TECHNICAL FIELD

This disclosure generally relates to illumination from light modulation devices, and more specifically relates to control of privacy display.

BACKGROUND

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 U.S. Pat. No. 9,519,153, which is incorporated by reference herein in its entirety.

In a known privacy display the privacy mode is provided by the addition of a removable louver film, such as marketed by 3M 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.

BRIEF SUMMARY

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; a control system arranged to control the display device; and an ambient light sensor arranged to detect the illuminance level of ambient light, wherein the control system is arranged to control luminance of the displayed image on the basis of the detected level of the ambient light in accordance with a transfer function, and when the privacy function is provided the transfer function maintains a relationship Ymax≤Yupper, where Ymaxis the maximum output luminance of the display device and Yupperis given by the equation:

Yupper=(ρ⁡(θ=4⁢5∘)/P⁡(θ=45⁢°))·I(1⁢0Smin-1)·π

where the equation for Yupperapplies for an observation direction having a polar angle θ of 45° from the normal to the display device at at least one azimuth angle around the normal to the display device, I is the detected level of the ambient light, the units of I being the units of Ymaxmultiplied by solid angle in units of steradian, ρ(θ=45°) is the reflectivity of the display device along the observation direction, P(θ=45°) is the ratio of the luminance of the display device along the observation direction to the maximum output luminance Ymaxof the display device, and Sminhas a value of 1.0 or more.

Control of the luminance of the displayed image on the basis of the detected level of the ambient light is desirable to optimise the visibility of the displayed image to an on-axis viewer. By performing such control in accordance with a transfer function that maintains the relationship Ymax≤Yupperwhen the privacy function is provided, the visual security level of operation of the display device may be maintained at or below the limit Sminin the observation direction even as the illuminance level of ambient light and the luminance of the display device are varied. By maintaining the visual security level at or below the limit Smin, an off-axis viewer in that observation direction cannot in practical terms perceive the displayed image.

Advantageously, Sminmay have a value of 1.5 or more. Such an increased limit of Sminachieves a higher level of visual security in which the image in invisible to an off-axis viewer, i.e. the viewer cannot perceive even that an image is being displayed, for most images and most observers.

Advantageously, Sminmay have a value of 1.8 or more. Such an increased limit of Sminachieves a higher level of visual security in which the image is invisible independent of image content for all observers.

Where the display device has a major axis and a minor axis of symmetry, the equation for Ymaxmay apply for an observation direction having a polar angle θ of 45° from the normal to the display device at an azimuth angle corresponding to either or both of the major axes in order to achieve the advantages with respect to an off-axis viewer with use of the display device in a landscape orientation, or the minor axis in order to achieve the advantages with respect to an off-axis viewer with use of the display device in a portrait orientation.

Advantageously, the control system may be arranged to control luminance of the displayed image when the privacy function is provided on the basis of the detected level of the ambient light in accordance with a transfer function that maintains a relationship Ymax≥Ylower, where Yloweris given b the equation

Ylower=(ρ⁡(Δθ=10⁢°)/P⁡(Δθ=10⁢°))·I(1⁢0Smax-1)·π

where the equation for Ylowerapplies for an observation direction having a polar angle θ of 10° from the direction of the maximum output luminance of the display device at at least one azimuth angle around the direction of the maximum output luminance of the display device, ρ(Δθ=10°) is the reflectivity of the display device along the observation direction having a polar angle θ of 10° from the direction of the maximum output luminance of the display device, P(Δθ=10°) is the ratio of the luminance of the display device along the observation direction having a polar angle θ of 10° from the direction of the maximum output luminance of the display device to the maximum output luminance Ymaxof the display device, and Smaxhas a value of 0.1 or less.

By further controlling the luminance of the displayed image when the privacy function is provided in accordance with a transfer function that maintains a relationship Ymax≥Ylower, the visibility of the displayed image to an on-axis viewer is maintained.

In some cases, the ambient light sensor may detect the illuminance level of ambient light incident on the display device in a non-directional manner. In such cases, the detected level I represents an average level, so that the effect of the present technique is achieved for off-axis viewers in varied locations.

In other cases, the ambient light sensor may detect the illuminance level of ambient light incident on the display device along an incident direction for reflection to the observation direction. In such cases, the effect of the present technique may be optimised specifically for an off-axis viewer in the observation direction.

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.

In cases where the display device comprises a backlight and a transmissive spatial light modulator arranged to receive light from the backlight, the control system may be arranged to control luminance of the displayed image by controlling the luminance of the backlight and/or by controlling transmission of light by the spatial light modulator.

In cases where the display device comprises an emissive spatial light modulator, the control system may be arranged to control luminance of the displayed image by controlling of emission of light by the spatial light modulator.

According to a second aspect of the present disclosure, there is provided a display apparatus comprising: a display device arranged to display an image and capable of operating in at least a public mode and a privacy mode, wherein the visibility of the image to an off-axis viewer is reduced in the privacy mode compared to the public mode; and the visibility of the image on-axis is maintained in the privacy mode; and a control system arranged to control the display device, the control system being capable of selectively operating the display device in the public mode or the privacy mode comprise an ambient light sensor arranged to detect the level of the ambient light.

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. The public mode can be used when discussing the contents on the screen with colleagues, for example within the corporate office.

Advantageously the operation of the privacy mode may be independent of user preference such that organisations may be provided with control of exposure of private data when used in a public place without the consent of the user.

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.

The control system may be arranged to selectively control any one or more of luminance, contrast, white point and spatial frequency of the image in the privacy mode in response to the detected level of the ambient light. The control system may be arranged to selectively control any one or more of luminance, contrast, white point and spatial frequency of the image in the public mode in response to the detected level of the ambient light. Advantageously the visual security level of operation of the display may be optimised for the illuminated environment. Further image appearance to the primary user during privacy display operation may be increased in response to the type of private image displayed.

The control system may be arranged to selectively control the display device to display information representing the visibility of the image to an off-axis viewer taking into account the detected level of the ambient light. Advantageously a display user may be provided with information regarding the visual security of the environment to achieve reliable decision making to view confidential data.

The control system may be arranged to control the display device to display information representing changes to user-controllable parameters capable of reducing the visibility of the image to an off-axis viewer. Advantageously the visual security level may be reduced while achieving comfortable viewing of image data for a primary user that is dependent on the user preference of image viewing.

The control system may be 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, using different transfer functions relating levels of luminance to detected levels of the ambient light in the public mode and in the privacy mode.

The transfer function in the public mode may relate higher levels of luminance to detected levels of the ambient light than the transfer function in the privacy mode. Advantageously a high visual security level may be provided in privacy mode to snoopers for a wide range of ambient light conditions and a high image visibility may be provided in public mode to users for a wide range of ambient light conditions.

The control system may be arranged to selectively control luminance of the displayed image in the privacy mode in response to the detected level of the ambient light, in accordance with a transfer function that maintains a relationship Ymax/I≥1 lux/nit, where Ymaxis the maximum output luminance of the display device measured in nits and I is the detected ambient light level measured in lux. Advantageously high visual security level is observed for varying ambient light levels and desirable image visibility is provided to the display user.

The control system may be arranged to selectively control luminance of the displayed image in the privacy mode in response to the detected level of the ambient light, in accordance with a transfer function that maintains a relationship Ymax≤Ylim, where Ylimis given by the equation:

Ylim=R⁢⁢θ+Vlim*K⁢⁢θ(Vlim-1)*P⁢θ
where Vlimhas a value of 10, Rθ is the reflected ambient illuminance at an observation angle θ of 45 degrees laterally and zero degrees in elevation from the normal to the display device, 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.

The control system may be arranged to control the display device to display information representing the visibility of the image to an off-axis viewer. Advantageously the visual security level of the display device may be optimised.

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

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

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

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

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

