DE102013004213A1 - Imaging colorimeter with high accuracy - Google Patents

Abstract

State-of-the-art digital color cameras are based on a set of color sensors whose actual spectral sensitivities do not meet the ideal requirement for derivation by linear combination from the spectral value functions. Such cameras are calibrated by the manufacturer in such a way that the device-specific color signals that result from the non-ideal filter gradients are first determined using a matrix using a matrix to determine the device-independent color values R, G and B in accordance with IEC 61966-2-1 (sRGB color space). With this calibration, the light type D65 (daylight) is usually used as illumination and white reference; The calibration is carried out with the aim of minimizing the errors resulting from the non-ideality of the spectral sensitivities of the RGB sensors for a large number of color patterns. The conversion of the sRGB color values into the color values X, Y and Z according to CIE 1931 is then carried out using a matrix multiplication in accordance with the requirements of the standard. However, if a measurement is carried out under incandescent light (illuminant A) or under illumination with fluorescent tubes (illuminant F) with a spectral distribution that differs from illuminant D65, its folding with the non-ideal sensor characteristics results in errors in the determined luminance and in the colors that cannot be corrected by a subsequent white balance. In order to minimize the described errors in the imaging measurement of luminance and color with digital SLR cameras, the following takes place according to the invention: 1. a correction of the recording of the camera by means of matrixing based on the prompt high-quality spectroradiometric determination of the color type of the scene, preferably by the optics of the camera; 2. A calibration of the described color measuring device, consisting of a combination of a reflex camera and spectroradiometer, on three or four colors of an electronic screen to be measured according to the well-known four-color method from Y. Ohno; 3. In addition, during the measurement of electro-optical screens, the conversion of the present linearly polarized into circularly polarized light takes place in order to avoid polarization-related errors in the measuring device described.

Description

State of the art

State-of-the-art digital cameras are based on a set of color sensors (usually R, G, B) whose actual spectral response does not meet the ideal requirement for linear combination derivation from the spectral value functions. Such cameras are calibrated by the manufacturer so that the device-independent color values R *, G * and B *, which result from the non-ideal filter curves, are first used to matrix the device-independent color values R, G and B via a matrix IEC 61966-2-1 (sRGB color space) can be determined. In this calibration (ie, determination of the transformation matrix), the illuminant D65 (daylight) is usually used as illumination and white reference; the calibration is done with the aim of minimizing the errors caused by the non-ideality of the spectral sensitivities of the RGB sensors for a large number of color samples. The conversion of the sRGB color values into the color values X, Y and Z after CIE 1931 then takes place via a matrix multiplication according to specifications of the standard. If a measurement is then made with incandescent light (type of light A) or under illumination with fluorescent tubes (types F) with a different spectral distribution from the type of light D65, their folding with the non-linear sensor characteristics results in errors in the luminance and in the colors determined.

• 1 eine Korrektur des Weißabgleichs der Aufnahme der Kamera durch Vergleich mit der spektroradiometrischen Ermittlung der Farbart der Szene bei einer spezifischen, von D65 (Tageslichtart) abweichenden, Beleuchtung;
• 2 die spektroradiometrische Kalibration des Farbmessgeräts auf drei oder vier Farben eines zu messenden elektronischen Bildschirms nach dem von Ohno angegebenen Verfahren;
• 3 die Umwandlung von linear in zirkular polarisiertes Licht.

To minimize the errors described in the imaging measurement of luminance and color is carried out according to the invention:

• 1 shows a correction of the white balance of the camera's image by comparison with the spectroradiometric determination of the chromaticity of the scene at a specific illumination deviating from D65 (daylight type);
• 2 the spectroradiometric calibration of the colorimeter on three or four colors of an electronic screen to be measured according to the method specified by Ohno;
• Figure 3 shows the conversion from linear to circularly polarized light.

Apparative realization of the invention

• 1 Für den ersten Anwendungsfall einer von der Lichtart D65 abweichenden Beleuchtung werden die Mittelwerte der Farbwerte des Kamerabilds R, G und B über das gesamte Bild oder über ausschnitte ermittelt und mit denen aus der spektroradiometrischen Messung verglichen. Dieser Abgleich erfolgt vorzugsweise mit einer farbneutralen Oberfläche, z. B einer photographischen Graukarte. Es wird eine Matrix berechnet, mit der die Farbkanäle der Kamera so gewichtet werden, dass die Abweichung zwischen der Messung mit der Kamera und dem Spektroradiometer minimal wird. Mit dieser Matrix werden alle folgenden Aufnahmen, die bei der gleichen Beleuchtung (Lichtart) erfolgen, korrigiert.
• 2 Für den zweiten Anwendungsfall, nämlich die qualitativ hochwertige farbmetrische Vermessung von elektronischen Bildschirmen, werden z. B. von drei Primärfarben des Bildschirms, meist Rot, Grün und Blau, oder anderen Farben, Aufnahmen gemacht und parallel dazu die Farbwerte mit dem Spektroradiometer ermittelt. Als vierte Farbe kann zur Kalibration noch der weiße Zustand des Bildschirms hinzugenommen werden. Aus diesen drei oder vier Messungen wird nach dem von Ohno angegebenen Verfahren eine Matrix zur Korrektur der Kamerafarbwerte ermittelt. Diese Matrix wird auf alle Farbwerte des Kamerabildes angewandt und so für jedes Bildelement (Pixel) die korrigierten Farbwerte X, Y und Z ermittelt, aus denen sich dann die Leuchtdichte und die Farbwertanteile mit hoher Genauigkeit berechnen lassen.
• 3 Zur Farbmessung an Bildschirmen, die mit Polarisatorfolien beschichtet sind, wird das Objektiv der Kamera mit einer drehbaren Viertelwellenlängenplatte versehen, um das in die Kamera fallende, zunächst linear polarisierte Licht, elliptisch zu polarisieren und somit den Einfluss der Ausrichtung der Kamera (Drehwinkel um die optische Achse) auf die Ergebnisse der Spektralanalyse zu unterdrücken. Dazu wird eine steckbar verbundene Kombination von Linearpolarisator und Viertelwellenlängenplatte verwendet, deren optische Achsen 45° zueinander ausgerichtet sind, um so zirkular polarisiertes Licht zu erzeugen. Dieser Zirkularpolarisator wird so eingestellt, dass das von dem Bildschirm kommende Licht ausgelöscht wird, dann wird der Linearpolarisator entfernt. Damit ist das vom Bildschirm kommende Licht so zur Viertelwellenlängenplatte ausgerichtet, dass zirkular polarisiertes Licht erzeugt wird.

The basis for the high-accuracy color measuring device described here is a digital camera with image decoupling from the imaging beam path (for example, by beam splitters, folding mirrors, etc.), which is combined with a spectrometer. In the case of the SLR camera, when the mirror is in the 45-degree position, light from the measurement object or scene to be measured is reflected by the lens into the camera's eyepiece. At the eyepiece, a device is mounted, which couples the light coming from the recorded scene in an optical fiber bundle and is guided by this to the entrance slit of a spectroradiometer. There, the light emanating from the scene is spectrally analyzed and the color values X _S , Y _S and Z _{S are determined} with high accuracy (ie by the measurement principle due to low measurement uncertainty).

• 1 For the first application of illumination other than illuminant D65, the mean values of the color values of the camera images R, G and B are determined over the entire image or over sections and compared with those from the spectroradiometric measurement. This adjustment is preferably carried out with a color neutral surface, for. B of a photographic gray card. A matrix is calculated that weights the color channels of the camera so that the difference between the camera and spectroradiometer measurements is minimal. With this matrix, all subsequent shots that take place at the same illumination (light type) are corrected.
• 2 For the second application, namely the high-quality colorimetric measurement of electronic screens, z. B. of three primary colors of the screen, usually red, green and blue, or other colors, taken pictures and determined parallel to the color values with the spectroradiometer. The fourth color can be added to the calibration of the white state of the screen. From these three or four measurements, a matrix for correcting the camera color values is determined according to the method specified by Ohno. This matrix is applied to all color values of the camera image and thus for each picture element (pixel) the corrected color values X, Y and Z are determined, from which the luminance and the color value parts can be calculated with high accuracy.
• 3 For colorimetry on screens coated with polarizer films, the lens of the camera is provided with a rotating quarter wavelength plate to elliptically polarize the initially linearly polarized light falling into the camera and thus to influence the orientation of the camera (rotation angle around the camera) optical axis) to suppress the results of spectral analysis. For this purpose, a pluggable combination of Linearpolarisator and quarter wavelength plate is used, the optical axes are aligned 45 ° to each other so as to produce circularly polarized light. This circular polarizer is adjusted so that the light coming from the screen is extinguished, then the linear polarizer is removed. Thus, the light coming from the screen is aligned with the quarter wavelength plate so that circularly polarized light is generated.

Functional components

• • Digital camera with image extraction from the beam path through beam splitters, folding mirrors, etc.
• • rotatable achromatic quarter wavelength plate with linear polarizer,
• Device for coupling the light emerging from the eyepiece into an optical fiber bundle (also with imaging optics),
• Fiber optic system (also with cross-section transducer),
• • spectroradiometer (also microspectro radiometer),
• • Computer with control and evaluation software.

Extraction from the viewfinder of the camera

Depending on the design of the camera, the following options are available for coupling the light entering from the observed scene through the objective lens via the beam splitter or mirror into an optical fiber bundle, which then directs the light to the entrance slit of the spectroradiometer.

If the optical system of the viewfinder can not be removed, there are three options:

• 1 depiction of the opening of the eyepiece into the entrance surface of the optical fiber bundle,
• 2 image of the image on the ground glass (focusing screen) of the viewfinder on the entrance surface of the optical fiber bundle,
• 3 Attach a light collector (eg Ulbricht sphere) to the eyepiece with the following coupling into the fiber optic bundle.

• 1 Abbildung des Bilds auf der Mattscheibe (Einstellscheibe) des Suchers auf die Eintrittsfläche des Lichtleiterbündels,
• 2 Abbildung von Ausschnitten des Bilds auf der Mattscheibe (Einstellscheibe) des Suchers über ein Zwischenbild, aus dem Bereiche der Szene ausgeblendet werden können, auf die Eintrittsfläche des Lichtleiterbündels.

If the optical system of the viewfinder can be removed, there are two options:

• 1 image of the image on the ground glass (focusing screen) of the viewfinder on the entrance surface of the optical fiber bundle,
• 2 Illustration of sections of the image on the ground glass (focusing wheel) of the viewfinder via an intermediate image, from which areas of the scene can be hidden, on the entrance surface of the optical fiber bundle.

Sequence of the measurement

During the adjustment of the parameters, which are a qualitatively high-quality recording of the scene by the camera (or a sequence of individual recordings to be offset later, eg with different exposure times) according to the prior art, that coming from the viewfinder optics, in the spectroradiometer guided light continuously spectrally analyzed. This can be done with specification of restrictions for the exposure time of the spectrometer, in particular if in the recorded scene light sources are included whose emission has temporal periodic modulations. The control of the spectrometer ensures that there is no overload. The state of correct exposure of the spectrometer can be visually indicated by a signal light or acoustically. In this condition, the camera's image pickup is triggered, which in the case of the SLR camera is associated with the flip up of the mirror in the camera, thereby interrupting the light path to the spectroradiometer. After completing the series of shots of the camera, the mirror returns to the position that reopens the path for the light from the scene to the spectroradiometer.

The evaluation of the spectral data continuously provided and recorded by the spectroradiometer is performed in such a way that the color values of the light coming from the scene are determined with high accuracy from the spectral distributions before and after the measurement of the camera, ie before and after the mirror is raised. The period during which the flipped mirror obstructs the light path is used to detect the dark signal of the spectrometer. The measuring signal of the spectrometer is corrected with respect to this dark signal.

In the case of a digital camera with a beam splitter, the dark signal of the spectroradiometer is measured by other suitable means (eg electromagnetic closure at the entrance slit of the spectrometer or elsewhere in the light path from the scene to the entrance slit of the spectrometer).

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• IEC 61966-2-1 [0001]
• CIE 1931 [0001]

Claims (1)

Imaging mobile, portable or handheld compact colorimeter characterized in that: 1 that a digital color camera is combined with a spectroradiometer; 2 that, optionally parallel to the image acquisition, shortly before or shortly after the image acquisition, a high-quality measurement of the chromaticity of the scene taken by the camera is performed by a spectroradiometer located close to the camera; 3 that the colorimetry with the spectroradiometer is done by the same lens as the image acquisition; 4 that the spectrally to be analyzed light is coupled out through the viewfinder from the imaging beam path; a. that the sum of the light from the scene is spectrally analyzed; b. that specific surface elements are selected from the scene to be recorded by means of a diaphragm (mask) and analyzed spectrally; 5 that the spectrally analyzed light is coupled out of the viewfinder beam path; a. that the sum of the light from the scene is spectrally analyzed; b. that specific surface elements are selected from the scene to be recorded by means of a diaphragm (mask) and analyzed spectrally; 6 that the colorimetry with the spectroradiometer takes place parallel to the image acquisition by a second objective directed to the same scene; 7 that the white balance of the camera shot due to the spectral analysis of the scene (eg, neutral gray card) is set or corrected to minimize color and luminance errors; 8 that corrective matrices for the camera are determined according to Ohno's method in order to be able to perform highly accurate, laterally resolved color and luminance measurements on electronic screens; Figure 9 shows that a separable combination of linear polarizer and achromatic quarter wave plate, whose optical axes are oriented 45 ° to each other, is used to convert linearly polarized light to circularly polarized light; 10 that after the adjustment of the combination according to claim 8 to extinction (i.e., minimal transmission) of the pluggable linear polarizer is removed.