Measuring device for measuring optical radiation sources and method for carrying out a measurement

Abstract

Measuring device for measuring an optical radiation source (1), comprising a support device (2) holding the radiation source (1) to be measured, at least one reflector (3), at least one camera and at least one evaluation unit, characterized in that the reflector (3) is designed as a planar, narrow reflector strip (3a) whose longitudinal dimension is at least ten times larger than its transverse dimension and which extends along its longitudinal axis (A), wherein the reflector strip (3a) and the radiation source (1) are arranged movably relative to each other, such that the direction of movement is transverse to the longitudinal axis (A) of the reflector strip (3a), and that the camera is designed as a line scan camera (4), multispectral camera or hyperspectral camera (7).

Description

[0001] The invention relates to a measuring device for measuring an optical radiation source, comprising a support device for holding the radiation source to be measured, at least one reference sensor, at least one reflector, at least one camera, and at least one evaluation unit. The invention further relates to a method for carrying out a measurement using such a measuring device.

[0002] The radiation sources to be measured include, for example, LEDs, LED arrays, headlights and other directional light sources.

[0003] To determine the luminous intensity distribution of optical radiation sources, especially vehicle headlights and similar light sources, so-called far-field goniophotometers are used. The measuring sensor, for example a photometer, is usually permanently installed. A goniometer serves as a support device, moving the respective radiation source to different positions in order to measure each direction of emission from the optical radiation source. Moving to each individual position makes the measurement very complex and time-consuming. To speed up the measurement, several sensors forming a matrix can be used instead of a single sensor. This can accelerate the measurement, but the required hardware is relatively complex and expensive.

[0004] Therefore, the use of measuring devices with a large-area reflector screen as a reflector and a camera, so-called screen photometers, has become common practice in order to achieve shorter measurement times with relatively low hardware requirements. The optical radiation source shines onto the large-area reflector screen. The reflection is recorded and measured using a suitable camera. The camera captures as much of the reflector screen as possible. While with a far-field goniophotometer each direction of emission from the optical radiation source must be addressed individually, the screen photometer delivers the measurement data simultaneously in a single image.

[0005] One problem, however, is the stray light reflected by the large reflector screen. After further reflection off the room walls, this light returns to the reflector screen, severely limiting the measurable dynamic range, especially when measuring car headlights with their pronounced cut-off lines. The problem of stray light reflected back from the reflector screen is primarily caused by its size. It is typically built as large as possible, usually as large as the dimensions of the light channel allow, so that the camera can capture the largest possible solid angle of the light distribution in a single image. Consequently, the projection screen reflects a significant amount of stray light back into the room. Another problem is the difficulty in achieving spectral alignment due to the non-selective reflecting properties of the reflector screen.

[0006] To reduce stray light, known implementations of screen photometers primarily employ stray light baffles between the optical radiation source and the reflector screen. These stray light baffles add to the complexity of the equipment. Furthermore, techniques exist to computationally compensate for the influence of stray light. For this purpose, correction values ​​are determined before the actual measurement with a special light source.

[0007] Such and similar measuring devices are known from the prior art. US 2012 / 0194820 A1 describes a goniophotometer with an arc-shaped reflector. The light source is essentially located in the center of the arc, and the reflected light is detected by a rotational movement in order to capture a three-dimensional light distribution.

[0008] From EP 2 700 921 A1, a device for measuring light distributions is known, in which a rotatable plane mirror deflects the light from a light source onto a detector unit. The detector unit and the mirror are positioned relative to the light source, scanning various spatial directions.

[0009] German patent DE 10 2015 201 093 A1 discloses a gonioradiometer with several stationary sensors or with a camera that captures light by reflection off a measuring wall. Additionally, an opening may be present in the measuring wall through which light for calibration directly strikes a sensor.

[0010] US patent 2010 / 0328672 A1 discloses a goniophotometer with a combination of a primary mirror and a synchronously rotating secondary mirror. The light source is mounted on a rotating stage, while the light, deflected by the mirrors, falls onto detector tubes. This arrangement allows for the measurement of light intensity in different spatial directions with a compact design and high accuracy. However, the setup is very complex.

[0011] The object of the invention is to further develop a measuring device of the type described above in such a way as to overcome the disadvantages of known screen photometers as far as possible. In particular, this involves the conflicting objectives of achieving fast, accurate, and meaningful measurements on the one hand, and particularly economical measurements on the other.

[0012] To solve this problem, the invention proposes, starting from a measuring device of the type mentioned at the outset, that the reflector is designed as a planar, narrow reflector strip whose longitudinal dimension is at least ten times larger than its transverse dimension and which extends along its longitudinal axis, wherein the reflector strip and the radiation source are arranged movably relative to each other, such that the direction of movement is transverse to the longitudinal axis of the reflector strip, and that the camera is designed as a line scan camera, a multispectral camera or a hyperspectral camera.

[0013] By reducing the reflector from a large reflector screen to a narrow reflector strip, the scattered light is drastically reduced. A narrow reflector strip is defined as one whose longitudinal dimension (along the longitudinal axis) is at least ten times, preferably at least one hundred times, larger than its transverse dimension (perpendicular to the longitudinal axis). According to the invention, during measurement, the radiation source is moved perpendicular to the reflector strip by means of the support unit, so that the light cone of the radiation source is guided transversely across the reflector strip. The position of the radiation source is transmitted from the support unit to the evaluation unit via a communication interface. The line scan camera or hyperspectral camera continuously or discretely acquires images of the reflector strip and transmits them to the evaluation unit.The evaluation unit synchronizes the recordings and the positional progression of the radiation source. The recordings are then combined, and the resulting two-dimensional image is used to determine the luminance and / or luminous intensity distribution of the radiation source. The measurement time remains similarly low compared to using a large-area reflector screen.

[0014] While multispectral cameras have 3 to 19 spectral channels for measurement, hyperspectral cameras have 20 to well over 200 spectral channels.

[0015] When using a multispectral or hyperspectral camera, the measurement can be spectrally resolved. This allows both the spectral reflection properties of the reflector to be calculated and the spatial spectral radiance distribution of the optical radiation source to be determined. During the measurement, a spectrum is acquired for each pixel. Depending on the required measurement accuracy, the measurement can be adjusted with the appropriate resolution by the camera used. This means that the spectral vector acquired for each pixel can have a very large number of spectral channels (e.g., over 100) when a hyperspectral camera is used, resulting in a quasi-continuous spectrum. However, in some cases, only eight or even fewer channels may suffice. In certain applications, even the use of an RGB camera with only three (color) channels may be sufficient for spectral matching.

[0016] It is particularly advantageous if the support device is designed to move the radiation source perpendicular to the reflector screen. When considering the mobility of the support device, special attention must be paid to ensuring that the entire light distribution of the optical radiation source can be projected onto the reflector screen during the movement.

[0017] It is particularly advantageous that the support device is designed as a goniometer. The goniometer allows for optimal movement of the radiation source to meet the requirements of the measurement. In this case, the movement is a rotation of the radiation source, with the axis of rotation running parallel to the longitudinal axis of the reflector strip. Furthermore, the support device can be configured to rotate the radiation source around other axes, e.g., transversely to the longitudinal axis of the reflector strip, for example, to measure the luminous intensity distribution over a larger beam angle range, which is not limited by the dimensions of the reflector strip.

[0018] Furthermore, it is particularly useful if the support device and the evaluation unit are connected via a communication interface. This allows the position and angle of the radiation source on the support device to be synchronized with the images from the line scan camera or hyperspectral camera in a very simple way during movement.

[0019] A preferred embodiment of the invention provides that the reflector strip has an opening (optionally closable) and that the reference sensor is arranged on the side of the reflector strip opposite the radiation source. Reference measurements of the light passing through the opening can be taken at suitable times using the reference sensor, which serve for calibration during the evaluation and analysis of the measurement data.

[0020] Exemplary embodiments of the invention are explained in more detail below with reference to the drawings. The drawings show: Fig. 1: schematically a measuring device according to the invention in a first embodiment; Fig. 2: schematically depicts the sequence of movements in the inventive method; Fig. 3: schematically a measuring device according to the invention in a second embodiment.

[0021] Fig. Figure 1 shows a measuring device according to the invention for measuring an optical radiation source (e.g., a car headlight), which is designated by reference numeral 1. The radiation source 1 is arranged on a goniometer 2a, which serves as a support device 2 for the radiation source 1. The radiation source 1 can be rotated by means of the goniometer 2a, as indicated by the double arrows. A reflector 3 is also shown, which, according to the invention, is designed as a narrow reflector strip 3a. The reflector strip 3a has a longitudinal axis A. The radiation source 1 projects light onto the reflector strip 3a, as indicated by the directional arrows, and the light is reflected by the reflector strip 3a. The reflected light is detected by a line-scan camera 4 directed at the reflector strip 3a, as indicated by the dashed arrows. The line camera is aligned such that the reflector strip 3a is imaged on the detector line of the line camera, i.e.The image of the reflector strip 3a extends longitudinally A along the detector array. The detector array captures the reflector strip 3a as completely as possible, i.e., along its entire longitudinal extent. Finally, a reference sensor 6 is arranged on the opposite side of the reflector strip 3a, which directly measures the emitted light from the optical radiation source 1 through a closable opening 5 in the reflector strip 3a. The line scan camera 4 also has an evaluation unit that communicates with the goniometer 2a via a suitable communication interface.

[0022] In the inventive method for measuring the radiation source 1, the source is moved transversely to the longitudinal axis A of the reflector strip 3a using the goniometer 2a, i.e., about an axis of rotation parallel to the longitudinal axis A of the reflector strip 3a. The movement should be designed such that, over the course of the movement, as much of the emitted light from the radiation source 1 as possible strikes the reflector strip 3a, so that the entire emission characteristic is scanned sequentially over time. The angular profile of the radiation source 1 is transmitted from the goniometer 2a to the evaluation unit via the communication interface. The line scan camera 4 continuously creates one-dimensional images of the reflector strip 3a. The images from the line scan camera 4 are synchronized with the position profile of the radiation source 1, so that each image can be assigned an angular position of the radiation source 1.Using this data, the evaluation unit can then create a two-dimensional image from the one-dimensional recordings of the line camera 4, in which the luminance and / or luminance distribution is represented as a function of the position.

[0023] Reference measurements can be taken at suitable times via the closable opening 5 and the reference sensor 6 (e.g., a photometer) by opening the normally closed opening 5. The reference data are also transmitted to the evaluation unit in order to calibrate the absolute values ​​of the data acquired by the line scan camera 4.

[0024] Fig. Figure 2 schematically shows the movement of a light cone 1a from the radiation source 1 relative to the reflector strip 3a. At time t1, the reflector screen 3a detects only the outer right edge of the light cone 1a. The radiation source 1 is rotated to the right, perpendicular to the longitudinal axis A of the reflector screen 3a, using only the goniometer 2a. At time t2, the reflector screen 3a detects the entire central axis of the light cone 1a, until finally, at time t3, only the outer right edge of the light cone is detected. Thus, all the emitted light from the radiation source 1 is detected by moving the entire light cone 1a once across the reflector strip 3a.

[0025] Fig. Figure 3 schematically shows a second embodiment of a measuring device according to the invention. Instead of the line scan camera 4 from the first embodiment, a hyperspectral camera 7 is now used for recording the reflector strip 3a. In this embodiment, the reflector strip 3a is also designed as a narrow strip, but somewhat wider than in the embodiment shown in Figure 3. Fig. 1. The wider design is possible because the remaining effects of scattered light can be calculated using the images from the hyperspectral camera 7.

[0026] The hyperspectral camera 7 acquires a spectrally resolved vector for each point of the reflector strip along its longitudinal extent, instead of a radiance value with a fixed spectral weighting (e.g., luminance). In this embodiment as well, the radiation source 1 is moved transversely to the longitudinal axis A of the reflector strip 3a, while the hyperspectral camera 7 acquires position-synchronously acquired images, which are subsequently combined to form a two-dimensionally spatially and additionally spectrally resolved radiance distribution. By convolving the spectral radiance distribution with a suitable spectral weighting function that takes into account the spectral properties of the reflector strip 3a, a two-dimensionally spatially resolved luminance distribution with a theoretically arbitrarily precise spectral match can be determined.

[0027] The imperfect spectral aselectivity of the reflector strip 3a can be compensated for, and, with a sufficiently high spectral resolution of the hyperspectral camera, the V(λ) weighting (according to the spectrally dependent brightness perception of the human eye) of the luminance can be maintained more accurately than is possible with full camera filtering in conventional screen photometers. Furthermore, the measured spectra can be directly evaluated, or any other color parameters can be calculated from them. Thus, the use of a hyperspectral camera 7 offers significant advantages even with reflector screens such as those used in conventional screen photometers or goniophotometers.

[0028] Hyperspectral cameras based on imaging spectrometers are generally built with an imaging transmission or reflection spectrometer, which generates the corresponding spectra for a strip-shaped area of ​​the image of the object being measured.

[0029] Depending on the application, a multispectral camera with up to 19 spectral channels can be used instead of a hyperspectral camera with 20 or more spectral channels. The spectral resolution of a multispectral camera is not as high as that of a hyperspectral camera, but it is perfectly adequate for the respective application.

[0030] The solution according to the invention is characterized by a very effective reduction of stray light. As described above, the solution according to the invention requires a reflector strip 3a with only a fraction of the size of reflector screens used in conventional screen photometers. Experiments and calculations have shown that the stray light caused by multiple reflections is reduced disproportionately by the approach according to the invention. This is a particularly important advantage for measuring automotive headlights, since the light distributions of car headlights often exhibit very high contrasts. When using screen photometers known from the prior art, the dark areas are easily overexposed by stray light and can then no longer be measured correctly. These disadvantages are overcome by the invention.

[0031] The additional movement of the radiation source 1 transverse to the longitudinal axis A of the reflector strip 3a required according to the invention, the parallel recordings by the line camera 4 or hyperspectral camera 7 and the subsequent merging of the individual recordings, does require slightly more time than with conventional screen photometers, but this is compensated for by avoiding measurement inaccuracies due to scattered light reflections. Reference symbol list: 1 radiation source 2 Support device 2a Goniometer 3 Reflector 3a Reflector strips 4-line camera 5 Opening 6 Reference sensor 7 Hyperspectral camera

Claims

[1] Measuring device for measuring an optical radiation source (1), comprising a support device (2) holding the radiation source (1) to be measured, at least one reflector (3), at least one camera and at least one evaluation unit, characterized by , that the reflector (3) is designed as a planar, narrow reflector strip (3a) whose longitudinal dimension is at least ten times larger than its transverse dimension and which extends along its longitudinal axis (A), wherein the reflector strip (3a) and the radiation source (1) are arranged movably relative to each other, such that the direction of movement is transverse to the longitudinal axis (A) of the reflector strip (3a), and that the camera is designed as a line scan camera (4), multispectral camera or hyperspectral camera (7). [2] Measuring device according to claim 1, characterized by, that the support device (2) is designed to move the radiation source (1) transversely to the longitudinal axis (A) of the reflector strip (3a). [3] Measuring device according to claim 1 or 2, characterized by , that the support device (2) is designed as a goniometer (2a). [4] Measuring device according to claim 3, characterized by , that the support device (2) is designed to rotate the radiation source (1) about at least one axis of rotation. [5] Measuring device according to one of the preceding claims, characterized by that the support device (2) and the evaluation unit are connected via a communication interface. [6] Measuring device according to one of the preceding claims, characterized by , that the reflector strip (3a) has an opening (5) and that a reference sensor (6) is arranged on the side of the reflector strip (3a) opposite the radiation source (1). [7] Method for measuring an optical radiation source (1) using a measuring device according to one of the preceding claims, wherein, a radiation source (1) is moved transversely to the reflector strip (3a) by means of the support device (2), the positional history of the radiation source (1) is transmitted from the support device (2) to the evaluation unit, the line scan camera (4), the multispectral camera or the hyperspectral camera (7) continuously or discretely in time create images of the reflector strip (3a) and transmit these to the evaluation unit, the recordings and the positional progression of the radiation source (1) are synchronized, The recordings are then combined to form a two-dimensional image and the luminance and / or luminous intensity distribution of the radiation source (1) is determined based on the two-dimensional image. [8] Method according to claim 7, characterized by, that a spectral alignment is performed. [9] Method according to claim 7 or 8, characterized by , that reference measurements are carried out using a reference sensor (6) and these are transmitted to the evaluation unit.

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