Producing colored light beams from white light

Details Producing colored light beams from white light Producing colored light beams from white light

1999-01-28

2001-05-29

Dowling, William (Department: 2851)

Optics: image projectors

Prism in light path

C353S033000, C359S634000

Reexamination Certificate

active

06238051

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to optical devices and prisms, and, more particularly, to separating a white light beam into several colored light beams.
2. Description of the Related Art
FIG. 1
illustrates a color-separating prism based on a cross cube
10
. The cross cube
10
has a square cross section and is composed of four glass prisms
12
,
14
,
16
,
18
. A first reflective layer
20
lies along a first principal diagonal of the cross cube
10
. A second reflective layer
22
lies along a second principal diagonal, and intersects the first layer
20
at a right angle. The first and second reflective layers
20
,
22
are multi-layers of dichroic material that selectively reflect certain wavelength or color ranges of light.
Referring to
FIG. 1
, a white light beam
24
enters the cross cube
10
through a first face
26
and is selectively reflected by the first and second reflective layers
20
,
22
. The first reflective layer
20
reflects red light
28
from the incoming white light beam
24
through a second face
30
of the cross cube
10
. The second reflective layer
22
reflects blue light
32
from the incoming white light beam
24
through a third face
34
of the cross cube
10
. Green light is not substantially reflected by either the first or second reflective layers
20
,
22
. Therefore, the green light
36
from the incoming white light beam
24
is transmitted through a fourth face
38
without substantial deviation. The cross cube
10
, therefore, separates the incoming white light beam
24
into separate red, blue and green light beams
28
,
36
,
32
, respectively, going in different directions.
FIG. 2
illustrates an exemplary percent reflectivity of the first and second reflective layers
20
,
22
of the cross cube
10
in
FIG. 1
as a function of wavelength in nanometers (nm). A solid line
39
shows exemplary values for the reflectivity of the first reflective layer
20
. The first layer
20
reflects substantially all visible red light and some infrared light at wavelengths greater than about 625 nm. At wavelengths below about 600 nm, the first reflective layer
20
is substantially transparent to visible light. A broken line
40
shows exemplary values for the reflectivity of the second reflective layer
22
. The second reflective layer
22
reflects substantially all visible blue light below wavelengths of about 460 nm. Above a wavelength of about 460 nm the second reflective layer
22
is substantially transparent to visible light. The reflectivities of the different dichroic materials making up the first and second layers
20
,
22
, give the cross cube
10
in
FIG. 1
its color-separating properties.
FIG. 3
illustrates a second color-separating prism that is generally referred to as a Philips prism
42
. The glass elements of the Philips prism
42
include first and second component prisms
44
and
46
, and a cover element
48
. A first reflective layer
50
is deposited on a back surface
52
of the first component prism
44
. A second reflective layer
54
is deposited between a back surface
56
of the second component prism
46
and the cover element
48
. Mountings
58
,
60
rigidly position the first and second component prisms
44
,
46
with respect to each other so that an air gap
62
exists between the first reflective layer
50
and a front surface
66
of the second component prism
46
. The Philips prism
42
separates an incoming white light beam
68
into color components, because the first and second reflective layers
50
,
54
selectively reflect blue and red light, respectively. The order of light (e.g., red first, then blue or vice versa) is changeable by changing the layers
50
and
54
.
The advantage of the Philips prism
42
over the cross cube
10
is that the incident angles of an incoming light ray to the prism interfaces are less steep. Thin-film coaters can optimize the coatings to get better performance than in the cross cube configuration. Referring to
FIG. 3
, the incoming white light beam
68
passes through a front surface
70
of the first component prism
44
. The first reflective layer
50
is constructed of layered dichroic material (e.g., thin film coating) as is the second reflective layer
22
in FIG.
1
. An incoming ray of blue light
72
is reflected by the first reflective layer
50
back towards the front surface
70
of the first component prism
44
. If the blue light
72
(e.g., a chief ray of a core of light) is substantially perpendicularly incident on the front surface
70
, reflected blue light
74
is re-incident on the front surface
70
at an angle that is greater than the critical angle for total internal reflection. Then, the reflected blue light
74
is reflected by the front surface
70
as blue light
92
toward a third surface
76
of the first component prism
44
. An incoming ray of red light
78
passes through the first reflective layer
50
without being substantially reflected. The ray of red light
78
is, however, reflected by the second reflective layer
54
. If the ray of red light
78
(e.g., collimated light or a chief ray of a core of light) is substantially perpendicularly incident on the front surface
70
of the first component prism
44
, a reflected ray of red light
80
is re-incident on the first surface
66
of the second component prism
46
at an angle that is greater than the angle for total internal reflection. Then, the reflected ray of red light
80
is reflected as red light
88
toward a second surface
82
of the second component prism
46
. An incoming ray of green light
84
incident on the front surface
70
of the first component prism
44
passes through the first and second reflective layers
50
,
54
substantially undeviated. The ray of green light
84
is transmitted through a back surface
86
of the cover element
48
as green light
90
. The Philips prism
42
thus separates the incoming white light beam
68
into the red light
88
, the green light
90
and the blue light
92
, all traveling in different directions.
Referring to
FIGS. 1 and 3
, the cross cube
10
and the Philips prism
42
, respectively, have several inconvenient properties. In the cross cube
10
, the first and second layers
20
,
22
make 45° angles with respect to the first surface
26
. The 45° arrangement simplifies the construction of the cross cube
10
, but may make the cross prism
10
inconveniently thick. Also, the reflectivities and transmissivities of the first and second layers
20
,
22
may differ for the two polarizations of the incoming white light beam
24
, because the light beam
24
is not perpendicularly incident on the first and second reflective layers
20
,
22
. The reflectivities are often polarization-dependent for non-perpendicular incidence. Further, though the Philips prism
42
in
FIG. 3
has less polarization-dependent reflectivities, due to the more perpendicular incidence of the white light beam
68
on the first and second reflective layers
52
,
54
, this same arrangement may also make the Philips prism inconveniently thick. Moreover, for the cross cube
10
, another disadvantage is that the center of the “X” may be projected (e.g., in a projection system) to a screen and seen as a line to a viewer of the screen.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.
SUMMARY OF THE INVENTION
In one aspect of the invention, a color-separating prism is provided. The color-separating prism includes first, second, and third component prisms that form first and second adjacent pairs of faces, and include nonadjacent faces. The third component prism has a front surface. The color-separating prism also includes a first reflective layer disposed in part between the first adjacent pair of faces and in part on one nonadjacent face and a second reflective layer disposed in part between the second adjacent pair of faces and in part on another nonadjacent face. The first reflective layer is

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