Patent Description:
Privacy displays provide image visibility to a primary user that is typically in an on-axis position and reduced visibility of image content to a snooper, that is typically in an off-axis position. A privacy function may be provided by micro-louvre optical films that transmit some light from a display in an on-axis direction with low luminance in off-axis positions. However such films have high losses for head-on illumination and the micro-louvres may cause Moiré artefacts due to beating with the pixels of the spatial light modulator. The pitch of the micro-louvre may need selection for panel resolution, increasing inventory and cost.

Switchable privacy displays may be provided by control of the off-axis optical output.

Control may be provided by means of luminance reduction, for example by means of switchable backlights for a liquid crystal display (LCD) spatial light modulator. Display backlights in general employ waveguides and edge emitting sources. Certain imaging directional backlights have the additional capability of directing the illumination through a display panel into viewing windows. An imaging system may be formed between multiple sources and the respective window images. One example of an imaging directional backlight is an optical valve that may employ a folded optical system and hence may also be an example of a folded imaging directional backlight. Light may propagate substantially without loss in one direction through the optical valve while counter-propagating light may be extracted by reflection off tilted facets as described in <CIT>.

In a known privacy display the privacy mode is provided by the addition of a removable louver film, such as marketed by <NUM> Corporation, which may not be fitted or removed by users reliably and therefore in practice, is not assiduously attached by the user every time they are outside the office. In another known privacy display the control of privacy mode is electronically activated but control is vested in the user who must execute a keystroke to enter privacy mode. <CIT> discloses background information.

According to a first aspect of the present disclosure, there is provided a privacy display apparatus as set out in claim <NUM>.

In this document, public and privacy refer to the mode of the display rather than the nature of the location. For example privacy (display) mode is typically selected in public places such as coffee shops and public (display) mode is typically selected in private location such as at home.

Any of the aspects of the present disclosure may be applied in any combination.

Embodiments of the present disclosure may be used in a variety of optical systems. The embodiments may include or work with a variety of projectors, projection systems, optical components, displays, microdisplays, computer systems, processors, self-contained projector systems, visual and/or audio-visual systems and electrical and/or optical devices. Aspects of the present disclosure may be used with practically any apparatus related to optical and electrical devices, optical systems, presentation systems or any apparatus that may contain any type of optical system. Accordingly, embodiments of the present disclosure may be employed in optical systems, devices used in visual and/or optical presentations, visual peripherals and so on and in a number of computing environments.

Before proceeding to the disclosed embodiments in detail, it should be understood that the disclosure is not limited in its application or creation to the details of the particular arrangements shown, because the disclosure is capable of other embodiments. Moreover, aspects of the disclosure may be set forth in different combinations and arrangements to define embodiments unique in their own right. Also, the terminology used herein is for the purpose of description and not of limitation.

These and other advantages and features of the present disclosure will become apparent to those of ordinary skill in the art upon reading this disclosure in its entirety.

Embodiments are illustrated by way of example in the accompanying FIGURES, in which like reference numbers indicate similar parts, and in which:.

Terms related to privacy display appearance will now be described.

A private mode of operation of a display is one in which an observer sees a low contrast sensitivity such that an image is not clearly visible. Contrast sensitivity is a measure of the ability to discriminate between luminances of different levels in a static image. Inverse contrast sensitivity may be used as a measure of visual security, in that a high visual security level (VSL) corresponds to low image visibility.

For a privacy display providing an image to an observer, visual security may be given as: <MAT>
where V is the visual security level (VSL), Y is the luminance of the white state of the display at a snooper viewing angle, K is the luminance of the black state of the display at the snooper viewing angle and R is the luminance of reflected light from the display.

The luminance of a display varies with angle and so the maximum luminance of the display Ymax occurs at a particular angle that depends on the configuration of the display.

In many displays, the maximum luminance Ymax occurs head-on, i.e. normal to the display. Any display device disclosed herein may be arranged to have a maximum luminance Ymax that occurs head-on, in which case references to the maximum luminance of the display device Ymax may be replaced by references to the luminance normal to the display device.

Alternatively, any display described herein may be arranged to have a maximum luminance Ymax that occurs at a polar angle to the normal to the display device that is greater than <NUM>°. By way of example, the maximum luminance Ymax may occur at a non-zero polar angle and at an azimuth angle that has for example zero lateral angle so that the maximum luminance is for an on-axis user that is looking down on to the display device. The polar angle may for example be <NUM> degrees and the azimuthal angle may be the northerly direction (<NUM> degrees anti-clockwise from easterly direction). The viewer may therefore desirably see a high luminance at typical non-normal viewing angles.

The off-axis relative luminance, P is sometimes referred to as the privacy level. However, such privacy level P describes relative luminance of a display at a given polar angle compared to head-on luminance, and in fact is not a measure of privacy appearance.

The illuminance, I is the luminous flux per unit area that is incident on the display and reflected from the display towards the observer location. For Lambertian illuminance, and for displays with a Lambertian front diffuser illuminance I is invariant with polar and azimuthal angles. For arrangements with a display with non-Lambertian front diffusion arranged in an environment with directional (non-Lambertian) ambient light, illuminance I varies with polar and azimuthal angle of observation.

Thus in a perfectly dark environment, a high contrast display has VSL of approximately <NUM>. As ambient illuminance increases, the perceived image contrast degrades, VSL increases and a private image is perceived.

For typical liquid crystal displays the panel contrast C is above <NUM>: <NUM> for almost all viewing angles. allowing the visual security level to be approximated to: <MAT>.

In the present embodiments, in addition to the exemplary definition of eqn. <NUM>, other measurements of visual security level, V may be provided, for example to include the effect on image visibility to a snooper of snooper location, image contrast, image colour and white point and subtended image feature size. Thus the visual security level may be a measure of the degree of privacy of the display but may not be restricted to the parameter V.

The perceptual image security may be determined from the logarithmic response of the eye, such that <MAT>.

Desirable limits for S were determined in the following manner. In a first step a privacy display device was provided. Measurements of the variation of privacy level, P(θ) of the display device with polar viewing angle and variation of reflectivity ρ(θ) of the display device with polar viewing angle were made using photopic measurement equipment. A light source such as a substantially uniform luminance light box was arranged to provide illumination from an illuminated region that was arranged to illuminate the privacy display device along an incident direction for reflection to a viewer positions at a polar angle of greater than <NUM>° to the normal to the display device. The variation I(θ) of illuminance of a substantially Lambertian emitting lightbox with polar viewing angle was determined by and measuring the variation of recorded reflective luminance with polar viewing angle taking into account the variation of reflectivity ρ(θ). The measurements of P(θ), r(θ) and I(θ) were used to determine the variation of Security Factor S(θ) with polar viewing angle along the zero elevation-axis.

In a second step a series of high contrast images were provided on the privacy display including (i) small text images with maximum font height <NUM>, (ii) large text images with maximum font height <NUM> and (iii) moving images.

In a third step each observer (with eyesight correction for viewing at <NUM> where appropriate) viewed each of the images from a distance of <NUM>, and adjusted their polar angle of viewing at zero elevation until image invisibility was achieved for one eye from a position near on the display at or close to the centre-line of the display. The polar location of the observer's eye was recorded. From the relationship S(θ), the security factor at said polar location was determined. The measurement was repeated for the different images, for various display luminance Ymax, different lightbox illuminance I(θ=<NUM>), for different background lighting conditions and for different observers.

From the above measurements S < <NUM> (V < <NUM>) provides low or no visual security, <NUM> ≤ S < <NUM> (<NUM> ≤ V < <NUM>) provides visual security that is dependent on the contrast, spatial frequency and temporal frequency of image content, <NUM> ≤ S < <NUM> (<NUM> ≤ V < <NUM>)provides acceptable image invisibility (that is no image contrast is observable) for most images and most observers and S ≥ <NUM> (V ≥ <NUM>) provides full image invisibility, independent of image content for all observers.

In practical display devices, this means that it is desirable to provide a value of S for an off-axis viewer who is a snooper that meets the relationship S ≥ Smin, where: Smin has a value of <NUM> or more to achieve the effect that the off-axis viewer cannot perceive the displayed image; Smin has a value of <NUM> or more to achieve the effect that the displayed image is invisible, i.e. the viewer cannot perceive even that an image is being displayed, for most images and most observers; or Smin has a value of <NUM> or more to achieve the effect that the displayed image is invisible independent of image content for all observers.

In comparison to privacy displays, desirably wide angle displays are easily observed in standard ambient illuminance conditions. One measure of image visibility is given by the contrast sensitivity such as the Michelson contrast which is given by: <MAT>
and so: <MAT>.

Thus the visual security level (VSL), V is equivalent (but not identical to) <NUM>/M. In the present discussion, for a given off-axis relative luminance, P the wide angle image visibility, W is approximated as <MAT>.

The above discussion focusses on reducing visibility of the displayed image to an off-axis viewer who is a snooper, but similar considerations apply to visibility of the displayed image to the intended user of the display device who is typically on-axis. In this case, decrease of the level of the visual security level (VSL) V corresponds to an increase in the visibility of the image to the viewer. During observation S ≤ <NUM> (V ≤ <NUM>, W ≥ <NUM>) may provide acceptable visibility of the displayed image. In practical display devices, this means that it is desirable to provide a value of S for an on-axis viewer who is the intended user of the display device that meets the relationship S ≤ Smax, where Smax has a value of <NUM>.

It would be desirable to provide control of a switchable privacy display.

<FIG> is a schematic diagram illustrating a front view of a privacy display apparatus <NUM> comprising a privacy display device <NUM> that is controlled by a privacy control system <NUM> operating in privacy mode with a first visual security level. The display device <NUM> displays an image.

Display apparatus <NUM> may comprise privacy mode capable display device <NUM> and control system <NUM>. The display device <NUM> is arranged to display an image and capable of operating in at least a public mode and a privacy mode, wherein in the privacy mode the privacy function is provided and the visibility of the image to an off-axis viewer is reduced compared to the public mode and the visibility of the image to the primary user in an on-axis position remains visible in both the privacy and public modes. The control system <NUM> selectively operates the display device <NUM> in the public mode or the privacy mode for at least one region of the displayed image, typically the entire displayed image.

The display device <NUM> may in general provide the privacy function in any way. Examples of suitable types of display device for use as the display device <NUM> are described further below.

Means to determine privacy mode operation will now be described.

For a head-on user in typical ambient illuminance environments, desirably the display device <NUM> provides a displayed image <NUM> that has a luminance to achieve high image visibility, W in both privacy and public modes of operation.

The display apparatus <NUM> may also comprise inputs related to desirable circumstances to provide privacy images, or conversely by undesirable circumstances to provide public images. Such desirable and undesirable circumstances may be determined by policy <NUM> that is provided for example by a corporate policy, government policy, medical ethical policy or by user preference settings.

The control system <NUM> may be arranged to selectively operate the display device <NUM> in the public mode or the privacy mode in response to the detected level of the ambient light. The display apparatus <NUM> has an ambient light sensor <NUM> that detected the illuminance level of ambient light. The ambient light sensor <NUM> may be of any suitable type, such as a photodiode which may have a photopic filter or a photopic light response current or voltage or digital value.

Some types of display have multiple optical effects to improve privacy performance, with exemplary optical effects described below. If more than one privacy optical effect is available, the mode that gives the widest viewing freedom for the primary user while still maintaining adequate visual security level at the ambient light level experienced can be selected by the control system <NUM>. Advantageously privacy is protected, and user productivity is maintained.

Airplane mode <NUM> may be selected, indicating that low light level ambient environments may be present and visual security level control adapted accordingly.

Advantageously in public mode the display device <NUM> may have greater image uniformity and viewing freedom for the primary user as well as being visible from multiple viewing locations.

Visual security level indicator <NUM> may be provided on the display which is a measure of the privacy level achieved. In the illustrative example of <FIG>, the indicator <NUM> may be an amber privacy warning that indicates there may be some residual image visibility to an off-axis snooper. When switched in to privacy mode the control system <NUM> may be arranged to control the display device <NUM> to display image <NUM> with information such as indicator <NUM> representing the visibility of the image to an off-axis viewer, for example to provide visual security level, V. Advantageously the user or their supervisor may be confident in the privacy level being achieved in the specific environment in which they are operating.

The display appearance in privacy mode as seen by a snooper will now be described together with further inputs for the control of visual security level.

<FIG> is a schematic diagram illustrating a look-down perspective of a privacy display comprising a privacy control system <NUM> operating in privacy mode with a first visual security level.

As will be described below, off-axis privacy may be provided by control of off-axis luminance, reflectivity and image contrast of the image <NUM> provided by a switchable privacy display device <NUM> to an undesirable snooper.

In one example, the display apparatus may comprise an emissive spatial light modulator. In this case, the privacy control system <NUM> may control luminance of the displayed image by controlling of emission of light by the spatial light modulator.

In another example, the display device may comprise a backlight and a transmissive spatial light modulator arranged to receive light from the backlight. In this case, the control system may be arranged to control luminance of the displayed image by controlling the luminance of the backlight and/or by controlling transmission of light by the spatial light modulator.

In operation in privacy mode, a limited output cone angle 402C that is typically centred on the optical axis <NUM> that is typically a surface normal to the display device <NUM> is provided. Off-axis luminance is reduced. Ambient light sources <NUM> illuminate the display surface with light rays <NUM>. Reflected light <NUM> from the display provides increased visual security level, V as described above.

Some light rays <NUM> may be incident on Ambient Light Sensor, ALS <NUM>. The ALS <NUM> may be a separate element or may be incorporated in the camera <NUM> detection system.

Ambient illuminance detection <NUM> provides a calculation of ambient illuminance and is input into the control system <NUM>. VSL calculation <NUM> is used to determine desirable display setting characteristics and output to display control <NUM>. The display control <NUM> may control display luminance setting <NUM> and may be further used to provide visual security level indicator <NUM> level <NUM>. Display control <NUM> is further described below in relation to an example of a privacy display.

Features of the embodiment of <FIG> not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

In public display mode, a larger solid angle output light cone 402D as illustrated in <FIG> may be provided from the switchable privacy display device <NUM> may be adjusted to be larger than in privacy mode, such that off-axis display luminance is increased.

Visual security level indicator <NUM> may be provided on the display which is a measure of the privacy level achieved.

Switching between exemplary privacy and public luminance profiles will now be described.

<FIG> is a schematic graph illustrating variation of output luminance with viewing angle for a typical collimated backlight arranged to cooperate with plural retarders <NUM> to provide high visual security level to a wide range of snooper locations. Features of the embodiment of <FIG> not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

<FIG> illustrates a desirable luminance profile <NUM> of a backlight <NUM> operated in privacy mode for use with the switchable liquid crystal retarder <NUM> of <FIG> in privacy mode. The profile <NUM> is modified by switchable liquid crystal retarder <NUM> to provide an illustrative profile <NUM> that advantageously achieves an off-axis relative luminance of less than <NUM>% at <NUM> degrees lateral angle and zero degrees elevation.

Control of visual security level will now be further described.

<FIG> is a schematic graph illustrating variation of visual security level with off-axis relative luminance of a switchable privacy display operating in privacy mode and with reference to the privacy displays of <FIG>, as exemplary embodiments of a switchable privacy display <NUM>. Features of the embodiment of <FIG> not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

<FIG> illustrates the profiles of visual security level, V (calculated in each illustrative embodiment from eqn. <NUM>, above) and with the illustrative embodiments as illustrated in TABLE <NUM> for varying privacy levels achieved at the target snooper viewing locations <NUM>, 26R. Display reflectivity of <NUM>% or more may be achieved for displays comprising reflective polariser <NUM>, while display reflectivity of approximately <NUM>% may be achieved for displays not comprising reflective polariser <NUM>.

At <NUM>% privacy level, various visual security level points <NUM> may be provided depending on display structure, ambient illuminance and display luminance. The present embodiments further provide indicator <NUM> for display of visual security level that may be provided for example by means of traffic light indicators.

The variation of display reflectivity with viewing angle will now be described.

<FIG> is a schematic graph illustrating variation of reflectivity with polar viewing angle (that may be the lateral angle for zero elevation) for two types of privacy display. Profile <NUM> illustrates the variation of reflectivity for an illustrative embodiment of <FIG> and profile <NUM> illustrates the variation of reflectivity for an embodiment without the reflective polariser <NUM> of <FIG>. Both profiles include Fresnel reflectivity at the outer polariser <NUM> and thus increase at high polar angles.

The variation of visual security level, V with viewing angle will now be described.

<FIG> is a schematic graph illustrating variation of visual security level with polar viewing angle for the two types of privacy display of <FIG>. VSL profile <NUM> illustrates an output for a display of the type of <FIG> with reflective polariser <NUM>, and VSL profile <NUM> illustrates an output for the display of <FIG> with the reflective polariser <NUM> omitted. VSL profiles are illustrated for the same ambient illuminance, I. The limit Vlim above which no image visibility is present is described further below. The angular range <NUM> of snooper locations for the profile <NUM> is thus greater than the angular range <NUM> for the profile <NUM>. The reflective polariser <NUM> achieves above threshold visual security level over a wider polar range, advantageously achieving increased protection from snoopers. Further, head-on luminance may be increased for a given ambient illuminance, increasing image visibility for the display user.

Selective control of the relationship between desirable display luminance and ambient light illuminance will now be described.

The control system <NUM> controls luminance of the displayed image on the basis of the detected level of the ambient light in accordance with a transfer function. The transfer function may be selected to optimise the visibility of the displayed image to an on-axis viewer. Similar techniques for optimisation of the visibility of a displayed image on the basis of the detected level of ambient light are commonly used for display devices for portable devices such as a mobile telephone and may be applied here. However, the transfer function may be adapted for use with the privacy display device <NUM> when a privacy function is provided, as follows.

Typically, the transfer function provides higher luminance of the displayed image in the public mode than in the privacy mode. Some illustrative examples will now be described.

<FIG> is a schematic diagram illustrating transfer function profiles <NUM>, <NUM>, <NUM>, <NUM> between display head-on luminance measured in nits and detected ambient illuminance measured in lux; and <FIG> is a schematic graph illustrating transfer function profiles <NUM>, <NUM>, <NUM>, <NUM> between the ratio of measured ambient illuminance to head-on display luminance measured in lux per nit and ambient illuminance measured in lux.

The control system <NUM> is arranged to selectively control luminance of the displayed image in the public mode and the privacy mode in response to the detected level of the ambient light, in accordance with different transfer functions <NUM>, <NUM>, <NUM>, <NUM>, <NUM> relating levels of luminance to detected levels of the ambient light in the public mode and in the privacy mode respectively.

Considering profile <NUM> of <FIG> and corresponding profile <NUM> of <FIG>, a linear variation of display luminance Y0 is provided compared to measured ambient illuminance with a constant ratio of <NUM> lux/nit for all illuminance levels. In operation, such a display has high luminance compared to background illuminance over all illuminance ranges.

Profiles <NUM>, <NUM> differ from profiles <NUM>, <NUM> by increasing the lux/nit ratio with increasing luminance. Advantageously such profiles achieve visually comfortable images with high image visibility and low perceived glare over a wide illuminance range.

In a switchable privacy display such as that described hereinbelow with respect to <FIG>, such profiles <NUM>, <NUM> and <NUM>, <NUM> may be desirable for a public mode of operation. The profiles <NUM>, <NUM>, <NUM>, <NUM> advantageously achieve high image visibility, (W ≥ <NUM> desirably) and low image security factor, (S ≤ <NUM> desirably) for on-axis and off-axis viewing locations over a wide polar region as will be described further hereinbelow.

Considering profile <NUM> of <FIG> and corresponding profile <NUM> of <FIG>, a linear variation of display luminance Y0 is provided compared to measured ambient illuminance with a constant ratio of <NUM> lux/nit for all illuminance levels. In operation, such a display has reduced luminance compared to background illuminance over all illuminance ranges in comparison to a display with the profiles <NUM>, <NUM>.

Profiles <NUM>, <NUM> differ from profiles <NUM>, <NUM> by increasing the lux/nit ratio with increasing luminance for luminance levels below <NUM> nits.

In a switchable privacy display such as that described hereinbelow with respect to <FIG>, such profiles may be desirable for a privacy mode of operation. The profiles <NUM>, <NUM>, <NUM>, <NUM> advantageously achieve high image visibility, (W ≥ <NUM> desirably) and low image security factor, (S ≤ <NUM> desirably) for on-axis and off-axis viewing locations over a wide polar region as will be described further hereinbelow. Advantageously such profiles <NUM>, <NUM> and <NUM>, <NUM> may achieve desirable luminance and image visibility to the display user at lower illuminance levels. Further such profiles <NUM>, <NUM>, <NUM>, <NUM> achieve increased image security at higher illuminance levels.

When operating in the privacy mode the privacy transfer function <NUM> is selected and the control system uses the measured ambient light level to control the display luminance so that a desirable visual security level, V at at least one off-axis snooper observation angle is provided for different ambient illumination levels. Advantageously display security may be maintained in different lighting conditions.

When operating in the public mode the public transfer function <NUM> may be selected to provide a desirable image visibility, W for different ambient illumination levels. Advantageously display visibility may be maintained in different lighting conditions for off-axis observers.

The variation of security factor with display control and ambient illuminance level will now be further described.

<FIG> is a schematic graph illustrating the variation of security factor with polar angle for an illustrative privacy display operating in privacy mode for a lux/nit ratio of <NUM>; <FIG> is a schematic graph illustrating the variation of security factor with polar angle for an illustrative privacy display operating in privacy mode for a lux/nit ratio of <NUM>; <FIG> is a schematic graph illustrating the variation of security factor with polar angle for an illustrative privacy display operating in public mode for a lux/nit ratio of <NUM>; and <FIG> is a schematic graph illustrating the variation of security factor with polar angle for an illustrative privacy display operating in public mode for a lux/nit ratio of <NUM>.

The profiles of <FIG> are provided by the illustrative embodiment of <FIG> hereinbelow, for different ratios of illuminance to head-on luminance Y0 as will now be described. The primary display user is located in the polar region near to lateral angle <NUM>°, elevation angle <NUM>°. Snoopers are typically located in polar locations with lateral angles > <NUM>° and more typically in polar locations with lateral angles > <NUM>°.

In <FIG>, the display <NUM> is arranged to provide low off-axis luminance (such as illustrated in the lateral direction by profile <NUM> of <FIG>), and high off-axis reflectivity (such as illustrated in the lateral direction by profile <NUM> of <FIG>). The display head-on luminance Y0 is controlled by control of the light sources <NUM> of the backlight <NUM> such that the luminance Y0 measured in nits is one-third of the illuminance (that is assumed to be the same for all polar angles) measured in lux. Around the on-axis directions, S ≤ <NUM> and an image is seen with high image visibility, W ≥ <NUM>. Advantageously the arrangement 8A is a desirable polar profile of security factor, S for privacy operation.

By way of comparison with <FIG> illustrates the variation of security factor, S with polar viewing angle for luminance Y0 measured in nits that is twice the illuminance measured in lux (that is the arrangement suitable for public mode viewing). Undesirably the polar region within which the security factor, S ≥ <NUM> is substantially reduced. Off-axis display users may see more image data than for the arrangement of <FIG>.

In <FIG>, the display <NUM> is arranged by control of polar control retarders <NUM> to provide increased off-axis luminance (such as illustrated in the lateral direction by profile <NUM> of <FIG>), and reduced off-axis reflectivity (such as illustrated in the lateral direction by profile <NUM> of <FIG>). The display head-on luminance Y0 in nits is controlled to be three times the illuminance measured in lux. Around the on-axis directions, S ≤ <NUM> and an image is seen with high image visibility, W ≥ <NUM>. The arrangement 8A is a desirable polar profile of security factor, S for privacy operation. Advantageously the polar region for S ≤ <NUM> is significantly increased such that off-axis observers can see an image on the display <NUM> with high image visibility.

By way of comparison with <FIG> illustrates the variation of security factor, S with polar viewing angle for luminance Y0 measured in nits that is one-third of the illuminance measured in lux (that is the arrangement suitable for privacy viewing). Undesirably the polar region within which the security factor, S > <NUM> is substantially reduced. Off-axis display users may undesirably see less image data than for the arrangement of <FIG>.

Advantageously the control system of the present embodiments achieves desirable performance in both privacy and public modes of operation for different illuminance levels.

It may be desirable for users, or control systems to select the transfer function to achieve desirable levels of luminance to the head-on user, to adjust the size of the polar region for head-on users in privacy mode, to adjust the size of the polar region for secure viewing by off-axis snoopers and/or to adjust the size of the polar region for public operation as will now be described.

<FIG> is a schematic graph illustrating user selectable transfer functions between head-on display luminance and ambient illuminance. In comparison to the arrangement of <FIG>, selectable profiles <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be provided, each of which is shaped as a step function of luminance of the displayed image with increasing detected levels of the ambient light.

Advantageously, the profile control of <FIG> may be provided at low cost and complexity due to the step function shape of the transfer function. The control system <NUM> could similarly provide a single one of the profiles to achieve the same benefit.

An illustrative example of operation will now be described. The display may be operated in a bright environment such as <NUM> lux. In such an environment the display may default to its maximum peak luminance of <NUM> nits provided to the head-on user. The user may further reduce the display luminance if desirable. Advantageously high visual security may be provided for a wide range of ambient illuminance. The profiles may be selected with a step function as illustrated to reduce the number of settings and reduce driving cost by selecting a different profile. Alternatively smooth profiles that vary continuously with ambient illuminance may be provided.

In a default setting <NUM>, when the ambient illuminance falls, for example between <NUM> lux and <NUM> lux, the display switches between <NUM> nits to <NUM> nits. Visual security level for snoopers is maintained above a threshold. A time constant may be applied to the switching of the profile so that the variation is not visible as a display flicker. The time constant may be several seconds for example.

At high illuminance levels a single display head-on luminance Ymax may be provided for all the profiles as illustrated, or the step functions may continue to vary with luminance.

In some environments, the user may prefer a brighter head-on image, with some limited reduction of visual security and so may select profile <NUM> in place of the default setting. In other environments for which high visual security level is desirable, profile <NUM> may be selected with a lower head-on luminance and increased visual security level.

In other words, the user may change the default display brightness setting from the profile illustrated by the default profile <NUM> in the figure. If the ambient illuminance changes the display may follow the brightness step profile selected e.g. profile <NUM> as shown.

During periods in which the ambient illuminance is varying or the user selection of profile is changed, switching between the profiles may be provided over an extended time period such as several seconds to achieve seamless variation of display appearance.

<FIG> is a schematic flowchart illustrating a method for operating user selectable transfer functions.

The display apparatus, for example a notebook computer, may have a system level PWM (Pulse Width Modulation) generator <NUM>. The input to the system level PWM generator <NUM> may include a setting for Global Brightness <NUM> set by the operating system and which may use as an input the output of a separate ambient light sensor (not shown).

The input to the Global Brightness <NUM> settings may also include user input which may bias or adjust the default display brightness. The PWM input <NUM> is received by the timing controller (TCON) board <NUM> which may include a microcontroller to perform processing functions. The TCON board <NUM> also includes input from a privacy enable <NUM> signal which determines if the display is in privacy mode or not. If the display is not in privacy mode the PWM output <NUM> may follow the PWM input <NUM>. The TCON board <NUM> further includes an input from an ambient light sensor ALS <NUM> which may be different from the ambient light sensor ALS provided by the system. In particular the ambient light sensor <NUM> may be provided with direct connection to the TCON <NUM> as illustrated. This connection may be independent of the operating system control. The PWM output <NUM> sent to the LED controller <NUM> is able to be modified by the TCON <NUM>. A time response function <NUM> takes input from the ALS <NUM> and enables the TCON <NUM> to provide PWM output <NUM> so that changes in ambient illuminance result in a change in signal to the LED controller <NUM> that varies gradually over time so that the user does not experience flicker or jumps in display brightness. The time response function <NUM> may also suppress the effects of frequency components of ambient illumination (e.g. <NUM> or <NUM>) that may result from fluorescent tubes or the like.

The LED controller <NUM> is connected to the LED bar <NUM> of the privacy display <NUM>, which may be a PCB or a flexible PCB incorporated within the backlight <NUM> of the privacy display <NUM> as illustrated in <FIG>, below for example. In other arrangements (not shown) the LED controller <NUM> may be provided by a display controller arranged to control the luminance of an emissive spatial light modulator <NUM> such as an emissive OLED display or emissive micro-LED display.

When the privacy function is provided in the privacy mode, the transfer function may maintain a relationship Ymax ≤ Yupper, where Ymax is the maximum output luminance of the display device and Yupper is given by the equation: <MAT>
where the equation for Yupper applies for an observation direction having a polar angle θ of <NUM>° from the normal to the display device at at least one azimuth angle around the normal to the display device, I is the detected level of the ambient light, the units of I being the units of Ymax multiplied by solid angle in units of steradian, ρ(θ=<NUM>°) is the reflectivity of the display device along the observation direction, P(θ=<NUM>°) is the ratio of the luminance of the display device along the observation direction to the maximum output luminance Ymax of the display device, and Smin has a value of <NUM> or more.

The formula for the value of Yupper is derived from eqn. <NUM>, considering both the reflectivity ρ and the ratio (relative luminance) P with respect to a viewer along an observation direction that has a polar angle θ of <NUM>° from the normal to the display device at at least one azimuth angle around the normal to the display device. By meeting the relationship Ymax ≤ Yupper, it is possible to ensure that the value of S for an off-axis viewer in the observation direction who is a snooper that meets the relationship S ≥ Smin, where Smin has a value of <NUM> or more, regardless of how the illuminance level of ambient light and the luminance of the display device vary. By maintaining the relationship S ≥ Smin, where Smin has a value of <NUM> or more, an off-axis viewer in that observation direction cannot in practical terms discern the visual security level at or below the limit Smin such an off-axis viewer cannot perceive the displayed image, as described above.

Advantageously, Smin may have a value of <NUM> or more. Such an increased limit of Smin achieves a higher level of visual security in which such the image is effectively invisible to an off-axis viewer along the observation direction for most images and most observers.

Advantageously, Smin may have a value of <NUM> or more. Such an increased limit of Smin achieves a higher level of visual security in which the image is invisible independent of image content for all observers.

Where the display device has a major axis and a minor axis of symmetry, the equation for Ymax may apply for an observation direction having a polar angle θ of <NUM>° from the normal to the display device at an azimuth angle corresponding to either or both of the major axes in order to achieve the advantages with respect to an off-axis viewer with use of the display device in a landscape orientation, or the minor axis in order to achieve the advantages with respect to an off-axis viewer with use of the display device in a portrait orientation.

The control system may also be arranged to control luminance of the displayed image when the privacy function is provided on the basis of the detected level of the ambient light in accordance with a transfer function that maintains a relationship Ymax ≥ Ylower, where Ylower is given by the equation: <MAT> where the equation for Ylower applies for an observation direction having a polar angle θ of <NUM>° from the direction of the maximum output luminance of the display device at at least one azimuth angle around the direction of the maximum output luminance of the display device, ρ(Δθ=<NUM>°) is the reflectivity of the display device along the observation direction having a polar angle θ of <NUM>° from direction of the maximum output luminance of the display device, P(Δθ=<NUM>°) is the ratio of the luminance of the display device along the observation direction having a polar angle θ of <NUM>° from the direction of the maximum output luminance of the display device to the maximum output luminance Ymax of the display device, and Smax has a value of <NUM> or less.

The formula for the value of Ylower is derived from eqn. <NUM>, considering both the reflectivity ρ and the ratio (relative luminance) P with respect to an observation directions that has a polar angle θ of <NUM>° from the direction of the maximum output luminance of the display device at at least one azimuth angle around the direction of the maximum output luminance of the display device. An on-axis viewer will typically be located along such an observation direction or at a smaller polar angle where the visibility is better.

By meeting the relationship Ymax ≥ Ylower, it is possible to ensure that the value of S for the on-axis viewer meets the relationship S ≤ Smax, where Smax has a value of <NUM> or less, regardless of how the illuminance level of ambient light and the luminance of the display device vary. By maintaining the relationship S ≤ Smax, where Smax has a value of <NUM> or less, the visibility of the displayed image to an on-axis viewer is maintained, as described above.

Desirable limits for head-on luminance of the display operating in privacy mode will now be described.

<FIG> is a schematic graph illustrating the variation <NUM> of perceived visual security with visual security level, V at an observation angle θ. Visual security level V is a measured quantity of any given display and varies with polar viewing angle.

By comparison with visual security level V, perceived visual security is a subjective judgement of the visibility of a displayed privacy image arising from the human visual system response at the observation angle.

In operation it has been discovered that above a threshold limit Vlim of visual security level V then no image information is perceived. This transition in the perceived visibility with changes in the visual security level V is very rapid, as shown by the steepness of the graph in <FIG> around the threshold limit Vlim. That is, as the visual security level V increases, initially the perceived visibility degrades only gradually and the image is essentially viewable. However, on reaching the threshold limit Vlim, the perceived image rapidly ceases to be visible in a manner that is in practice surprising to watch.

In observation of the surprising result, for a text document image that is of concern for privacy applications it was found that the perceived image seen by a snooper rapidly ceased to be visible for V of <NUM>. In the region <NUM> for values of V above <NUM>, all the displayed text had zero visibility. In other words the perceived text rapidly ceases to be visible in a manner that is in practice surprising to watch for V of <NUM> or greater.

In regions <NUM> below Vlim text was visible with low contrast and in region <NUM> below V' text was clearly visible.

It would be desirable to maximise head-on display luminance to achieve high image visibility to the primary display user. It would be further desirable to achieve high image security level for a snooper at the observation angle. The selective control of the head-on luminance will now be described in further detail.

For an observation angle θ, the maximum display output luminance Ymax (typically the head-on luminance) is prevented from exceeding a luminance limit Ylim at which the visual security level V is above the threshold limit Vlim so that the image is not perceived as visible at that observation angle θ, the luminance limit Ylim being given by: <MAT> where Rθ is the reflected ambient illuminance at the observation angle θ, Kθ is the display black state luminance at the observation angle, and Pθ is the relative luminance at the observation angle θ compared to the maximum display output luminance Ymax (typically the head-on luminance and is measured in nits). For display reflectivity ρθ and a Lambertian illuminant with illuminance Iθ measured in lux that is reflected by the display at the observation angle, the luminance limit Y0lim is also given by: <MAT>.

Since the illuminance Iθ is dependent on the amount of ambient light, the luminance of the display device may be controlled by the control system <NUM> in accordance with these relationships. Specifically, the privacy transfer function <NUM> used by the control system <NUM> as described above may be selected to maintain the relationship Ymax ≤ Ylim in order that the image is not perceived as visible at a desired observation angle θ, for example at an observation angle θ of <NUM> degrees laterally and zero degrees in elevation from the normal to the display device.

Subject to that limit, the luminance is preferably as high as possible in order to optimise the performance for the head-on view. Accordingly, the privacy transfer function <NUM> used by the control system <NUM> as described above may additionally be selected to maintain the relationship Ymax / Iθ ≥ <NUM> lux/nit as illustrated by the profile <NUM> in <FIG>. The illuminance Iθ may be the sensed ambient illuminance that is averaged from the illuminated scene.

Advantageously a display may be provided that has high image security to off-axis snoopers while achieving high image visibility for the head-on user for different illuminance levels.

Further description of the control of a privacy display will now be described.

<FIG> illustrates a flowchart of the privacy control system of <FIG> and <FIG>.

The display operating environment <NUM> may include (but is not limited to) network ID <NUM>, Date/Time <NUM>, GPS <NUM> data, Ambient Light Sensor <NUM> detection and Airplane mode <NUM> setting.

Corporate privacy policy <NUM> may include definitions under which the display should be operated in privacy mode including time and location; documents and applications; and visual security level specifications.

Other inputs may include display design parameters <NUM> and information on viewed documents and applications <NUM>.

Data processor <NUM> is used to analyse display operating environment <NUM>, display design parameters <NUM>, viewed documents and applications <NUM> and compare against corporate privacy policy <NUM>. The output determines whether to operate the display in privacy or public mode such that switch <NUM> is set for privacy or public mode operation based on data processor <NUM> output.

In the case of privacy mode operation the settings to apply to the display device <NUM> using display control system <NUM> and images <NUM> using image control system <NUM> in order to achieve desirable visual security level are provided. Further indication of visual security level using indicator <NUM> may be provided.

In the case of public mode operation, the appropriate illumination control including cone angle change by display control system <NUM> and luminance using LED driver <NUM> are provided to the display device <NUM>.

The controller <NUM> may continue to monitor the status of the display operating environment <NUM> and appropriate changes in policy <NUM> and adjust display device <NUM> and images <NUM> appropriately to maintain the target visual security level.

Advantageously the control system <NUM> may enable the visual security level, that may be the visual security level to be reliably calculated and compared to a corporate policy <NUM> level set for the device's current environment. The visual security level may be adjusted to the level required for the display device <NUM> environment so that the primary user retains optimal viewing freedom and comfort consistent with achieving the prescribed corporate privacy policy privacy level.

The ambient light sensor <NUM> may be of a type that detects the illuminance level of ambient light incident on the display device in a non-directional manner. In such cases, the detected level of illuminance I represents an average level, so that the effects described above are achieved for off-axis viewers in varied locations.

Alternatively, the ambient light sensor <NUM> may be of a type that detects the illuminance level of ambient light incident on the display device along an incident direction for reflection to the observation direction. In this case, it is possible to measure ambient illuminance in directions that correspond to locations in which ambient light reflectivity contributes to snooper security. This allows the effects described above to be optimised specifically for an off-axis viewer in the observation direction. Some examples of such an approach are as follows.

<FIG> is a schematic diagram illustrating a top view of a privacy display and off-axis ambient light sensor. Ambient light source <NUM> is reflected to snooper <NUM> by privacy display <NUM>. Ambient light sensor <NUM> is arranged to measure ambient illuminance in light cone 605R. In operation, the output of the ambient light sensor <NUM> is arranged to adjust the luminance to the user <NUM> to achieve a desirable visual security level for the ambient illuminance. In operation, the snooper only sees reflected ambient light from regions around the direction of light cone 605R for typical displays with no or limited diffuser (for example with front surface diffusers with a diffusion of AG50 or less).

<FIG> is a schematic graph illustrating polar regions for measurement of ambient illuminance for a privacy display. <FIG> thus indicates the polar locations 605R, <NUM> within which ambient light sources may be arranged to contribute to the visual security level as observed by off-axis snoopers. Ambient light sources that are located elsewhere do not contribute to visual security factor. It is undesirable to provide reduction of head-on luminance to compensate for ambient illuminance that is not providing increased visual security level, that is light sources outside the regions <NUM>, 605R.

Ambient light sensors that preferentially measure illuminance in the polar regions <NUM>, 605R will now be described.

<FIG>are schematic diagrams illustrating top views of off-axis ambient light sensors for measurement of the ambient illuminance in the polar regions of <FIG>.

<FIG> illustrates ambient light sensor <NUM> that comprises a mask <NUM> with apertures 241R, <NUM> that are separated by spacer <NUM> from the mask <NUM>. Sensor <NUM> measures ambient illuminance from off-axis ambient light source <NUM> while sensor 235R measures ambient illuminance from off-axis ambient light source 604R. Advantageously in privacy mode of operation, the visual security level provided to the snooper may be increased in response to appropriately placed ambient light sources 604R, <NUM>.

<FIG> is similar to <FIG> other than the two sensors <NUM>, 235R are replace by a single sensor 235C. Advantageously cost is reduced.

<FIG> illustrates an embodiment wherein the sensors <NUM>, 235R and masks <NUM>, 237R are tilted with respect to the normal direction to the display device <NUM>, with optical axes <NUM>, 299R that are directed towards the centres of the regions <NUM>, 605R. Advantageously in comparison to the arrangement of <FIG> stray light may be reduced and accuracy of measurement improved.

In the embodiments of <FIG> the apertures <NUM> and sensors <NUM> may be shaped to achieve matching measurement directions to the polar locations <NUM>, 605R of <FIG>.

The ALS may include a number of detector or detection channels that are able to detect different spectral bands or infra-red for example. One channel may also be used to detect flicker effects from, for example pulsed LED or fluorescent illumination. The individual detectors above may be multiplexed to an analogue to digital converter to reduce cost.

Illustrative examples of displays that are capable of switching between privacy mode and a public mode will now be described.

<FIG> is a schematic diagram illustrating in front perspective view a switchable directional display device <NUM> comprising a backlight <NUM>, switchable liquid crystal retarder <NUM> and a spatial light modulator <NUM>.

Display device <NUM> comprises a directional backlight <NUM> such as a collimated backlight arranged to output light, the backlight <NUM> comprising a directional waveguide <NUM>; and plural light sources <NUM> arranged to input input light into the waveguide <NUM>, the waveguide <NUM>, a rear reflector and light control films <NUM> being arranged to direct light from light sources <NUM> into solid angular extent 402A. Light control films <NUM> may comprise turning films and diffusers for example.

In the present disclosure a solid angular extent is the solid angle of a light cone within which the luminance is greater than a given relative luminance to the peak luminance. For example the luminance roll-off may be to a <NUM>% relative luminance so that the solid angular extent has an angular width in a given direction (such as the lateral direction) that is the same as the full-width half maximum (FWHM).

The backlight <NUM> may be arranged to provide an angular light solid angular extent 402A that has reduced luminance for off-axis viewing positions in comparison to head-on luminance.

Display control system <NUM> is arranged to provide control of light source driver <NUM>. Luminance of LEDs <NUM> may be controlled by control system, such that absolute off-axis luminance to a snooper may be controlled.

The spatial light modulator <NUM> may comprise a liquid crystal display comprising substrates <NUM>, <NUM>, and liquid crystal layer <NUM> having red, green and blue pixels <NUM>, <NUM>, <NUM>. The spatial light modulator <NUM> has an input display polariser <NUM> and an output display polariser <NUM> on opposite sides thereof. The output display polariser <NUM> is arranged to provide high extinction ratio for light from the pixels <NUM>, <NUM>, <NUM> of the spatial light modulator <NUM>. Typical polarisers <NUM>, <NUM> may be absorbing polarisers such as dichroic polarisers.

Optionally a reflective polariser <NUM> may be provided between the dichroic input display polariser <NUM> and backlight <NUM> to provide recirculated light and increase display efficiency. Advantageously efficiency may be increased.

The optical stack to provide control off-axis luminance will now be described.

Reflective polariser <NUM>, plural retarders <NUM> and additional polariser <NUM> are arranged to receive output light from the spatial light modulator <NUM>.

The plural retarders <NUM> are arranged between the reflective polariser <NUM> and an additional polariser <NUM>. The polarisers <NUM>, <NUM>, <NUM> may be absorbing type polarisers such as iodine polarisers while the reflective polariser <NUM> may be a stretched birefringent film stack such as APF from <NUM> Corporation or a wire grid polariser.

Plural retarders <NUM> comprise a switchable liquid crystal retarder <NUM> comprising a layer <NUM> of liquid crystal material, and substrates <NUM>, <NUM> arranged between the reflective polariser <NUM> and the additional polariser <NUM>. Retarder <NUM> further comprises a passive retarder <NUM> as will be described further below.

As described below, plural retarders <NUM> do not affect the luminance of light passing through the reflective polariser <NUM>, the retarders <NUM> and the additional polariser <NUM> along an axis along a normal to the plane of the retarders <NUM> but the retarders <NUM> do reduce the luminance of light passing therethrough along an axis inclined to a normal to the plane of the retarders <NUM>, at least in one of the switchable states of the switchable retarder <NUM>. This arises from the presence or absence of a phase shift introduced by the retarders <NUM> to light along axes that are angled differently with respect to the liquid crystal material of the retarders <NUM>.

Transparent substrates <NUM>, <NUM> of the switchable liquid crystal retarder <NUM> comprise electrodes arranged to provide a voltage across a layer <NUM> of liquid crystal material <NUM> therebetween. Control system <NUM> is arranged to control the voltage applied by voltage driver <NUM> across the electrodes of the switchable liquid crystal retarder <NUM>.

As will be described further below, the additional polariser <NUM>, plural retarders <NUM> and reflective polariser <NUM> may be arranged to provide polar control of output luminance and frontal reflectivity from ambient illumination <NUM>.

An example of an optical stack to provide control of off-axis luminance will now be described.

<FIG> is a schematic diagram illustrating in perspective side view an arrangement of the plural retarders <NUM> in a privacy mode of operation comprising a negative C-plate passive retarder <NUM> and homeotropically aligned switchable liquid crystal retarder <NUM> in a privacy mode of operation. In <FIG>, some layers of the optical stack are omitted for clarity. For example the switchable liquid crystal retarder <NUM> is shown omitting the substrates <NUM>, <NUM>.

The switchable liquid crystal retarder <NUM> comprises two surface alignment layers disposed on electrodes <NUM>, <NUM> and adjacent to the layer of liquid crystal material <NUM> and on opposite sides thereof and each arranged to provide homeotropic alignment in the adjacent liquid crystal material <NUM>. The layer of liquid crystal material <NUM> of the switchable liquid crystal retarder <NUM> comprises a liquid crystal material with a negative dielectric anisotropy. The liquid crystal molecules <NUM> may be provided with a pretilt, for example <NUM> degrees from the horizontal to remove degeneracy in switching.

The electric vector transmission direction of the reflective polariser <NUM> is parallel to the electric vector transmission direction of the output polariser <NUM>. Further the electric vector transmission direction <NUM> of the reflective polariser <NUM> is parallel to the electric vector transmission direction <NUM> of the additional polariser <NUM>.

The switchable liquid crystal retarder <NUM> comprises a layer <NUM> of liquid crystal material <NUM> with a negative dielectric anisotropy. The passive retarder <NUM> comprises a negative C-plate having an optical axis perpendicular to the plane of the retarder <NUM>, illustrated schematically by the orientation of the discotic material <NUM>.

The liquid crystal retarder <NUM> further comprises transmissive electrodes <NUM>, <NUM> arranged to control the liquid crystal material, the layer of liquid crystal material being switchable by means of adjusting the voltage being applied to the electrodes. The electrodes <NUM>, <NUM> may be across the layer <NUM> and are arranged to apply a voltage for controlling the liquid crystal retarder <NUM>. The transmissive electrodes are on opposite sides of the layer of liquid crystal material <NUM> and may for example be ITO electrodes.

Alignment layers may be formed between electrodes <NUM>, <NUM> and the liquid crystal material <NUM> of the layer <NUM>. The orientation of the liquid crystal molecules in the x-y plane is determined by the pretilt direction of the alignment layers so that each alignment layer has a pretilt wherein the pretilt of each alignment layer has a pretilt direction with a component 417a, 417b in the plane of the layer <NUM> that is parallel or anti-parallel or orthogonal to the electric vector transmission direction <NUM> of the reflective polariser <NUM>.

Driver <NUM> provides a voltage V to electrodes <NUM>, <NUM> across the layer <NUM> of switchable liquid crystal material <NUM> such that liquid crystal molecules are inclined at a tilt angle to the vertical. The plane of the tilt is determined by the pretilt direction of alignment layers formed on the inner surfaces of substrates <NUM>, <NUM>.

In typical use for switching between a public mode and a privacy mode, the layer of liquid crystal material is switchable between two states, the first state being a public mode so that the display may be used by multiple users, the second state being a privacy mode for use by a primary user with minimal visibility by snoopers. The switching may be by means of a voltage being applied across the electrodes. In general such a display may be considered having a first wide angle state and a second reduced off-axis luminance state.

Polar profiles of various elements of an illustrative embodiment of the stack of <FIG> will now be described.

<FIG> is a schematic graph illustrating the polar and azimuthal variation of output luminance of a collimated backlight and spatial light modulator.

<FIG> is a schematic graph illustrating the polar and azimuthal variation of transmission of a switchable retarder arranged between parallel polarisers for the illustrative embodiment of TABLE <NUM>.

<FIG> is a schematic graph illustrating the polar and azimuthal variation of relative reflection of a switchable retarder arranged between a reflective polariser and absorbing polariser for the illustrative embodiment of TABLE <NUM>.

<FIG> is a schematic graph illustrating the polar and azimuthal variation of total display reflectivity for the arrangement of <FIG> in a privacy mode of operation, that is the polar profile for the reflectivity ρ(θ,ϕ) where θ is the polar angle and ϕ is the azimuthal angle.

<FIG> is a schematic graph illustrating the polar and azimuthal variation of output luminance for the arrangement of <FIG> in a privacy mode of operation, that is the polar profile for the privacy level P(θ,ϕ).

<FIG> is a schematic graph illustrating the polar and azimuthal variation of visual security level, S(θ,ϕ) for the arrangement of <FIG> in a privacy mode of operation for a display head-on luminance, of value Ymax measured in nits that is half of the illuminance of value I measured in lux. Contour lines for S = <NUM>, S = <NUM> and S = <NUM> are illustrated to show polar regions of image privacy and image invisibility. Contour lines for S = <NUM> are illustrated to show polar regions of high image visibility.

<FIG> is a schematic graph illustrating the polar variation of visual security level, S for zero elevation for the arrangement of <FIG> in a privacy mode of operation for a display head-on luminance, of value Ymax measured in nits that is half of the illuminance of value I measured in lux. At <NUM> degrees the display is controlled such that the I/Ymax ratio (lux/nit) setting of the display is <NUM> and the image is invisible at polar angles of +/-<NUM> degrees.

Operation of the display of <FIG> in public mode will now be described.

<FIG> is a schematic diagram illustrating in perspective side view an arrangement of the retarders <NUM> in a public mode of operation. In the present embodiment, zero volts is provided across the liquid crystal retarder <NUM>, as in TABLE <NUM>.

In comparison to the arrangement of <FIG>, no voltage is applied and the molecules of the liquid crystal material <NUM> are substantially arranged normal to the alignment layers and electrodes <NUM>, <NUM>.

<FIG> is a schematic graph illustrating the polar and azimuthal variation of output luminance for the arrangement of <FIG> in a public mode of operation; and <FIG> is a schematic graph illustrating the polar variation of visual security level, S for zero elevation for the arrangement of <FIG> in a public mode of operation for a display head-on luminance, of value Ymax measured in nits that is half of the illuminance of value I measured in lux. In comparison to the arrangement of <FIG>, the display remains visible to users over a wide polar region with highest visibility near the axis.

<FIG> is a schematic graph illustrating the polar and azimuthal variation of output luminance for a backlight with a direction of maximum luminance Ymax that is not normal to the display. In comparison to <FIG>, which has Ymax at location <NUM> that is the display normal, <FIG> illustrates that Ymax is at location <NUM> that is above the axis. Advantageously display luminance may be increased for users that are looking down onto the display. The observation directions having a polar angle θ of <NUM>° from the direction of the maximum output luminance of the display device as described in eqn. <NUM> are illustrated by polar region <NUM>. Desirably at least within region <NUM> the image visibility of the image to the primary user is high, that is the security factor S is below <NUM>.

The propagation of polarised light from the output polariser <NUM> will now be considered for on-axis and off-axis directions for a display operating in privacy mode.

<FIG> is a schematic diagram illustrating in side view propagation of output light from a spatial light modulator through the optical stack of <FIG> in a privacy mode of operation.

When the layer <NUM> of liquid crystal material <NUM> is driven to operate in the privacy mode, the retarders <NUM> provide no overall transformation of polarisation component <NUM> to output light rays <NUM> passing therethrough along an axis perpendicular to the plane of the switchable retarder, but provides an overall transformation of polarisation component <NUM> to light rays <NUM> passing therethrough for some polar angles which are at an acute angle to the perpendicular to the plane of the retarders.

Polarisation component <NUM> from the output polariser <NUM> is transmitted by reflective polariser <NUM> and incident on retarders <NUM>. On-axis light has a polarisation component <NUM> that is unmodified from component <NUM> while off-axis light has a polarisation component <NUM> that is transformed by the retarders <NUM>. At a minimum, the polarisation component <NUM> is transformed to a linear polarisation component <NUM> and absorbed by additional polariser <NUM>. More generally, the polarisation component <NUM> is transformed to an elliptical polarisation component, that is partially absorbed by additional polariser <NUM>.

The polar distribution of light transmission illustrated in <FIG> modifies the polar distribution of luminance output of the underlying spatial light modulator <NUM>. In the case that the spatial light modulator <NUM> comprises a directional backlight <NUM> then off-axis luminance may be further be reduced as described above.

Advantageously, a privacy display is provided that has low luminance to an off-axis snooper while maintaining high luminance for an on-axis observer.

The operation of the reflective polariser <NUM> for light from ambient light source <NUM> will now be described for the display operating in privacy mode.

<FIG> is a schematic diagram illustrating in top view propagation of ambient illumination light through the optical stack of <FIG> in a privacy mode of operation.

Ambient light source <NUM> illuminates the display device <NUM> with unpolarised light. Additional polariser <NUM> transmits light ray <NUM> normal to the display device <NUM> with a first polarisation component <NUM> that is a linear polarisation component parallel to the electric vector transmission direction <NUM> of the additional polariser <NUM>.

In both states of operation, the polarisation component <NUM> remains unmodified by the retarders <NUM> and so transmitted polarisation component <NUM> is parallel to the transmission-axis of the reflective polariser <NUM> and the output polariser <NUM>, so ambient light is directed through the spatial light modulator <NUM> and lost.

By comparison, for ray <NUM>, off-axis light is directed through the retarders <NUM> such that polarisation component <NUM> incident on the reflective polariser <NUM> may be reflected. Such polarisation component is re-converted into component <NUM> after passing through retarders <NUM> and is transmitted through the additional polariser <NUM>.

Thus when the layer <NUM> of liquid crystal material is in the second state of said two states, the reflective polariser <NUM> provides no reflected light for ambient light rays <NUM> passing through the additional polariser <NUM> and then the retarders <NUM> along an axis perpendicular to the plane of the retarders <NUM>, but provides reflected light rays <NUM> for ambient light passing through the additional polariser <NUM> and then the retarders <NUM> at some polar angles which are at an acute angle to the perpendicular to the plane of the retarders <NUM>; wherein the reflected light <NUM> passes back through the retarders <NUM> and is then transmitted by the additional polariser <NUM>.

The retarders <NUM> thus provide no overall transformation of polarisation component <NUM> to ambient light rays <NUM> passing through the additional polariser <NUM> and then the retarder <NUM> along an axis perpendicular to the plane of the switchable retarder, but provides an overall transformation of polarisation component <NUM> to ambient light rays <NUM> passing through the absorptive polariser <NUM> and then the retarders <NUM> at some polar angles which are at an acute angle to the perpendicular to the plane of the retarders <NUM>.

The polar distribution of light reflection illustrated in <FIG> thus illustrates that high reflectivity can be provided at typical snooper locations by means of the privacy state of the retarders <NUM>. Thus, in the privacy mode of operation, the reflectivity for off-axis viewing positions is increased as illustrated in <FIG>, and the luminance for off-axis light from the spatial light modulator is reduced as illustrated in <FIG>.

In the public mode of operation, the control system <NUM>, <NUM>, <NUM> is arranged to switch the switchable liquid crystal retarder <NUM> into a second retarder state in which a phase shift is introduced to polarisation components of light passing therethrough along an axis inclined to a normal to the plane of the switchable liquid crystal retarder <NUM>.

By way of comparison, solid angular extent 402D may be substantially the same as solid angular extent 402B in a public mode of operation. Such control of output solid angular extents 402C, 402D may be achieved by synchronous control of the sets <NUM>, <NUM> of light sources and the at least one switchable liquid crystal retarder <NUM>.

Advantageously a privacy mode may be achieved with low image visibility for off-axis viewing and a large solid angular extent may be provided with high efficiency for a public mode of operation, for sharing display imagery between multiple users and increasing image spatial uniformity.

Additional polariser <NUM> is arranged on the same output side of the spatial light modulator <NUM> as the display output polariser <NUM> which may be an absorbing dichroic polariser. The display polariser <NUM> and the additional polariser <NUM> have electric vector transmission directions <NUM>, <NUM> that are parallel. As will be described below, such parallel alignment provides high transmission for central viewing locations.

A transmissive spatial light modulator <NUM> arranged to receive the output light from the backlight; an input polariser <NUM> arranged on the input side of the spatial light modulator between the backlight <NUM> and the spatial light modulator <NUM>; an output polariser <NUM> arranged on the output side of the spatial light modulator <NUM>; an additional polariser <NUM> arranged on the output side of the output polariser <NUM>; and a switchable liquid crystal retarder <NUM> comprising a layer <NUM> of liquid crystal material arranged between the at least one additional polariser <NUM> and the output polariser <NUM> in this case in which the additional polariser <NUM> is arranged on the output side of the output polariser <NUM>; and a control system <NUM> arranged to synchronously control the light sources <NUM>, <NUM> and the at least one switchable liquid crystal retarder <NUM>.

Control system <NUM> further comprises control of voltage controller <NUM> that is arranged to provide control of voltage driver <NUM>, in order to achieve control of switchable liquid crystal retarder <NUM>.

Advantageously, a privacy display is provided that has high reflectivity to an off-axis snooper while maintaining low reflectivity for an on-axis observer. As described above, such increased reflectivity provides enhanced privacy performance for the display in an ambiently illuminated environment.

Operation in the public mode will now be described.

<FIG> is a schematic diagram illustrating in side view propagation of output light from a spatial light modulator through the optical stack of <FIG> in a public mode of operation; and <FIG> is a schematic graph illustrating the variation of output luminance with polar direction for the transmitted light rays in <FIG>.

When the liquid crystal retarder <NUM> is in a first state of said two states, the retarders <NUM> provide no overall transformation of polarisation component <NUM>, <NUM> to output light passing therethrough perpendicular to the plane of the switchable retarder <NUM> or at an acute angle to the perpendicular to the plane of the switchable retarder <NUM>. That is polarisation component <NUM> is substantially the same as polarisation component <NUM> and polarisation component <NUM> is substantially the same as polarisation component <NUM>. Thus the angular transmission profile of <FIG> is substantially uniformly transmitting across a wide polar region. Advantageously a display may be switched to a wide field of view.

<FIG> is a schematic diagram illustrating in top view propagation of ambient illumination light through the optical stack of <FIG> in a public mode of operation; and <FIG> is a schematic graph illustrating the variation of reflectivity with polar direction for the reflected light rays in <FIG>.

Thus when the liquid crystal retarder <NUM> is in the first state of said two states, the retarders <NUM> provide no overall transformation of polarisation component <NUM> to ambient light rays <NUM> passing through the additional polariser <NUM> and then the retarders <NUM>, that is perpendicular to the plane of the retarders <NUM> or at an acute angle to the perpendicular to the plane of the retarders <NUM>.

In operation in the public mode, input light ray <NUM> has polarisation state <NUM> after transmission through the additional polariser <NUM>. For both head-on and off-axis directions no polarisation transformation occurs and thus the reflectivity for light rays <NUM> from the reflective polariser <NUM> is low. Light ray <NUM> is transmitted by reflective polariser <NUM> and lost in the display polarisers <NUM>, <NUM> or the backlight of <FIG> or optical isolator <NUM>, <NUM> in an emissive spatial light modulator <NUM> of <FIG>.

Advantageously in a public mode of operation, high luminance and low reflectivity is provided across a wide field of view. Such a display can be conveniently viewed with high contrast by multiple observers.

A display apparatus comprising an emissive display will now be described.

<FIG> is a schematic diagram illustrating a front perspective view a switchable directional display device comprising a directional backlight and two switchable liquid crystal retarders each arranged between a pair of polarisers. In comparison to the arrangement of <FIG>, the emissive display such as an OLED display or a micro-LED display comprises a further quarter waveplate <NUM> between the pixel layer <NUM> and output polariser <NUM>. Advantageously undesirable reflectivity from the backplane <NUM> is reduced.

<FIG> is a schematic graph illustrating the polar and azimuthal variation of output luminance of an emissive spatial light modulator.

<FIG> is a schematic graph illustrating the polar and azimuthal variation of transmission of a first switchable retarder arranged between a first pair of parallel polarisers for the illustrative embodiment of TABLE <NUM>.

<FIG> is a schematic graph illustrating the polar and azimuthal variation of relative reflection of the first switchable retarder 300A arranged between a reflective polariser <NUM> and absorbing polariser 318A for the illustrative embodiment of TABLE <NUM>.

<FIG> is a schematic graph illustrating the polar and azimuthal variation of total display reflectivity ρ(θ,ϕ) for the arrangement of <FIG> in a privacy mode of operation.

<FIG> is a schematic graph illustrating the polar and azimuthal variation of transmission of a second switchable retarder 300B arranged between a second pair of parallel polarisers for the illustrative embodiment of TABLE <NUM>.

<FIG> is a schematic graph illustrating the polar and azimuthal variation of output luminance P(θ,ϕ) for the arrangement of <FIG> in a privacy mode of operation.

<FIG> is a schematic graph illustrating the polar and azimuthal variation of visual security level, S for the arrangement of <FIG> in a privacy mode of operation for a display head-on luminance, of value Ymax measured in nits that is half of the illuminance of value I measured in lux.

<FIG> is a schematic graph illustrating the polar variation of visual security level, S for zero elevation for the arrangement of <FIG> in a privacy mode of operation for a display head-on luminance, of value Ymax measured in nits that is half of the illuminance of value I measured in lux. Desirably the security level, S is greater than <NUM> at +/-<NUM>°.

Other types of switchable privacy display will now be described.

A display device <NUM> that may be switched between privacy and public modes of operation comprises an imaging waveguide and an array of light sources as described in <CIT>. The imaging waveguide images an array of light sources to optical windows that may be controlled to provide high luminance on-axis and low luminance off-axis in a privacy mode, and high luminance with a large solid angle cone for public operation.

Switchable angular contrast profile liquid crystal displays are described in Japanese Patent Publ. No. <CIT> and in<CIT>. Such displays may provide out-of-plane tilt of liquid crystal molecules in the liquid crystal layer <NUM> of a liquid crystal display and may achieve reduced off-axis image contrast in privacy mode of operation. The display device <NUM> control system <NUM> may further comprise control of out-of-plane tilt of the liquid crystal molecules.

Claim 1:
A privacy display apparatus (<NUM>) comprising:
a display device (<NUM>) arranged to display an image and capable of operating in at least a public mode and a privacy mode, wherein the visibility of the image to an off-axis viewer is reduced in the privacy mode compared to the public mode; and
a control system (<NUM>) arranged to control the display device (<NUM>), the control system (<NUM>) being capable of selectively operating the display device (<NUM>) in the public mode or the privacy mode;
characterised in that the display apparatus (<NUM>) comprises a directional ambient light sensor (<NUM>) arranged to detect the illuminance level of the ambient light that is incident in a light cone directed along an incident direction for reflection towards an off-axis viewer.