DE102011103446B4 - Method and measuring device for brightness and / or color measurement - Google Patents

Abstract

Method for measuring the brightness and / or color of light emitted from at least one known and at least one unknown light source (14, 17, 18) from at least one light-emitting diode, characterized by the following steps: • for calibration of a measuring device (10) having a photo camera (11) ) starting from the at least one known light source (14) serving as a measurement standard, determine at least one first reference value for a brightness value and at least one second reference value for a color value; • assign the first reference value to the actual brightness value and the second reference value to the actual color value of the at least one known light source (14) serving as a measurement standard; • determine a measured brightness value and a measured color value from the at least one unknown light source (17, 18); • and carry out an extrapolation to determine an actual brightness value of the at least one unknown light source (17, 18), starting from the at least one first reference value for the brightness value and the measured brightness value, and an extrapolation to determine an actual color value of the unknown light source ( 17, 18), starting from the at least one second reference value for the color value and the measured color value, with the known and the at least one unknown light source (14, 17, 18) being used simultaneously to determine the brightness values and / or the color values be recorded with the photo camera (11); • the RGB values of the from. the light sources (14, 17, 18) emitted light are determined; • and the RGB values are converted into an x / y value according to a CIE standard color chart and / or into a wavelength and into a brightness value.

Description

The invention relates to a method for measuring the brightness and / or color of at least one light source, in particular of at least one light emitting diode, emitted light.

Measuring methods for measuring the brightness and color of the light-emitting diode in which video cameras or optical fibers are used in order to determine the light emitted by the light-emitting diode are known from the prior art. However, video cameras can not capture brightness and color accurately enough due to the short exposure time. Fiber optic fibers have the problem that the brightness decreases in proportion to the square of the distance between the optical fiber and the light source. This results in inaccurate measurement results of the brightness values even with small changes in distance. Fiber optic fibers can only accommodate a very limited number of pixels, typically 3x3 to 9x9 pixels, so that the light source is partially captured only over a very small area. This inevitably results in inaccuracies in terms of the brightness and color of the light source to be determined.

In the DE 102 16 395 B4 For example, a method for determining a color group of a light-emitting diode is described in order to be able to classify the light emitted by the light-emitting diode into the color group. For this purpose, a network of straight line sections is placed in a CIE color space, wherein the resulting network cells define a specific color location in which the light emitted by the light emitting diode can be located. Furthermore, the color location of the light-emitting diode and then the specific network cell in which the color locus of the light-emitting diode is determined are determined.

The US 2010/0207134 A1 discloses a lighting device with a plurality of lighting units which emit different light colors, which together produce a white light. The lighting units have a light emitting diode and phosphor. The phosphor glows in a predetermined color when excited by the light of the light emitting diode. The illumination device has a lighting unit with a blue light-emitting diode, a lighting unit with a UV-emitting light-emitting diode and a lighting unit with a violet light-emitting diode, wherein the phosphor of the respective lighting units emits different colors.

An apparatus for color measurement by means of a computer processor and an electronic image capture apparatus and a method of data collection in color measurement, the data being supplied by the electronic image capture apparatus, such as a chip memory, describes US Pat EP 2 317 305 A1 ,

• • Die Erfindung löst die gestellte Aufgabe mit einem gattungsgemäßen Verfahren gemäß dem kennzeichnenden Teil des Anspruchs 1. Diese Schritte können gleichzeitig oder nacheinander erfolgen. Selbstverständlich können auch nur einige dieser Schritte gleichzeitig durchgeführt werden. Bei der bekannten Lichtquelle können ein RGB-Wert und/oder ein x/y-Wert gemäß eines CIE-Normvalenzsystems und/oder eine Wellenlänge und ein Helligkeitswert bekannt sein, wohingegen bei der unbekannten Lichtquelle diese Parameter unbekannt sein können. Einer oder alle dieser Parameter können folglich mit dem erfindungsgemäßen Verfahren ermittelt werden. Somit können mit dem erfindungsgemäßen Verfahren absolute Helligkeits- und Farbwerte gemessen werden. Um Referenzwerte mit einer möglichst hohen Genauigkeit zu gewinnen, können mehrere bekannte Lichtquellen verwendet werden. In diesem Fall können die Lichtquellen zeitlich hintereinander oder gleichzeitig bestimmt werden. Nach der Bestimmung der Vielzahl der für die Referenzwertermittlung dienenden Helligkeits- und Farbwerte kann anschließend ein Mittelwert bestimmt werden, der einen Referenzwert höherer Genauigkeit darstellt. Bei der bekannten und der unbekannten Lichtquelle kann der Helligkeitswert ein physikalischer Wert sein oder der Wahrnehmung des menschlichen Auges entsprechen.

The object of the invention is to provide a method and a measuring device with which brightness and color values can be determined with a very high accuracy in the shortest possible time.

• • The invention solves the problem set by a generic method according to the characterizing part of claim 1. These steps can be carried out simultaneously or sequentially. Of course, only a few of these steps can be performed simultaneously. In the known light source, an RGB value and / or an x / y value according to a CIE standard valence system and / or a wavelength and a brightness value may be known, whereas in the case of the unknown light source these parameters may be unknown. One or all of these parameters can consequently be determined with the method according to the invention. Thus, absolute brightness and color values can be measured with the method according to the invention. To obtain reference values with the highest possible accuracy, several known light sources can be used. In this case, the light sources can be determined one after the other or simultaneously. After determining the plurality of brightness and color values used for the reference value determination, a mean value can then be determined which represents a reference value of higher accuracy. In the known and the unknown light source, the brightness value may be a physical value or correspond to the perception of the human eye.

The x / y value is the normalized two-dimensional representation of a CIE standard valence system. A background of the light source, for example a circuit board on which the light source can be arranged, does not flow into the measurement result when determining the RGB value, since the background of the light source appears black due to the contrast to the bright light source and thus an RGB value from zero or near zero.

The extrapolation for determining the brightness value and / or the extrapolation for determining the color value may be a linear extrapolation. Practice has shown that with a linear extrapolation, especially with small deviations from the reference value, satisfactory accuracies can be achieved.

In order to ensure the highest possible accuracy in determining the brightness value and the color value, the light can emitting at least one known and / or unknown light source are recorded completely and not just over a section. Thus, the recording of the light source over all pixels that contribute to the color value and the brightness value, provides stable results with repeated determination of the brightness value and the color value.

To calculate the brightness value, a linearized red value, a linearized green value and a linearized blue value can be added for each pixel of the light source and the sum over all pixels can be formed by these additions. This results in a very accurate brightness value of the light source.

The RGB value can be linearized as far as necessary by means of a gamma correction in order to be subsequently used for the calculation of the physical brightness value. To determine the perceived by the human eye brightness gamma correction can be omitted.

To be able to exclude brightness fluctuations as reliably as possible, a relatively long exposure time can be selected for the recording of the at least one light source. For this purpose, a filter can darken the light source. The exposure time can be constant for all measurements to obtain stable measurement results.

The at least one light source can be arranged at a precisely defined location on the board and the dimensions and position of the board can be determined. Thus, it is possible to define a reference board whose dimensions and their positioning serve as reference values. Subsequently, each additional board with the reference board in terms of their dimensions and Po sitioning compared and performed by means of a software computational correction of the dimensions and positioning. This process increases the accuracy of the determined color and brightness values.

The invention further relates to the measuring device according to claim 9. The camera is very well suited for emulating a human eye. It thus produces images as seen by the human eye. This is important because the light source can be used for example in the interior of a motor vehicle (appearance).

In order to emulate a human observer, the photo camera and the at least one light source may be spaced approximately one arm length of the observer to be emulated. This distance corresponds approximately to the distance of eyes of a person sitting behind a steering wheel of the motor vehicle, and displaying a dashboard. If fluctuations in the distance occur during a measurement, they can be determined by software and then corrected. As a result, the brightness value can be determined more precisely due to its dependence on the distance. Thus, in principle, distances of more or less than an arm's length are possible.

The measuring device has an evaluation unit with software for converting the RGB values into x / y values, in wavelengths and in brightness values and for carrying out the extrapolation.

Since in the case of the at least one light source, in particular in the case of the light-emitting diode, the brightness can not be produced exactly by the manufacturer, a series resistor can be connected in front of the light source or a duty cycle of a pulse width modulation of the light source can be changed to compensate for a brightness deviation. Therefore, from the brightness value of the unknown light source calculated during the extrapolation, the required series resistor required to compensate for the brightness deviation can be determined. If the calculated brightness value is too low, an existing series resistor is replaced by a smaller series resistor or the duty cycle of the pulse width modulation is increased. If the calculated brightness value is too large, an existing series resistor is replaced by a larger series resistor or the duty cycle of the pulse width modulation is reduced.

The method according to the invention is also very well suited for the production of the at least one light source, in particular for the production of the at least one light-emitting diode in electronic circuits, for quality control.

Hereinafter, an embodiment will be explained in more detail with reference to the accompanying drawings.

The single figure shows a measuring device 10 with a camera 11 , a lens 12 and a filter 13 , Below the measuring device 10 is a known light source used to determine a first reference value for a brightness value and a second reference value for a color value 14 on a circuit board 15 arranged. On a circuit board 16 there are unknown sources of light 17 and 18 whose brightness and color values are to be determined. The light sources 14 . 17 and 18 are in the case illustrated light emitting diodes.

With the camera 11 become the well-known light source 14 and the unknown light sources 17 and 18 recorded simultaneously. By means of software, the individual color and brightness values of the light sources 14 . 17 and 18 be determined.

The filter 13 darkens the light sources 14 . 17 and 18 , so that long exposure times are possible to compensate for brightness variations.

The accuracy of the measuring device 10 can be additionally increased by the lens 12 deliberately defocused. This can cause overexposure of the camera 11 through individual bright areas of the light sources 14 . 17 and 18 be prevented because of the intentional defocusing of the camera 11 the light on several pixels of the camera 11 distributed. Thus, a smoothing of the brightness intensity of the differently bright areas is achieved.

A distance 19 between the filter 13 and the light sources 14 . 17 and 18 is about one arm length of a human observer to be emulated. Thus, together with the photo camera 11 which is very well suited to emulating a human eye that emulates viewers reliably.

LIST OF REFERENCE NUMBERS

10

measuring device

11

Photo camera

12

lens

13

filter

14

light source

15

circuit board

16

circuit board

17

light source

18

light source

19

distance

Claims (10)

Method for measuring the brightness and / or color of at least one known and at least one unknown light source ( 14 . 17 . 18 ) light emitted from at least one light-emitting diode, characterized by the following steps: • for calibrating a photo camera ( 11 ) having measuring device ( 10 ) starting from the at least one known as a measurement standard known light source ( 14 ) determine at least a first reference value for a brightness value and at least a second reference value for a color value; The first reference value the actual brightness value and the second reference value the actual color value of the at least one known light source serving as a measurement standard ( 14 ) assign; A measured brightness value and a measured color value from the at least one unknown light source ( 17 . 18 ) determine; And an extrapolation for determining an actual brightness value of the at least one unknown light source ( 17 . 18 ), starting from the at least one first reference value for the brightness value and the measured brightness value, and an extrapolation for determining an actual color value of the unknown light source ( 17 . 18 ), wherein the at least one second reference value for the color value and the measured color value is assumed, for determining the brightness values and / or the color values the known and the at least one unknown light source (FIG. 14 . 17 . 18 ) simultaneously with the camera ( 11 ) are recorded; • from the recording the RGB values of the. the light sources ( 14 . 17 . 18 ) emitted light are determined; And the RGB values are converted into an x / y value according to a CIE standard color chart and / or a wavelength and a brightness value. A method according to claim 1, characterized in that the extrapolation for determining the brightness value and / or the extrapolation for determining the color value is a linear extrapolation. Method according to claim 1 or 2, characterized in that the light emitting at least one known light source ( 14 ) and / or unknown light source ( 17 . 18 ) are recorded completely over all pixels. Method according to one of claims 1 to 3, characterized in that for determining the brightness of a linearized red value, a linearized green value and a linearized blue value for each pixel are added and these additions the sum over all pixels is formed. A method according to claim 4, characterized in that the RGB value is linearized by means of a gamma correction. Method according to one of claims 1 to 5, characterized in that from the brightness value of the at least one unknown light source ( 17 . 18 ) a required series resistor is determined. Method according to one of claims 1 to 6, characterized in that a relatively long exposure time for receiving the at least one light source ( 14 . 17 . 18 ) is selected. Method according to one of claims 1 to 7, characterized in that the at least one light source ( 14 . 17 . 18 ) at a well-defined location on a board ( 15 . 16 ) and the dimensions and position of the board ( 15 . 16 ) be determined. Measuring device ( 10 ) for carrying out a method according to one of claims 1 to 8 with the camera ( 11 ) and a software unit having an evaluation unit, wherein of the known light source ( 14 ) at least the first reference value for the brightness value and at least the second reference value for the color value can be determined and the first reference value the actual brightness value and the second reference value the actual color value of the at least one light source ( 14 ), the measured brightness value and the measured color value from the at least one unknown light source ( 17 . 18 ) and an extrapolation for determining an actual brightness value of the at least one unknown light source ( 17 . 18 ), wherein the at least one first reference value for the brightness value and the measured brightness value can be assumed, and an extrapolation for determining the actual color value of the unknown light source (US Pat. 17 . 18 ) Can be carried out, wherein by the at least one second reference value for the color value, and to start from the measured color value, characterized in that for determining the brightness values and / or color values, the known and the at least one unknown light source ( 14 . 17 . 18 ) simultaneously with the camera ( 11 ), whereby the recording of the RGB values of the light sources ( 14 . 17 . 18 ) and the RGB values are converted into an x / y value according to a CIE standard color chart and / or a wavelength and a brightness value. Measuring device ( 10 ) according to claim 9, characterized in that the photo camera ( 11 ) and the at least one light source ( 14 . 17 . 18 ) a distance ( 19 ) of approximately one arm length of an observer to be emulated.

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