JP2001194621A - Optical device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical device, such as an optical write head which is capable of controlling the incident angle of incident light with a simple structure in order to suppress the spread of image formation dependent upon the incident angle which occurs in a lens array of an unimagnification image forming optical system. SOLUTION: An optical filter 8 which controls the incident angle on the unimagnification image forming optical system 8 is disposed on the incident side of the unimagnification image forming optical system, by which the image formation unsharpness on the image forming plane of a photoreceptor surface 5a, etc., may be improved. The optical device may therefore be realized with the sample structure without requiring any change at all for the light emitting element array 4 itself.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an LED used for an optical writing head of an electrophotographic optical printer, a digital copying machine, a facsimile or the like, or an optical reading unit such as a scanner.
The present invention relates to an optical device including an array, an EL array, an optical shutter array, or the like as a light emitting element array.

[0002]

2. Description of the Related Art With the development of information devices for office use and personal use in recent years, demands for higher resolution, compact and inexpensive devices for electrophotographic optical printers and scanners are increasing. As an example of a device satisfying such a demand, for example, an electrophotographic optical printer (LED) using an LED array in which a large number of LEDs are arranged
Printer). This type of printer is a solid-state scanning type using an LED array as a writing light source. Therefore, compared to a raster scanning type writing optical system using a semiconductor laser, the size of the device can be reduced, and vibration and heat can be reduced. Of the optical system due to the
Since D performs writing in parallel, there is an advantage that high-speed output can be achieved relatively easily.

FIG. 7 is a schematic cross-sectional view including a drum-shaped photosensitive member and an optical system of a writing system of a conventionally used optical printer.
Shown in The light 101 emitted from the LED array 100 passes through the same-magnification imaging optical system 102, and
Only the light that has entered the field of view 02 condenses on the photoconductor 103 and exposes the photoconductive surface 103a.

Here, the equal-magnification image forming optical system 102 is usually constituted by a lens array, the aberration varies depending on the incident angle with respect to the lens, and the image surface of the photosensitive member 103 as the image forming surface is blurred or misaligned. There is a problem that occurs. The light 101 emitted from the LED array 100 is incident on a lens (normally the lens closest to the LED) through which the principal ray of each LED passes and a plurality of lenses near the lens in the lens array (the same-magnification imaging optical system 102). At this time, the lens through which the principal ray passes is the brightest and has good image forming performance. As the lens moves away from the lens, the image forming performance deteriorates, and the image is blurred or displaced on the image forming surface, and unnecessary flare occurs.

In order to improve such optical characteristics,
There has been proposed a method for defining an incident angle of light incident on the unity magnification imaging optical system 102. It has been experimentally confirmed that the imaging aberration on the photoconductor 103 is improved by making the incident angle of the light 101 narrower than the viewing angle of the lens array (the same magnification imaging optical system 102).

As a structure for realizing this method, for example, a structure for narrowing the radiation angle of a light source proposed in Japanese Patent Application Laid-Open No. H11-78115 can be used. FIG. 8 shows an example of the structure. The LED array head 110 has a narrowing optical system 114 including a reflection mirror 113 for each light emitting element (LED) 112 of the light emitting element array 111, so that the light emission angle of the light emitting element 112 is narrowed and effectively equalized. The light is coupled to the imaging optical system 115 and is imaged on the imaging surface 116 of the photoconductor. At this time, light emitted from the light emitting element 112 is emitted as direct light 117a or indirect light 117b reflected by the reflection mirror 113. According to this configuration, since the radiation angle of the light source (the light emitting element 112) is narrow, light is made to enter only a limited lens of the equal-magnification imaging optical system 115 composed of a lens array.
6 is suppressed.

On the other hand, a light shielding plate having a large aspect ratio is arranged on an optical path through which light can be transmitted to remove light at an unnecessary angle.
An ED array head structure has been proposed in Japanese Patent Application Laid-Open No. 9-58052. FIG. 9 shows an example of the structure. This L
In the ED array head, light emitted from the LED element group 120a on the LED chip 120 enters the lens portion 121a of the lens array 121, inverts the image, and forms an image on the image forming surface 122 of the photoconductor. You. LED element group 1
The light blocking member 123 for limiting the angle of incidence is disposed between the lens 20a and each lens 121a, so that light in an unnecessary direction can be prevented from entering the lens portion 121a.

Further, as a similar structure, an example using a microlouver has been proposed in Japanese Patent Application Laid-Open No. 9-174932. The LED print bar of this proposal example is
The incident angle of the light emitted from the LED elements
The light is limited by a light-shielding microlouver array, is incident on an equal-magnification imaging optical system, and forms an image of an LED element on an image plane.

[0009]

However, in the case of the countermeasure shown in FIG. 8, the conventional light emitting element array cannot be realized as it is, and it is necessary to separately provide the narrowing optical system 114. That is, the light emitting element array itself needs to be changed.

In the case of the countermeasure shown in FIG. 9, the incident angle can be sufficiently limited by using the light shielding member 123 formed of a thin plate, but when the lens pitch P becomes small, for example, P When ≦ 1 mm, it becomes difficult to manufacture the light shielding member 123. Further, in the structure including the light shielding plate having such a lens pitch, the limiting characteristic of the incident angle of the incident light viewed from the optical system changes depending on the position of the light shielding plate, and the variation period of the incident angle of the light shielding structure varies. There is also a problem that imaging deterioration is caused.

Japanese Patent Application Laid-Open No. 9-174932 aims to improve the depth of focus with a micro louver structure, and the performance changes depending on the micro louver array. Therefore, the structure is not suitable for stably narrowing the incident angle.

Accordingly, the present invention provides a method for suppressing the spread of an image depending on an incident angle, which is generated in a lens array constituting a unit-magnification imaging optical system.
Another object is to provide an optical device such as an optical writing head that can control the incident angle of emitted light with a simple structure.

[0013]

According to the first aspect of the present invention,
In an optical device that forms an outgoing light from a light emitting element array on an image forming surface via an equal-magnification imaging optical system, an incident angle with respect to the equal-magnification imaging optical system or an emission angle from the equal-magnification imaging optical system Is disposed on at least one of the entrance side and the exit side of the same-magnification imaging optical system.

Since the same-magnification image forming optical system has an aberration depending on the incident angle, by combining an optical filter for controlling the incident angle (or the outgoing angle) with the same-magnification image forming optical system, the surface of the photosensitive member can be improved. Is improved on the imaging plane. At this time, no change is required for the light emitting element array itself. Incidentally, in many cases, the same-magnification imaging optical system is constituted by a lens array.
It is necessary to collect light with a wide radiation angle emitted from the array within the viewing angle of the lens, and the lens is designed so that the overall light-collecting performance does not decrease in consideration of the aberration depending on the incident angle. However, due to the spherical aberration of the lens, the aberration is not always reduced on the optical axis of the lens, and the imaging performance may be designed to be the maximum at an incident angle slightly shifted from the optical axis of the lens. The incident angle control by the optical filter of the present invention also takes such a case into consideration, and preferentially transmits light having an incident angle at which the aberration is minimized, and combines the same with an equal-magnification imaging optical system to form an image of a light emitting element array. Is minimized when the image is formed on the image plane.

According to a second aspect of the present invention, in the optical device according to the first aspect, the equal-magnification image-forming element comprises a lens array, and an incident angle controlled by the optical filter is smaller than a viewing angle of the lens array. Therefore, there is no dependency on the incident position.

In the conventional structure comprising a light shielding plate having a lens pitch, there has been a problem that the characteristic of limiting the incident angle of incident light viewed from the optical system changes depending on the position of the light shielding plate. For example, since the lens has characteristics that are almost independent of the incident position, the optical characteristics viewed from the individual lenses of the lens array, which is a unit-magnification imaging optical system, do not fluctuate with the lens pitch. The incident angle perceived by the light passing through the center and the light passing through the periphery of the lens does not change, and the imaging deterioration caused by the fluctuation cycle of the incident angle as in the case of the light shielding structure does not occur. As described above, since the optical filter that limits the incident angle and improves the imaging performance of the unit-magnification imaging optical system can control the incident angle almost independently of the incident position, the mounting position accuracy can be low and the optical device can be used with a low accuracy. The imaging performance can be improved while simplifying the manufacturing process.

According to a third aspect of the present invention, in the optical device according to the second aspect, the optical filter is configured such that light emitted from the light emitting element array is incident only on the closest lens of the lens array. Limit that angle.

Consider a case where a gradient index lens array generally used in an LED printer is used as an equal magnification imaging optical system. A gradient index lens array is usually 2
The three lenses adjacent to each other are arranged in a bales in a row, and three adjacent lenses are light emitting elements of a light emitting element array, for example, LE.
This is the closest lens seen from D. The light incident on the closest lens forms the brightest image with the least image deterioration. In the present invention, the characteristic of the optical filter is limited to the incident angle so that the light is incident only on the closest lens of the lens array. Image degradation on the image plane can be suppressed. That is, since only the lens having the best transmission efficiency and the best imaging performance in the lens array can be selectively used,
The imaging performance can be improved without significantly lowering the transmission efficiency.

The invention described in claim 4 is the first to third aspects of the present invention.
In the optical device according to any one of the above, the optical filter is disposed on the light emitting element array side with respect to the unit magnification imaging optical system.

Light emitting elements of a light emitting element array, for example, LE
The emission angle of D usually has a Lambert distribution, and has emission characteristics that are sufficiently wider than the viewing angle of the 1 × imaging optical system. Therefore, it is desirable to provide an optical filter on the incident side in order to prevent unnecessary light from being incident on the lens from the beginning. An optical filter is provided on the exit side of the unity imaging optical system, and even if the exit angle is restricted, image aberration on the image plane can be reduced to some extent, but by providing it on the entrance side, flare light is suppressed at the same time. it can.

[0021] The invention according to claim 5 is the invention according to claims 1 to 4.
In the optical device according to any one of the above, the optical filter includes a multilayer thin film.

By using a multilayer thin film having a transmittance that is dependent on the incident angle, a structure for controlling the incident angle independently of the incident position of light can be easily realized. In addition, uniform characteristics can be easily realized, and the alignment process of the optical filter can be greatly simplified.

According to a sixth aspect of the present invention, in the optical device according to the fifth aspect, the equal-magnification imaging element is formed of a gradient index lens array, and the multilayer thin film is formed on an end face of the gradient index lens array. It is directly laminated.

When manufacturing a gradient index lens array which is an equal magnification image forming optical system, an incident angle (or an outgoing angle) is simultaneously determined.
By depositing a multilayer thin film directly on the lens surface,
Since the optical filter and the lens are integrated, the optical device can be simplified both in manufacturing and in structure.

According to a seventh aspect of the present invention, in the optical device according to the fifth or sixth aspect, the optical filter is arranged such that the light filter is perpendicularly incident on the film at the shortest wavelength of the emission wavelength of the light emitting element array under a vertical incidence condition. It has a transmittance characteristic in which the transmittance is maximum and monotonically decreases as the wavelength becomes longer.

When the light incident on the dielectric multilayer thin film deviates from normal incidence, the wavelength dependence of the transmittance at normal incidence shifts to a shorter wavelength side (the incident wavelength shifts to a longer wavelength when viewed from the multilayer thin film). (Visible), if the wavelength dependence of the transmittance characteristics is generally attenuated as the wavelength becomes longer, the transmittance at oblique incidence becomes smaller than the transmittance under normal incidence conditions. Therefore, if a multilayer thin film having such a wavelength dependence of transmittance is used, the incident angle can be limited.

That is, at the incident angle θ where the normal incidence is 0 °, the apparent wavelength λm as viewed from the multilayer thin film becomes longer as compared with the actual wavelength λo, as in the general case of λm = λo / cosθ. If the incident angle changes from 0 degrees to θo, the incident wavelength looks from λo to λo / cosθo when viewed from the multilayer thin film. In other words, if the wavelength dependence of the transmittance characteristics of the multilayer thin film is generally attenuated from λo to λo / cos θo, the transmittance is high at normal incidence with respect to the incident light of the incident wavelength λo, and the oblique incidence at an angle θo It can be seen that the incidence angle is limited by this configuration because the transmittance is substantially reduced up to.

The light having the shortest wavelength as viewed from the multilayer thin film is light having the shortest wavelength of the emission wavelength of the light-emitting element array, which is incident under the vertical incidence condition, and the transmittance is maximized at this wavelength.
If the transmittance decreases monotonically as the wavelength becomes longer, the transmittance at oblique incidence with respect to vertical incidence always decreases monotonously over the entire wavelength range of the light emitting element array under consideration so that the incident angle can be limited. Become.

According to an eighth aspect of the present invention, in the optical device according to the fifth or sixth aspect, the optical filter has an incident angle dependency of transmittances of different wavelengths within a light emission wavelength range of the light emitting element array. Assuming that the incident angle is θ, the wavelength is λ1, λ2, the incident angle dependence of the transmittance is T, and the constant depending only on the wavelength is K, T (θ, λ1) = K (λ1, λ2) T (θ, λ2 ) It has a constant multiple relationship depending only on the wavelength.

If the incident angle dependence is not a constant multiple due to different wavelengths within the range of the emission wavelength of the light emitting element array, obliquely incident light has a high transmittance at a certain wavelength, and has a high transmittance at other wavelengths. A situation arises in which the transmittance is low. Since the same-magnification imaging optical system has an aberration that depends on the incident angle, a difference in the aberration depending on the incident wavelength occurs due to the difference in the incident angle dependence, and chromatic aberration occurs in the image formed on the image plane. become. If the incident angle dependence is a multiple of a constant depending only on the wavelength, chromatic aberration due to the incident angle dependence due to the multilayer thin film can be removed.

According to a ninth aspect of the present invention, in the optical device according to the eighth aspect, the optical filter has a wavelength dependence of a transmittance which can be approximated by an exponential function that attenuates in the longer wavelength direction with respect to the incident wavelength. .

Assuming that the wavelength λ dependency of the transmittance T is represented by T (λ), from the relationship λ (θi) between the wavelength λ and the incident angle θi,
The incident angle dependency of the transmittance is T (λ (θi)). If the ratio of the incident angle dependence does not depend on the angle even if the wavelength changes (constant times), the incident angle dependence is λa A (θ), where λa and λb are two wavelength perpendiculars, so T ( λa A (θ)) / T (λb A (θ)) = K (λa, λb) A (θ) = 1 / cos θ Since the right side has no θ dependency, the left side must be zero when differentiated. Therefore, dT (λa A (θ)) / dλ dA / dθ T (λb A (θ)) =
T (λa A (θ)) dT (λb A (θ)) / dλ dA / dθ holds. Divided into terms of λa and λb, dT (λaA (θ)) / dλ / T (λaA (θ)) = dT (λb
A (θ)) / dλ / T (λb A (θ)) must be satisfied irrespective of λa and λb, so that dT / dλ / T = αα is a constant independent of wavelength, and T (λ ) = C exp (α λ), and if the wavelength spectrum is an exponential function type, the shape of the incident angle keeps the constant times condition even if the incident wavelength changes. Since the wavelength dependence of the transmittance of the optical filter, which attenuates in the long wavelength direction and can be approximated exponentially with respect to the incident wavelength, is provided, the structure of the invention according to claim 8 can be used for a normal emission wavelength. Can be realized by designing a thin film that controls

According to a tenth aspect of the present invention, in the optical device according to any one of the first to ninth aspects, the imaging surface is a photoconductor surface, and the light emitting element array emits writing light to the photoconductor surface. .

Therefore, the present invention can be suitably applied to an optical writing head for performing optical writing on the surface of the photosensitive member.

[0035]

FIG. 1 shows a first embodiment of the present invention.
A description will be given with reference to FIG. The optical device of the present embodiment shows an application example to an optical writing head such as an LED printer.

First, an LED array 4 having individual LEDs 3 formed on a plurality of surface emitting LED array chips 2 mounted on a printed board 1 is provided as a light emitting element array. Between the LED array 4 and a drum-shaped photoreceptor 5 having a photoreceptor surface 5a serving as an image-forming surface, a lens array 7 which is an equal-magnification image-forming optical system in which a number of rod-shaped lenses 6 are arranged. Further, an optical filter 8 for controlling an incident angle with respect to the lens array 7 is provided on the incident side (the LED array 4 side) of the lens array 7.

As a result, roughly, each LED 3
Light 9a, 9b, etc., which is incident on the lens array 7 in a state almost perpendicular to the lens array 7, is incident on the lens array 7 without being reflected by the optical filter 8, and is formed on the photosensitive member surface 5a of the photosensitive member 5. An image is formed at an image position 10. On the other hand, when the emission angle of the light emitted from the LED 3 starts to deviate from the vertical, the light is reflected by the optical filter 8 as light 9c.
The light does not enter the lens array 7. Therefore, light that does not vertically enter the individual lenses 6 of the lens array 7 can be reduced, so that the spread of light at the image forming position 10 is suppressed, and an image (latent image) with less image blur can be formed. it can.

Next, details of each part will be described. The LED array chip 2 has an LED 3 of about 24 dots /
mm (= 600 dpi).
The emission wavelength of D3 is 700 nm to 720 nm in full width at half maximum. The lens array 7 constituting the unity magnification optical system includes:
Nippon Sheet Glass two-row lens array S of refractive index distribution type
LA20B is used and its working distance is 3.5m
m, the diameter of each lens 6 is 0.9 mm, and the viewing angle of the lens 6 is ± 22 degrees.

The optical filter 8 is a quartz substrate 1 having a refractive index of 1.42 and a thickness of 0.2 mm as shown in the enlarged sectional view of FIG.
1 has a laminated structure in which a non-reflective film 12 having a reflectivity of 2% is formed on one surface and a dielectric multilayer thin film 13 which is a bandpass filter designed to have a transmission peak wavelength of 700 nm is formed on the other surface. . The structure of the dielectric multilayer thin film 13 is 70
SiO ₂ film corresponding to ｎｍ wavelength of 0 nm (refractive index 1.
42) and TiO ₂ film (refractive index 2.5) 3 pairs, 1/2
Wavelength phase shift SiO ₂ film, and 1/4 wavelength Ti
Three pairs of the O ₂ film and the SiO ₂ film are laminated on the quartz substrate 11.

FIG. 3 shows a transmission spectrum of the optical filter 8 having such a configuration. The wavelengths under consideration are from 700 nm to 720 nm, and the transmittance is maximum at 700 nm, and the light amount decreases as the wavelength increases. The transmittance in FIG. 3 represents the transmittance in logarithm, and is 700 nm to 720 nm.
In the range of nm, it can be approximated by a substantially straight line, and the transmittance decreases exponentially with respect to the wavelength. For this reason, as shown in FIG.
Within the range of 20 nm, the dependence of the transmittance on the incident angle is almost constant times, and the transmittance changes for each wavelength. However, since the angle dependence for each wavelength is almost a constant times, it depends on the incident angle. It can be seen that the aberration improvement does not depend on the wavelength.

That is, at the incident angle θ where the vertical incidence is 0 °, the apparent wavelength λm as viewed from the dielectric multilayer thin film 13 is longer than the actual wavelength λo, as in the general case of λm = λo / cosθ. If the incident angle changes from 0 degree to θo, the incident wavelength is λo to λo as viewed from the dielectric multilayer thin film 13.
/ Cosθo. That is, the wavelength dependence of the transmittance characteristic of the dielectric multilayer thin film 13 is changed from λo to λo / cosθo.
If the incident light has a tendency to attenuate generally, the incident light at the incident wavelength λo has a high transmittance at normal incidence and the transmittance substantially decreases until the oblique incidence at an angle θo, so the incident angle is limited by this configuration. You can see that When viewed from the dielectric multilayer thin film 13, the light having the shortest wavelength is incident under normal incidence conditions.
The light having the shortest wavelength of the emission wavelength of the ED 3. If the transmittance is maximized at this wavelength and the transmittance monotonously decreases as the wavelength becomes longer, the entire wavelength range of the LED 3 under consideration (7
(00 nm to 720 nm), the transmittance at the time of oblique incidence with respect to vertical incidence always decreases monotonously, and the angle of incidence can be limited.

Each lens 6 of the lens array 7 has a diameter of 0.9 mm, and the distance between the lens 6 and the LED 3 is 3.5.
mm, the radiation angle covering the closest lens is about ± 14.
Degrees. As can be seen from FIG. 4, 1 for normal incidence.
At an angle of incidence of 4 degrees, the transmittance decreases to approximately 60%.
Therefore, it can be seen that the light obliquely incident on the lens disposed outside the closest lens is weakened by the optical filter 8 and the increase in aberration can be suppressed.

In the present embodiment, since the optical filter 8 limits the radiation angle (the incident angle with respect to the lens array 7), the effect of the optical filter 8 is the same everywhere between the LED 3 and the lens array 7. However, in the case of actual manufacturing, only two parts are combined instead of three separate parts by superimposing the LED array chip 2 and integrating it with the LED 3 or bonding and integrating the lens array 7 with the lens array 7. It is desirable to be able to perform positioning. The optical filter 8 itself has a simple structure in which the dielectric multilayer thin film 13 is formed on the quartz substrate 11. Even if the optical filter 8 is displaced in the direction perpendicular to the optical axis of the light 9 emitted from the LED 3, it has no effect. Therefore, the assembly of the optical filter 8 is a simple process that does not require alignment accuracy.

The optical filter 8 is connected to the L of the lens array 7.
Opposite side to ED3 (emission side of lens array 7 = photoconductor 5)
Side), the effect can be expected. It is desirable that the light with an unnecessary incident angle be shielded before entering the lens array 7 which is the same-magnification imaging optical system. However, due to the characteristics of the equal-magnification imaging optical system, the light is emitted at the same angle as the incident light. Therefore, unless the light does not satisfy the same-magnification condition due to imperfection of the lens 6, the emission angle (emission angle) is also restricted on the emission side of the equal-magnification imaging optical system, so that the light is provided on the LED 3 side. Similarly to the above, it is possible to suppress the imaging blur due to the light incident on the imaging system at a large angle. Of course, the optical filters 8 may be provided on both the entrance side and the exit side of the lens array 7.

A second embodiment of the present invention will be described with reference to FIG. The same portions as those described in the first embodiment are denoted by the same reference numerals, and description thereof is omitted (the same applies to the following embodiments).

In this embodiment, a gradient index lens array 14 is used as an equal-magnification image forming optical system, and an end face of the gradient index lens array 14, here, an incident end face on the LED 3 side, is provided with an optical filter. Dielectric multilayer thin film 15
Are directly laminated by an evaporation method. The configuration of the dielectric multilayer thin film 15 itself is the same as that of the dielectric multilayer thin film 13. Other configurations are the same as those in the first embodiment.

In such a configuration, the dielectric multilayer thin film 15 serving as an optical filter limits the incident angle to the gradient index lens array 14, and suppresses image degradation due to light having a large incident angle, while suppressing LED degradation. The advantage of using the array 4 is that no change is required in the arrangement or usage. In particular, according to the present embodiment, when the gradient index lens array 14 is manufactured, the dielectric filter and the lens array are integrally formed by simultaneously forming the dielectric multilayer thin film 15 for limiting the incident angle directly on the end face. Therefore, the optical writing head can be simplified both in manufacturing and in structure.

A third embodiment of the present invention will be described with reference to FIG. In the present embodiment, an LED array 17 having an edge emitting LED array chip 16 is used as a light emitting element array instead of the LED array 4 in the second embodiment. In this case, since the radiation direction of the LED changes with respect to the semiconductor chip, the mounting direction of the gradient index lens array 14 is changed, and the LED array 14 is mounted on the same printed circuit board 18.

As shown in FIGS. 3 and 4, the bandpass filter using the dielectric multilayer thin film 15 having the function of an optical filter has a smaller wavelength dependence as the emission wavelength width of the LED is smaller. In the present embodiment, an edge emitting LED array chip 16 having a slab-type waveguide structure having a light emission wavelength width smaller than that of a surface emitting LED is used, and a bandpass filter of the same design is more effective over the entire wavelength range of the LED. To make use of the amount of light. As shown in FIG. 4, when the emission wavelength extends from 700 nm to 720 nm, the transmittance decreases as the wavelength increases, and decreases to 25% at 720 nm. Since the stimulated emission component is increased by providing the waveguide structure, L
The emission wavelength of the ED is narrowed, for example, from 700 nm to 710
If the wavelength component up to nm is mainly used, the transmittance of the dielectric multilayer thin film 15 (optical filter) can be reduced to at least 50%, so that the light transmission efficiency of the optical filter increases and the image point 10 can be effectively reached. Light can be guided.

It is more desirable to use a semiconductor laser array instead of the edge emitting LED array chip 16 of the present embodiment, since the monochromaticity of light emission is further increased.

[0051]

According to the first aspect of the present invention, an optical filter for controlling an incident angle with respect to an equal-magnification imaging optical system or an emission angle from the same-magnification imaging optical system is provided on the incident side of the equal-magnification imaging optical system. And at least one of the output side, by combining an optical filter for controlling the incident angle or the output angle with the same-magnification image forming optical system, the image blur on the image forming surface such as the photoreceptor surface is reduced. The light emitting device array itself can be realized with a simple structure without any change.

According to the second aspect of the present invention, in the optical device according to the first aspect, since the unity-magnification imaging element is formed of a lens array and has a characteristic which is hardly dependent on the incident position, the mounting position accuracy is improved. The imaging performance can be improved while simplifying the manufacturing process of the optical device.

According to the third aspect of the present invention, in the optical device according to the second aspect, the optical filter is so arranged that the light emitted from the light emitting element array is made to enter only the closest lens of the lens array. By limiting the angle, only the lens having the best transmission efficiency and the best imaging performance in the lens array can be selectively used, so that the imaging performance can be improved without significantly lowering the transmission efficiency.

According to a fourth aspect of the present invention, in the optical device according to any one of the first to third aspects, the optical filter is provided on the light emitting element array side with respect to the unity magnification optical system. Therefore, unnecessary light can be prevented from being incident on the lens from the beginning, and flare light can be suppressed at the same time.

According to the fifth aspect of the present invention, in the optical device according to any one of the first to fourth aspects, by using a multilayer thin film having a transmittance having an incident angle dependence as an optical filter, the optical filter can be used. A structure for controlling the incident angle independently of the incident position can be easily realized, and uniform characteristics can be easily realized, and the alignment process of the optical filter can be greatly simplified.

According to a sixth aspect of the present invention, in the optical device according to the fifth aspect, when manufacturing a gradient index lens array which is an equal-magnification image forming optical system, a multilayer thin film for simultaneously limiting an incident angle is formed. By forming a film directly on the lens surface, the optical filter and the lens are integrated, and the optical device can be simplified in terms of manufacturing and structure.

According to a seventh aspect of the present invention, in the optical device according to the fifth and sixth aspects, the optical filter is arranged such that, under the vertical incidence condition, the optical filter is vertically incident on the film at the shortest wavelength of the emission wavelength of the light emitting element array. Since the transmittance is maximum and has a transmittance characteristic that monotonically decreases as the wavelength becomes longer, by using a multilayer thin film having such a wavelength dependency of the transmittance,
The angle of incidence can be limited.

According to an eighth aspect of the present invention, in the optical device according to the fifth or sixth aspect, the optical filter has an incident angle dependency of transmittances of different wavelengths within a light emission wavelength range of the light emitting element array. Assuming that the incident angle is θ, the wavelength is λ1, λ2, the incident angle dependence of the transmittance is T, and the constant depending only on the wavelength is K, T (θ, λ1) = K (λ1, λ2) T (θ, λ2 ) It has a constant multiple relationship depending only on the wavelength.

According to the ninth aspect of the present invention, in the optical device according to the eighth aspect, the optical filter has a wavelength dependence of a transmittance which can be approximated by an exponential function attenuating in the longer wavelength direction with respect to the incident wavelength. Therefore, the structure of the invention described in claim 8 can be realized by designing a thin film for controlling a normal emission wavelength.

According to the tenth aspect, the first aspect is provided.
10. The optical device according to any one of claims 9 to 9, wherein the imaging surface is a photoconductor surface, and the light emitting element array emits writing light to the photoconductor surface. It can be suitably applied to a head.

[Brief description of the drawings]

FIG. 1 is a schematic perspective view illustrating a configuration of a writing unit of an LED printer according to a first embodiment of the present invention.

FIG. 2 is an enlarged sectional view showing the structure of the optical filter.

FIG. 3 is a characteristic diagram showing a transmission spectrum of the optical filter.

FIG. 4 is a characteristic diagram showing the angle dependence of the transmittance of an optical filter.

FIG. 5 is a schematic side view illustrating a configuration of a writing unit of the LED printer according to the second embodiment of the present invention.

FIG. 6 is a schematic side view illustrating a writing unit configuration of an LED printer according to a third embodiment of the present invention.

FIG. 7 is a schematic front view showing a basic configuration example of the LED printer.

FIG. 8 is a schematic side view showing a configuration example of a first conventional example.

FIG. 9 is a schematic side view showing a configuration example of a second conventional example.

[Explanation of symbols]

 Reference Signs List 4 light emitting element array 5a image forming surface 7 lens array = equal magnification imaging optical system 8 optical filter 13 multilayer thin film 14 refractive index distribution type lens array = equal magnification imaging optical system 15 multilayer thin film = optical filter 17 light emitting element array

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat ゛ (Reference) H04N 1/036 F term (Reference) 2C162 AE28 FA04 FA17 FA43 FA45 FA50 5C051 AA02 CA08 DA03 DB02 DB22 DB23 DB29 DC04 DC05 DC07 FA01 5F041 AA31 CA12 CB22 DB07 EE11 EE22 FF13

Claims (10)

[Claims] 1. An optical device for imaging light emitted from a light-emitting element array onto an image forming surface via an equal-magnification imaging optical system, wherein an incident angle with respect to the equal-magnification imaging optical system or the equal-magnification imaging optics An optical device, wherein an optical filter for controlling an exit angle from a system is disposed on at least one of an entrance side and an exit side of the same-magnification imaging optical system. 2. The image pickup apparatus according to claim 1, wherein the equal-magnification imaging element is formed of a lens array, and an incident angle controlled by the optical filter is smaller than a viewing angle of the lens array, and has no dependency on an incident position. The optical device according to claim 1. 3. The optical filter according to claim 2, wherein the optical filter limits an angle of the outgoing light emitted from the light emitting element array so as to be incident only on a closest lens of the lens array. Optical device. 4. The optical device according to claim 1, wherein the optical filter is disposed on the light emitting element array side with respect to the unit magnification imaging optical system. 5. The optical device according to claim 1, wherein the optical filter is formed of a multilayer thin film. 6. The image forming apparatus according to claim 5, wherein the equal-magnification image-forming element comprises a gradient index lens array, and the multilayer thin film is formed directly on an end face of the gradient index lens array. Optical device. 7. The transmittance characteristic of the optical filter is such that the transmittance is maximum when vertically incident on the film at the shortest wavelength of the light emission wavelength of the light emitting element array under the vertical incidence condition, and monotonically decreases as the wavelength becomes longer. 6. The method according to claim 5, wherein
7. The optical device according to claim 6. 8. The optical filter according to claim 1, wherein, within the emission wavelength range of the light emitting element array, the incident angle dependence of the transmittance of different wavelengths is such that the incident angle is θ, the wavelength is λ1, λ2, and the transmittance is incident angle dependence. Where T is a property that depends only on the wavelength and K is a constant that depends only on the wavelength, T (θ, λ1) = K (λ1, λ2) T (θ, λ2) The optical device according to claim 5, wherein: 9. The optical device according to claim 8, wherein the optical filter has a wavelength dependence of transmittance that can be approximated by an exponential function type that attenuates in the longer wavelength direction with respect to an incident wavelength. 10. The optical device according to claim 1, wherein the image forming surface is a photoconductor surface, and the light emitting element array emits writing light to the photoconductor surface.