Light and method for operating a light

Abstract

The invention relates to a light, in particular for testing workpiece surfaces using a fluorescent marking means, which light has at least two lighting means (2, 3) which emit electromagnetic radiation with different wavelength ranges. According to the invention, the light is characterized in that the intensity with which the lighting means (2, 3) irradiates can be adjusted separately for at least one of the lighting means (2, 3). Expediently, the light (1) is configured to increase or reduce the intensity of at least one of the lighting means (2, 3) and at the same time to keep the intensity of at least one other of the lighting means (2, 3) constant, or to reduce it or increase it in the opposite way to the first-mentioned lighting means. In a refinement of the invention, the light (1) is configured to adjust the intensity at such a speed that the human eye can adapt to a change in the intensity during the adjustment without adverse effects on the person's sight.

Description

Description: "Lamp and method for operating a lamp" The invention relates to a lamp, in particular for testing workpiece surfaces, which has at least two light sources that emit electromagnetic radiation of different wavelength ranges. The invention further relates to a method for operating a lamp, in particular for testing workpiece surfaces, optionally using a fluorescent marking agent. Luminaires of the type mentioned above, which have white light and ultraviolet LEDs emitting both white light and ultraviolet radiation, are known from DE 10 2004 043 295 B4, EP 1 623 213 B l and US 8,61 6,722 B2. They are used for visual inspection of workpiece surfaces, in particular for the detection of contaminants, for penetrant testing and for fluorescent magnetic particle testing. Visual inspection, particularly when examining workpiece surfaces using the fluorescent penetrant method, involves using a fluorescent agent to make defects on workpiece surfaces visible through excitation with ultraviolet radiation. The inspection is usually carried out in the dark, at an ambient light level of less than 20 lux, because the human eye has difficulty seeing the surface. Mesopic vision (twilight vision) and scotopic vision (night vision) have higher contrast sensitivity in visual perception and can be tested more effectively when there is high contrast between the surface being examined and the excited fluorescence. A problem arises when performing the testing procedures with the known lamps: when switching between white light illumination, used to inspect and assess defects found on the workpiece under ultraviolet radiation, and ultraviolet illumination, a relatively large change in brightness occurs, to which the eyes must adjust. Since the testing often involves frequent changes in lighting, the eyes are subjected to a relatively high degree of strain. The invention is based on the objective of creating a lamp of the type mentioned above, with which the inspection of workpiece surfaces can be carried out more effectively. A luminaire that solves this problem is characterized by the fact that the intensity with which the luminaire emits light is adjustable for at least one of the light sources. The invention makes it possible to coordinate the intensities with which the respective light sources emit light when switching from one light source to another or when moving one light source to another, in such a way that abrupt changes in intensity are avoided. Advantageously, the eyes can adjust to such changes without abrupt changes in intensity. Adjusting the settings better to the change in intensity will result in improved visual perception. Sudden, potentially strong changes in intensity, such as those that occur with known lamps, especially when switching on and off the white light, particularly in low ambient light, and which are very unpleasant and tiring for the eyes and can even damage them, can be avoided. It is advantageous to have the intensity of at least one light source adjustable separately from the other light source. In one embodiment of the invention, one of the light sources is provided for emitting visible light, preferably white light, and another of the light sources is provided for emitting ultraviolet radiation, infrared radiation and / or blue-violet light. Preferably, the radiation emitted by the light source for visible light comprises a wavelength range between 380 and 780 nm, that of the light source for ultraviolet radiation a wavelength range between 200 and 400 nm, that of the light source for infrared radiation a wavelength range between 780 nm and 50 nm, and that of the light source for blue-violet light a wavelength range between 380 and 490 nm. In visual inspection, as explained above, ultraviolet radiation is used to detect defects or the like on the surface of a workpiece with a fluorescent agent, and under white light, possibly with simultaneous irradiation with ultraviolet radiation, the workpiece is aligned and / or defects or the like found under the ultraviolet radiation are inspected, analyzed, interpreted and / or assessed. Advantageously, the intensity with which the respective light source emits radiation is controllable and / or adjustable over the entire power range of the light source between a minimum intensity, at which the light source preferably emits no radiation, and a maximum intensity, which is preferably adjustable and may be changed within certain limits. In one embodiment of the invention, the luminaire is configured to increase or decrease the intensity of at least one of the light sources and at the same time to keep the intensity of at least one other of the light sources constant or to decrease or increase it in the opposite direction to the first-mentioned light source. Such changes in intensity allow, on the one hand, the addition of one light source to another, and on the other hand, the transition from one light source to another, and thus from one wavelength range of illumination to another. Advantageously, the intensity of individual or all of the light sources can be adjusted separately. In a further embodiment of the invention, the lamp is configured such that the speed at which the intensity is changed depends on the intensity at which the light source emits light. In a particularly preferred embodiment of the invention, the luminaire is provided such that the rate of change of the intensity with which the respective light source emits is lower in intensity ranges where relatively low intensity is emitted than in intensity ranges where comparatively high intensity is emitted. While it would be conceivable to change the rate of intensity change linearly as a function of time or proportionally to the intensity, it is preferably changed as an exponential function of the intensity. The different rates of intensity change allow the intensity change in the different brightness ranges to be adapted to the The eye's adaptability is adjusted, taking into account that the eye exhibits varying sensitivity to intensity changes across different intensity ranges and is more sensitive to changes in intensity in the dark than in bright light. This sensitivity is not linear to intensity but is described by the Weber-Fechner law or Stevens' power function, both of which show that the eye's sensitivity is proportional to the logarithm of the radiation intensity. Accordingly, in areas of lower radiation intensity, the intensity can only be changed at relatively slow rates without causing significant eye irritation, while in areas of higher radiation intensity, the intensity can be changed more rapidly without causing discomfort. In one embodiment of the invention, the lamp is configured to adjust the intensity at such a speed that the human eye can adapt to a change in intensity during the adjustment without or with only a minor impairment of visual perception, in particular visual acuity, preferably such that a person under one of the When adjusting the light source, any defects detected under different lighting conditions should be monitored. The difficulties often encountered with conventional lights – namely, detecting a defect found under UV light when illuminated with white light – are eliminated. This allows for faster and more accurate visual inspection than with traditional lights. In a further embodiment of the invention, the lamp is configured for adjusting the intensity such that the change in intensity is perceived by the human eye as uniform, in particular without sudden changes in intensity. Such a change in intensity is not perceived as unpleasant and is gentle on the eyes. Advantageously, the intensity is adjustable between 0% and 100% of a specified maximum intensity, preferably 20 to 2000 lux, such that the human eye can adapt to changes in intensity during the adjustment without impairing vision and / or that the change in intensity is perceived as smooth by the human eye. While it would be conceivable to provide such adjustability only in segments for specific intensity ranges, it is preferably provided across the entire intensity range to enable comfortable work with the light. The maximum intensity is advantageously adjustable, preferably separately for each of the luminaires, so that the luminaire can be adapted to individual needs. In a further embodiment of the invention, the average rate of change of the white light intensity is between 5 lux / s and 500 lux / s. The following durations are recommended for creating the intensity of the white light: - Increase in intensity from 0 to 20 lux / Decrease in intensity from 20 to 0 lux: Duration > 1 - 1.5 seconds - Increase in intensity from 0 to 50 lux / Decrease in intensity from 50 to 0 lux: Duration > 1.5 - 2.5 seconds - Increase in intensity from 0 to 100 lux / Decrease in intensity from 100 to 0 lux: Duration > 2 - 3 seconds - Increase in intensity from 0 to an intensity > 100 lux / Decrease in intensity from an intensity > 100 lux to 0 lux: Duration > 2.5 seconds Within the specified speed ranges, the light can be used at sufficiently high speeds; excessive eye strain is avoided, vision is not impaired when the intensity changes, and the intensity changes are perceived as smooth. perceived. In order to make the adaptation of the eye to the white light as comfortable as possible when adjusting the white light source from a switched-off state in which the light source does not emit light, the luminaire is expediently designed so that the white light source can be adjusted to intensities that are < 1%, preferably < 0.5%, particularly preferably < 0.1% of a maximum intensity provided for the white light. In a particularly preferred embodiment of the invention, the white light source can be adjusted from the switched-off state in steps of less than 1 lux, preferably less than 0.5 lux. The light can be configured such that it is controlled by the two Of the latter setting options, i.e., the percentage dependence on the maximum intensity and the steps in 1 or 0.5 lux, the one that results in smaller steps is selected. Preferably, the luminaire is configured such that the rate(s) of intensity change, preferably within certain limits, is / are variable to allow for individual adjustment. Advantageously, at least one of the light sources is formed by a discharge lamp and / or by at least one light-emitting semiconductor diode (LED), the light source(s) preferably being equipped with a filter for adjusting the respective wavelength range. Advantageously, the luminaire includes a control and / or regulation unit for adjusting the intensities. The control and / or regulation unit preferably comprises at least one constant current controller operating on the basis of electrical and electronic switching and control circuits.This allows the lights, especially the LEDs, to be operated particularly efficiently because the constant current control measures the operating current of the respective light source, particularly the LED or LED circuit, and keeps it constant within narrow limits. The constant current control can conveniently generate the constant current from an alternating and / or direct current voltage. The intensity of the LED is preferably changed using pulse-width modulation. The luminaire preferably incorporates a pulse-width modulation control for this purpose. Furthermore, the control and / or regulating device may include a control mechanism that can reduce or switch off the power of the luminaire or individual parts of the luminaire depending on the temperature, in order to protect the luminaire, in particular the light source and / or the control and / or regulating unit, from overheating. For cooling, the luminaire could additionally or alternatively be equipped with a fan, preferably controlled by the control and / or The control device is controllable and / or adjustable. The light is expediently designed such that the intensity of the light source, preferably emitting white light, is increased when the control element is actuated and decreased when the control element is released. Alternatively, it could be provided that the intensity of the light source, preferably for white light, is automatically increased up to the intended maximum intensity when the control element is activated, and that after reaching the maximum intensity, the intensity is automatically reduced or decreased again by pressing the control element again. The radiation is reduced again when the control element is activated, preferably to a predetermined minimum value at which preferably no radiation is emitted. Advantageously, the lamp is configured to indicate the operating status of at least one of the light sources. The operating status preferably includes the intensity with which the light source emits radiation and / or the duration during which the light source emits radiation, preferably at a predetermined intensity. Indicating the duration is particularly useful for ensuring that, during the aforementioned inspection of the workpiece surfaces, predetermined time periods are observed to allow the eye to adapt to specific intensities. Alternatively or additionally, the light may indicate whether the respective light source emits radiation or not, i.e., in particular whether it is switched on or off, and for how long it has been in the respective position. The operating state is indicated. Advantageously, it can be shown whether sufficient time has elapsed after reaching the respective operating state for the eyes to adapt to the respective intensity. Preferably, the lamp includes the standard values ​​for eye adaptation times for the respective operating states. In one embodiment of the invention, the lamp according to the invention is used as follows. First, preferably in the dark or at very low ambient light, the UV-emitting lamp is switched on. The intensity of the emitted radiation is preferably increased to a predetermined target intensity at one of the intensity change rates described above. However, it can also be set abruptly to the target intensity, since sudden changes in illumination resulting from the UV radiation are less unpleasant for the eye. The workpiece surface can then be examined under UV radiation. To inspect the workpiece surface under visible light as well, the white light emitter is switched on and its intensity is increased, preferably at one of the rates described above, to a predetermined target intensity. Optionally, the UV emitter can be switched off when a certain intensity of the white light emitter is reached or during the increase in intensity, or its intensity can be decreased, preferably at one of the rates described above. For further examination under UV radiation, the UV lamp is then switched on again and the intensity is increased. The radiation from the UV lamp is amplified, preferably at one of the speeds described above. Subsequently or simultaneously, the intensity of the white light lamp is increased again, preferably as described above, to a predetermined target intensity at which the workpiece surface is to be inspected with visible light. It is understood that the light fixture is designed to automatically perform the above-described changes in the intensity of the respective light sources, possibly in response to a command entered via a control element. In one embodiment of the invention, the luminaire has a monitoring device designed to detect an operational fault, in particular a defect in one of the light sources or other components of the luminaire, and preferably designed to shut down the luminaire upon detection of an operational fault. The monitoring device may also be designed to display the operational fault. In a further embodiment of the invention, the luminaire includes an adjustment device designed to regulate the intensity of the radiation emitted by the respective light source as a function of the device's temperature. Advantageously, this compensates for a reduction in intensity that occurs with increasing temperature. Advantageously, the size of the area that can be illuminated by the lamp is adjustable. Preferably, a first setting is provided in which a relatively small area can be illuminated and which serves for focused viewing, and a further setting with which a larger area can be illuminated. The lamp advantageously comprises at least one housing, preferably designed to hold the light source, at least one control element, at least one optical system, preferably comprising at least one lens, and / or at least a coolant, preferably a fan, a heat exchanger, and / or a cooling plate. In one embodiment of the invention, the lamp can be used in mobile and / or stationary applications. It can be a handheld and / or floor lamp, or a lamp that can be permanently installed, for example, on a bracket or a wall. While in a particularly preferred embodiment of the invention a device for operating the luminaire is integrated into the housing, it would also be conceivable to provide it outside the housing. The luminaire preferably comprises at least one housing for the light sources, at least one control cabinet or switch housing, and / or at least one operating element. Advantageously, the operating device includes at least one control element that can be operated manually or with the foot, by means of which the intensity of at least one of the light sources can be changed. The operating element, which is preferably formed by a push button or a rotary control, advantageously serves to switch the respective light sources on or off or to control and / or regulate the intensities with which the respective light sources emit light. The power supply for the luminaire can be provided by at least one external or integrated DC and / or AC power source. The term "luminaire" should also include a lighting system. The invention is explained in more detail below with reference to an exemplary embodiment and the accompanying drawings relating to that embodiment. The drawings show: Fig. 1 shows a luminaire according to the invention in a side section, Fig. 2 shows a front view of the lamp according to Fig. 1 , Fig. 3 shows a rear view of the lamp according to Fig. 1, and Figures 4 to 6 are diagrams illustrating the invention. A luminaire 1 according to the invention, as shown in Figures 1 to 3, has eight UV LEDs 2, which are provided for the emission of UV radiation (wavelength range 200–400 nm), and eight white light LEDs 3, which are provided for the emission of white light (wavelength range 380–780 nm). Optical components 4, 5 are arranged in front of the LEDs 2, 3 to influence the respective beam paths emanating from the LEDs. Viewed in the direction of radiation, a front lens 7 is arranged behind the optical components 4, 5. This lens is connected to a housing 9 of the luminaire 1 via fastening means 10 and is provided with filters 8 for each of the UV LEDs 2. The LEDs 2, 3 are soldered onto a carrier board 6, which is mounted on a heat sink 11. The heat sink 11 is connected via standoffs 12 to a circuit board 13, which carries several electronic components. A display LED 14 is also mounted on the circuit board 13, which, as explained in more detail below, is intended to indicate the operating status of the luminaire 1. A light guide 15 is arranged between a rear side of the housing 9 and the display LED 14, by means of which radiation from the display LED 14 can be directed to the rear side. The electronic components form a control and / or regulation device 26, which is designed to control and / or regulate the intensities with which the LEDs 2,3 emit light. The electronic components also form a monitoring device 27, which is designed to detect faults during the operation of the luminaires 1. The monitoring device 27 is configured to shut down the luminaire 1 upon detecting an operational fault, for example, if one of the LEDs 2 or 3 fails. Furthermore, it can be designed to indicate the operational fault, for example, by displaying a code via the indicator LED or another means. To characterize the display setup. Furthermore, an adjustment device 28 is formed on the circuit board 1 3 by the electronic components. This device is designed to regulate the power output of the LEDs 2, 3 depending on the temperature of the lamp 1, in order to compensate for changes in current that occur when the lamp 1 heats up, thus regulating the respective intensities to the intended values. A fan 1 6 is provided for cooling the lamp, by means of which air can be blown onto the circuit board 1 3 and the heat sink 1 1. The housing 9 is provided on its rear with a ventilation grille 1 7, which is designed to hold a filter and through which air is drawn in by the fan 1 6. The light fixture 1 also has a handle 18, at the lower end of which a cable 20 is routed via a strain relief 19, through which the light fixture 1 can be supplied with energy and, if necessary, controlled. As can be seen in particular in Fig. 3, the luminaire 1 is provided on its back with pushbuttons 21, 22, 23, 24 and a rotary control 25, which are intended for controlling intensities with which the LEDs 2, 3 light up and which interact with the control and / or regulating device 26. The control and regulating device 26 is designed to change the intensities with which the UV LEDs 2 and the white light LEDs 3 emit light separately from each other by means of pulse width modulation. The control and regulating device 26 changes the intensities at such a speed that the human eye can adapt to the changing intensity without or with only slight impairment of visual perception, in particular visual acuity, so that a Any detected defect can be monitored during changes in intensity. Furthermore, the intensities are adjusted so that the human eye perceives the change in intensity smoothly, i.e., without sudden jumps. To enable, on the one hand, a transition from UV to white light illumination that is as tiring as possible, and on the other hand, to perform this transition as quickly as possible, the control unit 26 is programmed such that the respective intensity is adjusted according to the adaptability of the The human eye perceives changes in intensity relatively slowly at low intensities and more rapidly at comparatively high intensities. The rate at which the intensity changes can be proportional or exponential to the intensity, or linear. The intensity of the UV LEDs and the white light LEDs 3 can be changed depending on the time. By pressing the buttons 21, 22, 23, 24, the control unit 26 is caused to change the intensity of the UV LEDs and the white light LEDs 3 as explained below with reference to Figures 4 to 6, which show diagrams depicting the time course of the intensity of the UV LEDs 2 (Y-axis luv plotted on the left) and the white light LEDs 3 (Y-axis lw plotted on the right). 1. Example (see Fig. 4): At time ti, the button 21 is pressed, thereby switching on the UV LED 2, which then shines with a target intensity IMAX, below which a workpiece can be examined. At time h, pressing one of the buttons 22 causes a switch from radiation by the UV LED 2 to radiation by the white light LED 3. This means that the intensity at which the white light LED 3 emits is increased to a target intensity IMAX, as explained above, while simultaneously the intensity at which the UV LED 2 emits is reduced to the point where it no longer emits light. At time h, the target intensity IMAX of the white light LED 3 is reached, and the intensity of the UV LED is reduced to zero. The workpiece can now be inspected under white light. If a further examination is to be carried out under UV illumination, the button 22 is released again and the light is switched to illumination by the UV LED 2 (time points and ts). By pressing the button 22 again, it is possible to switch back to white light (time points and h). When the light is switched back to UV light, button 22 is released again (time points ts and t?). To switch off the UV LED 2, the button 21 is pressed again. It goes without saying that the UV LED 2 and white light LED 3 can be switched back and forth as often as desired for examining the workpiece. It can be provided that a crossover to radiation using only white light is carried out only as long as one of the buttons 22 is pressed and, conversely, when one of the buttons 22 is released, the intensity of the white light LED 3 is reduced again and that of the UV LED is increased to the target intensity. Advantageously, a spot on the workpiece, e.g., a defect, viewed under ultraviolet or white light, can remain in view even during crossfading. In both directions of change, i.e., from white light to ultraviolet radiation and vice versa, the respective intensity changes are perceived as a film. Since the intensity changes occur at speeds that allow the eyes to adapt, visual perception, especially visual acuity, is not impaired or only minimally affected. As shown in Fig. 4a, the intensities of the UV LED 2 and the white light LED 3 can be adjusted such that the intensities change more rapidly at higher intensities than at lower intensities. As explained above, the human eye can then adapt better to the changing intensities. However, it would also be conceivable to adjust the intensities, as shown in Fig. 4b, in The dependence on time changes linearly. It goes without saying that the different intensity levels could also be combined. For example, the intensity of the white light LED 3 could be changed depending on the intensity level, while the intensity of the UV LED 2 could be changed proportionally to time. Example 2 (Figure 5) At time ti, pressing button 21 switches on the UV LED 2 and sets its intensity to the target intensity IMAX. Pressing one of the buttons 23 (time t2j) activates the white light LED 3 in addition to the UV LED 2. The intensity of the white light LED is gradually increased to a target value IMAX, as described above, until it reaches this value at time †3, at which point the intensity of the UV LED 2 remains constant. Similarly, the intensity of the white light LED 3 increases or remains constant at IMAX only as long as button 23 is held down. Releasing button 23 (time U) reduces the intensity of the white light LED 3 until it no longer emits light at time ts.If necessary, the white light LED 3 can then be switched back on with the UV LED 2 (time points up to ts and tio). Once the workpiece inspection is complete, the UV LED is switched off by pressing button 21 (time points h and tu). As explained above with reference to Fig. 5, the white light LED 3 can also, in this example, exhibit different characteristics depending on the respective intensity. Speeds can be changed (Fig. 5a) or the intensity can be carried out in a linear dependence on time (Fig. 5b). Example 3 (Figure 6) As Fig. 6 shows, the UV LED could also be switched off after the white light LED 3 is switched on (times and ts) and switched back on as needed. When white light is switched on again (time points† and ) to view the workpiece under UV radiation, the intensity of the white light LEDs 3 is then reduced again (time points ts and h) . Furthermore, as can be seen in Fig. 6, even when the UV LED 2 is switched on, the The intensity can be gradually increased to the target value IMAX, e.g. with one of the speeds described above. The push button 24 is designed to maintain the respective intensities of the UV LED 2 and the white light LED 3, i.e., to prevent a change in the illumination state achieved with the light 1. For example, by By pressing button 24, the light 1 can be kept in the state at time t3 according to Fig. 4a or in the state after time†3 according to Fig. 5a, without having to hold button 22 or button 22 or 23 pressed. The rotary control 25 is intended to change the target intensity with which the white light LED 3 emits within certain predetermined limits and to adjust it for use with the light 1.

Claims

Potency claims:

1. Lamp, especially for inspecting workpiece surfaces, which has at least two light sources (2,3) which electromagnetic emit radiation of different wavelength ranges, characterized by that for at least one of the light sources (2,3) an intensity with which the light source (2,3) emits is adjustable.

2. Luminaire according to claim 1 , characterized by that the luminaire (1 ) is designed to increase or decrease the intensity of at least one of the light sources (2,3) and at the same time to keep the intensity of at least one other of the light sources (2,3) constant or to decrease or increase it in the opposite direction to the first-mentioned light source.

3. Luminaire according to claim 1 or 2, characterized by that the luminaire ( 1 ) is arranged such that the speed at which the intensity is adjusted depends on the intensity with which the respective light source (2,3) emits light, wherein the speed is preferably lower at comparatively low intensity than at comparatively high intensity.

4. Luminaire according to one of claims 1 to 3, characterized by that the light (1 ) is used to adjust the intensity in such a way The speed is set so that the human eye can adapt to a change in intensity during the adjustment without impairing visual perception.

5. Luminaire according to one of claims 1 to 4, characterized by that the luminaire (1) is configured to adjust the intensity such that the change in intensity is perceived by the human eye as uniform, in particular without sudden changes in intensity. Luminaire according to any one of claims 1 to 5, characterized by that the speed at which the intensity is changed is between 5 lux / s and 500 lux / s. Luminaire according to one of claims 1 to <£>, characterized by that one of the light sources (2) is provided for the emission of visible light, preferably white light, and another of the light sources (3) is provided for the emission of ultraviolet radiation, infrared radiation and / or blue-violet light. Luminaire according to one of claims 1 to 7, characterized by a control and / or regulating device (26) for adjusting the intensities. Luminaire according to one of claims 1 to 8, characterized by that the light ( 1 ) is designed to indicate an operating state of at least one of the light sources (2,3). Luminaire according to one of claims 1 to 8, characterized by that the operating state includes the intensity with which the light source (2,3) emits the radiation and / or an emission duration, preferably an emission duration at a predetermined intensity. Luminaire according to one of claims 1 to 10, characterized by at least one operating device, preferably a hand and / or foot switch and / or control (21 ,22,23,24,25) , for changing the intensity. Luminaire according to one of claims 1 to 1 1 , characterized by a monitoring device designed to detect an operating fault, in particular a defect in one of the light sources (2, 3) or other components of the lamp (1), and preferably to indicate the operating fault and / or to shut down the lamp (1) upon detection of an operating fault. Method for operating a lamp (1), in particular for inspecting workpiece surfaces, especially using a fluorescent marking agent, which has at least two light sources (2, 3) emitting electromagnetic radiation of different wavelength ranges. characterized by that for at least one of the light sources (2,3) an intensity with which the light source (2,3) emits is adjusted. Method according to claim 13, characterized by that the intensity is adjusted at such a speed that the human eye can adapt to a change in intensity during the adjustment without impairment of vision. Method according to claim 13 or 14, characterized by that the luminaire (1) includes a control element (21, 22, 23, 24) and that the intensity with which one of the light sources (2, 3) emits, preferably the white light emitting light source (2, 3), is increased or decreased by actuating the control element (21, 22, 23, 24) and the intensity with which the other light source (2, 3) emits is simultaneously kept constant or, conversely, decreased or increased relative to the respective first light source (2, 3).