JP2003040634A - Annealing method for optical glass element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To shorten an annealing time for releasing a gradient refractive index and a stress during manufacturing procss such as a molding process without impacting to an optical property of an optical glass element. SOLUTION: A cooling rate of an annealing from a maximum temperature of heating a formed optical glass element is adjusted between 20 deg.C and 50 deg.C per hour. And the maximum temperature of an annealing furnace is kept above a distortion temperature of a nitrate material to be annealed and below an annealing temperature; furthermore a retention time of the maximum temperature is kept from 5 minutes to an hour.

Description

Detailed Description of the Invention

The present invention relates to a technique for manufacturing a precision optical glass element at low cost by molding, and more particularly to an annealing process for removing distortion inside the molded optical glass element and adjusting the refractive index.

[0002]

2. Description of the Related Art As the needs for high performance and miniaturization of optical systems have expanded, an efficient manufacturing method for aspherical glass lenses has been emphasized.

Although many mass production means for aspherical glass lenses have been devised, recently, a manufacturing method in which an aspherical glass lens is formed by cooling and solidifying an optical glass material that has been softened by heating while being molded between molding dies, has become mainstream. ing. This manufacturing method is currently most suitable for products that require compact, high-performance and inexpensive lenses such as digital cameras, but as the required performance improves, the refractive index generated inside the lens during the cooling and solidification process of the glass material There is a growing concern about the influence of distribution and distortion on optical performance.

The annealing process is most effective as a means for removing the strain inside the optical glass element and adjusting the refractive index, and is a well-known technique introduced by glass material makers and the like. Precision annealing is essential to bring out the original performance of optical glass materials, but the annealing conditions recommended by glass manufacturers generally take a long time, such as about one week for each batch. It takes time to find defective products in the process, and a large number of defective products may occur. In this case, it is also a problem that it takes twice as much time from re-introduction to production of a product. Further, a long tact time is not preferable because it also affects the product cost. From the above, a technique for shortening the annealing time is desired.

Japanese Unexamined Patent Publication (Kokai) No. 11-35330 discloses a high-speed annealing method that shortens the annealing process that conventionally takes several days. In this method, the optical glass element is designed on the assumption that precision annealing is performed, and in the actual manufacturing process, the optical glass element is annealed at a speed faster than the cooling rate of the precision annealing, or the annealing is not performed at all. It is something that should not be done.

[0006]

However, in the case where the annealing is not performed at all as described above, the influence of the flare caused by the birefringence phenomenon due to the distortion when the optical system becomes precise and the reduction in the resolution due to the refractive index distribution. Is often not negligible, and annealing is not unnecessary for all optical glass elements. Also, in terms of increasing the cooling rate, the cooling rate in the annealing step is an important step for determining the refractive index. Therefore, if the cooling rate is unnecessarily increased, the characteristics of the entire optical system may not be satisfied.

The present invention has been made in consideration of the above problems, and reduces the distortion and the refractive index distribution to a level that does not affect the lens performance while keeping the surface accuracy at a high level. An object of the present invention is to provide an annealing method for an optical glass element that shortens the takt time of the annealing step.

[0008]

In order to achieve the above object, the optical glass element annealing method according to the invention of claim 1 is an optical glass element annealing method for removing distortion and refractive index distribution due to the influence of a molding step or the like. The cooling rate is 20 ° C. to 100 ° C. per hour.

According to the present invention, when the molded optical glass element is cooled, the time required for the annealing treatment can be shortened while maintaining the internal homogeneity of the glass.

A second aspect of the present invention is the method for annealing an optical glass element according to the first aspect, wherein the maximum temperature in the annealing furnace is not less than the strain point temperature of the glass material to be annealed and not more than the annealing point temperature, and the maximum temperature holding time is maintained. Is set to 5 minutes to 1 hour.

In the present invention, the temperature applied to the optical glass element is set to the strain point temperature of the glass material to be annealed or higher and the annealing point temperature or lower, and the time is set to 5 minutes to 1 hour to minimize the deformation amount of the optical glass element. Internal strain and refractive index distribution can be removed while suppressing.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail with reference to FIGS.

[First Embodiment] Prior to the description of the present embodiment, a conventional annealing method will be described. The optical glass element 1 shown in FIG. 1 was molded by a molding device (not shown). This optical glass element 1 is a biconvex lens having a diameter (φ) of 12 mm and a wall thickness of 3.2 mm, and is s-BAL41 (refractive index catalog value n _d = 1.56384, strain point temperature 48).
It is molded using a glass material having a temperature of 6 ° C. and an annealing point temperature of 523 ° C.). The surface precision of the optical glass element 1 before the annealing treatment is 2 Newton fringes on both sides, the Talysurf PV value is 0.15 μm, and the optical characteristics are such that the refractive index near the center is n _d = 1.56010, the radial direction. Has a refractive index distribution of about 3
It was 0 / 100,000.

This optical glass element 1 is put in an electric furnace (annealing furnace) with a program temperature adjusting mechanism equipped with a muffle (not shown) and sealed. Then, as shown in FIG. Heat up to 500 ° C, which is the "highest temperature". By heating to the "maximum temperature", the viscosity of the glass material decreases and strain is alleviated.

In this state, the "maximum temperature" is maintained for 600 minutes, that is, the "holding time" of 10 hours, to absorb the time difference until the distortion due to the variation in the temperature in the furnace or the difference in the individual elements of the optical glass element 1 is removed. . After "holding time" has passed, 1
Annealing (gradual cooling) is performed at a “cooling rate” of 2 ° C. per hour. The "cooling rate" in this case is set so as not to generate new strain inside the optical glass element 1 from which the strain has been removed.

In this way, cooling proceeds at a constant "cooling rate", and when the temperature inside the furnace has dropped to "outside air cooling temperature", outside air is introduced into the furnace. Wait for the temperature in the furnace to fall to a temperature at which the optical glass element 1 can be taken out, and then the optical glass element 1
Then, the annealing process is completed. The time required for the treatment was about 160 hours, that is, less than one week.

Mounting evaluation and refractive index measurement of the optical glass element 1 thus obtained were carried out. This result is Experiment 1
Is as shown in Table 2. The annealing conditions are shown in Table 1 as Experiment 1.

Next, in the first embodiment, the same glass material as that used in the conventional annealing method is similarly shaped and processed,
The optical glass element 1 of FIG. 1 is obtained. Then, in the same manner, put it in the annealing furnace, seal it, and similarly "maximum temperature" 500 ° C.
Heat up to.

In this embodiment, thereafter, cooling was performed at a "cooling rate" of 100 ° C. per hour after a "holding time" of 20 minutes. This cooling is different from the conventional annealing method. When the temperature inside the furnace has dropped to the "outside air cooling temperature", the outside air is introduced into the furnace. After that, when the temperature inside the furnace has dropped to a temperature at which the optical glass element 1 can be taken out, the optical glass element 1 is taken out and the annealing process is completed. To do. In this embodiment, the time required for the annealing treatment was about 8 hours.

For comparison with Experiment 1 which is a conventional annealing method, the annealing conditions of this embodiment are shown in Table 2 as Experiment 2, and the characteristics of the optical glass element after annealing are shown in Table 2 as Experiment 2.

[0021]

[Table 1]

[0022]

[Table 2]

As can be seen from Table 2, in Experiments 1 and 2, the inside could be made almost homogeneous. That is, even if the annealing rate was increased, the glass material performance other than the absolute value of the refractive index could be maintained.

Further, the surface accuracy could be improved as compared with the conventional annealing condition. It is considered that this is because under the conditions of Experiment 1, it takes too long to expose the lens to the high temperature. As a result, the surface accuracy was slightly deteriorated, and thus the resolution was further inferior to that of the lens of Experiment 2.

As described above, when the annealing method according to the present embodiment is compared with the conventional annealing method, the optical characteristics of the optical glass element are excellent in the annealing method according to the present embodiment, and the annealing time is about 1 / It is 20 and is advantageous in terms of cost.

The annealing method of the present embodiment uses s
-It is similarly effective for glass materials other than BAL41. When changing the glass material, it is possible to set the maximum temperature and the cooling rate in accordance with the change in the refractive index characteristics due to the strain point temperature of the glass material and the cooling rate.

The cooling rate is from 20 ° C./h to 10
0 ° C./h is the best from the viewpoint of the balance between productivity and performance. Even if the cooling rate is slower than 20 ° C./h, the characteristic improvement commensurate with the increase in the processing time cannot be obtained, and the cost is too high, contrary to the performance. In addition, the cooling rate is 100 ℃
If it is faster than / h, it is technically difficult to uniformly cool the inside of the furnace and the optical glass element, and the quality may vary.

In this embodiment, it goes without saying that the shape of the molded optical glass element does not matter, but the maximum temperature and its holding time can be selected according to the shape and volume of the optical glass element. It is effective for maintaining accuracy. For example, when the volume (outer diameter) of the optical glass element increases, it is preferable to slightly increase the holding time in order to ensure homogeneity.

[Embodiment 2] In this embodiment, the optical glass element 1 and the annealing furnace used are the same as those in Embodiment 1, but the maximum temperature during the annealing condition is different from that in Embodiment 1. There is. That is, as shown in Table 3, in Experiment 3, the temperature was set to 520 ° C., which is higher than the maximum temperature of Experiment 1 of 500 ° C. and close to the annealing point temperature (523 ° C.). Further, in Experiment 4, the temperature was higher than the annealing point temperature, lower than the transition point temperature, and higher than that in Experiment 3 by 540.
The maximum temperature was set to ° C. The cooling rate is 100 ° C./hour as in the first embodiment.

On the optical glass element 1 thus obtained, mounting evaluation and refractive index measurement of the optical glass element were carried out as in the first embodiment. Table 3 shows the annealing conditions and Table 3 shows the measurement results.

[0031]

[Table 3]

[0032]

[Table 4]

As can be seen from Table 4, in Experiments 2, 3, and 4, the inside could be made substantially homogeneous. That is, even if the annealing rate was increased, the glass material performance other than the absolute value of the refractive index could be maintained. Regarding the surface accuracy, deterioration can be confirmed in Experiments 3 and 4, and as a result, the resolution is inferior to that of the optical glass element of Experiment 2. From this, it is considered that the vicinity of the annealing point temperature is the limit as the maximum annealing temperature of the optical glass element.

In the annealing method according to this embodiment,
Compared with the conventional annealing method, it is advantageous in both product quality and cost, and this annealing method is the same as in the first embodiment.
Similarly, it is effective for glass materials other than s-BAL41. When the glass material is changed, the maximum temperature is set according to the characteristics of the glass material near the strain point temperature. Further, the shape of the optical glass element does not matter, but it is effective from the viewpoint of maintaining the surface accuracy that the maximum temperature and the holding time thereof are selected according to the shape and volume of the optical glass element. For example, when the volume (outer diameter) of the optical glass element increases, it is preferable to slightly increase the maximum temperature holding time in order to ensure homogeneity.

[0035]

As described above, according to the first aspect of the present invention, when cooling the optical glass element, the time required for annealing can be shortened while maintaining the internal homogeneity of the glass, so that the annealing can be performed efficiently. be able to.

According to the second aspect of the invention, the internal strain and the refractive index distribution can be removed in a short time while minimizing the deformation amount of the optical glass element, and the annealing can be efficiently performed.

[Brief description of drawings]

FIG. 1 is a side view of an optical glass element that is annealed.

FIG. 2 is a characteristic diagram showing changes in temperature during annealing.

Claims (2)

[Claims] 1. An annealing method for an optical glass element, wherein a cooling rate is 20 ° C. to 100 ° C./hour in an annealing method for an optical glass element that removes distortion and a refractive index distribution due to an influence of a molding process or the like. 2. The optical glass element according to claim 1, wherein a maximum temperature in the annealing furnace is set to a strain point temperature of the glass material to be annealed or higher and an annealing point temperature or lower, and a maximum temperature holding time is set to 5 minutes to 1 hour. Annealing method.