Optical: systems and elements – Single channel simultaneously to or from plural channels
Reexamination Certificate
1999-04-23
2001-06-19
Epps, Georgia (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
C359S799000, C385S115000
Reexamination Certificate
active
06249381
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an illuminating method and an illuminating device for producing an illuminating light beam for a display device, a measuring device, a microscope, an exposure device, or the like.
2. Description of the Related Art
Conventionally, as a light source for illuminating light used in an illuminating device of, for example, a projection type liquid crystal display or a measuring device, an incoherent light source such as a lamp or light emitting diode (LED) has been used.
On the contrary, trials to use laser light from a laser light source, such as a solid-state laser, a gas laser, or a semiconductor laser, for illuminating light have been carried out. The laser light is superior in directionality and has high intensity, and is a light beam of high coherence. However, speckle resulting from high coherence becomes the most difficult technical problem.
For example, a semiconductor laser is a light source which has a very high photoelectric conversion efficiency and emits laser light with excellent directionality. However, it has been hardly used as an illuminating light source because of the problem of speckle due to high coherence.
In the 1970s, although studies of a display (hereinafter referred to as a laser display) using laser light were carried out at various places, in addition to problems of insufficient output of the light source and the modulation method, the problem of speckle was one of the problems which became obstacles against realization in practical use.
In recent years, the development of elementary techniques as key components of a laser display, such as a high output laser using wavelength conversion of a solid-state laser, semiconductor lasers capable of oscillating three primary colors of red (R), green (G), and blue (B), and a spatial modulator (light valve) using a liquid crystal or a micromachine, have been made at a high pace.
It is known that the contrast of speckle highly depends on the spectrum width of a light beam emitted from a light source. When the center wavelength of the light beam is &lgr;
0
, the full width at half maximum of the wavelength spectrum is &Dgr;&lgr;, the spectrum width of the wave number is W, and the spectrum of the wave number is S(k), that is,
(
numerical
expression
13
)
W
=
-
π
2
·
Δ
k
λ
0
2
S
(
k
)
=
Exp
[
-
(
k
-
k
0
)
2
2
·
W
2
]
2
π
·
W
an object is illuminated with this light, and in an optical system for forming an image of the object, when the light path length difference from one point on the object to its corresponding image has fluctuation of a standard deviation of &sgr;
Z
, it is known that the standard deviation &sgr;
I
of speckle intensity to the mean value <I> of illumination intensity is obtained by
(
numerical
expression
14
)
σ
l
2
⟨
I
⟩
2
=
1
1
+
(
2
·
W
·
σ
Z
)
2
(see G. Parry, “Laser Speckle and Related Phenomena”, pp. 93, Springer-Verlag, 1984).
When N speckle patterns which are incoherent from each other (that is, they do not interfere with each other) and have no correlation, are superimposed, the sum becomes an intensity sum of the respective speckle patterns. At this time, the contrast of speckle is lowered to 1/N.
For example, when N optical fibers are bundled, and the length of each of the optical fibers is changed by a length not less than a coherence length, it becomes possible to neglect the interference between the respective optical fibers. The speckle at this time is superposition of intensities of speckle patterns I
1
, I
2
. . . I
N
formed by the respective optical fibers. Thus, the contrast of speckle is lowered by averaging (unifying).
This will be described with reference to a document (E. G. Rawson, J. W. Goodman, R. E. Norton, J.O.S.A. 70, 968-976,1980, Appendix).
First, when the spatial mean intensity of speckle patterns from each optical fiber is arbitrary, and the contrast of each speckle is L, the following expression is established.
(
numerical
expression
15
)
I
=
∑
k
=
L
N
I
k
At this time, the contrast C of speckle produced as a result of superposition, which is expressed by the following expression, and the multiplication Neff (N
eff
) of effective speckle are obtained. The contrast C of speckle is expressed by
(
numerical
expression
16
)
C
=
1
N
eff
and the ensemble mean intensity of a total sum is obtained as a sum of each ensemble mean intensity like the following expression.
(
numerical
expression
17
)
I
_
=
∑
k
=
L
N
I
k
_
At this time, variance &sgr;
I
2
expressed by
(
numerical
expression
18
)
σ
I
2
=
I
2
_
-
(
I
)
_
2
=
∑
I
k
∑
I
L
_
-
(
∑
I
k
_
)
(
∑
I
L
_
)
=
∑
k
∑
L
(
I
k
I
L
_
-
I
k
_
I
L
_
)
When the correlation coefficient of a k-th speckle and an L-th speckle pattern is &rgr;
kL
, the following expression is established.
(
numerical
expression
19
)
ρ
kL
=
I
k
I
L
-
I
k
I
L
_
(
I
k
-
I
k
_
)
)
2
(
I
L
-
I
L
_
)
2
Here, from the relation of the assumption of L-th contrast and
(
numerical
expression
20
)
σ
2
=
u
2
_
-
(
u
)
_
2
the following expression is obtained.
(
numerical
expression
21
)
I
k
2
_
=
2
(
I
k
)
_
2
(
I
k
-
I
k
_
)
2
=
I
k
2
_
-
(
I
k
_
)
2
=
I
k
_
σ
I
2
=
∑
k
∑
L
ρ
kL
I
k
I
L
_
Thus, the contrast of the sum of patterns of all speckles becomes
(
numerical
expression
22
)
C
=
σ
I
I
_
=
∑
k
∑
L
I
k
I
L
_
ρ
kL
∑
k
I
k
_
That is, the multiplicity Neff (N
eff
) of effective speckle becomes
(
numerical
expression
23
)
N
eff
=
1
C
2
=
∑
k
∑
L
I
k
I
L
_
∑
k
∑
L
I
k
I
L
_
ρ
kL
Here, if N speckle patterns are incoherent with each other,
(
numerical
expression
24
)
ρ
KL
=
{
1
(
k
=
L
)
0
(
others
)
}
and at this time,
(
numerical
expression
25
)
N
eff
=
1
C
2
=
∑
k
∑
L
I
k
I
L
_
(
∑
k
I
_
k
)
2
→
(
for
any
k
,
L
)
I
k
_
=
I
L
_
N
2
N
=
N
is established. That is, if N speckle patterns which are incoherent and have equal intensity are superimposed on each other, the contrast becomes 1/N.
Fujii et al studied the contrast of speckle produced on an image of a random phase object by spatially partial coherent illumination (see H. Fujii, T. Asakura, opt. Comm. 12, 32-38, 1974).
According to that study, the speckle contrast C is expressed by
(
numerical
expression
26
)
C
=
∫
∫
Γ
(
χ
1
,
χ
2
)
Γ
(
χ
3
,
χ
4
)
K
(
χ
′
-
χ
1
)
K
*
(
χ
′
-
χ
2
)
K
(
χ
′
-
χ
3
)
K
*
(
x
′
-
χ
4
)
x
exp
[
R
(
x
1
-
x
2
)
+
R
(
x
3
-
x
4
)
]
x
{
exp
[
-
R
(
x
1
-
x
3
)
-
R
(
x
2
-
x
4
)
+
R
(
x
1
-
x
4
)
+
R
(
x
3
-
x
2
)
]
-
1
x
ⅆ
x
1
ⅆ
x
2
ⅆ
x
3
ⅆ
x
4
∫
∫
Γ
(
χ
1
·
χ
2
)
K
(
χ
′
-
χ
1
)
K
*
(
χ
′
-
χ
2
)
exp
[
R
(
χ
1
-
χ
2
)
]
ⅆ
χ
1
ⅆ
χ
2
Here, &Ggr; is the correlation function of the electric field amplitude between two points of the object, K is the amplitude transfer function, and R is the correlation function of the phase
Epps Georgia
Limbach & Limbach L.L.P.
Seyrafi David
Sony Corporation
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