Optical: systems and elements – Projection screen – With reflector or additional screen
Reexamination Certificate
2001-03-22
2003-03-25
Adams, Russell (Department: 2851)
Optical: systems and elements
Projection screen
With reflector or additional screen
C359S443000, C359S452000, C359S459000, C349S088000
Reexamination Certificate
active
06538814
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a screen for reflecting a projected image, in particular to an active screen for reflecting a projected image having improved contrast, and to a projection system incorporating such a screen.
When an image is to be viewed by a group of people, it is common to magnify the image by projecting it with a projector onto a screen having a diffusely reflective surface. Screens for indoor use typically have an area of a few square meters, and screens for outdoor use may have an area of 10 m
2
or more.
However, ambient light reflected by the screen can cause the image to suffer from poor contrast, requiring a more powerful projector to be used. Although the amount of ambient light can be reduced by viewing the image in a darkened room, this is not always possible and can be inconvenient.
It is known to limit the amount of ambient light reflected towards a viewer by using a reflective material whose reflectance decreases at angles of incidence below about 45 degrees. However, this may result in an undesirable reduction in the range of satisfactory viewing angles. Also, some ambient light will come from the same direction as the projected light, limiting the effectiveness of this approach.
To achieve a high contrast, plasma addressed liquid crystal displays and field emission displays have been used, but these can be expensive when used to form a display whose area is of the order of 1 m
2
or above.
SUMMARY OF THE INVENTION
According to one aspect of the invention, there is provided a projection screen for reflecting from a first side a projected image with enhanced contrast, comprising: an active layer whose reflectance can be changed under the influence of an electric field; at least a first electrode for applying an electric field across the active layer; and a photosensitive material whose electrical or dielectric properties can be changed under illumination, wherein the photosensitive material is arranged such that the electrical field across local regions of the active layer is dependent on the illumination incident upon the photosensitive material in each such local region with the result that reflectance of the active layer is locally dependent upon an intensity of light incident upon the first side of the projection screen in that local region.
Preferably, the photosensitive material is a photoconductor adapted to control current through the first electrode. In another advantageous arrangement, there is also a second electrode, whereby the electric field across the active region is applied between the first electrode and the second electrode. In this case, the photosensitive material may be a photoconductor adapted to control current through either electrode.
One of the electrodes may be situated between the first side of the projection screen and the photosensitive material, in which case this electrode is preferably transparent. It will be understood that the term transparent includes the possibility that some absorption will occur.
Because the optical properties of the screen can be changed in localised areas without changing the optical properties of the whole screen, an image with improved contrast and sharpness can be obtained.
Since the screen has a layer structure, it can conveniently be made flat over a large area and mounted vertically, for example on a wall, so that it can be viewed by a group of people.
Since the projection screen does not generate light, it will have a lower power consumption than emissive displays such as field emission displays or cathode ray tubes, and will be more suitable for displaying an image over a large area.
The local reflectance of the screen may be a function of the light incident upon it, in which case an image will be projected onto the front of the screen so that a reflected image can be viewed. In one embodiment, the local reflectance of the screen at a point will increase when the intensity of light incident on the screen at that point is increased. This will cause the screen to reflect more light in the bright areas of a projected image, thereby improving the contrast of the reflected image relative to the contrast which would be obtained with a screen having a uniform reflectance.
An active layer will preferably be used whose reflectance can be changed under the influence of an electric field, such that the active layer can be in a reflective state in which it reflects light, or in a less reflective state in which it reflects less or no light. In those regions of screen where the active layer is in the reflective state, visible light incident on the front of the screen will be reflected, so as to form an image that can be viewed. The active layer may be a normal mode active layer which is in a non reflective state when an electric field above a threshold value is applied across it and in a reflective state when the electric field applied across it is below the threshold value. However, a reverse mode active layer may be used which is in a reflective state only when an electric field above a threshold value is applied.
It will be understood that the bulk and/or interface optical properties of the active layer may be responsible for the changes in reflectance. Preferably, the reflectance of the active layer will be due to light scattering within the bulk of the active layer, and scattering will be reduced when the active layer is in the non reflective state. The active layer may comprise a liquid crystal material. Preferably, the active layer will comprise regions of liquid crystal material or other optically active material embedded in a structural matrix. In a preferred embodiment, the active layer will be a Polymer Dispersed Liquid Crystal material, comprising liquid crystal micro droplets or pockets embedded in a polymer matrix.
Preferably, the active layer will be an electrically insulating layer so that an electric field can be applied across it when desired. However, in one embodiment, the active layer comprises a transparent photoconductor material.
The threshold electric field value at which the active layer makes a transition from a reflective state to a non reflective state will preferably be well defined, in order to selectively switch off the reflectance of the screen in regions where the incident light is below a pre-set value. However, it will be appreciated that the transition will in practice occur within a range of electric field values if for example the microdroplets are not of a uniform size, or there are other inhomogeneities in the active layer.
Preferably, the pre-set level of incident light intensity at which the active layer changes state will be chosen such that it is above the ambient light level. In the dark areas of the projected image where only ambient light is incident on the screen, the screen will remain dark, thereby improving the contrast of the image. Preferably, a potentiometer or other adjusting means will be provided to adjust the threshold light level at which the active layer changes state, so that the screen can provide good contrast under a range of ambient light conditions. The adjusting means may be manual, or alternatively automatic adjusting means may be provided, comprising an electronic circuit having a light sensor, for example.
The active layer may be transparent when it is in the non reflective state if the screen is used to display a reflected image, in which case an absorbing surface will preferably be located behind the active layer. This will cause the screen to have a dark appearance in the dark areas of the image incident on the screen.
The photosensitive material will preferably be a photoconductor that only conducts under illumination within a predetermined frequency range, at typical operating temperatures such as room temperature. However, a material with a light dependent dielectric constant may be used in the case where current is not required to pass through the photosensitive material.
The photosensitive material may be in the form of a layer situated adjacent to the active layer, between the first and second
Hunter Andrew Arthur
Rudin John Christopher
Adams Russell
Cruz Magda
Hewlett--Packard Company
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