Optical: systems and elements – Single channel simultaneously to or from plural channels – By partial reflection at beam splitting or combining surface
Reexamination Certificate
2000-09-28
2001-12-11
Mack, Ricky (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By partial reflection at beam splitting or combining surface
C348S337000, C348S338000
Reexamination Certificate
active
06330113
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a color-separation prism assembly, and more particularly relates to a color-separator prism with adjustable path lengths.
2. The Prior Art
A number of color-separation prism designs have been in widespread use. A well-known design is the so-called “Philips” prism assembly. A lateral cross-section of a prior art Philips prism assembly is shown in
FIG. 1. A
Philips prism assembly is formed of three individual prisms, a first prism
10
, a second prism
12
and a third prism
14
, each of which has certain flat, optically polished surfaces. First prism
10
has optical surfaces
16
,
18
and
20
. Second prism
12
has optical surfaces
22
,
24
and
26
. Third prism
14
has optical surfaces
28
and
30
. In addition to their optical finish, surfaces
18
and
22
(or
28
) have multi-layer dichroic coatings that cause them to have graded reflection versus wavelength to make filter functions that are approximately color matching functions. Some unwanted far-off-axis rays may find one-bounce or three-bounce paths to the exit surface of the first prism
10
. A flare-stop notch
32
is provided in first prism
10
to block such unwanted light paths that would otherwise cause flare without blocking desired ray paths, as is readily understood by persons of ordinary skill in the art.
The Philips prism assembly operates in the following manner: a ray
40
of light from the incoming image enters first prism
10
at normal incidence through surface
16
. Most of the blue light in the ray (wavelengths shorter than about 500 nanometers) is reflected to form ray
42
, while the remainder of the light is transmitted to form ray
44
. Ray
42
is totally internally reflected from surface
16
to form ray
46
, which passes through surface
20
at normal incidence to form the blue output of the assembly. Most of the red light in the ray
44
(wavelengths greater than about 580 nanometers) is reflected off surface
22
to form ray
48
, which is totally internally reflected from surface
24
to form ray
50
, which passes through surface
26
at normal incidence to form the red output of the assembly. The remainder of the light passes into third prism
14
to form ray
52
, which passes through surface
30
at normal incidence to form the green output of the assembly. Other rays entering at non-normal incidence follow corresponding nearby paths.
It is understood that the wavelengths quoted are typical of industrial practice, and can vary somewhat in different applications, as is well understood by those of ordinary skill in the art. Such skilled persons are aware that the crossover wavelength (the wavelength of transition between transmission and reflection) of a typical dichroic coating is insensitive to polarization at normal incidence (0 degree angle-of-incidence, ray perpendicular to surface), but becomes increasingly sensitive to polarization as the angle of incidence is increased. A first advantage of the Philips prism assembly is based on this fact. Both surface
18
and surface
22
are operating at less than 30° angle of incidence. For this reason, the crossover wavelength for polarizations in and perpendicular to the page are nearly equal, and excellent color separation results. A second advantage of the Philips prism assembly is that the green ray
52
exits the assembly having encountered no reflections, while red ray
50
and blue ray
44
exit the assembly having encountered two reflections so that none of the exiting rays are mirrored by having an odd number of reflections. Another advantage of the Philips prism assembly is that the individual prisms can be slid relative to each other to adjust the lengths of the three optical paths to be the same.
The disadvantages of a Philips assembly stem from the total internal reflection of ray
48
on surface
24
to form ray
50
. The requirement of total internal reflection from surface
24
requires an air gap
54
between the two surfaces. This air gap
54
must be fairly uniform and securely held, and must be protected from moisture accumulation. The two-reflection paths of rays
44
and
50
also lengthen the optical length to over 2 times the width of an exit face, which requires the use of lenses with a longer back working distance.
Additionally, the third prism
14
of the Philips prism assembly has limited slide movement along the second prism
12
toward the first prism
10
, since the surface
18
of the first prism
10
creates an obstruction. In order to increase the adjustable optical path length to the output faces for the prism assembly, the third prism
16
needs to be able to traverse further across the second prism
14
toward the first prism
10
than is possible in the prior art.
BRIEF DESCRIPTION OF THE INVENTION
A color-separation prism assembly according to the present invention comprises first, second and third prisms. The first and second prisms have entrance surfaces, exit surfaces, and partially-reflecting surfaces. The entrance surface of the second prism is separated by an air gap from the partially-reflecting surface of the first prism. The third prism has an entrance surface and an exit surface. The entrance surface of the third prism is adjacent to the partially-reflecting surface of the second prism.
The first prism has a cut-out serving as a flare-stop, and providing relief so as to allow the entrance surface of the third prism to slide across a plane disposed over the cutout into a volume defined by the cut-out. The plane is formed as an extension of the partially-reflecting surface of the first prism. An optical axis passes through the first, second and third prisms. The optical axis passes at normal angles through the entrance surface of the first prism, and the exit surfaces of the first, second and third prisms. The partially reflecting surfaces each have angles-of-incidence of less than 30° relative to the optical axis.
The modified Philips prism assembly of the present invention allows the third prism greater slide movement thereby increasing the ability of the prism assembly to adjust the optical path lengths to the output faces.
REFERENCES:
patent: 5134468 (1992-07-01), Ohmuro
patent: 5777674 (1998-07-01), Ohmuro
patent: 6238051 (2001-05-01), Huang
patent: 02000266915A (2000-09-01), None
Lyon Richard F.
Ruda Mitchell C.
Slagle Timothy M.
Stuhlinger Tilman W.
Foveon, Inc.
Mack Ricky
Sierra Patent Group Ltd.
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