Optical: systems and elements – Projection screen
Reexamination Certificate
1999-03-19
2002-04-30
Metjahic, Safet (Department: 2858)
Optical: systems and elements
Projection screen
Reexamination Certificate
active
06381068
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a reflection-type projection screens and projection systems using reflection-type projection screens. More particularly, the present invention provides projection screens including a reflective polarizing element in combination with a diffusing element and/or glare suppressing element.
BACKGROUND OF THE INVENTION
Conventional projection screens for use with, e.g., overhead projectors, slide projectors, and liquid crystal projectors, typically include transparent or translucent porous fine particles held in a transparent medium and a reflective material located behind the particles. The projection screens reflect substantially all of the light incident on them, i.e., they reflect ambient light, as well as light from the imaging source. Because a portion of the ambient light is reflected towards the viewers, the image contrast and/or the apparent brightness of the image is often reduced, particularly in areas with relatively high levels of ambient light.
To enhance the brightness of the reflected image, some projection screens include retroreflective elements such as glass beads, etc. to retroreflect the ambient light back in the direction from which it approached the screen. The addition of retroreflective elements, however, narrows the range of angles over which the image can be viewed because the imaging light is also retroreflected. Furthermore, if the source of ambient light is aligned with the viewers, the ambient light can also be retroreflected towards the viewers along with the image.
The brightness of images produced by liquid crystal projectors, in particular, can be relatively low because light of only one polarization state is projected onto the screen due to the nature of the liquid crystal displays used to form the images. When the projection screen reflects ambient light along with the reduced brightness projected image, the image contrast can be significantly reduced. As a result, liquid crystal projectors are used primarily in areas with low levels of ambient light, such as rooms in which the windows are covered with curtains and/or in which artificial lighting is dimmed, to limit the contrast reducing effects of the ambient light. That is undesirable, however, because it can impair the ability of observers in the room to consult written materials, take notes, etc. during presentations.
Attempts to address the brightness and contrast problems associated with liquid crystal projectors have included the use of absorptive polarizers in combination with reflective materials. By incorporating absorbing polarizers in the screens, about one-half of the ambient light can be absorbed by the projection screen rather than reflected as in conventional screens that do not include absorptive polarizing materials.
The absorptive polarizing materials used in the projection screens preferentially allow the transmission of light with a first polarization state and block the transmission of light with a second polarization state. The transmitted light is then reflected back through the absorptive polarizing material by the reflective material. Because the liquid crystal projectors use light of only one polarization state to form images, that light is preferentially reflected by the projection screens. Ambient light, however, typically includes light having both polarization states and, therefore, a significant portion of the ambient light incident on the projection screens is absorbed rather than reflected. As a result, the contrast and apparent brightness of the images formed by the liquid crystal projectors on projection screens including absorptive polarizing materials can be improved as compared to conventional projection screens that reflect light of both polarization states.
Although ideal absorptive polarizing materials transmit all of the incident light of the first polarization state and absorb all of the incident light of the second polarization orientation, actual absorptive polarizing materials absorb at least some of the incident light with the first polarization state along with absorbing light of the second polarization state. As a result, some of the image light is absorbed rather than reflected, thereby reducing image contrast and brightness. Furthermore, in projection screens using absorptive polarizing materials, the absorptive polarizing material is located in front of the reflector. Because of that arrangement, incident light and the imaging light having the preferentially transmitted first polarization state must pass through the absorptive material two times before reaching the viewer. In each of those passes, the absorptive polarizing material can absorb a significant portion of the light with the first polarization state, thereby reducing image brightness.
In addition to the above problems, projection screens with absorptive polarizers that include other elements such as diffusing materials, etc. may also suffer from reduced image brightness and/or contrast if those additional elements cause some of the image light to change polarization states. The portion of the image light that changes to the polarization state absorbed by the absorptive polarizing materials is not available for the viewer. The result is reduced image brightness and/or contrast.
SUMMARY OF THE INVENTION
The present invention provides a reflective front projection screen capable of projecting an image with enhanced contrast and a wide viewing angle in the presence of relatively high levels of ambient light, and a projection system using this screen. The projection screens provide the desired combination of effects by using a reflective polarizing element in combination with a diffusing element and/or glare suppressing element. The reflective polarizing element transmits light of one polarization state and reflects light of a different polarization state.
The reflective polarizing element may be diffusely reflective or specularly reflective. The optical properties of the diffusing element and/or glare suppressing element may be selected based on the optical properties of the reflective polarizing element to further enhance image brightness and contrast.
The projection systems of the present invention preferably project an image using light of the polarization state that is reflected by the reflective polarizing elements in the screens to further enhance brightness and contrast of the image.
The front projection screens of the present invention preferably exhibit improved optical gain as compared to known screens over a desired range of viewing angles. As used herein, “optical gain” of a front projection screen is determined as the ratio of measured screen luminance at a given angle divided by the expected luminance of an ideal Lambertian screen at that angle, where the incident light is directed at the screen along a normal axis. An ideal Lambertian screen diffusely reflects light uniformly in all directions from zero to 90 degrees from the normal axis.
In one aspect, the present invention provides a reflective front projection screen having a front surface facing a viewer, the screen including a specular reflective polarizing element substantially reflecting light having a first polarization state and substantially transmitting light having a second polarization state; and a diffusing element scattering the light having the first polarization state that is reflected by the reflective polarizing element, the diffusing element located between the reflective polarizing element and the front surface of the screen.
In another aspect, the present invention provides a reflective front projection screen having a front surface facing a viewer, the screen including a diffuse reflective polarizing element substantially reflecting light having a first polarization state and substantially transmitting light having a second polarization state; and a glare suppressing element suppressing specular glare from the diffuse reflective polarizing element, the glare suppressing element located between the diffuse reflective polarizing element and the f
Bruzzone Charles L.
Harada Takashi
Ishikawa Makoto
Moshrefzadeh Robert S.
Nevitt Timothy J.
3M Innovative Properties Company
LeRoux Etienne
Miller William D.
Raasch Kevin R.
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