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>, on which the two-part form of the independent claims is based, discloses privacy display apparatus comprising a display device arranged to display an image, the display device being capable of providing privacy functions in which the visibility of the image to an off-axis viewer is reduced compared to the visibility of the image to an on-axis viewer. These include a luminance-privacy function providing reduction of the luminance of the displayed image to the off-axis viewer and a contrast-privacy function providing reduction of the contrast of the displayed image to the off-axis viewer. The display apparatus also comprises a control system arranged to control the display device. <CIT> discloses further relevant prior art.

According to a first aspect of the present disclosure, there is provided a privacy display apparatus comprising: a display device arranged to display an image, the display device being capable of providing a privacy function in which the visibility of the image to an off-axis viewer is reduced compared to the visibility of the image to an on-axis viewer; at least one privacy light source arranged to provide external illumination from an illuminated region onto the display device along an incident direction for reflection from the display device to a predetermined viewer position at a polar angle of greater than <NUM>° to the normal to the display device; and a control system arranged to control the display device, wherein the control system is arranged to control the luminous flux of the at least one privacy light source when the privacy function is provided.

Thus, when the privacy function is provided, illumination is provided from the illuminated region onto the display device along the incident direction and reflected in the direction of an off-axis viewer in the predetermined viewer position. As a result, the illuminated region provides suppression of the visibility of the displayed image to the off-axis viewer by providing an external illuminance. In general terms, the suppression occurs because the external illuminance reduces the contrast of the displayed image, and thereby improves the visual security level. By comparison, the illumination from the illuminated region is not reflected towards an on-axis viewer who therefore perceives no degradation of the visibility of the image.

The privacy display apparatus may be applied in a range of situations. By way of non-limitative example, the privacy display apparatus may be applied in an automotive vehicle. In this case, conveniently, the illuminated region may be part of the door of the automotive vehicle.

The display apparatus may further comprise an ambient light sensor arranged to detect the illuminance level of ambient light.

The control system may derive a measure of the illuminance of light on the display along the incident direction using the illuminance level of ambient light detected by the ambient light sensor and selectively control the luminous flux of the at least one privacy light source on the basis of the derived measure.

In a case where the ambient light sensor is a directional sensor arranged to detect the illuminance level of ambient light incident on the display device along the incident direction, the measure of the illuminance Iθ of light on the display along the incident direction is the illuminance level of ambient light detected by the ambient light sensor.

In a case where the ambient light sensor is arranged to detect the illuminance level of ambient light from a range of directions, the control system may derive a measure of the illuminance of light on the display along the incident direction using both the illuminance level of ambient light detected by the ambient light sensor and the luminous flux of the at least one privacy light source.

Advantageously, the control system is arranged to control the luminous flux of the at least one privacy light source to maintain a relationship Iθ ≥ Iθmin where Iθ is the derived measure of the illuminance of light on the display along the incident direction and Iθmin is given by the equation <MAT>
where Ymax is the maximum output luminance of the display device, the units of Ymax being the units of Iθmin divided by solid angle in units of steradian, ρ(θ) is the reflectivity of the display device for light along the incident direction, P(θ) is the ratio of the luminance of the display device along the incident direction to the maximum output luminance of the display device, and Smin has a value of <NUM> or more.

By controlling the luminous flux of the at least one privacy light source on the basis of the derived measure of the illuminance of light on the display along the incident direction in a manner that maintains the relationship Iθ ≥ Iθmin when the privacy function is provided, the perceptual security level of operation of the display device may be maintained at or above the limit Smin in the predetermined viewer position at a polar angle of greater than <NUM>° to the normal to the display device, even as the illuminance level of ambient light and the luminance of the display device are varied. By so maintaining the perceptual security level at or above the limit Smin, an off-axis viewer cannot perceive the displayed image.

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 in invisible to an off-axis viewer, i.e. the off-axis viewer cannot even perceive that an image is being displayed, 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.

The at least one privacy light source may be located in various locations to provide illumination from the illuminated region, for example as follows.

One possibility is that the illuminated region is a surface and the at least one privacy light source is arranged to illuminate the illuminated region. In this case the illumination from the illuminated region is provided by reflection from the surface.

Another possibility, is that the at least one privacy light source is arranged in the illuminated region so as to provide said illumination as light output thereby.

With either of these possibilities, advantageously the privacy light source is not visible by the on-axis viewer.

The display device may be 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, the control system being capable of selectively operating the display device in the public mode or the privacy mode for at least one region of the display device. This provides for selective operation in the public mode or the privacy mode depending on the usage of the display device. By way of example, the privacy mode can be used in public places such as cafes or trains in order to enable the primary user to keep working but preventing onlookers or snoopers from being able to see or photograph data from the screen and the public mode can be used when discussing the contents on the screen with colleagues, for example within the corporate office.

The control system may be arranged to selectively operate the display device in the public mode or the privacy mode in response to the detected level of the ambient light.

According to a second aspect of the present disclosure, there is provided a method of controlling a display device that is arranged to display an image and is capable of providing a privacy function in which the visibility of the image to an off-axis viewer is reduced compared to the visibility of the image to an on-axis viewer; the method comprising providing at least one privacy light source arranged to provide external illumination from an illuminated region onto the display device along an incident direction for reflection from the display device to a predetermined viewer position at a polar angle of greater than <NUM>° to the normal to the display device; and controlling the luminous flux of at least one privacy light source when the privacy function is provided.

The second aspect of the present invention corresponds to a method of operation of the display apparatus in accordance with the first aspect of the present disclosure and provides similar advantages.

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.

Panel contrast ratio is given as: <MAT>
so the visual security level may be further given as: <MAT>
where: Ymax is the maximum luminance of the display; P is the off-axis relative luminance, typically defined as the ratio of luminance at the snooper angle to the maximum luminance Ymax; C is the image contrast ratio; ρ is the surface reflectivity; and I is the illuminance. The units of Ymax are the units of I divided by solid angle in units of steradian.

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 device. 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>° anticlockwise 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 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 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(q=<NUM>), for different background lighting conditions and for different observers.

From the above measurements S < <NUM> provides low or no visual security, <NUM> ≤ S < <NUM> provides visual security that is dependent on the contrast, spatial frequency and temporal frequency of image content, <NUM> ≤ S < <NUM> provides acceptable image invisibility (that is no image contrast is observable) for most images and most observers and S ≥ <NUM> provides full image invisibility, 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>. In the present discussion, for a given off-axis relative luminance, P the wide angle image visibility, W is approximated as <MAT>.

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.

The display apparatus <NUM> comprises privacy mode capable display device <NUM> and a 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.

Examples of suitable types of display device 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 display apparatus <NUM> has a primary ambient light sensor <NUM> that detects the illuminance level of ambient light. 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 primary 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. 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.

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 comprises 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 a typically a surface normal <NUM> to the display apparatus <NUM> is provided. Off-axis luminance is reduced. In other embodiments (not shown) the cone 402C may be tilted with respect to the surface normal <NUM>, such as for use in off-axis displays such as centre console mounted displays.

Ambient light sources <NUM> illuminate the display surface with light rays <NUM>. Reflected ambient light rays 610A provide a reflected region <NUM> from the display <NUM> that provides light rays <NUM> that contribute to increased visual security level, V as described elsewhere herein.

Some light rays <NUM> may be incident on the primary ambient light sensor <NUM>. The primary ambient light sensor <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.

<FIG> further illustrates privacy light source <NUM> that is arranged to provide light rays <NUM> that are reflected as light rays <NUM> after reflection from the privacy display <NUM>. In the present embodiments the privacy light source <NUM> is controlled by the display control <NUM> to adjust the luminance of reflected light rays <NUM>, <NUM> in correspondence to the display luminance in the cone 402C as will be described further herein.

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. Further the illuminance from the privacy light source <NUM> may be reduced or removed to advantageously achieve increased image visibility.

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, that may be the target snooper viewing polar locations <NUM>, 27R illustrated in <FIG> for example.

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 display of <FIG>, as an exemplary embodiment 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 for 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.

Variations <NUM>, <NUM> are illustrative profiles for public mode of operation and variations <NUM>, <NUM> are illustrative profiles for privacy mode of operation.

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> relating levels of luminance to detected levels of the ambient light in the public mode and in the privacy mode respectively. The transfer function <NUM> in the public mode relates higher levels of luminance to detected levels of the ambient light than the transfer function <NUM> in the privacy mode.

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.

As will be described further hereinbelow with respect to <FIG>, the illuminance levels may be varied by means of control of ambient light sources, the display luminance may be varied or the display luminance and ambient illuminance may be varied to achieve desirable image visibility to the primary user <NUM> and desirable visual security to the snooper <NUM> in a privacy mode of operation.

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 <NUM> 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 <NUM>/<NUM> 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 <NUM>/<NUM> 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 to select the transfer function to achieve desirable level of luminance.

<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 maximum 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 ambient light sensor <NUM> which may be different from the ambient light sensor 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 ambient light sensor <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.

Advantageously the profile control of <FIG> may be provided at low cost and complexity.

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 Viim 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 Viim. 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 Viim, 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 Ylim 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, primary 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.

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.

It may be desirable to provide enhanced visual security at low ambient illuminance levels, for example in an automotive vehicle at night.

<FIG> is a schematic diagram illustrating a look-down view of an automotive cabin operated in a low external light (dark) environment and a control system comprising in-vehicle illumination arranged to provide desirable visual security.

Vehicle <NUM> comprises a privacy display device <NUM> that is part of a privacy display apparatus <NUM> as described herein. In this example, the privacy display device <NUM> is located in front of the passenger <NUM> who is not the driver (the vehicle being left-hand drive in this example), and the driver being the snooper <NUM> in a privacy mode of operation. Thus, the passenger <NUM> is the on-axis viewer. The driver <NUM> is therefore an off-axis viewer. The privacy mode is selected when it is desired that displayed image is not visible to the driver <NUM>, typically including times when the vehicle is being driven so that the driver <NUM> is not distracted by the displayed image.

The display apparatus <NUM> also includes a dashboard privacy light source <NUM> and/or a door privacy light source <NUM> each of which provide illumination from an illuminated region <NUM>, as follows. In this example, illuminated region <NUM> is a surface of panel of a door of the vehicle.

The dashboard privacy light source <NUM> is mounted in the dashboard of the vehicle, so is remote from the illuminated region <NUM>, but is arranged to illuminate the illuminated region <NUM>. The illumination from the dashboard privacy light source <NUM> is reflected as light rays <NUM> and so illumination is provided from the illuminated region <NUM> by reflection. The dashboard privacy light source <NUM> may comprise one or more light emitting elements, and may be directional so as to direct light predominantly towards the illuminated region <NUM>. The dashboard privacy light source <NUM> is controllable to vary its luminous flux.

The door privacy light source <NUM> is mounted in the illuminated region <NUM>, i.e. the panel that is part of a door of the vehicle itself. Thus, illumination is provided from the illuminated region <NUM> by the light output directly by the second privacy light source <NUM>. The door privacy light source <NUM> may be a diffuse source. The door privacy light source <NUM> may comprise one or more light emitting elements. The door privacy light source <NUM> is controllable to vary its luminous flux.

The display apparatus <NUM> may include either one or both of the dashboard privacy light source <NUM> and the door privacy light source <NUM>, as convenient within the configuration of the vehicle. Whichever is provided, the dashboard privacy light source <NUM> and the door privacy light source <NUM> are located so that they are not visible by the on-axis viewer <NUM>.

The illumination region <NUM> is located so that it provides illumination of the display device <NUM> along the incident direction for reflection to the predetermined viewer position of the driver <NUM> who is the off-axis viewer in this example. The predetermined driver <NUM> position may be at an angle of <NUM> degrees to the normal to the display <NUM>. Alternatively, it may be desirable to provide a high security factor for a driver leaning towards the passenger <NUM> side with the intention to observe the display, such as an angle of less than <NUM> degrees, or less than <NUM> degrees.

Thus, the incident direction and the reflection from the display device <NUM> are both at a polar angle of greater than <NUM>° to the normal to the display device. In this example, the direction is set by the configuration of the vehicle, but in other examples the incident direction and reflection may be designed with reference to the location of an off-axis viewer who is desired not to view the displayed image. For example, the polar angle may be <NUM>°.

When the privacy function of the display device <NUM> is provided in the privacy mode, the control system <NUM> controls the luminous flux of the dashboard privacy light source <NUM> and the door privacy light source <NUM> so that illumination is provided from the illuminated region <NUM> onto the display device <NUM> along the incident direction. The light is reflected in the direction of the driver <NUM>. As a result, the illuminated region <NUM> provides suppression of the visibility of the displayed image from the display device <NUM> by providing an external illuminance. In general terms, the suppression occurs because the external illuminance reduces the contrast of the displayed image. This effect may similarly be recognised in the fact that the visual security level V shown in eqn. <NUM> is increased by increasing the illuminance I. By comparison, illumination from the illuminated region <NUM> is not reflected towards the passenger <NUM> who therefore perceives no degradation of the visibility of the image.

As described above, the display apparatus includes a primary ambient light sensor <NUM> and control system <NUM> uses the illuminance level of ambient light detected by the primary ambient light sensor <NUM> to control the luminance of the displayed image. Thus, in operation, the primary ambient light sensor <NUM> is used to determine a desirable image luminance for the passenger <NUM>. The primary ambient light sensor <NUM> is of a type that detects the illuminance level of ambient light incident on the display device in a non-directional manner. Thus, the primary ambient light sensor <NUM> measures general cabin illuminance by ambient illumination <NUM> for example.

Optionally, the display apparatus <NUM> also includes a directional ambient light sensor <NUM> of a type that detects the illuminance level of ambient light incident on the display device <NUM> along the incident direction for reflection to the predetermined viewer position of the driver <NUM> at a polar angle of greater than <NUM>° to the normal to the display device <NUM>. Thus the directional ambient light sensor <NUM> detects the illuminance level of light on the display device <NUM> from the illuminated region <NUM> of the interior of the vehicle. By way of example, the directional ambient light sensor <NUM> may be provided to collect light from illuminated region <NUM> only, over cone angle θa, by providing a capture lens <NUM> in the directional ambient light sensor <NUM>. By means of calibration, the illuminance of the display device <NUM> resulting from the illuminated region <NUM> may be determined from the output of the directional ambient light sensor <NUM>.

At low light levels, natural ambient lighting may fall and the luminance of reflected light seen by the driver <NUM> may be inadequate to achieve desirable visual security level. As described elsewhere herein, the luminance of the display device <NUM> may be reduced to achieve reduction in image visibility to the driver <NUM>. However image luminance may be reduced to a level that is undesirable for the passenger <NUM>. Thus, the control system <NUM> may use either the illuminance level of ambient light detected by the primary ambient light sensor <NUM>, or the illuminance level of ambient light detected by the directional ambient light sensor <NUM>, if provided, in order to selectively control the luminous flux of the dashboard privacy light source <NUM> and the door privacy light source <NUM>, for example as follows. In each case, a measure of the illuminance Iθ of light on the display device <NUM> along the incident direction is determined using the detected illuminance level of ambient light. Then, the luminous flux of the dashboard privacy light source <NUM> and the door privacy light source <NUM> is controlled on the basis of the derived measure.

In the case of using the directional ambient light sensor <NUM>, as this provides measurement of illuminance level of ambient light incident on the display device <NUM> along the incident direction from the illumination region <NUM>, the measure of the illuminance Iθ of light on the display device <NUM> along the incident direction derived by the control system <NUM> is the illuminance level of ambient light detected by the directional ambient light sensor <NUM>, due to its directional nature.

In the case of using the primary ambient light sensor <NUM>, as this detects the illuminance of light on the display device <NUM> in a non-directional manner but there is increased illuminance along the incident direction from the illuminated region <NUM>, the output of the primary ambient light sensor <NUM> does not directly provide a reliable measure of the illuminance Iθ of light on the display device <NUM> along the incident direction. However, the increased illuminance along the incident direction from the illuminated region <NUM> is dependent on the luminous flux that is output by the dashboard privacy light source <NUM> and the door privacy light source <NUM>. Accordingly, the control system derives the measure of the illuminance Iθ of light on the display device along the incident direction using both the illuminance level of ambient light detected by the primary ambient light sensor <NUM> and the luminous flux of the dashboard privacy light source <NUM> and the door privacy light source <NUM>.

The dependence of the measure of the illuminance Iθ along the incident direction on the illuminance level of ambient light detected by the primary ambient light sensor <NUM> and the luminous flux of the dashboard privacy light source <NUM> and the door privacy light source <NUM> may be derived for a given display apparatus <NUM> in a given situation by taking calibration measurements of the directional illuminance along the incidence direction and the output of the primary ambient light sensor <NUM> while varying the amount of ambient light and the luminous flux of the dashboard privacy light source <NUM> and the door privacy light source <NUM>.

The visual security level for the driver <NUM> is then determined from the measure of the illuminance Iθ of light on the display device <NUM> along the incident direction.

When the measure of the illuminance Iθ of light on the display device <NUM> along the incident direction falls below a threshold, the control system <NUM> is arranged to provide at least one of (i) reduction of display <NUM> luminance by means of control of display backlight and/or spatial light modulator transmission or emission and (ii) increase of illuminance of the display <NUM> from the illumination region <NUM> by means of control of the dashboard privacy light source <NUM> and the door privacy light source <NUM>.

In operation, as the naturally occurring ambient illuminance falls, for example at night time, the ambient light sensor <NUM> detects a reduced luminance from the illuminated region <NUM>.

The display luminance to the passenger <NUM> is adjusted to provide a comfortable image as the light level adjusts. However, such a comfortable image may provide undesirable perceptual image visibility to the driver <NUM>. To achieve desirable visual security level, the luminous flux of the dashboard privacy light source <NUM> and the door privacy light source <NUM> is controlled to provide an illuminance Iθ of light on the display device <NUM> along the incident direction to provide perceptual image security level S to the driver <NUM> that is <NUM> or more, preferably <NUM> or more and most preferably <NUM> or more. Specifically, this may be achieved by the control system <NUM> maintaining a relationship Iθ ≥ Iθmin where Iθ is the derived measure of the illuminance of light on the display device along the incident direction and Iθmin is given by the equation: <MAT> where Ymax is the maximum output luminance of the display device <NUM>, the units of Ymax being the units of Iθmin divided by solid angle in units of steradian, ρ(θ) is the reflectivity of the display device for light along the incident direction, P(θ) is the ratio of the luminance of the display device along the incident direction to the maximum output luminance of the display device <NUM>, and Smin has the desired value. <NUM> is derived from eqn. <NUM>, considering both the reflectivity ρ and the ratio (relative luminance) P with respect to the driver <NUM>.

Where Smin has a value of <NUM> or more, the driver <NUM> cannot perceive the displayed image.

Where Smin has a value of <NUM> or more, the driver <NUM> cannot even perceive that an image is being displayed, for most images and most observers.

Where Smin has a value of <NUM> or more, the image is invisible to the driver <NUM> independent of image content for all observers.

Advantageously, therefore, an image with desirable luminance may be provided to the passenger <NUM> and desirable perceptual image security may be provided to the driver <NUM>.

Further, the position of the driver <NUM> may be monitored by means of head detection sensor <NUM>. The portion <NUM> of the illuminated region <NUM> that provides illumination to the driver may be calculated from the head position of the driver <NUM> and only that portion <NUM> may be illuminated, for example by means of light source <NUM> in the portion <NUM>. Advantageously the size of the illuminated region <NUM> that is illuminated may be reduced and the total background light provided within the vehicle reduced.

<FIG> illustrates a flowchart of the method of controlling the privacy light sources <NUM>, <NUM> that is implemented by the privacy control system <NUM> of <FIG>.

In step S1, the illuminance level of ambient light is detected by the primary the ambient light sensor <NUM> and optionally also the directional ambient light sensor <NUM>, if provided.

In step S2, the luminance level of the display device <NUM> is set using the illuminance level of ambient light is detected by the primary the ambient light sensor <NUM>. This step is performed as described above to optimise the displayed image for the passenger <NUM>.

In step S3, the measure Iθ of the illuminance of light on the display device along the incident direction is derived. This is performed as described above, for example being the illuminance level of ambient light detected by the directional ambient light sensor <NUM>, if provided, or being derived from both the illuminance level of ambient light detected by the primary ambient light sensor <NUM> and the luminous flux of the privacy light sources <NUM>, <NUM>.

In step S4, which is optional, the location of driver <NUM> is measured by sensor <NUM> and a portion <NUM> of the illuminated region <NUM> that provides illumination to the driver <NUM> is determined. If step S4 is performed, then step S5 described below is carried out in respect of the incident direction from that portion <NUM> of the illuminated region <NUM>. Otherwise, the driver <NUM> is taken to be at a predetermined position known from the configuration of the vehicle, so step S5 described below is performed in respect of the incident direction corresponding to that position.

In step S5, the visual security level V in respect of the driver <NUM> and the corresponding incident direction is calculated using eqn. This calculation uses the maximum output luminance Ymax of the display device <NUM> and the measure Iθ of the illuminance of light on the display device <NUM> along the incident direction derived in step S3 as the illuminance I, because the driver <NUM> is being considered. This calculation also uses the reflectivity ρ(θ) of the display device <NUM> for light along the incident direction, and the luminance roll-off P(θ) of the display device <NUM> along the incident direction for reflection towards the driver <NUM>, that is the ratio of the luminance of the display device along the incident direction to the maximum output luminance of the display device <NUM>. The perceptual security level S is then derived from the visual security level V in accordance with eqn.

In step S6, it is determined whether the privacy light sources <NUM>, <NUM> are currently on. In the case of a positive determination in step S6, the lights remain on so the method proceeds to step S9. In the case of a negative determination in step S6 then the method proceeds to step S7.

In step S7 it is determined whether to turn on the privacy light sources <NUM>, <NUM>, based on a comparison of the perceptual security level S derived in step S5 with minimum level Smin. If the perceptual security level S derived in step S5 is above the minimum level Smin then the method proceeds to step S8 in which the privacy light sources <NUM>, <NUM> are turned on so as to increase the visual security, and thereafter the method proceeds to step S9. Otherwise, the method reverts to step S1, so the privacy light sources <NUM>, <NUM> remain off.

As described above, the minimum level Smin of the perceptual image security level S for the driver <NUM> may be selected to have a value of <NUM> or more in order to achieve the effect that the driver <NUM> cannot perceive the displayed image, a value of <NUM> or more in order to achieve the effect that the driver <NUM> cannot even perceive that an image is being displayed, for most images and most observers, or a value of <NUM> or more in order to achieve the effect that the image is invisible to the driver <NUM> independent of image content for all observers.

In step S9, the luminous flux of the privacy light sources <NUM>, <NUM> is adjusted based on the perceptual security level S derived in step S5. If the privacy light sources <NUM>, <NUM> are currently off, then they are turned on. If the privacy light sources <NUM>, <NUM> are currently turned on then their luminous flux is adjusted based on the difference between the perceptual security level S and the minimum level Smin. If the perceptual image security S is less than the minimum level Smin, then the luminous flux of the privacy light sources <NUM>, <NUM> is increased. Conversely, if the perceptual image security S is more than the minimum security level Smin, then the luminous flux of the privacy light sources <NUM>, <NUM> is decreased.

This control may be implemented by a feedback loop using the measure of the illuminance Iθ along the incident direction as a feedback parameter. In particular, the control system <NUM> may adjust the luminous flux of the privacy light sources <NUM>, <NUM> so as to a relationship Iθ ≥ Iθmin where Iθ is given by eqn. <NUM> above. In this manner, the perceptual security level S may be maintained at or above the minimum level Smin.

Advantageously passenger <NUM> may see a comfortable image and driver <NUM> may see an image with low or no distraction in a wide range of ambient lighting conditions.

While the example of <FIG> is a display apparatus <NUM> in a vehicle <NUM>, similar techniques may be applied to a display apparatus <NUM> to be used in any environment where it is desired to limit the visibility of an image to an off-axis viewer. By way of example, there will now be described an example where the display apparatus <NUM> is arranged to increase image security for snoopers in an office environment.

<FIG> is a schematic diagram illustrating a look-down view of a privacy display apparatus <NUM> intended for desktop use and including privacy light sources light sources <NUM> arranged to increase image security for off-axis snoopers in response to ambient lighting conditions. 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.

The privacy display apparatus <NUM> is provided in laptop computer <NUM> which is shown situated on the surface of a desk <NUM> for a seated user <NUM> who is the on-axis viewer. The off-axis viewer is a standing snooper <NUM> who sees a reflection from at least part of the display device <NUM>. The display apparatus <NUM> includes privacy light sources <NUM> that are arranged in (or near) to the laptop computer <NUM> to illuminate an illuminated region <NUM> of the desk <NUM>. This corresponds to the illuminated region <NUM> in the example of <FIG> and is seen reflected in the display device <NUM> by the snooper <NUM>. The display apparatus <NUM> includes a primary ambient light sensor <NUM> and a directional light sensor <NUM> and operates in the same manner, and to provide the same effect, as in the example of <FIG> described above.

<FIG> is a schematic diagram illustrating a front view of a privacy display <NUM> and light sources <NUM>; and <FIG> is a schematic diagram illustrating a top view of the privacy display <NUM> of <FIG>. 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 comparison to the embodiment of <FIG>, the alternative of <FIG> illustrate that the light sources <NUM> may be arranged to illuminate the front surface of the display <NUM> to provide reflected light <NUM> to the snooper <NUM> that may be controlled as described elsewhere herein. Advantageously the security factor of the display <NUM> may be increased in environments where other sources of ambient light are limited or absent.

There will now be described some possible arrangements for the directional ambient light sensor <NUM> that measure ambient illuminance in directions that correspond to locations in which ambient light reflectivity contributes to visual security for an off-axis viewer.

<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>.

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 a 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 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.

As may be used herein, the terms "substantially" and "approximately" provide an industry-accepted tolerance for its corresponding term and/or relativity between items. Such an industry-accepted tolerance ranges from zero percent to ten percent and corresponds to, but is not limited to, component values, angles, et cetera. Such relativity between items ranges between approximately zero percent to ten percent.

Claim 1:
A privacy display apparatus (<NUM>) comprising:
a display device (<NUM>) arranged to display an image, the display device (<NUM>) being capable of providing a privacy function in which the visibility of the image to an off-axis viewer is reduced compared to the visibility of the image to an on-axis viewer; and
a control system (<NUM>) arranged to control the display device (<NUM>),
characterised in that:
the privacy display apparatus further comprises at least one privacy light source (<NUM>) arranged to provide external illumination from an illuminated region (<NUM>, <NUM>) onto the display device (<NUM>) along an incident direction for reflection from the display device (<NUM>) to a predetermined viewer position at a polar angle of greater than <NUM>° to the normal to the display device (<NUM>); and
the control system (<NUM>) is arranged to control the luminous flux of the at least one privacy light source (<NUM>) when the privacy function is provided.