System for inspecting surfaces of an optical wave using a graduated density filter
Systems for inspecting a surface of an optical wave originating from an optical device are provided. The optical device includes an exit pupil, and the inspection system includes an optical measuring head and a computer for processing the images from said optical measuring head. The optical measuring head includes a density gradient filter, the density varying periodically in the two directions in space, a matrix frame having at least four identical lenses of square shape of the same focal length and being arranged symmetrically, and a photodetector array, each of the four lenses forming an image of the pupil in the plane of this array. The image processing computer includes computing means for computing the partial derivatives ∂ Δ ∂ x ( x , y ) and ∂ Δ ∂ y ( x , y ) of the wave surface Δ(x, y) in the plane of the exit pupil.
1. A system for inspecting a surface of an optical wave originating from an optical device, said optical device comprising an exit pupil, said inspection system comprising an optical measuring head and a computer for processing the images from said optical measuring head, wherein:
the optical measuring head comprises:
a density gradient filter, in a plane referenced (x′, y′) perpendicular to the optical axis of the optical measuring head, the transmission T(x′, y′) of said filter being governed by the equation:
T
(
x
′
,
y
′
)
=
1
+
sin
(
2
2
π
(
x
′
-
y
′
)
/
p
x
)
sin
(
2
2
π
(
x
′
+
y
′
)
/
p
y
)
2
p x and p y representing the periods of the two sinusoidal functions dependent respectively on (x′−y′) and (x′+y′);
a matrix frame of identical lenses of square shape and of the same focal length, said matrix frame comprising at least four lenses, each center of one of the four lenses being arranged on an axis passing through the center of the exit pupil and a point O′M′(i, j) of the density gradient filter such that, in the plane referenced (x′, y′),
O
′
M
i
,
j
′
=
[
i
(
m
+
0.25
)
p
x
/
2
j
(
n
+
0.25
)
p
y
/
2
]
i and j being able to adopt the values −1 and +1, m and n being positive integers; and
a photodetector array, each of the four lenses forming an image of the pupil in the plane of this array, these images being referenced I″ k (x, y), k varying from 1 to 4; and
the image processing computer comprises computing means for computing the partial derivatives
∂
Δ
∂
x
(
x
,
y
)
and
∂
Δ
∂
y
(
x
,
y
)
of the wave surface Δ(x, y) in the plane (x, y) of the exit pupil, these partial derivatives being equal to
∂
Δ
∂
x
(
x
,
y
)
=
Ax
+
B
sin
-
1
(
I
4
″
(
x
,
y
)
-
I
3
″
(
x
,
y
)
+
I
2
″
(
x
,
y
)
-
I
1
″
(
x
,
y
)
C
)
∂
Δ
∂
y
(
x
,
y
)
=
Ay
+
B
sin
-
1
(
I
4
″
(
x
,
y
)
+
I
3
″
(
x
,
y
)
-
I
2
″
(
x
,
y
)
-
I
1
″
(
x
,
y
)
C
)
A, B and C being constants dependent on the geometric parameters of the optical measuring head.
2. The inspection system according to claim 1 , wherein, when the optical device is an objective, its focal plane is located in the plane of the matrix frame of the lenses.
3. The inspection system according to claim 1 , wherein, when the optical device is an afocal system, the optical measuring head comprises an additional optic arranged in the exit pupil, such that the focal plane of said additional optic is located in the plane of the matrix frame of the lenses.
4. The inspection system according to claim 1 , wherein the matrix frame comprises at least a second quadruplet of lenses.
5. The inspection system according to claim 1 , wherein the two periods p x and p y are equal.