JPH02103159A - Optical writing head - Google Patents

Abstract

PURPOSE:To reproduce a left and right symmetric figure by forming a light emitting region into a shape having a symmetric axis in a sub-scanning direction. CONSTITUTION:A narrow strip like light emitting part 21 is provided in the hermetically sealed case 20 of an optical writing head H along a main scanning direction X and one filamentous cathode 22 is provided under tension in parallel to said light emitting part 21. When the cathode 22 is heated, thermoelectrons are generated and attracted to an acceleration grid electrode 28 to which positive voltage is applied to be accelerated and a part thereof passes through an acceleration slit 28a. At this time, positive voltage is applied to eight anode electrodes 23 successively from a single side and, when the thermoelectrons are attracted to the anode electrodes 23 to which positive voltage is applied, said thermoelectrons pass only through a selection slit 26a to which positive voltage is applied to reach a fluorescent body 24. Each pixel of an image is reproduced using a light emitting region 24a formed into an oval shape having a symmetric axis in a sub-scanning direction Y so that both of the straight line whose intersecting angle with a main scanning direction X is theta' and a straight line of (pi-theta') have the same continuity. By this method, a left and right symmetric figure can be recorded on a photosensitive body and a left and right symmetric image can be reproduced and, therefore, image qualty is enhanced.

Description

DETAILED DESCRIPTION OF THE INVENTION A6 Object of the Invention (1) Industrial Application Field The present invention relates to an optical writing head for electrostatic recording, and particularly to an optical writing head using a vacuum fluorescent tube.

Such an optical writing head is used when recording an electrostatic latent image by selectively irradiating light onto a photoreceptor of a digital electronic copier or printer.

(2) Prior Art Conventionally, this type of optical writing head has been disclosed in, for example, Japanese Patent Application Laid-Open No. 62-47258 and Japanese Patent Application Laid-Open No. 62-272452.
No. 2, etc. are known.

The conventional optical writing head H0 described in these publications is used by being arranged so as to face a photoreceptor O2 for recording electrostatic latent images with a SELFOC lens O1 in between, as shown in FIG. be done.

As shown in FIG. 15.16, this optical writing head H6 has a side wall 0 formed of glass on a glass substrate Fi04 adhered to a support plate 03 formed of aluminum or iron.
5a and a box-shaped sealed case 05 having a transparent wall 05b
are glued together, and the inside is kept in a vacuum state. The light-transmitting wall 05b is arranged to face the SELFOC lens 01. Said sealed case 05
Inside, one filament-shaped hybrid 06 is stretched along the main scanning direction (direction perpendicular to the paper surface in FIG. 15.16) X. Also, as shown in Figure 16.17,
On the surface of the glass substrate 04, eight thin strip-shaped anode electrodes 07 are arranged parallel to the main scanning direction X and at equal pitches PV (see FIG. 17) in the sub-scanning direction Y.
A phosphor 08 is coated on the entire surface of each anode electrode 07.

Between the cathode 06 and the anode electrode 07 (that is, below the anode electrode 07 in FIG. 16), a plurality of selection grid electrodes 09 (see FIG. 17) are arranged along the main scanning direction X. Each of the selection grid electrodes 09 includes the cathode 06 to the anode electrode 07.
A selection slit 09a is formed to selectively block or pass thermoelectrons moving to. This selection slit 09a is arranged in the main scanning direction (that is, the plurality of anode electrodes 07 and the phosphor 0 on the surface thereof).
8) are formed so as to intersect at an angle of intersection θ (see FIG. 17) with respect to X (longitudinal direction of 8). An accelerating grid electrode 0 formed of a single plate between the selection grid electrode 09 and the cathode 06 for accelerating thermoelectrons moving from the negative 8i06 to the anode electrode 07.
10 (see FIG. 16), and as shown in FIG. 17, the acceleration grid electrode 010 has a selection slit 09a at a position opposite to the selection slit 09a.
An accelerating slot 7) 010a is formed which is slightly larger than the above. Therefore, when viewed from below the acceleration grid electrode 010, the acceleration slit 0IOa, the selection slit 09
The phosphor 08 on the surface of the anode electrode 07 can be seen through a.

The shape of the visible phosphor 08 is a parallelogram, as is clear from FIG. Each of the parallelogram-shaped phosphors 08 forms one light emitting region 08a. The positions of the light-emitting portions 08a on the surface of the plurality of anode electrodes io7 facing one selection ring 09a are sequentially shifted by a pitch PX in the main scanning direction X when viewed from the sub-scanning direction Y. The selection grid electrodes 09 are arranged such that the interval between the selection grid electrodes 09a is eight times the pitch Px. Therefore, as is clear from FIG. 17, light emitting portions 08a are formed on the surface of one anode electrode 07 at a pitch of 8×(P8), and when viewed from the sub-scanning direction Y,
The light emitting parts 08a are continuously arranged from end to end along the main scanning direction X at constant pitches P and l.

Further, the position of each light emitting portion 08a on the surface of the eight anode electrodes 07 facing one selection slit 09a is determined in the sub-scanning direction Y when viewed from the main-scanning direction X.
are shifted by pitch Pv.

(3) Problems to be Solved by the Invention By the way, in the optical writing head H0, since the light emitting portion 08a is formed in a rhombic shape, the photoreceptor 02
The shape of the upward writing pixel S becomes a slanted ellipse as shown in FIG.

Therefore, as shown in FIG.
), the straight line in the direction of θ′ (θ′ζθ) continues relatively well, but the intersection angle with the main scanning direction (X) is (
Straight lines in the π-θ' direction tend to be discontinuous.Therefore, with the conventional optical writing head H0, it is difficult to reproduce left-right symmetrical figures, resulting in a problem of deterioration in image quality.

In view of the above-mentioned problems, the present invention reproduces a symmetrical figure in an optical writing head in which light-emitting portions formed on a plurality of anode electrodes are made to selectively emit light using a plurality of selection grids. That is the issue.

B1 Structure of the Invention (1) Means for Solving the Problems In order to solve the above problems, the optical writing head of the present invention is formed along the main scanning direction and arranged at a predetermined pitch in the sub-scanning direction. A selective opening for passing thermionic electrons is formed along a direction intersecting the main scanning direction at a predetermined intersecting angle between the plurality of rows of anode electrodes and the cathode that emits thermionic electrons. A plurality of selection grid electrodes for controlling the movement of thermoelectrons emitted from the cathode to the seven node electrodes are arranged at a predetermined pitch along the main scanning direction, and a surface of the anode electrode facing the selection aperture is provided. In the optical writing head in which a light emitting portion that emits light by collision of the thermoelectrons is formed, the light emitting portion is formed in a shape having an axis of symmetry in the sub-scanning direction.

(2) Effects According to the optical writing head of the present invention having the above-described configuration,
Since the light emitting portion is formed in a shape having an axis of symmetry in the sub-scanning direction, a symmetrical figure can be recorded on the photoreceptor. Therefore, the reproduced image will also be a symmetrical image.

(3) Embodiments Hereinafter, embodiments of the optical writing head according to the present invention will be described based on the drawings.

FIG. 1 is a diagram showing a schematic configuration of an electrostatic latent image recording device equipped with a first embodiment of the optical writing head of the present invention, and FIG. 2 is a diagram showing the positions of reading pixels of a document and read data of the image. 3 is an enlarged view of the main part of FIG. 1, FIG. 4 is a perspective view of the optical writing head H used in the electrostatic latent image recording device described above, and FIG. 5 is a side sectional view of the optical writing head H. , FIG. 6A is an enlarged sectional view taken along the VIA-VIA line in FIG. 5, and FIG. 6B is an enlarged sectional view of FIG.
An enlarged view of the VIB part in the figure, FIG. 7 is an explanatory diagram of an image reproduced using the light emitting part of the shape shown in FIGS. 6A and B,
8 is a sectional view taken along the line ■-■ in FIG. 6A, FIG. 9 is a sectional view taken along the line IX--IX in FIG. 6A, and FIG. 10 is an enlarged view of the section X in FIG. 12 and 12 are the second embodiment of the present invention.
13 and 14 are explanatory diagrams of the third embodiment of the present invention, which correspond to FIGS. 6A, B, and 7, respectively, and FIGS. This is a diagram corresponding to FIG. 7.

In the electrostatic latent image recording apparatus shown in FIG.
Reflected light from a lamp 2 for irradiating a document that moves along the lower surface of a document (not shown) placed above is focused onto an image reading unit 6 via a moving mirror system 3, a lens 4, and a fixed mirror system 5. and converts it into an electrical signal. Further, this electric signal is converted into binary data for each reading pixel in the image processing section 7, and data for 15 lines of the read original is stored. FIG. 2 is a diagram showing the position of a reading pixel of a document and the reading data of that pixel, and shows the reading data 1ra (n=1
．． 2. ...) is the read data of the first line of the read document, and each value is 0 or 1. The light from the optical writing head H, which is activated by the values of these read data (the details of this "activation" will be described later), is focused through the SELFOC lens array 9 (see Figure 1), and the surrounding area is charged. During charging, an electrostatic latent image is formed on the surface of the photosensitive drum 14 on which the developing unit 11, transfer charger 12, and cleaner unit 13 are disposed.

After the electrostatic latent image on the photosensitive drum 14 is transferred to the paper supplied from the paper tray 15, the paper passes through the fixing unit 16 and is ejected to the paper ejection tray 17.

As shown in FIGS. 3 to 5, the optical writing head H is disposed so as to face the photosensitive drum 14, which has a photosensitive member formed on its surface, with the SELFOC lens array 9 in between, and is made of aluminum. Or support plate 18 made of iron etc.
A glass side wall 20a is attached to the glass substrate 19 bonded to the glass substrate 19.
and a box-shaped sealed case 20 having a transparent wall 20b are bonded together. The interior thereof is maintained in a vacuum state.

Section 5.6. As shown in Figures El to 10, the interior of the sealed case 20 has a longitudinal direction, that is, a main scanning direction (fifth direction).
In the figure, a thin strip-shaped light emitting section 21 is provided along the direction (direction perpendicular to the plane of the drawing)

Further, as shown in FIG. 6A, eight anode electrodes 231 to 23° are provided on the surface of the glass substrate 19 (if there is no need to distinguish between these eight anode electrodes 23. to 23-, the following (hereinafter referred to as "anode electrodes 23") are arranged along the main scanning direction X and in the sub-scanning direction Y at equal pitches Pv. A phosphor 24 (see FIG. 5) is deposited on the surface of the anode electrode 23.

No. 5.8 of the eight anode electrodes 23 on the glass substrate 19
~lO In the lower part of the figure, a plurality of selection grid electrodes 26 supported by spacers 25 provided on both sides of the eight anode electrodes 23 are arranged. As shown in FIG. 6A, the plurality of selection grid electrodes 26 are arranged in the main scanning direction
The selection grid electrodes 26 are arranged so that their adjacent side edges are parallel to each other and intersect with each other at an angle of θ with respect to the main scanning direction. Each selection grid electrode 26 is formed with a selection slot 7) 26a for selectively blocking or passing thermoelectrons moving from the cathode 22 to the anode electrode 23. This selection slit 26a is the 6th A, B
As shown in the figure, the grid electrodes 26 are formed parallel to both sides of the selection grid electrode 26, and are arranged to intersect with the main scanning direction X at an angle of intersection θ, similar to the sides of the selection grid electrodes 26. . The selection slit 26a is formed in a portion facing the anode electrode FfA23, that is, in the sub-scanning direction Y.
An oval-shaped selection opening 26a having long sleeves at the top and a connection opening 26a connecting between each selection dose slot 26a1! It is formed from. The short axis dimension of the selection opening 26al is Xt, the long axis dimension is F+, and the connection opening 26al
The width a2 is set smaller than the short axis dimension Xt.

Below the selection grid electrode 26 in FIGS. An acceleration grid electrode 28 is formed from the body. The accelerating grid electrode 2B is arranged so as to cover the entire surface of the light emitting section 21, and as shown in FIG. An acceleration slit 28a having a slightly larger width is formed.

This acceleration slit 28a is connected to the selection opening 26at.
An elliptical acceleration aperture 28a+ having a symmetry axis in the sub-scanning direction Y, that is, a portion facing the acceleration aperture 28a.
1 and a connection opening 28aX that connects the two. The dimension of one of the symmetric axes of the acceleration opening 28an' is x8 and the dimension of the other symmetric axis is yt, and the width of the connection opening 28a is the dimension of the shorter axis of the symmetric axis x8.
It is set smaller than t.

By the way, the dot shape of the reproduced image using the RAD H for optical writing in boat fishing is determined by the dimensions x and y of the selection aperture 26a1.
, and the dimension of the acceleration opening 28a1 relative to it X*+! t
The quality of the reproduced image also varies depending on the size of the image. Therefore, in this embodiment, the selection opening 26a
1, the dimensions X1 and Fg of the acceleration opening 28a1 are set within a range that satisfies the following formula.

4 x, > x, > x l・----(1)
4F+ >yl >y, ・−・・・・
・・・・・・・・・−・・・・・・・・・・・・−(2) And the radiation of the anode electrode 23=”120#m.

Xt-9QgmSxt-L35am,yt=120
am, yt -180#m.

In this case, when viewed from below the acceleration grid electrode 28,
The eight anode electrodes 2 are passed through the acceleration slit 28a of the acceleration grid electrode 28, the acceleration opening 28a of the selection slit 26a, and the selection opening 26a+.
3. The phosphor 24 on the surface is visible. The shape of the visible phosphor 24 is an ellipse having an axis of symmetry in the sub-scanning direction Y, as is clear from FIGS. 6A, 6B and 7. Each of the elliptical phosphors 24 forms one light emitting region 24a, the shape of which is determined by the shape of the selection aperture 26a. The positions of the light emitting portions 24a on the surfaces of the eight anode electrodes 23 within one selection slit 26a are sequentially shifted by a pitch P8 when viewed from the sub-scanning direction Y. The selection grid electrodes 26 are arranged such that the interval between the selection slits 26a is eight times the pitch PII. Therefore, as is clear from FIG. 6A, the surface of one anode electrode 23 has a light emitting portion 24a with a pitch BP.
When viewed from the sub-scanning direction Y, the light-emitting portions 24a are continuously arranged at a constant pitch P from end to end along the main scanning direction X.

Further, the positions of the light emitting portions 24a on the surfaces of the eight anode electrodes 23 within one selection slit 26a are shifted by the pitch Pv of the anode electrodes 23 when viewed from the main scanning direction X.

Next, the overall operation of the first embodiment of the optical writing head according to the present invention having the above-mentioned configuration will be explained, and then the light-emitting portion 24a will be shaped into an elliptical shape having a symmetrical axis in the sub-scanning direction Y as described above. The effect when formed will be explained.

The optical writing head H equipped with the light emitting section 21 configured as described above generates thermoelectrons when the cathode 22 is heated by a current flowing therethrough, and these thermoelectrons are transferred to an accelerating grid to which a positive voltage is applied. It is attracted to the electrode 28 and accelerated, and a part of it passes through the acceleration slit 28a. At this time, 8
A positive voltage is applied to the anode electrode 23 of the book sequentially from one side, and at the same time as the positive voltage is applied to each anode electrode 23, the selection grid electrode 26 is applied with a positive voltage and a negative voltage according to the image signal. A voltage is selectively applied. Therefore, when the thermoelectrons that have passed through the acceleration pickpocket 7) 28a of the acceleration grid electrode 28 are attracted to the anode electrode 23 to which a positive voltage is applied, the selection grid electrode 26 to which a negative voltage is applied is selected. The selection grid electrode 2 to which a positive voltage is applied cannot pass through the selection slit 26a.
It passes through only the No. 6 selection slit 26a and reaches the phosphor 24.

Therefore, the first writing line on the surface of the photosensitive drum 14 shown in FIG. 1 (that is, the data r of the first reading line shown in FIG. 2 of the read original).

(line where n=1, 2...) is to be written is the anode electrode 23. When facing the data lr1, lr9+ Irl? shown in FIG. ＋ ...＋ l'
1R-ff+...(n-12,...) according to the value of the anode electrode 23. When the upper light emitting portion 24a is made to emit light, writing is performed on the 1.9th, 17°, . . . , 8n-7th pixel from the left of the first writing line on the surface of the photosensitive drum 14.

Next, when the photosensitive drum 14 rotates so that the first writing line on the surface of the photosensitive drum 14 faces the anode electrode 238 and the second writing line faces the anode electrode 23, the data in FIG. )'t, l'll1, 1rl1, ・
・'+ IF@11-4+ ”' (n = 1 +
2 + .
! r 啼 + zr + '11 ”・l tr@n-7,
... (n=1.2...) according to the value of the anode electrode 23. The upper light emitting portion 24a is caused to emit light. Then, the 2.10.1B, . The 1.9°17th, . . . , 8n-7th pixels from the left are written.

At this point, the first writing line on the surface of the photosensitive drum 14 is 1.9.1? from the left. ,..., 8n-7th pixel and 2.10.1B from the left.

..., 8n-6th pixel is written.

In this way, while rotating the photosensitive drum 14, each of the anode electrodes 231 to 23. The upper light emitting portion 24a is selectively caused to emit light to write on the surface of the photosensitive drum 14 according to the read data.

Then, the first writing line on the surface of the photosensitive drum 14 is connected to the anode electrode 23. The eighth write line is opposite to the anode electrode 23. When you are facing the data? + +r+s+ trt2+...+ 1rll
a1,... (n=1, 2...) according to the values of the anode electrodes 23. When the upper light emitting portion 24a is made to emit light, all data I is written to the first writing line on the surface of the photosensitive drum 14.
r1r IrZr 1r3. ..., ge1. ...(n
=1.2. ...) is finished writing.

In this way, one writing line on the surface of the photosensitive drum 14 corresponds to the eight anode electrodes 231 to 231 of the optical writing head H.
When the electrostatic latent image has been sequentially opposed to 23 angles, writing of the electrostatic latent image to that one writing line is completed.

Next, the effect when the light emitting portion 24a is formed into an elliptical shape having a symmetry axis in the sub-scanning direction Y as described above will be described.

Each pixel of the image reproduced using the elliptical light emitting portion 24a as described above has a shape having an axis of symmetry in the sub-scanning direction Y, so as shown in FIG. Both the straight line whose angle is θ' (ζθ) and the straight line whose angle is (π-θ') are reproduced with similar continuity.

FIGS. 11A and 11B and FIG. 12 are explanatory diagrams of a second embodiment of the present invention.

11A and B correspond to FIGS. 6A and B of the first embodiment, and FIG. 12 corresponds to FIG. 7, and this second embodiment has the configuration other than those shown in these figures. The configuration is the same as that of the first embodiment.

Each selection grid 26 of this second embodiment has a circular selection opening 2 with a radius of X that determines the shape of eight light emitting parts.
6a1° is formed opposite to the anode electrode 23. Further, the accelerating grid electrode 28 has a radius x that is slightly larger overall than the selection openings 26a, ′, as shown in FIG.
4 acceleration openings 28a1° are formed. Therefore,
When viewed from below the acceleration grid electrode 28, the phosphor 24 on the surface of the eight anode electrodes 23 is seen through the acceleration opening 28al' of the acceleration grid electrode 28 and the selection opening 26a+' of the selection grid 26. appear. The shape of the visible phosphor 24 is a circle with a radius of X, as is clear from Figures 11A and 11B. Each of the circular phosphors 24 forms one light emitting region 24a.

Next, the effect when the light emitting portion 24a is formed into a circular shape as described above will be explained.

Each pixel of the image reproduced using the circular light emitting portion 24a as described above has an axis of symmetry in the sub-scanning direction Y, so as shown in FIG. Both the straight line whose intersection angle is θ' and the straight line whose intersection angle is (π-θ') are reproduced with similar continuity.

FIGS. 13A and 14 are explanatory diagrams of a third embodiment of the present invention.

13A and 13B correspond to FIGS. 6A and 6B of the first embodiment, and FIG. 14 corresponds to FIG. 7, and this third embodiment has the configuration other than those shown in these figures. The configuration is the same as that of the first embodiment.

Each selection grid 26 of this third embodiment has a rectangular selection opening 26 a 1 that determines the shape of eight light emitting parts.
', 26a1'' are formed to face the anode electrode 23. Furthermore, the acceleration grid electrode 28 has selection openings 26a1 at positions facing the selection openings 26a, 11 as shown in FIG. 13A. A slit-shaped accelerating opening 28a+'' whose overall width is slightly larger than the diameter of the opening 28a+'' is formed. Therefore, when viewed from below the acceleration grid electrode 28, the acceleration opening 2 of the acceleration grid electrode 28 is
8 a 1″ and selection opening 2 of selection grid 26
The phosphors 24 on the surfaces of the eight anode electrodes 23 can be seen through 6a+'. The shape of the visible phosphor 24 is rectangular, as is clear from FIG. 13A. Each of the rectangular phosphors 24 forms one light emitting region 24a.

Next, the effect when the light emitting portion 24a is formed into a rectangular shape as described above will be explained.

Each pixel of the image reproduced using the rectangular light emitting portion 24a as described above has an axis of symmetry in the sub-scanning direction Y, so as shown in FIG. A straight line whose intersection angle is θl (!=:θ) and (π−θ′)
Both straight lines are reproduced with similar continuity.

Although the embodiments of the optical writing head according to the present invention have been described in detail above, the present invention is not limited to the above embodiments.
Various minor design changes may be made without departing from the invention as claimed.

For example, the selection aperture 2 that determines the shape of the light emitting part 24a
6a+, 26a+', 26a+" can be any other suitable shape as long as it has an axis of symmetry in the sub-scanning direction Y. In this case, in the reproduced image, the main scanning direction The line whose intersection angle is θ′ and (π−θ
′) can be reproduced with similar continuity. Further, the shape of the light emitting portion 24a can also be determined by the shape of the MfiJf region of the phosphor 24 on the anode electrode 23. Furthermore, in each of the above-described embodiments, the filament cathode 22 is arranged at one point along the main scanning direction
Although an example is shown in which only a book is stretched, it is also possible to stretch a plurality of filament cathodes 22 in parallel with each other.

C0 Effects of the Invention According to the optical writing head of the present invention having the above-described configuration,
Since the light emitting portion is formed in a shape having an axis of symmetry in the sub-scanning direction, a symmetrical figure can be recorded on the photoreceptor. Therefore, a symmetrical image can be reproduced, resulting in improved image quality.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a schematic configuration of an electrostatic latent image recording device equipped with a first embodiment of the optical writing head of the present invention, and FIG. 2 is a diagram showing the positions of reading pixels of a document and read data of the image. 3 is an enlarged view of the main part of FIG. 1, and FIG. 4 is a perspective view of the optical writing head H used in the electrostatic latent image recording device described above.
6A is an enlarged sectional view taken along line VIA-VIA in FIG. 5, and FIG.
Fig. 7 is an explanatory diagram of an image reproduced using the light-emitting portion of the shape shown in Figs. 6A and B. Fig. 8 is a cross section taken along the line ■-■ in Fig. 6A. Figure 9 is 6A
10 is an enlarged view of the section X in the direction of the arrow in FIG. 9, and FIGS. 6A, B and FIG. 7, and FIGS. 13 and 14 are explanatory views of the third embodiment of the present invention, and FIGS. FIG. 18 is an explanatory diagram of the prior art. X... Main scanning direction, Y... Sub-scanning direction 22... Cathode, 23... Anode electrode, 24a... Light emitting part,
26...Selection grid electrode, 26al126a+°
+26a+'... Selection aperture, 28... Acceleration grid electrode, 28a+ + 28ar', 2Bar"
・Acceleration opening

Claims (3)

[Claims] (1) A row of a plurality of anode electrodes (23) formed along the main scanning direction (X) and arranged at a predetermined pitch (P_Y) in the sub-scanning direction (Y) and a cathode (23) that emits thermoelectrons ( 22), the selection apertures (26a_1, 26a_1', 26a_
A selection grid electrode (26) having a pitch of 1") and controlling the movement of thermoelectrons emitted from the cathode (22) to the anode electrode (23) is arranged at a predetermined pitch along the main scanning direction (X). A plurality of selection openings (8P_X) are arranged, and the selection opening (
26a_1, 26a_1', 26a_1''), in which a light emitting part (24a) is formed on the surface of the anode electrode (23) that emits light by collision of the thermoelectrons, the light emitting part (24a) is a sub-surface. An optical writing head formed in a shape having an axis of symmetry in the scanning direction (Y). (2) The selection grid (26) and the cathode (22)
between the selection openings (26a_1, 26a_
Acceleration openings (28a_1, 28a_1', 28a_1") for accelerating thermionic electrons are located opposite to the openings (28a_1, 28a_1', 28a_1").
) and is provided with an accelerating grid electrode (28) that accelerates the movement of thermoelectrons emitted from the cathode (22) toward the anode electrode (23). . (3) The optical writing head according to item 1 or 2, wherein the shape of the light emitting portion (24a) is determined by the selection aperture (26a_1, 26a_1', 26a_1'').