Control program for a device for photorefractive corneal surgery of the eye
In a control program, according to which a laser-beam spot is guided, while being controlled with respect to position and time, over a cornea to be corrected photorefractively, so as to ablate a predetermined ablation profile therefrom, the effect of the angle between the laser earn and the corneal surface on the energy density of the laser-beam spot incident on the corneal surface and/or on the fraction of the laser-beam energy incident on the corneal surface which is reflected away, is taken into account when generating the control program.
1. A computer readable media including a control program, according to which a laser-beam spot is guided, while being controlled with respect to position and time, over a cornea to be corrected, so as to ablate a predetermined ablation profile therefrom, comprising:
instructions to take into account the effect of the angle between the laser beam ( 68 ) and the corneal surface on the energy density of the laser-beam spot incident on the corneal surface,
wherein the time interval the laser-beam spot is incident on the corneal surface at an incident point ( 58 ) of the corneal surface is increased as a function of the distance (r) of the incident point ( 58 ) of the laser-beam spot centre on the cornea ( 54 ) from an axis running parallel to the laser-beam direction which axis meets the corneal surface at a right angle (z axis),
wherein the effect of the distance r of the incidence point ( 58 ) of the laser-beam spot centre on the cornea ( 54 ) from an axis running parallel to the laser-beam direction which meets the corneal surface at a right angle (z axis), is taken into account, and account is taken of the fact that the energy density F of the emitted laser-beam spot of radius r s is reduced to F/kl(r), in the case of a cornea assumed to be hemispherical with radius R, when incident on its curved surface ( 54 ), where
k1
(
r
)
=
A
eff
(
r
)
A
0
=
A
eff
(
r
)
π
·
r
s
2
and
A
eff
(
r
)
=
∫
-
rs
rs
∫
rs
2
-
x
2
+
r
-
rs
2
-
x
2
+
r
1
+
(
ⅆ
ⅆ
x
f
(
x
,
y
)
)
2
+
(
ⅆ
ⅆ
y
f
(
x
,
y
)
)
2
ⅆ
x
ⅆ
y
with
z=f ( x,y )= f ( r )=√{square root over ( R 2 −x 2 −y 2 )}=√{square root over ( R 2 −r 2 )},
r =( x 2 +y 2 ) 1/2
where x, y, z are the coordinates of the incidence point ( 58 ) of the laser-beam spot centre in a Cartesian coordinate system, in which the origin lies at the sphere centre of the cornea which is assumed to be hemispherical.
2. The computer readable media according to claim 1 , wherein the formula is applied for the ablation depth achieved owing to a particular laser-beam spot pulse, in that it is reduced to d·kor 1 (r) in relation to the ablation depth d in the case of normal incidence of the laser-beam spot when the laser-beam spot is incident on the curved surface ( 54 ), where
kor1
(
r
)
=
ln
(
F
k1
(
r
)
F
th
)
ln
(
F
F
th
)
and F th is the energy-density threshold above which ablation takes place, and in that, when generating the control program, this formula is used to adjust the control of the laser beam in accordance with the desired ablation depth.
3. The computer readable media according to claim 1 , wherein account is taken of the fact that, in the case of the cornea assumed to be hemispherical, the unreflected fraction of the energy density F/k 1 (r) of the laser-beam spot incident on the curved surface is given as (1-k 2 (r))·F/k 1 (r), where
k2
(
r
)
=
q
⊥
2
(
r
)
+
q
∥
2
(
r
)
2
with
q
⊥
(
α
1
)
=
n
2
-
sin
2
(
α
i
)
-
cos
(
α
i
)
1
-
n
2
q
∥
(
α
1
)
=
n
2
cos
(
α
i
)
-
n
2
-
sin
2
(
α
i
)
n
2
cos
(
α
i
)
+
n
2
-
sin
2
(
α
i
)
where Tπ/2-α 1 is the angle between the laser beam and the corneal surface, where
α
1
(
r
)
=
α
tan
(
r
R
2
-
r
2
)
with 0≦r 2 <R 2 ,
and n is the empirically determined refractive index of the cornea at the of wavelength of the laser beam which is used.
4. The computer readable media according to claim 1 wherein account is also taken of the fact that a fraction of the laser-beam energy incident on the corneal surface is reflected away.
5. The computer readable media according to claim 4 , wherein the formula is applied for the ablation depth due to a particular laser-beam spot pulse, in that it is reduced to d·kor(r) in relation to the ablation depth d in the case of normal incidence of the laser-beam spot when the laser-beam spot is incident on the curved surface ( 54 ), where
kor
(
r
)
=
ln
(
(
1
-
k2
(
r
)
)
·
F
k1
(
r
)
·
F
th
)
ln
(
F
F
th
)
and F th is the energy-density threshold above which ablation takes place, and in that, when generating the control program, this formula is used to adjust the control of the laser beam in accordance with the desired ablation depth.
6. A computer readable media including a control program, according to which a laser-beam spot is guided, while being controlled with respect to position and time, over a cornea to be corrected photorefractively, so as to ablate a predetermined ablation profile therefrom, comprising:
instructions to take into account the effect of the angle between the laser beam and the corneal surface on the fraction of the laser-beam energy incident on the corneal surface which is reflected away,
wherein the effect of the distance r of the incidence point ( 58 ) of the laser-beam spot centre on the cornea from an axis running parallel to the laser-beam direction, which meets the corneal surface at a right angle (z axis) is taken into account,
wherein account is taken of the fact that, in the case of the cornea assumed to be hemispherical with radius R, the unreflected fraction of the energy density F of the laser-beam spot incident on the curved surface is given as (1-k 2 (r))·F, where
k2
(
r
)
=
q
⊥
2
(
r
)
+
q
∥
2
(
r
)
2
with
q
⊥
(
α
1
)
=
n
2
-
sin
2
(
α
i
)
-
cos
(
α
i
)
1
-
n
2
q
∥
(
α
1
)
=
n
2
cos
(
α
i
)
-
n
2
-
sin
2
(
α
i
)
n
2
cos
(
α
i
)
+
n
2
-
sin
2
(
α
i
)
where π/2-α 1 is the angle between the laser beam and the corneal surface, where
α
1
(
r
)
=
α
tan
(
r
R
2
-
r
2
)
with 0≦r 2 <R 2 ,
and n is the empirically determined refractive index of the cornea at the wavelength of the laser beam which is used.
7. The computer readable media according to claim 6 , wherein the formula is applied for the ablation depth due to a particular laser-beam spot pulse, in that it is reduced to d·kor 2 (r) in relation to the ablation depth d in the case of normal incidence of the laser-beam spot when the laser-beam spot is incident on the curved surface, where
kor2
(
r
)
=
ln
(
(
1
-
k2
(
r
)
)
·
F
F
th
)
ln
(
F
F
th
)
and F th is the energy-density threshold above which ablation takes place, and in that, when generating the control program, this formula is used to adjust the control of the laser beam in accordance with the desired ablation depth.
8. A computer readable media including a control program according to which an intensity of a laser-beam is controlled, while being controlled with respect to position and time, over a cornea to be corrected, comprising:
instructions for determining a reduction in an energy density of the laser-beam incident on a corneal surface due to an angle between the laser beam and the corneal surface;
instructions for determining a fraction of the laser-beam energy incident on the corneal surface that is reflected away; and
instructions for adjusting the intensity in response to the determined fraction and reduction;
wherein the time interval the laser-beam spot is incident on the corneal surface at an incident point of the corneal surface being increased as a function of he distance of the incidence point of the laser-beam spot center of the cornea from an axis running parallel to the laser-beam direction which axis meets the corneal surface at a right angle;
wherein the energy density F of the emitted laser-beam spot of a radius r s is calculated as being reduced to F/k 1 (r), in the case of a cornea assumed to be substantially hemispherical with radius R, when incident on its curved surface ( 54 ), where
k1
(
r
)
=
A
eff
(
r
)
A
0
=
A
eff
(
r
)
π
·
r
s
2
and
A
eff
(
r
)
=
∫
-
rs
rs
∫
rs
2
-
x
2
+
r
-
rs
2
-
x
2
+
r
1
+
(
ⅆ
ⅆ
x
f
(
x
,
y
)
)
2
+
(
ⅆ
ⅆ
y
f
(
x
,
y
)
)
2
ⅆ
x
ⅆ
y
with
z=f ( x,y )= f ( r )=√{square root over ( R 2 −x 2 −y 2 )}=√{square root over ( R 2 −r 2 )},
r =( x 2 +y 2 ) 1/2
where x, y, z are the coordinates of the incidence point ( 58 ) of the laser-beam spot centre in a Cartesian coordinate system, in which the origin lies at the sphere centre of the cornea.
9. A computer readable media including a control program according to which an intensity of a laser-beam is controlled, while being controlled with respect to position and time, over a cornea to be corrected, comprising:
instructions for determining a reduction in an energy density of the laser-beam incident on a corneal surface due to an angle between the laser beam and the corneal surface;
instructions for determining a fraction of the laser-beam energy incident on the corneal surface that is reflected away; and
instructions for adjusting the intensity in response to the determined fraction and reduction;
wherein the time interval the laser-beam spot is incident on the corneal surface at an incident point of the corneal surface being increased as a function of the distance of the incidence point of the laser-beam spot center of the cornea from an axis running parallel to the laser-beam direction which axis meets the corneal surface at a right angle;
wherein an unreflected fraction of the energy density F of the laser-beam spot incident on the curved surface is calculated by the formula (1-k 2 (r))·F, where
k2
(
r
)
=
q
⊥
2
(
r
)
+
q
∥
2
(
r
)
2
with
q
⊥
(
α
1
)
=
n
2
-
sin
2
(
α
i
)
-
cos
(
α
i
)
1
-
n
2
q
∥
(
α
1
)
=
n
2
cos
(
α
i
)
-
n
2
-
sin
2
(
α
i
)
n
2
cos
(
α
i
)
+
n
2
-
sin
2
(
α
i
)
where π/2-α 1 is the angle between the laser beam and the corneal surface, where
α
1
(
r
)
=
α
tan
(
r
R
2
-
r
2
)
with 0<r 2 <R 2 ,
and n is the empirically determined refractive index of the cornea at the wavelength of the laser beam which is used.