According to some aspects of the present description, an optical reflector includes a plurality of polymeric layers numbering at least 200 in total. In some embodiments, each of the polymeric layers may have an average thickness of less than about 500 nm. In some embodiments, a layer thickness gradient of the polymeric layers may include a first portion and a second portion joined by a step portion. In some embodiments, each of the first and second portions may extend across at least 30 of the polymeric layers, and the step portion may extend across less than about 10 of the polymeric layers. In some embodiments, a change in thickness across the step portion may be at least 5 times greater than a change in thickness across each of the first and second portions. In some embodiments, the step portion may have a negative slope. In some embodiments, the step portion may have a positive slope. In some embodiments, the step portion may be substantially vertical.

In some embodiments, for a substantially normally incident light, the plurality of polymeric layers may have an average optical reflectance of greater than about 80%, or greater than about 85%, or greater than about 90%, in a visible wavelength range extending from about 420 nm to about 700 nm, and may have an optical transmittance versus wavelength that comprises a bandpass segment comprising a global peak transmittance of between about 2% and about 80%, or between about 2% and about 60%, or between about 2% and about 40%, at a global peak wavelength in the visible wavelength range and a corresponding full width at half maximum (FWHM) of between about 10 nm and about 50 nm, or between about 10 nm and about 40 nm, or between about 10 nm and about 30 nm, or between about 10 nm and about 20 nm.

In some embodiments, the optical reflector may have an average optical reflectance of greater than about 80%, or greater than about 85%, or greater than about 90%, in the visible wavelength range for each of mutually orthogonal polarization states.

In some embodiments, the optical reflector may be part of an optical system. In some embodiments, the optical system may include a light source and the optical reflector. In some embodiments, the light source may be configured to emit emitted light toward a measurement subject (e.g., the finger of a user) and receive reflected light reflected by the measurement subject. In some embodiments, the optical reflector may be disposed between the light source and the measurement subject.

According to some aspects of the present description, an optical reflector includes a plurality of polymeric layers numbering at least 200 in total. In some embodiments, each of the polymeric layers may have an average thickness of less than about 500 nm. In some embodiments, for a substantially normally incident light, the plurality of polymeric layers may have an average optical reflectance of greater than about 80%, or greater than about 85%, or greater than about 90%, in a visible wavelength range extending from about 420 nm to about 700 nm, and an optical transmittance versus wavelength that includes a bandpass segment with a global peak transmittance of greater than about 2% at a global peak wavelength in the visible wavelength range and a corresponding full width at half maximum (FWHM) of at least 5 nm. In some embodiments, when a substantially white Lambertian light having first color coordinates a1* and b1* in a CIE Lab color space illuminates the optical reflector, the optical reflector may reflect the illuminating white light, with the reflected light having respective second color coordinates a2* and b2* averaged over all angles of reflectance, wherein a color difference ΔE*(ab) between the first and second color coordinates is less than about 10.

In some embodiments, the optical reflector may have an average optical reflectance of greater than about 80%, or greater than about 85%, or greater than about 90%, in the visible wavelength range for each of mutually orthogonal polarization states.

In some embodiments, the optical reflector may be part of an optical system. In some embodiments, the optical system may include a light source and the optical reflector. In some embodiments, the light source may be configured to emit the substantially white Lambertian light toward a measurement subject (e.g., a finger or skin of a user) and receive the reflected light. In some embodiments, the optical reflector may be disposed between the light source and the measurement subject.

According to some aspects of the present description, an optical reflector includes a plurality of polymeric layers numbering at least 200 in total. In some embodiments, each of the polymeric layers may have an average thickness of less than about 500 nm. In some embodiments, for a substantially normally incident light, the plurality of polymeric layers may have an average optical reflectance of greater than about 80%, or greater than about 85%, or greater than about 90%, in a visible wavelength range extending from about 420 nm to about 700 nm, and may have an optical transmittance versus wavelength that includes a bandpass segment with a global peak transmittance at a global peak wavelength in the visible wavelength range and a corresponding full width at half maximum (FWHM). In some embodiments, when a substantially white Lambertian light is incident on the optical reflector, the optical reflector may reflect the incident white light, wherein a difference ΔE*(ab) between color coordinates (a1*, b1*) of the incident light in a CIE Lab color space and color coordinates (a2*, b2*) of the reflected light in the CIE Lab color space averaged over all angles of reflectance, is between about 0.01 and about 0.5.

In some embodiments, the optical reflector may have an average optical reflectance of greater than about 80%, or greater than about 85%, or greater than about 90%, in the visible wavelength range for each of mutually orthogonal polarization states.

In some embodiments, the optical reflector may be part of an optical system. In some embodiments, the optical system may include a light source and the optical reflector. In some embodiments, the light source may be configured to emit the substantially white Lambertian light toward a measurement subject (e.g., a finger or skin of a user) and receive the reflected light. In some embodiments, the optical reflector may be disposed between the light source and the measurement subject.

Turning now to the figures, FIGS. 1A and 1B show side views of an optical system including an optical film for two-factor authentication, according to the present description. The embodiments shown in each of FIGS. 1A and 1B are similar and share many like-numbered components. The function of like-numbered components should be assumed to be the same in each figure unless specifically stated otherwise, and thus the descriptions of these components may not be duplicated for each figure. Starting with FIG. 1A, an optical system 200 includes a display 50, a backlight 60 including a plurality of light sources 65 (e.g., light-emitting diodes), and an optical film 100 disposed between display 50 and backlight 60. In some embodiments, the backlight 60 (via light sources 65) is configured to emit light 30 having blue, green, red, and near infrared emission spectra.

Turning temporarily to FIGS. 6A-6B, these figures provide additional detail on the emission spectra emitted by light sources 65. FIG. 6A shows the typical emission spectra for backlight light sources such as light-emitting diodes. FIG. 6A shows the emission spectra for blue light 90b, green light 90g, and red light 90r. Each of the spectra 90b, 90g, and 90r has a corresponding full width at half maximum (FWHM) value, as shown in FIG. 6A. In this example, the FWHM for the blue spectra 90b is about 25 nm, the FWHM for the green spectra 90g is about 40 nm, and the FWHM for the red spectra 90r is about 25 nm. FIG. 6B shows a typical emission spectra for an infrared light emitting diode 90ir. The infrared spectra 90ir has a corresponding FWHM value of about 45 nm. The shape (including the height and width) of each of the spectra curves in FIGS. 6A and 6B are relevant to the functional performance of the optical film 100, to be discussed elsewhere herein.

Returning to FIG. 1A, optical film 100 may include a plurality of polymeric layers numbering at least 200, or at least 300, or at least 400, or at least 500, or at least 600 in total disposed between, and co-extruded with, opposing first and second outer layers. In some embodiments, each of the polymeric layers may have an average thickness of less than about 150 nm, and may be arranged in a layer thickness gradient with a thinnest polymeric layer disposed closer to the first outer layer and a thickest polymeric layer disposed closer to the second outer layer. In some embodiments, the layer thickness gradient of optical film 100 may be configured such that the optical film has a first average transmission percentage in a first wavelength range (e.g., a near infrared wavelength range) and a peak transmission percentage in a different second wavelength range (e.g., the peak of a narrow “spike” or notch of transmission in a visible wavelength range). Stated another way, the optical film 100 may be so configured as to substantially allow the transmission of wavelengths in at least two distinct wavelength ranges to enable the use of the film in applications requiring biometric authentication using two different wavelengths of light. Additional details on the construction and performance of optical film 100 are provided elsewhere herein.

FIG. 1B shows an optical system 200B that is similar to optical system 200 of FIG. 1A, except that an optical stack 150 replaces the single optical stack 100 of the embodiments in FIG. 1A. Optical stack 150 includes two or more optical films 100a and 101, both of which may be substantially similar to optical film 100 of FIG. 1A. At least one of the optical films 100a and 101 may be configured to exhibit different optical characteristics based on the polarization type of the light passing through the film. For example, optical film 101 may be substantially identical to optical film 100 of FIG. 1A except that it substantially transmits light 30 of one polarization type (e.g., light with a linear p-pol polarization type) and substantially (or partially) blocks light 30 of a second, orthogonal polarization (e.g., light with a linear s-pol polarization type). In some embodiments, the other optical film 100a may be substantially identical to optical film 100 of FIG. 1A and may substantially transmit both orthogonal polarization types of light. In some embodiments, at least one of the films 100a and 101 may be configured to either transmit or reflect light based on the polarization type of the light 30. The use of linear s-pol and linear p-pol polarization types in this description are examples only, and other polarization types (e.g., linear vs. circular polarization type) may also be appropriate and within the scope of the description. Also, it should be noted that other configurations of the optical stack are possible, and that the optical film 100 (or films 100a, 101) may be used to implement optical films with additional functions, including, but not limited to, films such as a reflector, a reflective polarizer, an absorbing polarizer, partial reflector, and a diffuser.

FIG. 2 provides additional details on the construction of optical film 100 of FIG. 1A, according to the present description. In some embodiments, optical film 100 includes a plurality of polymeric layers 40 (including layers 40a-40d and 40w-40z) numbering at least 200 in total disposed between, and co-extruded and co-stretched with, a first outer layer 11 and an opposing second outer layer 12. In some embodiments, each of the polymeric layers 40 may an average thickness of less than about 150 nm. In some embodiments, a thinnest polymeric layer 40a in the plurality of polymeric layers may be disposed closer to first outer layer 11, and a thickest polymeric layer 40z may be disposed closer to second outer layer 12.

The plurality of polymeric layers 40 may be divided into two or more sections 43 and 45. Within each of sections 43 and 45, a layer thickness gradient for each section may be substantially linear. In some embodiments, in between sections 43 and 45, there may be a discontinuity 20 (e.g., a non-linear step in layer width between layer 40d and 40w). This step in width for discontinuity 20 may be at least about 5 times greater than the corresponding step changes in thickness across each of section 43 and 45. Additional detail on the layer thickness gradient of optical film 100 can be seen in FIG. 5 discussed elsewhere herein.

FIG. 3 provides a plot 90 of transmission percent versus wavelength for an optical film for two-factor authentication, according to the present description. Plot 90 is an example plot for one embodiment of optical film 100 (see, for example, FIG. 1A). Other embodiments consistent with or providing variants of the present description are possible. Plot 90 may be divided into three primary wavelength ranges of interest. A first wavelength range 71 which exhibits a first average transmission percentage TA1, a second wavelength range 72 exhibiting a peak transmission percentage Tp, and a third wavelength range 73 exhibiting a third average transmission percentage TA3. In the embodiment of FIG. 3, first wavelength range 71 is a near infrared wavelength range and second wavelength range 72 is a narrow notch transmission disposed in the visible wavelength range. However, other embodiments may be configured where first wavelength range 71 and second wavelength range 72 are substantially swapped (i.e., that is, where first wavelength range 71 extends across visible wavelengths and the narrow notch of second wavelength range 72 is disposed within the near infrared wavelengths).

In some embodiments, the first average transmission percentage TA1 may be greater than peak transmission percentage Tp. In some embodiments, peak transmission percentage Tp may be greater than third average transmission percentage TA3. In some embodiments, peak transmission percentage Tp may be greater than 30 times TA3, or greater than 40 times TA3, or greater than 50 times TA3.

In some embodiments, optical film 100 as represented by plot 90 may substantially transmit light in a near infrared wavelength range. In some embodiments, this near infrared wavelength range (i.e., first wavelength range 71) may extend from about 700 nm to about 2000 nm, or from about 800 nm to about 1500 nm, or from about 840 nm to about 1300 nm. In some embodiments, optical film 100 as represented by plot 90 may also substantially transmit light in a narrow visible wavelength range. In some embodiments, this visible wavelength range (i.e., second wavelength range 72) may extend from about 550 nm to about 700 nm, or from about 600 nm to about 700 nm, or from about 610 nm to about 680 nm.

In some embodiments, plot 90 may have a peak Tp at a peak wavelength with a corresponding full width at half maximum (FWHM) in the visible wavelength range. The peak Tp may separate a lower wavelength range from a higher wavelength range. In some embodiments, such as the example of FIG. 3, the lower and higher wavelength ranges may extend across M1 nanometers, such that the ratio of M1/FWHM is greater than or equal to about 1.5, or about 2.0, or about 2.5, or about 3.0. Stated another way, the FWHM associated with peak Tp may be configured such that it is relatively narrow, in order to minimize the amount of light in the visible wavelength range that is transmitted and which may contribute to a negative effect on the display image quality. In some embodiments, there is a second average transmission percentage TA2 exhibited across the wavelength range including the lower wavelength range and higher wavelength range (such as those shown in FIG. 3). In some embodiments, optical film 100 (see, for example, FIG. 1A) may be configured such that the ratio Tp/TA2 is greater than or equal to about 30, or about 40, or about 50.

As the optical films described herein may be configured to allow transmission of light in two separate wavelength ranges to enable two-factor authentication, a near infrared range and a visible range, care must be taken that any light transmitted in the visible wavelength range does not have a significant (negative) effect on the image quality seen at the display. That is, if visible wavelengths are used in an authentication scheme (e.g., light around 620 nm for use in determining information related to an absorptive property of blood), that visible light may create a visible effect that may be perceived on the display. To mitigate this negative defect, the visible wavelengths may be limited in either wavelength range (e.g., a very narrow transmission spike) or transmission percentage (e.g., a shorter spike with less transmission “power”). FIGS. 4A and 4B provide example plots of transmission percentage versus wavelength for two different embodiments of an optical film. In FIG. 4A, the optical film is configured such that it exhibits a single, narrow “spike” transmission 75a, which is relatively tall (i.e., has a nearly 70% transmission at the peak wavelength) but quite narrow (i.e., extends across only a small range of wavelengths, from about 615 nm to about 640 nm). In FIG. 4B, the optical film is configured such that it exhibits a shorter, wider transmission region 75b, which is relatively short (i.e., about 28% transmission peak) but which extends across a larger range of wavelengths (i.e., extends about 590 nm to about 660 nm). The transmission curve and peak of the optical film may be configured as required for a particular application. In some applications, for instance, a transmission peak around 30% may be adequate to be detected for use in the authentication procedure, depending on the performance of the sensor used. Both of the embodiments of FIG. 4A and FIG. 4B provide ways of limiting the effect of the visible light transmitted on the visual appearance of the display.

FIG. 5 is a plot showing a layer thickness profile for an embodiment of the optical films described herein. Layer thickness gradient 80 is a plot of layer thickness in nanometers (nm) on the y-axis versus layer number on the x-axis. The layer thickness gradient 80 includes a first portion 82 and a second portion 88. In some embodiments, each of first portion 82 and second portion 88 are substantially linear portions. In some embodiments, for example, a best linear fit to each of the first portion 82 and second portion 88 may have an R-squared value greater than or equal to about 0.8, or about 0.85, or about 0.90. In some embodiments, each of the first portion 82 and second portion 88 may extend across at least 30, or at least 40, or at least 50, or at least 100, or at least 200, polymeric layers. In some embodiments, the first portion 82 and second portion 88 are separated by a step portion 85 which may extend across less than about 10 of the polymeric layers. The change in thickness across the step portion 85 may be at least 5 times greater than a change in thickness across each of the first portion 82 and second portion 88. In some embodiments, the step portion 85 may be configured such that, for a target wavelength, there are no layers with an optical thickness close to a quarter-wave at the target wavelength (i.e., the multi-layer optical film is not strongly reflective as the target wavelength since none of the layers of the film are resonant at the target wavelength). That is, by positioning step portion 85 so that certain layer thicknesses are avoided in the overall layer thickness gradient, the optical film will not block or reflect (i.e., they will substantially transmit) wavelengths of light that would normally resonate with the omitted layers).

FIGS. 6A-6C provide plots of alternate layer thickness profiles for an optical film for two-factor authentication. Similar to the plot shown in FIG. 5, each of the plots in FIGS. 6A-6C represent a layer thickness gradient 80a, 80b, 80c which is a plot of layer thickness in nanometers (nm) on the y-axis versus layer number on the x-axis. Each of the layer thickness gradients 80a, 80b, 80c includes a first portion 82a, 82b, 82c and a second portion 88a, 88b, 88c which are substantially linear portions, separated by a step portion 85a, 85b, 85c. In FIG. 6A, showing embodiments 80a, the first portion 82a shows a gradual decrease in layer thickness as the layer number increases, step portion 85a shows a much steeper drop in layer thickness, and second portion 88a shows a gradual increase in layer thickness. The embodiment shown in FIG. 6B, showing embodiments 80b, the first portion 82b shows a gradual increase in layer thickness as the layer number increases, step portion 85b shows a steep drop in layer thickness, and second portion 88b shows a gradual decrease in layer thickness. In each of the embodiments 80a and 80b, the step portions 85a and 85b may be configured such that, for a target wavelength, there are no layers with an optical thickness close to a quarter-wave at the target wavelength, similar to the embodiment