Optical property measurement using a sensor behind a display

Abstract

An example of the present application discloses a method for measuring an optical property of a target, comprising illuminating the target with an illumination area using a display screen in contact with the target, and analyzing a signal reflected from the target and transmitted through the display screen back to a sensor located behind the display screen to determine the optical property of the target.

Description

Technical Field

[0001] The present disclosure relates to optical property measurement using sensors behind a display screen, and particularly, but not exclusively, to methods for measuring the color and translucency level of an object. Background Art

[0002] The present disclosure relates to a method for measuring optical properties of an object by using an illuminated display screen and a sensor located behind the display screen. In particular, but not exclusively, the method is used to determine the color and translucency level of the object.

[0003] When light reflects from an object, it is reflected, scattered, or absorbed in some combination, depending on the properties of the object. The interpretation of color is the human interpretation of the combination of scattered and reflected light from the object, which itself consists of a specific distribution of wavelengths or spectra in the visible spectrum (VIS).

[0004] To measure an object's true color, all reflected light (specular component) and scattered light (diffuse component) from the object must be captured. The perceived color of an object cannot be measured absolutely, but rather relative to another perceived color. In this method, the specular component is excluded, making the diffuse component the primary signal. This diffuse component relates to the object's surface condition and provides information about gloss, finish, and texture. To measure the relative perceived color of an object, standard measurement geometry is used to remove specular reflections.

[0005] An example configuration known from the prior art consists of a (0° / 45° or 45° / 0°) setup. In this setup, the illumination source is positioned at 45° to the surface normal (0°), and the sensor is positioned along the surface normal. However, for diffusely reflecting targets, which produce a larger portion of reflected diffuse light than specular light, this geometric restriction is less stringent.

[0006] When measuring the color of a translucent object, light penetrates the object but is also scattered by it. This scattered light can scatter multiple times before exiting the object at angles different from its entry angle. The amount of light scattered depends on its wavelength. In some objects, the longer wavelengths of the incident light are scattered more. In other objects, the shorter wavelengths of the incident light are scattered more.

[0007] Currently, existing technologies face challenges capturing all reflected light from translucent objects in near-field setups. In these conditions, light from the illuminated area on the translucent object will be diffusely scattered, with some wavelength regions being scattered too much or too little and failing to return through the sensor aperture to be captured. Consequently, accurately determining the true color of translucent objects under these conditions is difficult due to the inevitable loss of spectral information. The scattering processes within the material are complex and cannot be easily compensated for.

[0008] Currently, the known solutions in the prior art for avoiding the loss of spectral information in the true color measurement of translucent materials are to use either a small illumination area and a large measurement spot on the target, or a large illumination area and a small measurement spot on the target. Figure 1A and 1B , respectively. In the first approach, the measurement spot must be large enough to capture all scattered light. This requires a large sensor and equipment. In the second approach, the illumination area needs to be large enough so that a combination of many different scattered events is used to produce the full spectrum in the measurement spot. This is only feasible for small measurement spot areas and depends on the degree of scattering or translucency within the object.

[0009] It is therefore an object of the present disclosure to provide a solution to one or more of the above problems or at least to provide a useful alternative. Summary of the Invention

[0010] Generally, the present disclosure proposes overcoming the aforementioned issues by illuminating a target using a display screen having a non-zero transmission function across at least a portion of the visible spectrum. This non-zero transmission function allows the target to be placed in contact with the display, thereby producing a large illumination area, yet a small measurement spot standard, for reliable color measurement. Furthermore, a single illumination device can be used to apply a color pattern to the target without significant separation between the light source and the target, thereby reducing the size of the configuration. This arrangement enables color measurement of the target, translucency level measurement, and reconstruction of the re-emission spectrum.

[0011] According to a first aspect of the present invention, a method for measuring optical properties of an object is provided. A display screen is provided. When the object is in contact with the display screen, the object is illuminated using an illumination area on the display screen. A sensor is provided, positioned behind the display screen, to receive light reflected from the object and transmitted back through the display screen. Signals obtained from the sensor during the illumination period are analyzed to determine the optical properties of the object.

[0012] In the prior art, specific configurations are required to measure the perceived color of an object. For example, a (0° / 45° or 45° / 0°) setting. However, these methods require a large amount of space. In a near-field setting, translucent materials diffusely scatter light, which results in a loss of spectral information, which usually dominates on one side of the optical range. The reflection process in translucent materials is complex, so spectral information cannot be easily compensated. Therefore, color measurement of translucent materials requires either a small illumination spot and a large measurement spot to ensure that the spectral information is not scattered and lost, or a large illumination spot and a small measurement spot. In the prior art, both methods require considerable space.

[0013] Compared to known systems, this optical property measurement method using a sensor behind a display screen offers the following advantages. It reduces the size of the setup area for optical property measurement, thereby simplifying prior art methods. It also enables novel methods for target illumination using patterned and / or colored areas.

[0014] Finally, the present optical property measurement method disclosed herein using a sensor behind a display screen utilizes a novel approach in that at least the display screen is in contact with the target and the sensor is located behind the lighting device and is capable of measuring optical properties such as the color of the target, the translucency level of the target, and reconstructing the re-emission spectrum of the target. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Some examples of the present disclosure will now be described, by way of example only, and with reference to the accompanying drawings, in which:

[0016] Figure 1A and 1B A simplified schematic diagram of a measurement setup compatible with determining color measurements of translucent materials is shown.

[0017] Figure 2 A simplified schematic diagram of an apparatus for performing a method of measuring optical properties of a target material according to the present disclosure is shown.

[0018] Figure 3A and 3B The correction of the system sensitivity based on the transmission function of the OLED display according to the present disclosure is shown.

[0019] Figure 4A and 4B Exemplary emission and transmission functions are shown, respectively, for an OLED display according to the present disclosure. DETAILED DESCRIPTION

[0020] To provide improved color measurement of target materials, particularly translucent materials, a process is proposed below in which the target material is illuminated with a display screen in contact with the target, and the reflected signal is detected by a sensor located behind the display screen and analyzed.

[0021] Figure 1A and 1B The constraints on the illumination required to measure transparent materials are shown. There are broadly two possible approaches. Figure 1A The first approach shown in FIG is a wide illumination spot 111 on the target 101, where measurement 112 occurs only in the center of the spot (i.e., where the illumination is brightest). This is inefficient because most of the reflected light is not measured, but it allows all parts of the reflection to be picked up (e.g., the sensor will receive direct reflections of the central rays of illumination, as well as indirect reflections of the outer rays). Figure 1BThe second approach shown in is to use a narrow illumination spot 121, where the measurement 122 surrounds the illumination spot to capture all reflected light. This is more energy efficient because all light is captured, but it requires a large sensor.

[0022] Figure 2 is a schematic diagram of an apparatus according to the present disclosure. The apparatus includes a display screen 201 having a backing 202 and an optional diffuser or filter 203 located behind the backing. Behind the display screen, backing, and diffuser or filter is a sensor assembly 204 comprising a housing 205 having an aperture 206 (e.g., 1 mm in diameter) and a light sensor 207. To perform measurements of a target material, the display screen 201 is placed in contact with the material 210 and illumination 211 is provided to the material. Light 212 reflected from the material passes through the display screen and the aperture and is detected by the light sensor 207. The signal received by the sensor during the illumination period can then be used to determine the optical properties of the target.

[0023] The illumination provided by the display screen is a colored area (e.g., primary color, secondary color, or white). The reflected light can be used to determine how the target material scatters light and, therefore, how transparent the material is.

[0024] The size, shape, and position of the illumination provided by the display can be varied, and analysis is performed for each successive illumination pattern and by comparing the results between illumination patterns. This can be used, for example, to detect defects in the target material and determine their location (since these defects will reflect differently, but only when illuminated by the display). Typically, the illumination area will be centered around the sensor's axis, but this is not required.

[0025] As is common with optical sensors, the sensor output will need to be calibrated based on the sensor sensitivity. Figure 3A A graph of the sensitivity of an example multispectral sensor is shown in FIG. As can be seen from the figure, the sensor has several transmission peaks 311, 312, 313, 314, 315, 316, 317 and 318, each corresponding to one of the sensor channels. Calibration will also be based on the light output - that is, the emission function of the display screen (in Figure 4A ) and the displayed pattern.

[0026] In the above device, it is possible to Figure 4B The calibration can be improved by using the transmission function of the display (and any other components in front of the sensor) shown for the OLED in . Figure 3B shows that when placed with Figure 4B The transmission function is shown behind the display, with Figure 3AEach of the peaks 321, 322, 323, 324, 325, 326, 327, 328 corresponds to Figure 3A , but their height and shape have changed significantly (note the different scales on the two graphs) to account for the reduced light transmission of the display.

[0027] To aid calibration, the transmission function can be measured relative to a material of known reflectivity. For example, take a material with known optical properties, measure that material using the methods described above, and then adjust the calibration so that the output matches the material's known optical properties. This calibration, or any other suitable recalibration, can be done periodically to account for changes in the display's transmission or sensor's sensitivity over time, or it can be performed before each measurement of a target material with unknown properties.

[0028] Other techniques known in the art, such as taking multiple measurements and averaging them, may be used to improve the accuracy of the measurements.

[0029] Obtaining a broad spectrum measurement of a material's optical properties can be accomplished with white light, or alternatively, can be accomplished by sequentially illuminating the target material with different colors of light (e.g., from each pixel color in a display) and concatenating all measured reflectance functions (or, during calibration, all measured transmittance functions of a display screen).

[0030] Whether the lighting includes a colored pattern or a white pattern, the rest of the display will typically be black - although this is not strictly required.

[0031] The shape of the illumination on the display screen can be a circle, or another pattern with circular symmetry, such as a ring, or a series of concentric rings of different thicknesses and / or colors. However, in general, the pattern can be any image that can be formed by the display screen, and the sensor can be calibrated accordingly.

[0032] In cases where the target material has some pre-existing feature or division (e.g., a target composed of two materials attached together at a join, or a target with a defect in its material), the shape of the illumination can be changed to account for that feature or division (e.g., illuminating the material on each side of a join differently, or selecting a pattern to locate a defect).

[0033] The sensor can be a three-channel sensor or a multispectral sensor. Suitable multispectral sensors include the AS7341 multispectral sensor (which has 8 channels) or other sensors with at least 4 channels.

[0034] The measured optical properties of the target may include color measurements, which can be converted to a color space such as RGB or XYZ. The conversion to the color space can be performed by multiplying the output signal vector from the sensor by an [N×M] matrix, where N is the number of sensor channels and M is the number of dimensions in the color space. For example, a conversion between a 3-channel sensor and RGB color space would require a 3×3 matrix, and a conversion between an 8-channel sensor and XYZ color space would require an 8×3 matrix. The matrix can be determined by calibration of the sensor as described above.

[0035] The measured optical properties may also include a reconstructed emission spectrum, which may be similarly calculated using an [N x M] matrix, where M is the number of "bins" in the emission spectrum model.

[0036] The measured optical properties can include translucency, which can be determined by varying the size and / or shape of the illuminated area to determine changes in scattered light. For example, the illumination shape can be a circle centered on the sensor, and the size can vary. With small illumination sizes, all scattered light is captured. With larger illumination sizes, less scattered light is captured, but the spectrum and proportion of the captured scattered light will depend on the translucency properties of the material. By analyzing and comparing each measured signal for each different illumination size, a translucency level can be assigned to the target material. For example, this can be based on the increased contribution of specific frequency regions within the scattered light spectrum for certain sizes.

[0037] Other standard algorithms known to those skilled in the art may be used to convert the measured reflectance in each band of the sensor into a color, emission spectrum, translucency measurement, or other optical property.

[0038] The display screen may be an organic light emitting diode (OLED) screen, or any other suitable illumination source capable of emitting light and having a non-zero transmission function over at least a portion of the visible spectrum. Preferably, this will have a non-zero transmission function over the entire visible spectrum, but this is not required.

[0039] As mentioned previously, additional optical devices such as filters or diffusers can be placed between the display and the sensor, behind or in front of the aperture. Diffusers can be used to reduce the effects of scattering inhomogeneities in the target material, allowing for determination of average optical properties. Filters can be used to block near-infrared, ultraviolet, or other unwanted light.

[0040] When the target material is not in place (and this would not require lighting), the sensor behind the display can also be used as an ambient light sensor.

[0041] As an example use case, the target material could be skin, and the method could be used by placing a display on a person's skin to obtain color, translucency, and other optical information. This data could then be used, for example, for skin tone matching in the cosmetics industry.

[0042] Examples of the present disclosure may be used in many different applications, including color measurement in the textile and coatings industries, skin tone matching in the cosmetics industry, defect detection measurement in the machinery industry, and are particularly well suited for optical property measurement of translucent materials, as well as other industries.

[0043] The present invention is defined by the appended claims and is specifically not limited by the examples disclosed above. "Comprising" should be taken to mean that the relevant object includes the listed features, but may also include other features. Where appropriate, any reference to the singular includes the plural, and where the action is performed by a processor or other computing device, the functionality of several processors may be combined in one processor, and vice versa. Features disclosed in separate examples or in separate dependent claims may be combined as appropriate, and any reference signs appearing in the claims should not be construed as limiting their scope.

[0044] List of reference numerals:

[0045] 101 Target materials for measurement

[0046] Illumination spot on 111 target

[0047] 112 measurement areas

[0048] 121 Illumination spot on target

[0049] 122 measurement areas

[0050] 201 display screen

[0051] 202 display screen backing

[0052] 203 Diffuser or filter (optional)

[0053] 204 sensor assembly

[0054] 205 shell

[0055] 206 holes

[0056] 207 light sensor

[0057] 210 target materials

[0058] 211 Lighting

[0059] 212 reflected light

[0060] Sensitivity of channels in multispectral sensors 311 to 318

[0061] 321 to 328 Sensitivities of channels in a multispectral sensor corrected using the transmission function of a display screen

Claims

1. A method for measuring optical properties of an object, comprising: Provide display screen; illuminating the target using an illumination area on the display screen, wherein the target is in contact with the display screen; providing a sensor located behind the display screen to receive light reflected from the target and transmitted back through the display screen; calibrating a signal from the sensor using a calibration function comprising a sensor sensitivity function and a transmission function of the display screen, wherein the transmission function is non-zero in at least a portion of the visible spectrum; as well as The calibrated signal is analyzed to reconstruct the re-emission spectrum of the target. The method of claim 1 , wherein the illumination area is centered about the sensor axis. 3 . The method according to claim 1 , further comprising varying the size of the illuminated area and performing analysis based on measurements at each size.

4. The method of claim 1 or 2, wherein the illuminated area comprises a white pattern and the remainder of the display screen is black.

5. The method of claim 1 or 2, wherein the illuminated area comprises a colored pattern and the remainder of the display screen is black.

6. The method of claim 5, wherein the illumination area sequentially includes emission from each pixel color in the display screen.

7. The method according to claim 1 or 2, wherein the shape of the illumination area on the display screen is substantially circular.

8. The method according to claim 1 or 2, wherein the sensor comprises one of the following: Three-channel color sensor; Multispectral sensor.

9. The method of claim 8, wherein the multispectral sensor comprises eight channels.

10. The method of claim 1 or 2, wherein the optical characteristic comprises a color measurement. The method of claim 10 , wherein the color measurements are converted into a color space.

12. The method of claim 11, wherein the color space is one of: RGB; and XYZ.

13. The method of claim 1 or 2, wherein the optical property comprises a level of translucency.

14. The method according to claim 1 or 2, wherein the display screen comprises an organic light emitting diode (OLED) display screen.

15. The method of claim 1 or 2, wherein additional optical equipment is placed between the display screen and the sensor, wherein the additional optical equipment comprises one or both of a diffuser and a filter.

16. The method of claim 15, wherein the filter blocks in the near infrared (NIR) range.

17. The method of claim 1 or 2, wherein the target comprises skin.

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