A scanning optical apparatus

Abstract: A scanning optical apparatus has a source (1) of light for producing a light beam as a light flux (5) in a given direction, a polariscope (2) for polarizing the light flux (5) of the light beam in another given direction, a member or medium which is scanned by the light flux (5) of the light beam while the light flux (5) of the light beam is polarized by the polariscope (2), and a plastic lens (3) placed between the polariscope (2) and the scanned member or medium (4) for forming an image spot on the scanned member or medium (4). The plastic lens (3) has two kinds of surfaces, one of which is a polarization surface or optical surface through which the light flux (5) of the light beam passes, and the other of which is a non-polarization surface or non-optical surface through which the light flux (5) of the light beam does not pass. All of the surfaces of the plastic lens (3) have dampproofing characteristics.

Description: The present invention relates to a scanning optical apparatus comprising a source of light for generating a light beam as a light flux, a deflector for deflecting the light beam generated by said source of light, a scanned member which is scanned by the light flux of the light beam deflected by said deflector, and an image forming optical means placed between said scanned member and said deflector, said image forming optical means including at least a plastic lens, wherein the entire surface of the lens, including the surface portions through which the deflected light is transmitted and the surface portions on which no deflected light is incident, is coated with a protection film so as to achieve dampproofing characteristics without having any substantial effect on the optical characteristics of said plastic lens. Such a scanning optical apparatus is known from JP-A-1 100 510. The present invention relates generally to a scanning optical system or apparatus which has an image forming optical means including a lens system at least a part of which is made of a plastic material. A prior art scanning optical system comprises a source of light, a deflector for deflecting a beam of light generated from the source of light as a light flux, a scanned member or medium which is scanned by the light beam of the light flux deflected by the deflector, and an image forming optical means placed between the deflector and the scanned member or medium. Lenses made of a plastic material can be employed in the image forming optical means. A plastic lens can be manufactured at a cost much lower than that of a glass lens. In addition, plastic lenses of a special shape can also be mass-produced as well. Therefore, plastic lenses are now widely used in a scanning optical system. Acrylic is a typical material for producing a plastic lens. However, the plastic material has such a shortcoming that it is effected by the temperature and humidity more easily than glass. With respect to humidity in particular, the plastic material has a property of absorbing moisture so that, in an environment with a high humidity, it absorbs moisture. As a result, its index of refraction varies largely due to absorbed moisture. As known, the greater the quantity of moisture absorbed, the greater the change in index of refraction. As described above, a plastic lens has a property of absorbing moisture. The absorbed moisture largely changes its index of refraction and the shape of the lens as well. Accordingly, the image forming performance of the lens is effected by the humidity with ease. As a result, the scanning optical system has a shortcoming that its characteristic is not stable. A problem with protection films, as used for the plastic lens of the above-mentioned prior art optical system according to JP-A-1 100 510, is that the optical characteristics of the plastic lens may be adversely affected. From Patent Abstracts of Japan, Vol. 7, No. 48 (P-178), 24.02.83 & JP-A-57 197501 it is known to deposit a vinylidene chloride resin film as a moisture proof film on a prism consisting of acrylic resin. Further, it is known from Patent Abstracts of Japan, Vol. 12, No. 60 (E-584) 23.02.88 & JP-A-62 202437 to provide a film made of a polymer containing vinylidene chloride on a surface of a lens made of plastic material. In view of the problems of the conventional technology described above, it is an object of the present invention to provide a scanning optical apparatus or system which employs a plastic lens with an image forming performance hardly affected by humidity. The above object is achieved by a scanning optical apparatus, mentioned at the outset, wherein said protection film comprises a crystalline high-polymer material which is a solvent-soluble crystalline material selected from a vinylidene chloride and a vinylidene chloride copolymer. According to the present invention, a scanning optical apparaus generally comprises a deflector for deflecting a light beam originated from a source of light as a light flux, a scanned medium or member which is scanned by the light flux of the light beam deflected by the deflector, and an image forming optical means placed between the scanned member or medium and the deflector. The image forming optical means has at least a plastic lens. The plastic lens has at least two surfaces. One surface is an optical surface or, in other words, a surface on which the deflected light is incident through which the light beam passes. The other surface is non-optical surface or, in other words, a surface on which the deflected light is non-incident. Both of the two surfaces have a dampproofing character or characteristic over the entire area thereof. The present invention will become more apparent from the following detailed description of a preferred embodiment with reference to accompanying drawings in which: Fig. 1 is a conceptual diagram showing simply a scanning optical apparatus according to a preferred embodiment of the present invention; Fig 2. shows a process of absorbing and draining moisture which occurs on a non-optical surface perpendicular to an optical surface of a plastic lens; Fig. 3 shows a distribution of the index of refraction on the non-optical surface perpendicular to the optical surface of the plastic lens shown in Fig. 2; Fig. 4 shows a process of absorbing and draining moisture which occurs on a optical surface of a plastic lens; Fig. 5 shows a distribution of the index of refraction on the optical surface of a plastic lens shown in Fig. 4; Fig. 6 shows day-to-day changes in spot size of a plastic lens not subjected to a dampproofing process; and Fig. 7 shows day-to-day changes in spot size of a plastic lense experiencing or subjected to a dampproofing process. An example of a scanning optical apparatus according to a preferred embodiment of the present invention is shown in Fig. 1. A source 1 of light is provided in a conventional manner so as to produce a light beam in a given direction toward a polariscope 2. The light beam originating from the source 1 of light is deflected by the deflector 2 so as to pass through a plastic lens 3 toward a medium or member 4 to be scanned as a light flux 5. The light flux 5 is deflected by the deflector 2 and then passes through the plastic lens 3. The light beam of the light flux 5 finally forms an image forming spot on the scanned medium or member 4. The configuration of the scanning optical apparatus allows the image forming spot to scan over the entire area of the scanned member or medium 4, accompanying the rotation of the deflector 2. The lens 3 constitutes a part of an image forming optical means. The image forming optical means may include a set of plural lenses comprising at least one plastic lens although not shown. As shown conceptually in Fig. 1, the plastic lens 3 comprises two opposite surfaces on which deflected light is incident (in the following referred to as optical surfaces) through which the light flux 5 passes, and four surfaces on which deflected light is non-incident (in the following referred to as non-optical surfaces) through which the light flux 5 does not pass. The plastic lens 3 is elongate, and the length of the plastic lens 3 in a direction parallel to the optical surfaces is larger than its length in a direction perpendicular to the optical surfaces. All surfaces of the plastic lens 3 have dampproofing characteristics over the entire area thereof. The optical surfaces and the non-optical surfaces thereof are subjected to a dampproofing process or treatment. In a typical dampproofing process, the entire area of all surfaces of the plastic lens 3 are coated with a solvent-soluble crystalline high-polymer material such as vinylidene chloride. Preferably, such a crystalline high-polymer material for the coating purpose that does not have any effect on the optical characteristics of the plastic lens is selected. In case the solvent-soluble crystalline high-polymer material has some effect on the optical characteristics, the effect has to be so small that no problem is resulted in. As described above, in addition to the vinylidene-chloride, a copolymer of the vinylidene-chloride family can also be used as the solvable crystalline high-polymer material. These substances can be used as a shield for protecting the elaborateness of the lens structure against moisture molecules. The plastic lens 3 is subjected to a dampproofing process or treatment over the entire area of all surfaces, that is, the two optical surfaces and the four non-optical surfaces thereof. Examples to be used for comparison, which undergo a dampproofing process over a part or parts of the entire area of the surfaces of the plastic lens will be explained. For example, either the optical surface or the non-optical surface thereof is subjected to a dampproofing treatment. The effect of the entire dampproofing process in case of the present invention will be compared to that of the partial dampproofing process in case of the comparative examples. In Figs. 2 to 5, the reference d designates depth from the surface of the plastic lens. A typical result of the comparative example shown in Fig. 3 is obtained from a process shown in Fig. 2. Fig. 2 shows a state in which the non-optical surfaces of the plastic lens 3 undergo a moisture absorbing and draining process as indicated by directions of arrows. Fig. 3 shows a distribution of the index of refraction in the plastic lens 3 the non-optical surfaces of which experienced the moisture absorbing and draining process indicated in Fig. 2. The state shown in Fig. 2 is equivalent to a case in which only the optical surfaces of the plastic lens 3 undergo a dampproofing process and then is exposed to an environment with a high humidity. In such a case, the index of refraction of the plastic lens 3 is distributed in a non-uniform manner as shown in Fig. 3 so as to depend on the depth from the non-optical surfaces of the lens. The index of refraction varies, depending upon the depth from the non-optical surface. As described above, the index of refraction in the plastic lens 3 has a nonuniform distribution due to absorbed moisture, affecting the beam of light that passes through the plastic lens 3 as the light flux 5. It is thus understood that, by applying the dampproofing process only to the optical surface of the plastic lens 3, a desired effect cannot be obtained. Another typical result of the comparative example shown in Fig. 5 is obtained from a process shown in Fig. 4. Fig. 4 shows a state in which the optical surfaces of the plastic lens 3 undergo a moisture absobing and draining process as indicated by directions of arrows. Fig. 5 shows a distribution of the index of refraction in the plastic lens 3, the optical surface of which experienced the moisture absorbing and draining process indicated in Fig. 4. The state indicated in Fig. 4 is equivalent to a case in which only the non-optical surface of the plastic lens 3 undergoes a dampproofing process and then is exposed to an environment with a high humidity. Also in such a case, the index of refraction of the plastic lens 3 is distributed nonuniformly as shown in Fig. 5. As shown in the figure, the index of refraction varies, depending upon the depth from the optical surface. As described above, the index of refraction of the plastic lens 3 has a nonuniform distribution due to absorbed moisture, affecting the beam of light that passes through the plastic lens 3. It is thus understood that, by applying the dampproofing process only to the non-optical surface of the plastic lens 3, a desired effect cannot be obtained. As described above, if only either the non-optical surface or the optical surface of the plastic lens 3 undergoes the dampproofing process, the desired effect cannot be obtained. On the other hand, if both the non-optical surface and the optical surface of the plastic lens 3 do not experience the moisture absorbing and draining process, the index of refraction of the plastic lens 3 becomes uniform. The state of the plastic lens 3 with both the non-optical surface and the optical surface of the plastic lens 3 experiencing no moisture absorbing and draining process is equivalent to the case of exposing the plastic lens 3 after both the non-optical surface and the optical surface of the plastic lens 3 have undergone a dampproofing process. Next, experiments will be explained in which two plastic lenses undergoing no dampproofing process and five plastic lenses experiencing a dampproofing process on the entire areas of both the optical surfaces and the non-optical surfaces thereof were exposed to an environment with a high humidity. Note that the two plastic lenses and the five plastic lenses are all of the same type as shown in Fig. 1. After the exposure to the high-humidity environment, variations in spot sizes were observed. Both experiments were carried out under all the same conditions except the time of exposure to the high-humidity environment and the initial spot sizes. Results of experiments without and with the dampproofing process are shown in Figs. 6 and 7 respectively. It is observed from Fig. 6 that a plastic lens undergoing no dampproofing process exhibits large variations in spot size even before 20 days have lapsed since the start of the exposure to the environment with a high humidity. On the other hand, it is observed from Fig. 7 that a plastic lens experiencing a dampproofing process on the entire area of both the optical surface and the non-optical surface thereof exhibits a stable spot size even after 50 days have lapsed since the start of the exposure to the environment with a high humidity. In a scanning optical apparatus according to the present invention, a plastic lens employed in an image forming optical means has dampproofing characteristics on the entire area of both the optical surfaces and the non-optical surfaces thereof to give stable image forming characteristics not effected by humidity over a long period of time.

Claims: Scanning optical apparatus, comprising: a source of light (1) for generating a light beam as a light flux; a deflector (2) for deflecting the light beam generated by said source of light (1); a scanned member (4) which is scanned by the light flux of the light beam deflected by said deflector (2); and an image forming optical means placed between said scanned member (4) and said deflector (2), said image forming optical means including at least a plastic lens (3), wherein the entire surface of the lens (3), including the surface portions through which the deflected light is transmitted and the surface portions on which no deflected light is incident, is coated with a protection film so as to achieve dampproofing characteristics without, having any substantial effect on the optical characteristics of said plastic lens (3), characterized in that said protection film comprises a crystalline high-polymer material which is a solvent-soluble crystalline material selected from a vinylidene chloride and a vinylidene chloride copolymer. The scanning optical apparatus of claim 1, characterized in that said plastic lens (3) has a long shape extending in a direction which is substantially parallel to the surface portions of the lens through which the deflected light is transmitted. The scanning optical apparatus of any of claims 1 - 2, characterized in that said plastic lens (3) has two opposite surfaces through which the deflected light is transmitted, and four surfaces on which no deflected light is incident. The scanning optical apparatus as defined in any of claims 1 - 3, characterized in that said deflector is a rotatable polygonal mirror (2).

Inventors: TAKAHASHI TAKASHI, TAKAHASHI, TAKASHI
Applicants: TOPCON CORP, KABUSHIKI KAISHA TOPCON
Application year: 1993
Main classification: G02B26
IPC codes: G02B1, G02B7, G02B26