Image projector and optical assembly
View Patent ↗An image projector is disclosed that can include a light source and a MEMS mirror to receive a light beam emitted by the light source and oscillate to scan the light beam across multiple-beam-generators that each include a planar beam splitter arranged to receive the light beam and generate multiple beams to project an image.
1. An image projector comprising:
a light source to provide a light beam;
a MEMS mirror arranged to receive the light beam and to reflect the light beam;
one or more multiple-beam-generators, each of the one or more multiple-beam-generators comprising:
a planar beam splitter arranged to receive the light beam reflected by the MEMS mirror and transmit at least part of the light beam; and
a planar reflector to receive the part of the light beam which is transmitted through the planar beam splitter to generate multiple beams, the planar beam splitter and the planar reflector arranged in parallel; and
a focusing lens arranged to receive the multiple beams generated by the one or more multiple-beam-generators, for each of the one or more multiple-beam-generators, the distance (h) between the planar beam splitter and planar reflector is such that the optical distance (OD) between the planar beam splitter and planar reflector is greater than, or equal to, half the coherent length of the light beam.
2. The image projector of claim 1 , the MEMS mirror to oscillate about at least one oscillation axis to scan the light beam across the one or more multiple-beam generators.
3. The image projector of claim 1 , the one or more multiple beam generators comprising a planar beam splitter element defining the planar beam splitter and a planar reflector element defining the planar reflector, the planer beam splitter element and the planar reflector element arranged in parallel and spaced apart such that there is an air-gap between the planar beam splitter element and the planar reflector element, wherein the optical distance (OD) is defined as:
OD
=
t
cos
(
θ
2
)
+
h
cos
[
sin
-
1
(
n
sin
θ
2
n
3
)
]
θ
2
=
sin
-
1
(
n
1
sin
θ
i
n
)
wherein ‘t’ is the thickness of the planar beam splitter element, ‘θ i ’ is the incidence angle of the light beam on the planar beam splitter element, ‘n1’ is the refractive index of the material in which the light beam passes before it is incident on the planar beam splitter element, ‘n’ is the refractive index of planar beam splitter element, ‘h’ is the distance between the planar beam splitter element and planar reflector element, ‘n3’ is the refractive index of the air in the air-gap.
4. The image projector of claim 1 , the one or more multiple beam generators comprising:
a block of transparent material having a first surface and a second surface opposite the first surface;
a planar coating of semi-reflective material on the first surface, the planar coating of semi-reflective material defining the planar beam splitter; and
a planar coating of reflective material on the second surface, the planar coating of reflective material defining the planar reflector, wherein the optical distance (OD) is defined as:
OD
=
h
cos
(
θ
2
)
wherein h is this distance between the coating of semi-reflective material and the coating of reflective material, and θ 2 is
θ
2
=
sin
-
1
(
n
1
sin
θ
i
n
)
wherein ‘θ i ’ is the incidence (AOI) of the light beam 5 on the planar coating of semi-reflective material ( 82 ), ‘n1’ is the refractive index of the material in which the light beam 5 passes before it is incident on planar coating of semi-reflective material, ‘n’ is the refractive index planar coating of semi-reflective material.
5. The image projector of claim 1 , comprising a plurality of multiple-beam-generators arranged in optical communication, wherein the planar beam splitter and the planar beam reflector of each of the plurality of multiple-beam-generators lie on differently orientated planes.
6. The image projector of claim 1 , the planar beam splitter configured to have a beam splitting ratio of 40% reflection to 60% transmission.
7. The image projector claim 1 , the planar beam splitter arranged such that the part of the light beam to be transmitted through the planar beam splitter and reflected by the planar reflector passes directly to the focusing lens without passing back through the planar beam splitter.
8. An optical assembly comprising,
an image projector comprising:
a light source to provide a light beam;
a MEMS mirror arranged to receive the light beam and to reflect the light beam;
one or more multiple-beam-generators, each of the one or more multiple-beam-generators comprising:
a planar beam splitter arranged to receive the light beam reflected by the MEMS mirror and transmit at least part of the light beam; and
a planar reflector to receive the part of the light beam which is transmitted through the planar beam splitter to generate multiple beams, the planar beam splitter and the planar reflector arranged in parallel; and
a focusing lens arranged to receive the multiple beams generated by the one or more multiple-beam-generators and transmit the light through the focusing lens, for each of the one or more multiple-beam-generators, the distance (h) between the planar beam splitter and planar reflector is such that the optical distance (OD) between the planar beam splitter and planar reflector is greater than, or equal to, half the coherent length of the light beam;
a screen to receive light transmitted through the focusing lens and to diffuse the received light; and
a head-up-display to receive light diffused by the screen.
9. The optical assembly of claim 8 , comprising a numerical aperture which is greater than a predefined numerical aperture value representing the numerical aperture of a viewer, wherein the numerical aperture of the optical assembly is defined as (d/2)/(F·M), wherein M is the magnification of the head-up-display, and F is the focal length of the of the focusing lens 11 , and d is the distance between two consecutive multiple light beams output from the multiple beam generator and prior to said multiple light beams being incident on the focusing lens.
10. The optical assembly of claim 9 , the one or more multiple beam generators comprising a planar beam splitter element defining the planar beam splitter and a planar reflector element defining the planar reflector, the planer beam splitter element and the planar reflector element arranged in parallel and spaced apart such that there is an air-gap between the planar beam splitter element and the planar reflector element, wherein the optical distance (OD) is defined as:
OD
=
t
cos
(
θ
2
)
+
h
cos
[
sin
-
1
(
n
sin
θ
2
n
3
)
]
θ
2
=
sin
-
1
(
n
1
sin
θ
i
n
)
wherein ‘t’ is the thickness of the planar beam splitter element, ‘θ i ’ is the incidence angle of the light beam on the planar beam splitter element, ‘n1’ is the refractive index of the material in which the light beam passes before it is incident on the planar beam splitter element, ‘n’ is the refractive index of planar beam splitter element, ‘h’ is the distance between the planar beam splitter element and planar reflector element, ‘n3’ is the refractive index of the air in the air-gap, wherein ‘d’ satisfies the condition:
d
=
2
(
t
tan
θ
2
+
h
tan
θ
3
)
cos
θ
i
θ
2
=
sin
-
1
(
n
1
sin
θ
i
n
)
θ
3
=
sin
-
1
(
n
1
sin
θ
2
n
3
)
wherein ‘t’ is the thickness of the planar beam splitter element, ‘θ i ’ is the angle of incidence (AOI) of the light beam on the planar beam splitter element, ‘n1’ is the refractive index of the material in which the light beam passes before it is incident on the planar beam splitter element, ‘n’ is the refractive index of planar beam splitter element, ‘h’ is the distance between the planar beam splitter element and the planar reflector element, ‘n3’ is the refractive index of the material which occupies the space between planar beam splitter element and planar reflector element.
11. The optical assembly of claim 9 , the one or more multiple beam generators comprising:
a block of transparent material having a first surface and a second surface opposite the first surface;
a planar coating of semi-reflective material on the first surface, the planar coating of semi-reflective material defining the planar beam splitter; and
a planar coating of reflective material on the second surface, the planar coating of reflective material defining the planar reflector, wherein the optical distance (OD) is defined as:
OD
=
h
cos
(
θ
2
)
wherein h is this distance between the coating of semi-reflective material and the coating of reflective material, and θ 2 is
θ
2
=
sin
-
1
(
n
1
sin
θ
i
n
)
wherein ‘θ i ’ is the incidence (AOI) of the light beam 5 on the planar coating of semi-reflective material ( 82 ), ‘n1’ is the refractive index of the material in which the light beam 5 passes before it is incident on planar coating of semi-reflective material, ‘n’ is the refractive index planar coating of semi-reflective material, wherein ‘d’ satisfies the condition:
d
=
2
h
tan
[
sin
-
1
(
sin
θ
i
n
)
]
cos
θ
i
wherein ‘h’ is the distance between the coating of semi-reflective material ( 82 ) and the coating of reflective material ( 84 ), n is the refractive index of the block of transparent material ( 46 , 81 ), θ i is the angle of incidence of the light beam on the of semi-reflective material ( 82 ).
12. The optical assembly of claim 9 , the screen comprising a microlens array, wherein ‘d’ satisfies the condition:
d
=
F
tan
[
(
2
k
+
1
)
θ
P
2
]
wherein F is the focal length of the of the focusing lens, k is an integer which is the order of the multiple-beam-generator, and θ p is equal to:
θ
P
=
λ
P
wherein λ is the wavelength of the light beam and ‘P’ is the pitch of the microlens array.
13. The optical assembly of claim 11 , wherein the distance ‘h’ between the planar beam splitter and the planar reflector satisfies the condition:
h
=
d
2
tan
[
sin
-
1
(
sin
θ
i
n
)
]
cos
θ
i
wherein ‘n’ is the refractive index of material between the planar beam splitter and the planar reflector, θ i is the angle of incidence of the light beam on the beam splitter, and ‘d’ is the distance between two consecutive multiple light beams which are output from the multiple beam generator and prior to said multiple light beams being incident on the focusing lens.
14. The optical assembly of claim 11 , wherein the screen comprises a microlens array and wherein the microlens array comprises different sized microlens so that the microlens array has a plurality of different pitch lengths between microlens, or wherein the screen comprises a micromirror array wherein the micromirror array comprises different sized micromirrors so that the micromirror array has a plurality of different pitch lengths between micromirrors.
15. The optical assembly of claim 14 , wherein the pitch (P) between consecutive microlens' is equal to:
P =( P eff /cos θ scan )
wherein P eff is a predefined effective pitch value and Θ scan is the angle of incidence of the light from the projector device on the microlens′.
16. The optical assembly of claim 14 , wherein the pitch (P) between consecutive microlens' is equal to:
P
=
(
2
k
+
1
)
λ
2
tan
-
1
{
2
h
F
tan
[
sin
-
1
(
sin
θ
i
n
)
]
cos
θ
i
}
wherein k is an integer which is the order of the multiple beam generator, Θi is the angle of incidence of the light beam of the planar beam splitter, F is the focal length of the collimating lens, λ is the wavelength of the light beam, ‘h’ is the distance between the planar beam splitter and the planar reflector, n is the refractive index of material between the planar beam splitter and the planar reflector.