JP2003212569A - Optical element and method of manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical element having a smooth surface and a method of manufacturing the same. <P>SOLUTION: The method has a process step of previously heating a glass blank to a temperature corresponding to a range of 10<SP>10.5</SP>to 10<SP>7</SP>dPa.s in glass viscosity, a process step of previously heating dies to a temperature corresponding to a range of 10<SP>14</SP>to 10<SP>9</SP>dPa.s in glass viscosity of the glass blank, a process step of pressing the glass blank by the dies, a process step of heating up the dies to a range of 10<SP>12</SP>to 10<SP>10</SP>dPa.s in the temperature corresponding to the viscosity of the glass blank while maintaining the pressing state, a process step of maintaining the temperature of the dies for the prescribed time in the state of the heated up temperature, a process step of maintaining the temperature of the dies for the prescribed time, then cooling the dies, and a process step of parting the optical element after the temperature of the dies attains the temperature below the glass transition temperature of the glass blank. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical element manufactured by pressing a heat-softened glass material and a method for manufacturing the same.

[0002]

2. Description of the Related Art In recent years, a method for obtaining an optical element by pressing a heat-softened glass material with a molding die has been widely used.

Japanese Unexamined Patent Publication No. 2001-97729 discloses a technique of forming the surface of a molding die with a material containing gold as a main component.

[0004]

The surface roughness of the molding die or the molding function surface of the molding die is processed into an optical element obtained by pressing a glass material which has been softened by heating with a molding die. The crevices and undulations that occur at the time are transferred. Here, the shape waviness refers to continuous large and small fine waves on the shape of the optical element. Hereinafter, the terms have the same meaning. The shape waviness adversely affects the optical system because it causes a diffraction effect. FIG. 7 shows the surface shape and shape waviness of the molding function surface of the molding die. For the measurement, Talysurf manufactured by Rank Taylor Hobson was used.

The heating temperature of the glass is 10 ⁹ d in terms of glass viscosity.
The surface shape and waviness of the optical element molded by heating the temperature of the molding die to a temperature equal to or higher than Pa · s and heating the temperature of the molding die to a temperature equal to or higher than the temperature corresponding to 10 ⁹ dPa · s in glass viscosity before starting pressing 8 shows. The shape waviness of the molding function surface of the molding die is directly transferred. The optical element on which the surface roughness of the molding die or the grind on the molding function surface of the molding die is transferred, the transmittance is reduced, and flare is generated, and the optical element on which a large undulation is transferred is reduced in contrast. generate.

The mold described in Japanese Patent Laid-Open No. 2001-97729 uses a film mainly composed of gold on the mold base, but the film mainly composed of gold has scratches due to normal handling. There is a drawback that it is easy to enter. JP 2001-9
In the film forming technique described in Japanese Patent No. 7729, a method of increasing the hardness of the film by injecting ions into the surface of the molding surface is described, but when the glass is pressed, it consists of minute dust on the glass surface. Scratches occurred when the foreign matter was molded. Since the scratches generated in the mold are transferred to the optical element by molding, an optical defect is generated on the surface of the optical element.

Further, if the film thickness is increased during film formation for the purpose of eliminating roughness and grinds during processing of a molding die, stress may cause unevenness on the surface or cracks in the film.

The present invention has been made in view of the above problems of the prior art, and an object thereof is to provide an optical element having a smooth surface and a method for manufacturing the same.

[0009]

In order to achieve the above object, the optical element of the first invention is such that the glass material that has been softened by heating is brought into direct contact with the molding function surface of the heated mold at least once or more. The optical element obtained by the above is characterized in that the surface roughness or shape waviness of the surface of the optical element after molding is smoother than the surface roughness or shape waviness of the molding function surface of the molding die.

The optical element of the second invention is the optical element of the first invention, wherein the evaluation value of the surface roughness or shape waviness of the molding function surface of the mold is X, and the surface roughness or shape waviness of the surface of the optical element after molding. When the evaluation value of is Y, 0.5 ≦ Y / X ≦
The relationship is 0.95.

According to the first and second aspects of the present invention, an optical element having a small surface roughness or shape waviness can be obtained without directly transferring the surface roughness or shape waviness of the press mold.

In the method for manufacturing an optical element according to the third invention, the glass material has a glass viscosity of 10 ^10.5 to 10 ⁷ dPa · s.
And a step of preheating to a temperature corresponding to the range of 10 ¹⁴ to 10 ⁹ in terms of the glass viscosity of the above glass material.
The step of preheating to a temperature corresponding to the range of dPa · s, the step of pressing the glass material with the molding die, and the step of maintaining the pressed state in the molding die at a temperature corresponding to the viscosity of the glass material are performed. ^{12 ~10 10} d
A step of raising the temperature of the molding die to a range of Pa · s, a step of maintaining the temperature of the molding die at the elevated temperature for a predetermined time, a step of maintaining the temperature of the molding die for a predetermined time and then cooling, And releasing the mold after the temperature reaches a temperature not higher than the glass transition temperature of the glass material.

A method of manufacturing an optical element according to a fourth aspect of the present invention is the third aspect.
In the invention described above, the molded optical element is 0.015 dP.
The glass viscosity is 10 ¹⁵ dPa · s from the glass transition temperature at a rate of not less than a · s / min and not more than 0.020 dPa · s / min.
It is characterized in that it is gradually cooled at least once to a temperature corresponding to.

In the third and fourth inventions described above, when the glass material that has been softened by heating is brought into contact with a molding die in which the temperature of the molding die is set to be lower than the temperature of the glass material and pressed, the glass surface becomes Be cooled rapidly. At this time,
Since stress is generated on the glass surface, shrinkage occurs on the glass surface. Therefore, it is possible to obtain an optical element in which the roughness of the surface of the mold and the crevices of the mold are not transferred. After the pressing step, the mold is heated and pressed to maintain the inner thickness of the optical element to a desired thickness. Then, in the step of gradually cooling, the internal strain of the optical element is removed.

In FIG. 1, the heating temperature of the glass material is set equal to or higher than the temperature corresponding to the glass viscosity of 10 ⁹ dPa · s, and the temperature of the molding die is 10 ¹³ dPa in terms of the glass viscosity before the start of pressing.
-The surface shape and shape waviness of an optical element formed by heating to a temperature corresponding to s are shown. As compared with FIG. 8, the shape waviness of the molding surface is smaller and the surface is smoother.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (Embodiment 1) Embodiment 1 of the present invention will be described with reference to FIG. FIG. 2 is a schematic configuration diagram showing a molding unit of a molding apparatus used in the method of manufacturing an optical element according to the present embodiment.

In FIG. 2, a glass heater 4 for heating the glass material 3 held by the arm 2 is arranged in the molding chamber 1 of the molding apparatus, and is provided above and below the glass material 3, respectively. ing. In this embodiment,
A heater that uses radiant heat to heat is used as the glass heater 4, but an infrared heater, electromagnetic induction heating,
There is essentially no difference in using other heating means such as a ceramic heater or laser irradiation heating.

The glass material 3 has a columnar shape and can be placed on the holder 5 and held by the arm 2. The glass material 3 may be a lens or a glass gob that has been polished to an approximate shape according to the shape of the optical element to be molded.

The holder 5 is formed by processing a tungsten-based alloy material into a cylindrical shape, and is provided with a flange for mounting the glass material 3 on its inner peripheral surface. The inner diameter of the flange allows the lower die 8 described later to be inserted therethrough. Thus, it is slightly larger than the outer diameter of the lower mold 8.

The arm 2 is attached to a rotary shaft 6, and by rotating the rotary shaft 6, the glass material 3 held via a holder 5 is placed above and below the glass heater 4 for glass.
It can be conveyed to a heating position between them and a molding position between the upper mold 7 and the lower mold 8.

The upper die 7 and the lower die 8 form a molding die and are arranged so as to face each other vertically. The upper mold 7 is made of WC,
It is fixed to the molding chamber 1 with the molding function surface 7a provided at the tip end facing the lower mold 8. The molding functional surface 7a of the upper die 7 is composed of a convex surface portion and a flat surface portion provided on the outer peripheral side of the convex surface portion, and the convex surface portion and the flat surface portion forming the molding functional surface 7a are
Both are mirror-finished with a surface roughness Rmax of 0.08 μm or less. Further, the upper die 7 is entirely covered with a CrN film.

The lower mold 8 is made of WC, and is attached to the cylinder 9 with the molding function surface 8a provided at the tip end facing the molding function surface 7a of the upper mold 7. It is movable. The molding function surface 8a of the lower die 8 is processed into a concave aspherical surface having a surface roughness Rmax of 0.08 μm or less, and the outer periphery of the molding function surface 8a is chamfered with C0.1 to prevent burrs. It has been subjected. The lower die 8 is entirely covered with a CrN film.

The upper mold 7 and the lower mold 8 can be controlled to be heated to desired temperatures by an upper mold infrared heater 10 and a lower mold infrared heater 11, respectively. The upper mold infrared heater 10 and the lower mold infrared heater 11 have a circular shape and are arranged around the upper mold 7 and the lower mold 8 so as to surround the upper mold 7 and the lower mold 8.

The molding chamber 1 is provided with a suction valve 12 and a leak valve 13. A non-oxidizing atmospheric gas is introduced into the molding chamber 1 from a suction valve 12 connected to a non-oxidizing gas supply pipe (not shown). When the pressure is higher than the set value, leak valve 1
The structure is such that it can be purged so as to leak from No. 3. In the present embodiment, nitrogen gas was used as the non-oxidizing gas at the time of molding. The oxygen concentration inside the molding chamber 1 after the start of heating was set to 200 ppm or less.

Next, a method of manufacturing an optical element by the molding apparatus having the above structure and its operation will be described. The glass material 3 is placed on the holder 5, held by the arm 2 and arranged between the glass heaters 4.

Nitrogen gas is sucked into the molding chamber 1 through the suction valve 12. At the time when the oxygen concentration in the molding chamber 1 becomes 200 ppm or less, the glass heater 4 sets the glass material 2
The infrared heater 10 for the upper mold heats the upper mold 7 and the infrared heater 11 for the lower mold heats the lower mold 8, respectively.

The glass material 3 has a glass viscosity of 10 ⁹ dP.
When the temperature equal to or higher than a · s is reached, the arm 2 is rotated by the rotating shaft 6 and the glass material 3 is arranged between the glass heater 4 and the upper mold 7 and the lower mold 8. At this time, the upper mold 7 and the lower mold 8 have a glass viscosity of 10 ^13.5 dPa ·
It is heated to a temperature corresponding to s.

Next, the cylinder 9 is driven to raise the lower mold 8. The pressing of the glass material 3 is started when the lower mold 8 lifts the glass material 3 from the holder 5 and the upper mold 7 contacts the glass material 3.

When the pressing of the glass material 3 is started, the glass surface is rapidly cooled by the lower mold 7 and the lower mold 8. For this reason, the glass does not flow to the details forming the fine roughness of the molding function surface of the molding die, which is the molding function surface 7a of the upper mold 7 and the molding function surface 8a of the lower mold 8. Moreover, since a temperature difference occurs between the inside of the glass and the glass surface,
The contracting action acts on the glass, and a gap is formed between the molding function surface of the molding die and the glass material 3. For this reason, the transfer effect of the molding die to the glass is weakened, and it becomes difficult to transfer the shape waviness, crevices, roughness, and scratches of the molding function surface of the molding die.

While maintaining this pressed state, the upper mold 7 and the lower mold 8 were made to have a glass viscosity of the glass material 3 of 10 ¹³ to 10 10.
^The temperature is raised to a corresponding temperature range of ¹⁰ dPa · s and the desired thickness of the optical element is pressed. In this embodiment,
While maintaining the pressed state, the temperature was raised to 10 ^10.5 dPa · s at a temperature corresponding to the viscosity of the glass material 3. At this time, if too much pressure is applied, the glass surface may be broken, so it is necessary to determine an appropriate pressure depending on the viscosity of the glass to be heated and the shape of the optical element. In the present embodiment, the initial pressure of the press is 11 Pa, and the press pressure after the temperature rise is 0.
It was set to 2 Pa.

After holding the upper mold 7 and the lower mold 8 at a temperature corresponding to the viscosity of the glass material 3 of 10 ^10.5 dPa · s for 20 seconds, cooling was started.

After cooling the upper mold 7 and the lower mold 8 to a temperature not higher than the glass transition point of the glass material 3, the lower mold 8 was lowered by the cylinder 9 and released to obtain a desired optical element.

In the present embodiment, since a temperature difference occurs between the surface and the central portion of the glass material 3 during pressing, internal strain occurs, but it is gradually cooled after releasing the mold or is gradually cooled while the mold is in contact. The internal strain can be removed by any of the following methods: first performing slow cooling and then releasing. In the present embodiment, 0.
At a cooling rate of 020 dPa · s / min, the glass was gradually cooled from the glass transition temperature or lower to a temperature corresponding to 10 ¹⁵ dPa · s and released.

The surface roughness of the optical element manufactured according to this embodiment was measured by using Tallysurf manufactured by Rank Taylor Hobson. The cut-off filter is Gaussian, the cut-off length is 0.08 μm, and the measurement results for 10 sections are shown in FIG.
Shown in. In addition, the measurement result of the lower mold 8 measured in the same manner is shown in FIG.
Shown in. Center average roughness Ra of the molding function surface 8a of the lower die 8
With respect to 0.078 μm, the center average roughness of the optical element press-molded on the molding function surface 8a could be Ra 0.062 μm. Further, the shape waviness of the molding function surface (molding function surface 8a of the lower mold 8 in the present embodiment) shown in FIG. 7 and the optical element shown in FIG. When an optical simulation was carried out for the shape waviness of the molded surface), an improvement of 18% in MTF was observed. In the present embodiment, the quality of shape undulation is determined by simulation, but it may be determined by spectrum analysis by Fourier analysis.
Further, in the present embodiment, the surface roughness was also measured using the Talysurf, but a measuring instrument capable of analyzing irregularities such as an atomic force microscope, a scanning tunneling microscope, a three-dimensional surface analysis microscope and an interferometer. If so, other measuring instruments may be used. Also,
In the present embodiment, the central average roughness is used as the evaluation index, but even if it is changed to an index such as rms or Rz, it is particularly specified if the same measurement index and measurement method as the molding functional surface of the molding die are carried out. You don't have to.

According to the present embodiment, it is possible to obtain a good optical element even from a molding die having a score and a waviness. Further, even if the molding function surface of the molding die is rough, the surface of the optical element becomes smooth, so that the molding die that has been roughened by stacking the molding numbers can be used as it is.

(Embodiment 2) In this embodiment, a lens used as an objective lens of a microscope is molded. Objective lenses for microscopes are
Since the mirror surface property is required, the molding function surface of the molding die provided in the molding device is processed to have Ra of 0.0020 μm or less. The rest of the configuration of the molding apparatus used in this embodiment is the same as that of the first embodiment, and therefore its explanation is omitted.

Next, with reference to FIG. 2, a method of manufacturing the optical element of the present embodiment and its operation will be described. In the present embodiment, since the glass material 3, the upper mold 7, and the lower mold 8 are the same as those in the first embodiment before the heating is started, the description thereof will be omitted.

The glass material 3 placed on the holder 5
Is held by the arm 2 and heated by the glass heater 4, and when the glass viscosity reaches a temperature equal to or higher than 10 ⁸ dPa · s, the arm 2 is rotated by the rotating shaft 6 to move the glass material 3 to the upper mold. It is arranged between 7 and the lower mold 8.
At this time, the upper mold 7 and the lower mold 8 are the infrared heater 1 for the upper mold.
0 and infrared heater 11 for the lower mold, glass viscosity is 1
It is heated to a temperature corresponding to 0 ¹⁰ dPa · s.

The steps immediately after the pressing of the glass material 3 by the upper mold 7 and the lower mold 8 are the same as those in the first embodiment,
The description is omitted.

While maintaining the pressed state of the glass material 3, the upper mold 7 and the lower mold 8 are pressed to the desired inside thickness of the optical element.

In the present embodiment, the upper mold 7 and the lower mold 8 are heated to a temperature corresponding to a glass viscosity of 10 ¹⁰ dPa · s, so that the desired inside meat can be obtained without raising the temperature. I was able to. After pressing as it was, it was cooled to a temperature not higher than the glass transition point and released from the mold to obtain a desired optical element.

Also in the present embodiment, a temperature difference occurs between the surface and the central portion of the glass material 3, so that internal strain occurs.
If there is internal distortion, birefringence occurs inside the optical element and the optical performance deteriorates. In this embodiment, since the objective lens used in the microscope is molded, it is required that the internal strain be smaller. So, after releasing the mold, take out the optical element,
Put in a heating furnace for slow cooling (not shown), and set the glass viscosity to 1
After raising the temperature to 0 ^13.8 dPa · s,
In 0.015dPa · s / minute cooling rate of ^{10 1} 5 dPa
-Slowly cooled to a temperature corresponding to s.

The surface roughness of the optical element manufactured according to this embodiment was measured by using Tallysurf manufactured by Rank Taylor Hobson. Fig. 5 shows the measurement results of the cutoff filter in Gauss, cutoff length 0.08 μm, and 10 sections.
Shown in. In addition, as a comparative example, FIG. 6 shows the measurement results of the same measurement of the optical element formed by heating the glass material and the molding die to the same temperature in the conventional method and molding them. As for the surface roughness measurement result, the optical element molded by the conventional method was better, but when the mounting evaluation with a microscope was performed, the optical element molded by the conventional method had a lower resolution capability.

Photo 1 is the result of observing the mold with a field emission scanning microscope (hereinafter referred to as FESEM).
Photo 2 is a FESEM observation photo of the surface of the optical element manufactured by the conventional method using the mold of Photo 1. Photo 3
[Fig. 3] is an FESEM observation photograph of the surface of the optical element manufactured according to the present embodiment using the mold of photograph 1.

Comparing Photograph 2 and Photograph 3, Photograph 3
It was possible to judge that the surface was smooth, but it could not be identified because it was a mirror surface that exceeded the resolution of the roughness meter. Therefore, Photo 2
The images of Photograph 3 and Photograph 3 were image-processed and compared. The color tone range of the images of Photo 2 and Photo 3 was calculated to calculate the range of color values. As a result, the photograph of Photo 2 was 15.09 and the photograph of Photo 3 was 13.54. Image processing similarly,
The calculated molding die (Photo 1) was 16.46. Of the optical elements created based on this embodiment, 13.9
The following items also passed the mounting evaluation.

According to the present embodiment, in addition to the effects of the first embodiment, it is influenced by the cutoff value and section during measurement, the curvature of the optical element, and exceeds the resolution of the roughness measuring instrument. In particular, the quality of the surface of the optical element having Ra of 0.005 μm or less can be determined.

The technical idea of the following configuration is derived from the above-described specific embodiments. (Supplementary Note) (1) The glass viscosity of the glass material is 10 ^10.5 to 10 ⁷
The step of preheating to a temperature corresponding to the range of dPa · s, and the mold having a glass viscosity of the above glass material of 10
A step of preheating to a temperature corresponding to a range of ¹⁴ to 10 ⁹ dPa · s, a step of pressing the glass material with the molding die, and a step of maintaining the pressed state with the molding die corresponding to the viscosity of the glass material. At a temperature of 10 ¹²
10 to 10 ¹⁰ dPa · s, a step of maintaining the temperature of the molding die for a predetermined time at a lower press pressure than the pressing step while maintaining the temperature of the molding die at the elevated temperature, and maintaining the temperature of the molding die for a predetermined time. And a step of cooling after that, and a step of releasing the mold after the temperature of the molding die reaches a temperature not higher than the glass transition temperature of the glass material, the method for producing an optical element.

(2) The glass material has a glass viscosity of 10
A step of preheating to a temperature corresponding to a range of ^10.5 to 10 ⁷ dPa · s, and a mold is preheated to a temperature corresponding to a range of 10 ¹² to 10 ¹⁰ dPa · s as the glass viscosity of the above glass material. A step, a step of pressing the glass material with the molding die, a step of maintaining the molding die at a lower pressing pressure than the pressing step for a predetermined time, and a step of cooling the molding die after maintaining the temperature for the predetermined time, And a step of releasing the mold after the temperature of the molding die reaches a temperature not higher than the glass transition temperature of the glass material, the method for producing an optical element.

According to the method for producing an optical element of appendix (1), the glass material is pressed with a molding die at a temperature lower than that of the glass material,
It is possible to mold an optically excellent surface in which transfer of surface roughness or shape waviness of the molding function surface of the molding die is reduced.
Further, by pressing at a pressing pressure lower than that in the pressing step, it is possible to obtain a desired inside thickness without causing cracks or the like on the glass surface.

According to the method of manufacturing the optical element of the supplement (2), the manufacturing process of the supplement (1) can be simplified and the same effect as that of the supplement (1) can be obtained.

[0051]

As described above, according to the optical elements of claims 1 and 2 of the present invention, the transfer of the surface roughness or the undulation of the molding function surface of the molding die is reduced, and it is optically excellent. An optical element with a surface is obtained.

According to the method for manufacturing an optical element of claims 3 and 4 of the present invention, the surface of the molding functional surface of the molding die or the transfer of shape waviness is reduced by pressing with a molding die at a temperature lower than that of the glass material. As a result, it is possible to manufacture an optical element having an optically excellent surface and removing internal strain.

[Brief description of drawings]

1 shows a surface state of an optical element of the present invention, and FIG.
1A shows the surface shape, and FIG. 1B shows the shape waviness.

FIG. 2 is a schematic configuration diagram showing a molding unit of a molding apparatus used for carrying out Embodiments 1 and 2 of the present invention.

FIG. 3 is a diagram showing the surface roughness of the lower mold used in the first embodiment of the present invention.

FIG. 4 is a diagram showing the surface roughness of the optical element molded according to the first embodiment of the present invention.

FIG. 5 is a diagram showing the surface roughness of an optical element molded according to the second embodiment of the present invention.

FIG. 6 is a diagram showing the surface roughness of an optical element as a comparative example with respect to the second embodiment of the present invention.

FIG. 7 shows a molding function side of a molding die according to the prior art,
FIG. 7A shows a surface shape, and FIG. 7B shows a shape waviness.

FIG. 8 shows a surface state of an optical element in the prior art,
FIG. 8A shows the surface shape, and FIG. 8B shows the shape waviness.

[Photo 1] A photograph showing a result of observing a mold with a field emission scanning microscope.

Photo 2 is a photograph showing the result of observing the surface of an optical element manufactured by a conventional method with a field emission scanning microscope.

FIG. 3 is a photograph showing the result of observing the surface of an optical element manufactured according to the second embodiment of the present invention with a field emission scanning microscope.

[Explanation of symbols]

1 molding room Two arms 3 glass material 4 Glass heater 5 holder 6 rotation axes 7 Upper mold 8 Lower mold 9 cylinders 10 Infrared heater for upper mold 11 Infrared heater for lower mold 12 Inhalation valve 13 Leak valve

Claims (4)

[Claims] 1. An optical element obtained by directly contacting and pressing a heat-softened glass material with a molding function surface of a heated molding die at least once, wherein the surface roughness or shape of the molding function surface of the molding die. Than swell
An optical element characterized in that the surface roughness or shape undulation of the surface of the optical element after molding is smooth. 2. When the evaluation value of the surface roughness or waviness of the molding function surface of the molding die is X, and the evaluation value of the surface roughness or waviness of the optical element surface after molding is Y, 0.5 ≦ Y
2. The relationship of /X≦0.95 is satisfied.
The optical element described. 3. A glass material having a glass viscosity of 10 ^10.5.
A step of pre-heating to a temperature corresponding to a range of 10 ⁷ dPa · s, and a molding die having a glass viscosity of 10 ¹⁴ to 10 10
A step of preheating to a temperature corresponding to a range of ⁹ dPa · s, a step of pressing the glass material with the forming die, and a temperature corresponding to the viscosity of the glass material while maintaining the pressed state. 10 ¹² to 10 ¹⁰ dPa · s
The step of raising the temperature of the forming die to a temperature of the forming die, the step of maintaining the temperature of the forming die at the elevated temperature for a predetermined time, the step of cooling the temperature of the forming die for a predetermined time and then the temperature of the forming die And a step of releasing the glass material after reaching a temperature equal to or lower than the glass transition temperature of the glass material. 4. A molded optical element of 0.01 is formed in the step of gradually cooling or after the step of releasing.
The glass viscosity is 10 ¹⁵ dP from the glass transition temperature at a speed of 5 dPa · s / min or more and 0.020 dPa · s / min or less.
The method for producing an optical element according to claim 3, wherein the optical element is gradually cooled to a temperature corresponding to a · s at least once.