlling multiple LED channels, each comprising at least one LED, and for generating a mixed light from the light generated by the respective LED channels. A look-up table is provided for each target color location achievable by the LED module, in which the operating current for each LED channel is stored.

[0003] WO 2017 / 162 323 A1 discloses an efficient control arrangement and a control method that enable particularly efficient data transmission, particularly for LED control units. The document also addresses a corresponding protocol that causes control units to execute the corresponding method steps.

[0004] WO 2017 / 162 324 A1 discloses a method and a device for bidirectional communication between a command unit and a plurality of LED control units connected to it. This makes it possible to transmit control commands to several serially connected LED control units at high speed and to return execution results from these control units to a command unit.

[0005] WO 2017 / 153 026 A1 shows a method and a device for brightness compensation of a light-emitting diode, whereby a constant brightness of the light-emitting diode is always achieved regardless of the temperature fluctuation.

[0006] Known methods utilize pulse width modulation (PWM), which takes advantage of the inertia of the components used, resulting in a uniform brightness even when the LED is switched on or off in a certain proportion. The brightness is then adjusted depending on the ratio of the on state to the off state. Such pulsing of the LED is typically not perceived by the human eye, and a uniformly adjustable brightness results from this control.

[0007] Furthermore, it is possible to integrate a pulse generator into the constant current source circuit, whereby the supply voltage remains constant and the lamps are clocked using the current source itself, which operates in pulsed mode. Control circuits are known for this purpose, by which the LEDs are regulated to an adjustable setpoint, with the setpoint being adjustable by a controller. Dimming of LEDs is achieved according to known methods directly by dimming the current through the LEDs. Control logic for regulating the current supply to the LED, also depending on the LED's temperature, is also known.

[0008] Light-emitting diodes (LEDs) are used in many application scenarios where they are at least not considered to be inferior to incandescent bulbs. While incandescent bulbs can be easily dimmed in terms of brightness, methods are known for LEDs that, for example, control these LEDs using a predetermined control pattern, thereby enabling optical dimming. In contrast, however, it is often desired that an LED be brightened when, for example, the ambient temperature rises. This is the case because LEDs typically exhibit a luminous behavior that reduces the emitted luminosity as the temperature rises.

[0009] It is generally known that light-emitting diodes, which are typically provided as red, green, or blue-emitting LEDs, are susceptible to brightness or color fluctuations due to temperature development. Thus, according to the state of the art, it is disadvantageous that the color variations depending on the temperature development or brightness variations can be so pronounced that they are perceptible to the human eye, thus resulting in undesirable optical effects. Such optical effects can relate to comfort functions, for example, in a vehicle, although application scenarios also stipulate that the LEDs perform a safety function. For example, LEDs are also used as optical warning signals, and the disadvantage of brightness or color variations can be safety-critical.

[0010] Particularly problematic, given the current state of the art, is the technical effort required to manufacture LEDs. The corresponding LEDs must undergo tests, resulting in increased reject rates due to the LEDs' inability to achieve predetermined target values ​​depending on temperature. This situation is particularly detrimental in the automotive application scenario. A particular disadvantage arises here: the installed LEDs cannot be replaced at any time, meaning the end customer would have to take their vehicle in for maintenance. In addition to the high logistical effort required, this disadvantage of the current state of the art reduces end customer acceptance of corresponding optical devices.

[0011] It is therefore an object of the present invention to propose an improved method for setting a constant wavelength of a light-emitting diode, which enables the light-emitting diode to achieve a color that is as consistent as possible without requiring significant technical effort. Furthermore, it is an object of the present invention to propose a correspondingly configured system arrangement and a computer program product with control commands that execute the method or operate the system arrangement.

[0012] The problem is solved by the features of patent claim 1. Further advantageous embodiments are specified in the subclaims.

[0013] Accordingly, a method for setting a constant wavelength of a light-emitting diode is proposed, comprising controlling the light-emitting diode by means of a preset current value, measuring an actually prevailing temperature of a control unit arranged in the immediate vicinity of the controlled light-emitting diode, providing an empirically determined wavelength variation of the light-emitting diode as a function of the temperature of the light-emitting diode, and adjusting the preset current value as a function of the actually prevailing temperature and the empirically determined wavelength variation to set the constant wavelength of the light-emitting diode.

[0014] Those skilled in the art will recognize that individual method steps can be performed iteratively and / or in a different order. In particular, method steps can comprise further substeps. Typically, the control of the LED is performed iteratively, and the prevailing temperature at the control unit is measured iteratively. In a preparatory method step, an empirically determined wavelength variation is provided. The preset current value is adjusted at a specific rate or within preset intervals.

[0015] Using the proposed method, a constant wavelength of an LED is set by detecting the error rate of the LED and then adjusting the current value accordingly. The constant wavelength is an essentially constant wavelength, with the human eye serving as the reference point for the constant wavelength. From a technical perspective, the proposed method actually makes it possible for the wavelength to not be constant, but to adjust it so that it is constant with respect to the naked human eye. Thus, the constant wavelength results in a color value that is consistent for the human observer. However, using technical aids, it can be recognized that the constant wavelength is merely an essentially constant wavelength that varies slightly.

[0016] A light-emitting diode can be a red, green, blue, or white light-emitting diode. It is known to combine these different individual light-emitting diodes into light-emitting diode units, so that, depending on the design, three or four individual light-emitting diodes, for example, form one light-emitting diode unit. In this case, additional technical devices are required, which, for example, control the individual light-emitting diodes in such a way that a specific wavelength or brightness is achieved.

[0017] The proposed control units serve this purpose by indirectly applying a certain current to the LEDs or performing pulse-width modulation. Pulse-width modulation adjusts the brightness or luminosity of each individual LED, and then the wavelength is adjusted based on the current value. The proposed current value is therefore the current value used to control the LED. This does not contradict the fact that no current is supplied, at least temporarily, during pulse-width modulation.

[0018] This provision of current occurs as part of controlling the LED using a preset current value. This generally involves operating the LED according to a provided specification. This method step is also carried out according to the prior art, with the disadvantage that the constant preset current value leads to a wavelength variation, which becomes apparent to the observer as the color of the LED changes. This occurs due to the changing temperature conditions within the LED. The preset current value is typically stored in a memory unit of the LED unit or is provided by the control unit.

[0019] In a further method step, an actual prevailing temperature is measured by a control unit arranged in the immediate vicinity of the controlled LED. According to the invention, it is therefore recognized that the temperature does not have to be measured directly at the LED, but that the control unit can be used for this purpose. The invention therefore results in a design that allows the temperature to be measured at an alternative location and that the measuring sensor or temperature sensor can also be arranged on the control unit. Since the temperature is not measured directly at the LED, but rather at the control unit, the proposed method takes this distance into account according to one aspect and varies the current value accordingly. Since the control unit is arranged in the immediate vicinity of the LED, a conclusion can be drawn about the temperature of the LED during runtime.

[0020] In this context, immediate proximity is to be interpreted as meaning that the proximity is essentially immediate, such that only one layer, for example as described later, is arranged between the sensor and the control unit. Thus, "immediate" is to be interpreted as meaning that no further active components are installed. Consequently, only passive components, such as connecting layers or heat-conducting layers, are arranged between the LED and the control unit. Generally, the feature of "immediate" proximity is optional in that no further active, heat-generating units are arranged between the LED and the control unit. Thus, the method step can also be carried out by measuring an actual prevailing temperature of a control unit arranged near the controlled LED.In particular, distances of less than one millimetre are also considered to be immediate.

[0021] An empirically determined wavelength variation of the LED is then provided as a function of the LED's temperature. This is also referred to as providing a characteristic curve for the LED. The empirically determined wavelength variation indicates the extent to which the LED's wavelength changes with increasing or decreasing temperature. This is also referred to as the LED's error rate and indicates a technically determined value that corresponds to a delta of the wavelength value that occurs when the LED's temperature rises or falls. This empirical value can be stored in a data memory.

[0022] Since the length variation is now known, as is a temperature, which can be used to determine the temperature of the LED, the preset current value is adjusted. The process then branches iteratively back to a first step, which involves controlling the LED. The LED is controlled in such a way that the constant wavelength, or essentially a constant wavelength, of the LED is established.

[0023] Thus, in this process step, the wavelength variation is compensated for by temperature, and the current value is adjusted in such a way that the color value of the LED is always constant.

[0024] In general, the invention allows for the fact that the actual prevailing temperature is measured at the control unit and not at the LED, and that the empirically determined wavelength variation provided relates to a temperature of the LED. It is therefore advantageous to calculate a compensation factor that takes into account the fact that the actual measurement is not taken directly at the LED, but rather at the control unit. Consequently, the invention allows for an alternative design and to operate the method accordingly.

[0025] In a final, iterative process step, the preset current value is adjusted, and the LED is actually controlled based on this adjusted current value. This ensures that the LED emits a constant wavelength over time and temperature development.

[0026] According to one aspect of the present invention, the method is implemented for a red, blue, green, or white-emitting LED. This has the advantage that the proposed method not only allows the colors to be adjusted, but also allows the luminosity to be adjusted using a white-emitting LED, eliminating the need for a separate method for brightness compensation. Thus, the brightness of the LED can be controlled with minimal technical effort.

[0027] According to a further aspect of the present invention, the method is carried out iteratively such that the adjustment of the preset current value occurs essentially every 2 seconds. This has the advantage that an adjustment of the wavelength actually takes place every time, but this requires little computational effort and the underlying components can then also be designed efficiently. According to the invention, it was recognized that adjusting the current value every two seconds is so advantageous with regard to human perception that no significant error, i.e. a deviation of the actual wavelength from the target wavelength, occurs within such a time interval and thus only negligible error rates occur. In this respect, it is ensured that the human eye does not detect any deviation in the wavelength and therefore perceives a constant wavelength overall.From a technical perspective, it is only possible to use tools to determine that the wavelength varies within the 2 seconds, which is then promptly adjusted. Thus, the invention creates a suitable balance between hardware complexity and human perception.

[0028] According to a further aspect of the present invention, the preset current value specifies a current pulse of a pulse-width modulation. This has the advantage that the preset current value can be switched on and off during the pulse-width modulation, allowing the brightness to be varied. Thus, when controlling the LED using a preset current value, no current can be applied temporarily, thus implementing pulse-width modulation.

[0029] According to the present invention, the preset current value is adjusted using a stored error function. This has the advantage that a function can be empirically determined that multiplies or adds the inverse of the wavelength error to the current strength, thus canceling or compensating for the resulting error, i.e., the wavelength deviation. The error function thus determines a value by which the preset current value must be adjusted to restore the original wavelength.

[0030] According to a further aspect of the present invention, the error function provides a compensation value that compensates for the wavelength variation of the LED. This has the advantage that, depending on the prevailing temperature, a delta is created with respect to the current value, and this delta is applied to the preset current value in such a way that the desired constant wavelength is again established.

[0031] According to a further aspect of the present invention, the compensation value is present as a compensation factor and / or compensation summand. This has the advantage that a compensation value can be multiplied and / or summed, whereby a combination of both options is also proposed according to the invention. Thus, the current value can be adjusted at any time such that the desired constant wavelength is established or the error in the wavelength deviation is compensated.

[0032] According to the present invention, the error function determines the temperature of the LED depending on the actual prevailing temperature of the control unit. This has the advantage that the temperature value does not have to be taken directly from the LED; rather, according to the invention, the temperature of the control unit is measured and the temperature of the LED is then inferred. In this respect, an alternative design can be implemented and empirical values ​​can be consulted which indicate which temperature values ​​prevail at the LED at which temperature of the control unit. Furthermore, conclusions can be drawn about the wavelength based on the temperature, which in turn allows the current value to be adjusted so that the desired wavelength is again set. This is the case because, for technical reasons, the wavelength varies with the prevailing temperature.

[0033] According to a further aspect of the present invention, the preset current value is adjusted when an actual wavelength deviates from the target wavelength by more than a threshold value. This has the advantage that not every wavelength deviation needs to be corrected immediately; rather, a threshold value can be defined that corresponds, for example, to the accuracy of the unaided human eye. If this threshold value is undershot or exceeded, the current value is adjusted, and the underlying hardware components can be designed particularly efficiently. This is the case because not every deviation needs to be compensated for immediately; rather, the threshold value can be selected to be so large that the variation is just invisible to the human eye.In this respect, the threshold can also take the underlying hardware into account, and this in turn can be designed efficiently.

[0034] According to a further aspect of the present invention, the empirically determined wavelength variation specifies a characteristic curve of the light-emitting diode. This has the advantage that a technical specification, also referred to as a characteristic curve, can already be provided by the manufacturer. The characteristic curve describes the characteristics of the light-emitting diode, and thus a wavelength variation as a function of temperature can also be provided, which is then corrected according to the invention.

[0035] According to a further aspect of the present invention, the immediate proximity is less than 1 mm. This has the advantage that the underlying unit is chosen to be so small that it can still be considered immediate proximity. However, it has been found according to the invention that larger deviations are complex to calculate. Thus, a proximity of less than 1 mm typically does not lead to a significant temperature error, and the temperature of the control unit can be used as the basis for the method according to the invention, rather than the temperature of the LED.

[0036] According to a further aspect of the present invention, the close proximity is adjusted by means of a thickness of an adhesive layer, a silicone layer, a polymer layer, a heat-conducting layer, an aluminum layer and / or a copper layer. Furthermore, an air gap or casting resins can be used for this purpose. This has the advantage that the distance between the LED and the control unit or alternatively the distance between the measuring sensor and the control unit is adjusted such that at least one of the listed layers is used. This is generally a close proximity because no electronic components are arranged between the proposed nominal units and thus no new heat source is created. Thus, according to the invention, close proximity is referred to despite the introduction of a layer.According to the invention, the current value is adjusted taking such a layer into account and thus compensates for the fact that, according to the invention, the prevailing temperature is measured at the control unit and not at the light-emitting diode.

[0037] According to a further aspect of the present invention, the control unit is provided as a controller, a controller chip, a logic circuit, a logic gate, or a microcontroller. This has the advantage that efficient computing units are used as control units that control the LED(s). Using a corresponding control unit, the LED can be controlled using pulse width modulation. In particular, according to the invention, the LED is controlled using a preset current value, which can be regulated, for example, by the control unit.

[0038] The object is also achieved by a system arrangement for setting a constant wavelength of a light-emitting diode, comprising a control unit configured to control the light-emitting diode by means of a preset current value, at least one measuring sensor configured to measure an actually prevailing temperature of the control unit arranged in the immediate vicinity of the controlled light-emitting diode, an interface unit configured to provide an empirically determined wavelength variation of the light-emitting diode as a function of the temperature of the light-emitting diode and a compensation interface configured to adapt the preset current value as a function of the actually prevailing temperature and the empirically determined wavelength variation for setting the constant wavelength of the light-emitting diode.

[0039] The problem is also solved by a computer program product with control commands which execute the proposed method or operate the proposed system arrangement.

[0040] According to the invention, it is particularly advantageous that the method is configured to operate the proposed system arrangement, and the system arrangement is configured to execute the proposed method. Thus, the method comprises method steps that can be functionally replicated based on the structural features of the system arrangement. Furthermore, the system arrangement comprises functional components that provide a function according to the proposed method steps.

[0041] The computer program product serves both to carry out the method steps and to operate the system arrangement.

[0042] Further advantageous aspects are explained in more detail in the attached figures. They show: Fig. 1: a development of a wavelength of a light-emitting diode as a function of temperature as a starting point for the present invention; Fig. 2: a development of a wavelength of a light-emitting diode as a function of a set current value as a further starting point for the present invention; Fig. 3: a compensation of a wavelength according to one aspect of the present invention; Fig. 4: a system arrangement according to a further aspect of the present invention; and Fig. 5: a schematic flow diagram of the proposed method for setting a constant wavelength according to the present invention.

[0043] Fig. 1 shows a diagram on the left, with the temperature of the LED plotted on the x-axis and the resulting wavelength emitted by the LED on the y-axis. Typically, a constant wavelength is required, which, however, varies disadvantageously with temperature. As shown in the present diagram, the wavelength increases with increasing temperature, causing the viewer to perceive an undesirable color variation. An analogous example is shown on the right for a specific value. The present invention aims to compensate for this variation in wavelength.

[0044] Fig. 2 The left-hand diagram shows a current plotted on the x-axis and a wavelength plotted on the y-axis. As can be seen here, the wavelength varies depending on the supplied current, and thus the wavelength decreases with increasing current. A characteristic curve development is also shown on the right-hand side, with the wavelength again plotted on the y-axis and the current on the x-axis. According to the invention, the disadvantages of the wavelength varying depending on temperature developments are overcome, whereby in this case it is exploited that the wavelength can also be changed depending on the supplied current or current value.

[0045] Fig. 3 shows one aspect of the present invention, namely that it is possible to determine at which temperature which wavelength predominates, and for this purpose, it is also possible to calculate how a corresponding error function should be designed. For example, temperatures of 20 °C and 110 °C are considered.

[0046] On the right-hand side, a corresponding diagram is shown, which again plots the provided current value on the x-axis and the wavelength on the y-axis. According to the invention, these two diagrams are now combined according to Fig. 3 combined, and the increasing wavelength on the left side as a function of temperature is eliminated with the decreasing wavelength on the right side as a function of the supplied current value.

[0047] According to the invention, both diagrams are combined, and the current value increases with increasing temperature. Thus, the wavelength increases with temperature, which is compensated for according to the invention by the error function increasing the set current value in such a way that the increase on the left side results in a reduction in the wavelength according to the right side. A constant wavelength is then superimposed on both curves, which is created according to the invention.

[0048] Consequently, according to the invention, the current value is adjusted depending on the prevailing temperature or the wavelength variation. This method can be carried out iteratively in such a way that the diagrams are created for each of the LEDs, i.e., the red, green, blue, and white LEDs.

[0049] Fig. 4 shows the proposed system layout, with a temperature sensor located at the top left, which measures the temperature at the control unit or in the immediate vicinity of the LED and then transmits the measured value in analog form to an analog-to-digital converter. This component then provides the digital measured value to the error function component. A so-called one-time programmable component, i.e., a non-volatile memory, also known as OTP for short, is located on the left side. The error function component then transmits the set value to a digital-to-analog converter, which then controls the LED.

[0050] Fig. 5shows in a schematic flow diagram the proposed method for setting a constant wavelength of a light-emitting diode, comprising controlling 100 the light-emitting diode by means of a preset current value, measuring 101 an actually prevailing temperature of a control unit arranged in the immediate vicinity of the controlled 100 light-emitting diode, providing 102 an empirically determined wavelength variation of the light-emitting diode as a function of the temperature of the light-emitting diode and adjusting 103 the preset current value as a function of the actually prevailing temperature and the empirically determined wavelength variation for setting 104 the constant wavelength of the light-emitting diode.

[0051] According to one aspect of the present invention, at least one sensor is provided for measuring the temperature value at at least one measuring location. Multiple measuring locations are suitable for this purpose, for example, a measuring location at exactly one LED, a measuring location at each LED, a measuring location at a microcontroller connected to an LED, or a measuring location in the immediate vicinity of an LED. For example, the proposed method is used with several interconnected LEDs. In this case, it is possible for several LEDs to be connected in series, for example. If this plurality of LEDs is installed in an automobile, different temperatures may prevail at different locations. This means that the LEDs can not only heat up on their own, but heat can also be radiated by adjacent components.Thus, the invention makes it possible to take this into account and determine a temperature value at multiple measurement locations. An immediate environment here describes an environment that allows a conclusion to be drawn about the temperature of the LED. Therefore, this temperature does not have to be measured directly at the LED; instead, a temperature sensor can be spaced from the LED such that any temperature input from neighboring components is negligible. In particular, this means that there is no need for physical contact in the sense of touching the temperature sensor and the LED.

[0052] According to a further aspect of the present invention, the light-emitting diode is present as a triplet of three light-emitting diode units, and the light-emitting diode units each emit a different color. Individual LEDs are also possible according to the invention. This has the advantage that colored LEDs can be used. In particular, the invention makes it possible to continue using conventional LEDs and only control the current regulator of these LEDs in such a way that the advantage according to the invention is achieved. Furthermore, the proposed method has the advantage that brightness compensation can be carried out independently of the color setting of the light-emitting diode. Those skilled in the art are aware of other light-emitting diodes which have light-emitting diode units that can be reused according to the invention. For example, a light-emitting diode unit is present as a semiconductor component or as any other light-emitting component.Emitting different colors, or light in different wavelengths, serves to set a predetermined color value.

[0053] According to a further aspect of the present invention, a memory module provides a plurality of temperature values, each of which is associated with a current value. This has the advantage that a plurality of temperature values ​​can be taken into account, and the temperature values ​​can be predetermined with respect to the current values ​​in such a way that the same brightness value of the LED is always established. In particular, the number of current value / temperature value pairs can be determined in a preparatory method step.

[0054] Accordingly, the memory module, or the storage of the current values, must be interpreted in such a way that any type of memory module or storage is possible. Thus, the memory module does not have to be configured dynamically in such a way that it must be writable during runtime, i.e., while the current controller is being controlled. Rather, storage merely requires the relevant information to be incorporated into a hardware module in some way. It may also be necessary to provide not a single memory module, but rather to provide additional components that enable the current value to be provided.

[0055] In the present case, a light-emitting diode should be understood as a device which can also comprise further LED chips. Thus, the light-emitting diodes according to the invention in turn consist of further light-emitting diode units or semiconductor chips. For example, the known red, green and blue light-emitting diode units can be used for this purpose, which are adjusted with respect to the so-called RGB color space. These individual light-emitting diode units are combined in an light-emitting diode housing in such a way that their light combines to form a predetermined color value. For example, it is possible to adjust a mixing ratio such that the light-emitting diode as a whole emits white light. Further devices, such as a diffuser, can also be provided for this purpose. When individual light-emitting diodes or light-emitting diode units are combined, any desired colored light can be achieved by appropriately controlling the individual components.This makes it possible to create color transitions, for example. According to the invention, so-called multi-LED components can be used, for example.

Claims

1. A method for adjusting a constant wavelength of a light-emitting diode, comprising; - control (100) of the light-emitting diode by means of a preset current value; - measuring (101) an actual prevailing temperature of a control unit arranged in the immediate vicinity of the driven (100) light-emitting diode, exclusively characterised by: components are arranged between the light-emitting diode and the control unit; - providing (102) an empirically determined wavelength variation of the light-emitting diode as a function of a temperature of the light-emitting diode; and - adjustment (103) of the preset current value as a function of the actually prevailing temperature and the empirically determined wavelength variation for setting (104) the constant wavelength of the light-emitting diode, wherein the adjustment (103) of the preset current value is carried out by means of a stored error function and the error function determines the temperature of the light-emitting diode as a function of the actually prevailing temperature of the control unit.

2. The method according to claim 1, wherein the method is carried out in each case for a red, blue, green or white emitting light-emitting diode.

3. The method according to claim 1 or 2, wherein the method is carried out iteratively such that the adjustment (103) of the preset current value takes place substantially every two seconds.

4. The method according to one of the preceding claims, wherein the preset current value specifies a current pulse of a pulse width modulation.

5. The method according to claim 1, wherein the error function provides a compensation value which compensates for the wavelength variation of the light-emitting diode.

6. The method according to one of the preceding claims, wherein the compensation value is present as a compensation factor and / or compensation summand.

7. The method according to one of the preceding claims, wherein the adjustment (103) of the preset current value takes place when an actual wavelength deviates from the target wavelength by more than a threshold value.

8. The method according to one of the preceding claims, wherein the empirically determined wavelength variation specifies a characteristic curve of the light-emitting diode.

9. The method according to one of the preceding claims, wherein the close proximity is less than one millimeter.

10. The method according to one of the preceding claims, wherein the close proximity is adjusted by means of a thickness of an adhesive layer, a silicone layer, a polymer layer, a thermally conductive layer, an aluminium layer and / or a copper layer.

11. The method according to any one of the preceding claims, wherein the control unit is provided as a controller, a controller chip, a logic circuit, a logic gate or a microcontroller.

12. A system arrangement for adjusting a constant wavelength of a light-emitting diode, comprising; - a control unit set up to control (100) the light-emitting diode by means of a preset current value; - at least one sensor arranged to measure (101) an actual prevailing temperature of the control unit arranged in the immediate vicinity of the driven (100) light-emitting diode, exclusively characterised by: components are arranged between the light-emitting diode and the control unit; - an interface unit arranged to provide (102) an empirically determined wavelength variation of the light-emitting diode as a function of a temperature of the light-emitting diode; and - a compensation interface set up for adjusting (103) the preset current value as a function of the actually prevailing temperature and the empirically determined wavelength variation for setting (104) the constant wavelength of the light-emitting diode, wherein the adjustment (103) of the preset current value can be carried out by means of a stored error function and the error function determines the temperature of the light-emitting diode as a function of the actually prevailing temperature of the control unit.

13. A computer program product comprising control instructions which cause the apparatus of claim 12 to perform the method steps of any one of claims 1 to 11 when executed on a computer.