Patent Application
20160363697
This application claims priority from Japanese Patent Application No. 2015-117175 filed on Jun. 10, 2015, the disclosure of which is incorporated herein by reference in its entirety.
Methods disclosed herein relate to manufacture of a scanning lens made of plastic.
Scanning lenses molded of plastic are known in the art. Among materials available for such a scanning lens, rigid materials of high flexural strength which resists fracture during the molding process are preferred, but many of such rigid materials have high photoelastic coefficients. Therefore, a scanning lens as molded of such a material would exhibit a high level of birefringence (due to residual stress resulting from the molding process) which should be detrimental to its imaging characteristics.
In order to reduce the detrimental effect of birefringence, the molded scanning lens may be annealed. However, annealing, if excessively applied, would disadvantageously cause thermal deformation of a lens surface. On the other hand, insufficient annealing would leave some residual stress unremoved which would disadvantageously fail to sufficiently reduce birefringence, thus deteriorating the imaging characteristics.
In this respect, it would be desirable to provide a method for manufacturing a scanning lens which exhibits improved properties in terms of birefringence to thereby enhance imaging characteristics.
In one aspect, a method for manufacturing a scanning lens elongate in a main scanning direction is disclosed herein. The method comprises a first step of forming a lens by injecting a molten plastic material into a mold, and a second step of annealing the lens formed in the first step under conditions to modify optical properties of the lens such that:
for each position to be scanned along the main scanning direction,
|R×Δφ×Ws|≦19.248λ−2020.7 (1)
where R is a maximum of retardation by birefringence [nm], Δφ is a variation in optic axis orientation per unit length in a sub scanning direction [deg/mm], Ws is a beam width in the sub scanning direction at an incident surface of the lens [mm], and λ is a wavelength of a beam [nm].
The above and other aspects, their advantages and further features will become more apparent in detail illustrative, non-limiting description with reference to the accompanying drawings, in which:
A detailed description will be given of an illustrative, non-limiting embodiment with reference made to the drawings where appropriate. In the following description, a general setup of a light-scanning optical system with a scanning lens as an illustrative example will be described briefly with reference to
As shown in
The coupling lens 2 is provided between the semiconductor laser 1 and the polygon mirror 5 and configured to convert the light beam emitted from the semiconductor laser 1 into a light beam converging slightly in a main scanning direction and focused near a reflecting surface 5A of the polygon mirror 5 in a sub scanning direction. Herein, the main scanning direction is a direction in which the beam is deflected by the polygon minor 5, that is, the lateral (right-left) direction in
The aperture stop 3 is a member having an aperture defining sizes, in the main scanning direction and in the sub scanning direction, of the light beam coming from the coupling lens 2 (beam widths or diameters).
The polygon mirror 5 has a plurality of reflecting surfaces (specular surfaces) 5A disposed equidistantly from an axis 5B of rotation of the polygon mirror 5; the polygon mirror 5 shown in
The scanning lens 6 is arranged to allow a laser beam reflected and thus deflected by the polygon minor 5 to pass therethrough. This scanning lens 6 is configured to converge the laser beam in the main scanning direction and the sub scanning direction into a spot-like image to be focused on the image surface 9A to be scanned. The scanning lens 6 is also configured to correct an optical face tangle error of each reflecting surface 5A of the polygon minor 5. The scanning lens 6 has f-theta characteristics such that the laser beam deflected at a constant angular velocity by the polygon minor 5 is converted into a laser beam that scans the image surface 9A at a constant linear velocity.
The scanning lens 6 is made of plastic material and has an elongate shape (elongate in the main scanning direction). The plastic material for the scanning lens 6 preferably has a glass transition temperature between or equal to 135° C. and 145° C. Such a relatively high glass transition temperature of the plastic material used for molding the scanning lens 6 serves to enhance the flexural strength of the scanning lens 6.
The plastic material for the scanning lens 6 may preferably have a photoelastic coefficient (absolute value) equal to or greater than 1.5×10−14 [Pa−1]. The photoelastic coefficient (absolute value) of the plastic material for the scanning lens 6 may preferably be equal to or smaller than 1.0×10−12 [Pa−1]. For example, the plastic material may be cyclic polyolefin.
A method for manufacturing a scanning lens 6 will be described hereafter.
First, the scanning lens 6 is formed by injecting a molten plastic material into a mold (first step).
Next, in order to reduce the residual stress in the scanning lens 6 for improved properties in terms of birefringence (i.e., in favor of reduced birefringence), the scanning lens 6 formed in the first step is annealed (second step). In this step, if the annealing is insufficient, the birefringence would still be at such a non-negligible level that the imaging characteristics at the image surface 9A with a laser beam passing through the scanning lens 6 could become deteriorated as compared with the case in which no birefringence is observed. Contrariwise, if the annealing is subjected excessively, the shape of the lens surface of the scanning lens 6 would be distorted, which could also result in deterioration of its imaging characteristics. With this in view, the second step of annealing the scanning lens 6 is executed under conditions specified (optimized) to modify the optical properties of the scanning lens 6 to those which satisfy the following conditions (1) and (2):
for each position to be scanned along the main scanning direction,
|R×Δφ×Ws|≦19.248λ−2020.7 (1)
where R is a maximum of retardation by birefringence [nm], Δφ is a variation in optic axis orientation per unit length in the sub scanning direction [deg/mm], Ws is a beam width in the sub scanning direction at an incident surface of the scanning lens [mm], and λ is a wavelength of a beam [nm], and
5000≦R×Δφ×Ws|max (2)
where |R×Δφ×Ws|max is a maximum of values of |R×Δφ×Ws| obtained in positions to be scanned along the main scanning direction within a main scanning range.
The inequality (1) represents the condition under which a laser beam (with any of wavelengths of 850, 788, 650 and 515 [nm]) passing through the scanning lens 6 and striking the image surface 9A forms a beam spot having a diameter not increased more than 10% as compared with the case with a lens in which no birefringence is observed. Therefore, the annealing step executed under this condition (1), even if it is subjected to a scanning lens 6 formed of a material with a high photoelastic coefficient, will result in improved imaging characteristics of the scanning lens 6.
The inequality (2) represents the lower limit of |R×Δφ×Ws|max with which distortion in the shape of the lens surface of the scanning lens 6 as would be caused by annealing process falls within a permissible range. As annealing process proceeds, |R×Δφ×Ws|max decreases. However, as shown in
Variables employed in the inequalities (1) and (2) will be described hereafter with reference to
As shown in
As shown in
As shown in
The beam width Ws in the sub scanning direction at an incident surface of the scanning lens 6 varies with the positions to be scanned along the main scanning direction. For instance, the beam width Ws in the sub scanning direction at the incident surface of the scanning lens 6 increases toward outside in the main scanning direction, as shown in
As described above, the value of |R×Δφ×Ws| for each position to be scanned along the main scanning direction becomes smaller after annealing than before annealing and the variation thereof as compared among the positions to be scanned along the main scanning direction becomes smaller after annealing than before annealing.
In this specific example, the annealing step is executed to modify the optical properties of the lens such that, after annealing, the value of |R×Δφ×Ws| for each position to be scanned along the main scanning direction satisfies |R×Δφ×Ws|≦19.2482λ−2020.7 and 5000≦|R×Δφ×Ws|max.
Next, the method of annealing in the second step will now be described in detail.
In the second step, as shown in
The predetermined period of time tc is determined with a temperature condition fixed within the predetermined range, such that the optical properties of the scanning lens 6 achieved after the annealing process satisfy both of the conditions (1) and (2). To be more specific, the predetermined period of time tc is determined such that the area of a shaded region in
To give an example, the target maximum temperature T1 may be a temperature lower than the glass transition temperature by 1.5 to 4.5° C., for example, by 3° C., the takeout temperature T2 may be a temperature lower than the heat distortion temperature by a degree not less than 7° C., and the cooling rate may be 2.0° C./min. or less.
It is to be understood that various modifications and changes may be made to the specific structure and process steps as described above by way of example.
In the above description, the scanning lens 6 is made of cyclic polyolefin. The plastic material for the scanning lens 6 may be cyclic polyolefin polymer or cyclic polyolefin copolymer.
Furthermore, the plastic material for the scanning lens 6 is not limited thereto; for example, the plastic material for the scanning lens 6 may be other plastic material such as polycarbonate.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015-117175 | Jun 2015 | JP | national |
