JPS59135416A - Forming device of modulated light - Google Patents

Abstract

PURPOSE:To hold the output of a photoelectric transducer constant approximately, by controlling a modulator so that the output of the photoelectric transducer becomes constant and leading out a strong beam from a modulator for a part corresponding to black and leading out a weak beam from it for a part corresponding to white. CONSTITUTION:A beam 12 from a laser generator 11 is modulated in an optical modulator 13 in accordance with a voltage applied to the modulator, and a beam 12' is condensed by a lens 17 and is irradiated to a beam splitter 14 and is separated to two beams, and a beam 16 is irradiated onto a material 18 having a picture. The reflected light is converted to an electric signal by a photoelectric transducer 20 and is applied to a modulated signal forming circuit 21. Consequently, such control is performed that a large quantity of beam is emitted from the optical modulator 13 when the picture on the material reflects weak light and a small quantity of beam is emitted from the modulator 13 when the picture is white to reflect strong light, and the beam emitted from the modulator has picture information; and therefore, a beam 15 separated by the beam splitter 14 is used for information recording as it is.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a modulated light forming device that forms light modulated by information on an irradiated object such as a document.

Devices that irradiate a certain amount of light onto a document, read information on the document, control a light generator with the read information, and obtain modulated light modulated by the information on the document are widely known. It is something.

However, such conventional devices require a part that reads information from a document and a part that modulates light based on the read information.

The present invention eliminates the above-mentioned conventional drawbacks by using a modulated light forming device having an extremely simple configuration.

Before explaining the present invention in detail, first, an example applied to a copying apparatus will be briefly explained.

In FIG. 1, reference numeral 11 indicates a laser oscillator which is a light generating means, and a beam 12 from this laser generator 11 is transmitted to a modulator 13, and the voltage applied to the modulator 13 is After being modulated accordingly, this beam 12' is focused by a condenser lens L7t/C and irradiated onto a beam splinter 14 to form two beams 15.
After being separated into 16 beams, the beam 16 is irradiated onto a material 18 having an image, which is arranged on a converging surface by the lens 17 .

Therefore, from this material 18, reflected light 19 corresponding to the image on the material can be obtained, but such light 19
is converted into an electric signal by the photoelectric conversion element 20 and then applied to the modulation signal forming circuit 21. This modulation signal forming circuit 21
is used to form a modulation signal to be applied to the modulator 13, and this modulation signal is such that the signal detected by the photoelectric conversion element 20 is at a constant level with respect to a predetermined reference level. This is a signal that controls a modulator. Therefore, if the image on the document is black with little light reflection,
The beam emitted from the modulator 13 is controlled so that a large amount of the beam is emitted from the modulator 13, and when the image on the material is white with a lot of light reflection, a small amount of the beam is emitted from the modulator 13. It has image information, and therefore, the beam 15 divided by the beam splinter 15 can be used as it is as a beam for recording trench information.

To further explain the photoelectric conversion element 20 and the modulation signal forming circuit 21 with reference to FIG. 2, the photoelectric conversion element consists of a first diode 22, and the light received by this photodiode 22 is converted into an electric signal. The signal is amplified by an amplifier 23, and the amplified signal is applied to one input terminal 25 of an operational amplifier 24. The reference numeral 26 indicates an ordinary voltage regulator which has a voltage selection means therein and can derive a desired voltage by switching the means. is applied to the other input terminal 27 of the operational amplifier 24.

Therefore, if the signal level from the output terminal 28 to the input terminal 25 becomes low, a high level signal 75≦, or input terminal 2
When the signal level to 5 becomes high, a low level signal is derived.

Therefore, if the modulator 13 is configured so that the beam 12 is easily transmitted when the modulation signal level is high and is difficult to transmit when the modulation signal level is low, the beam 12' can be weakened when there is a lot of reflected light. If there is little reflected light, the beam 12' is weakened, and the output signal 75 from the amplification 23 is controlled to be a certain difference from the output voltage from the voltage regulator. . (However, if there are 75 reflected lights, the level of the output signal of the amplifier 23 will be high, and if there are few reflected lights, the level of the output signal of the amplifier 23 will be low.) Therefore, the level of the output signal from the amplifier 23 is always maintained at a constant level, and therefore the beam 12' is intensity-modulated in accordance with the image in the modulator 13.

Figure 3 shows the signal waveforms at various parts when the beam 12' in Figures 1 and 2 is deflected and scanned over the material shown in Figure 3(a). As shown in (a), if a material having a black area 30, 31 on a white background is scanned with the heam spont 32 in the direction of the arrow p along the dotted line, then in FIG.
As explained in Figure 2, the modulator is controlled so that the output of the photoelectric conversion element is constant, so as shown in (b), a strong beam is modulated in the area corresponding to black, and a weak beam is modulated in the area corresponding to white. In this way, the output of the photoelectric conversion element becomes approximately constant as shown in (c).

As described above, since the beam from the modulator contains image information, a copying apparatus can be formed by irradiating the recording medium with the beam 15.

FIG. 4 shows the above-described copying apparatus in more detail. A laser beam emitted from a laser oscillator 34 is guided to an input aperture of a modulator 36 via a reflecting mirror 35. The reflecting mirror 35 is inserted to bend the optical path in order to reduce the space of the apparatus, and can be removed if unnecessary.

For the modulator 36, an acousto-optic modulation element using a known acousto-optic effect or an electro-optic element using an electro-optic effect is used.

At the modulator 36, the laser beam is subjected to strong or weak modulation according to the input signal to the modulator 36.

However, if the laser oscillator 34 is a semiconductor laser, a gas laser, etc. that can perform current modulation, or an internal modulation type laser that incorporates a modulation element in the oscillation optical path, the modulation The beam expander 36 is omitted and the beam is guided directly to the beam expander 37.

After the laser beam from the modulator 36 passes through the reflecting mirror 35, the beam diameter is expanded by the beam expander 37 while remaining a parallel beam. Furthermore, the laser beam whose beam diameter has been expanded is incident on a polyhedral rotating mirror 38 having a plurality of mirror surfaces. The polyhedral rotating mirror 38 is mounted on a shaft supported by a high-precision bearing (for example, an air bearing) and is driven by a motor 39 that rotates at a constant speed (for example, a hysteresis non-chronous motor or a DC servo motor). The beam emitted from the panda is swept horizontally and is further imaged as a spot on a photosensitive tram 41 by an imaging lens (or condensing lens) 40 . When collimated light is imaged into a spot by an imaging lens, the minimum diameter of the spot dmin is given by λ; wavelength A of the light used; incident aperture of the imaging lens; when f and λ are constant, A is If it is made larger, a smaller spot diameter dmin can be obtained.

The beam expander 37 mentioned above is used to provide this effect. Therefore, if the required drnin can be obtained by the beam diameter of the laser oscillator, the beam expander 37 is omitted.

A beam splinter 42 having a length that covers the entire width of the beam sweep is arranged between the photosensitive drum 41 and the imaging lens 400.

Therefore, a portion 43 of the beam from the rotating polygon mirror 38 is irradiated onto the photosensitive drum 41, and the other portion 44 is irradiated onto the transparent glass 46 for placing the document on the document table 45. Therefore, the beam 44 moves on the glass 46 as shown by arrow Q by rotating the rotating polygon mirror 39.

Reference numeral 47 denotes a light receiving element that receives reflected light generated by irradiating the beam 44 onto the original when the original is placed on the glass 46 and converts it into an electrical signal.

Since the document table 45 is moved at a constant speed on the rail 48 in the direction of arrow P by a drive device (not shown), the glass 4
6, and while rotating the rotating polygon mirror 39,
By moving the document table 45 in the direction of arrow P, the entire surface of the document can be scanned.

At this time, it goes without saying that the photosensitive drum 41, which has a photosensitive layer provided on its entire surface, rotates in synchronization with the movement of the document table 45.

Since the light receiving element 46 and the modulator 36 correspond to the photoelectric conversion element 20 and the optical modulator 13 in FIGS. 1 and 2, respectively, the output of the light receiving element 46 causes the By controlling the modulator 36 with a circuit such as
The same information as the material on the document table 45 is recorded on the photosensitive drum 41.

Thereafter, the photosensitive drum 41 is processed by an electrophotographic processing process to visualize the recorded information, and this image is transferred and fixed onto plain paper 47 and output as a copy.

Next, the printing section will be explained with reference to FIG. 5 as well. As an example of the electrophotographic process applied to this embodiment, as described in Japanese Patent Publication No. 42-23910 of the present applicant, a photosensitive plate 49 whose basic constituents are a conductive support, a photoconductive layer, and an insulating layer. The surface of the insulating layer is uniformly charged positively or negatively using a first corona charger, and a charge having a polarity opposite to the charging polarity is placed at the interface between the photoconductive layer and the insulating layer or inside the photoconductive layer. Then, the surface of the insulating layer to be charged is irradiated with the laser beam 43 and at the same time an AC corona discharge is applied by the AC corona discharger 51 to form a pattern due to the difference in surface potential that occurs according to the light and dark pattern of the laser beam 43. formed on the surface of the insulating layer,
The entire surface of the insulating layer is uniformly exposed to light to form a high-contrast electrostatic image on the surface of the insulating layer, and the electrostatic image is further developed with a developing device 5 using a developer mainly composed of charged colored particles.
2, the visible image is transferred to a transfer material 53 such as paper using an internal or external electric field, and then the transferred image is transferred using a fixing means 54 such as an infrared lamp or a hot plate. After fixing to obtain an electrophotographic print image and transferring, the surface of the insulating layer is cleaned by a cleaning device 55.
The photosensitive plate 49 is cleaned to remove remaining charged particles, and the photosensitive plate 49 is used repeatedly.

Next, in the embodiments described so far, the surface of the insulating layer of the photoreceptor, which has been uniformly charged in advance, is subjected to an alternating current corona discharge to attenuate the electric charge on the surface of the insulating layer, and at the same time, the photoreceptor is irradiated with laser light. The phenomena occurring in the body will be further explained in detail with reference to FIG.

FIG. 6 shows the state of change in surface potential on the surface of the insulating layer of the photoreceptor.

FIG. 6(a) shows a case where the frequency of the alternating current of the alternating current corona discharge is relatively low. At this time, the potential of the surface of the insulating layer during AC neutralization can take an intermediate value between the curve shown by the solid line and the curve shown by the dotted line due to the difference in the phase of the AC voltage. However, the laser beam irradiation is performed for a very short time on a specific location on the photoreceptor, for example, in this example, it is
It is 50 nanoseconds. Therefore, due to the difference in the potential of the surface of the insulating layer when the laser beam is irradiated, the potential of the electrostatic image obtained after the entire surface is exposed remains constant even though the irradiation amount of the laser beam is constant. It will stop happening. Therefore, unevenness synchronized with the frequency of the alternating current occurs in the developed image. This phenomenon is caused by the fact that in applications such as copying machines, exposure is performed over the entire AC static elimination area.
Phase effects are averaged out and do not appear.

In order to eliminate the phenomenon of unevenness, when the frequency of AC static elimination is increased (Fig. 6 (b)), the overall static elimination time does not change, but the insulating layer surface potential fluctuates in synchronization with the AC frequency. Therefore, the potential difference on the surface of the insulating layer during laser beam irradiation is reduced, and the unevenness of the developed image becomes practically negligible. This is explained by the equivalent circuit shown in FIG. In Figure 7, E is the voltage applied to the discharge electrode of the AC corona discharger, RC is the resistance when the corona current flows between the discharge electrode and the photoreceptor, and cp is the photoreceptor regarded as a capacitance-only load. It shows the capacitance of the photoreceptor at the time.

At this time, if the potential on the surface of the insulating layer immediately before entering AC static elimination due to -order charging is fc Vo and the voltage applied to the AC corona discharge electrode is E = Eo cos (wt + θ), then the surface of the insulating layer during AC static elimination is The potential vp is +Vod
-Shishi-(4) It is expressed as ψ-jan' (-knee) CpRc.

From equation (4), the static elimination time is given by the second term on the right side, and its time constant τ is CpRe.

It also varies due to the frequency of the alternating current corona discharge.

Also, from Fig. 8, the AC static elimination time td is td=’=(5) V; Drum circumferential speed ■ β: Given by the width of the static elimination area.

Furthermore, the amount corresponding to Cp in the equivalent circuit of FIG. 7 is proportional to the surface area of the photoreceptor that passes through the static elimination area per unit time.

Cp=Av-(6) A; constant of proportionality, where cp
= Cp+ + Rc = RC+ + v = v
Assuming that static electricity is sufficiently removed under the + condition,
The time constant for static elimination in equation (4) is T'+ = Cp+
RC+ (7) At this time, the AC discharge frequency W. The amplitude Wo of the fluctuation caused by this amplitude W. is large enough to cause density unevenness in the developed image. By setting w = W+ (Wl > WQ), we have the following equation, and it is assumed that Wl is sufficiently small so as not to cause the density unevenness.

In this way, by changing the frequency of AC corona discharge, the density unevenness can be removed without changing the static elimination time.

Next, assuming that the circumferential speed of the drum is -αVl""V2, C20-αCp+QO) The static elimination time is td2=knee"-12-(11)v2
αVI α β td+ = − l Therefore, the static electricity removal time must be τ2−1.

α Therefore, in order to remove static electricity sufficiently within td2, the static electricity removal time constant τ1Cp1°RC+ τ2 = C20'RC2= =
It can be seen that it is necessary to use the formula 02) α α 00).

In practice, changing Re is achieved by changing the distance between the discharge electrode wire and the photoreceptor. At this time, the amplitude W2 of the fluctuation due to the frequency of the AC corona discharge is as follows, and the condition for w2 for W2 = W'+ is determined as follows: W2 C1)2RC2= W+ Cp+Rc+0
From equation 5), it is necessary to apply an AC corona frequency higher than a certain value in order to prevent the unevenness of the developed image, and that value is proportional to the circumferential speed of the drum.

In this example, the circumferential speed V of the drum is 30 cm/8e
6. The width of the static elimination area was 3 cm x 3 Q cm, the capacitance C of the photosensitive plate was 5PF/7, the current for AC static elimination was 75 μArm5, the voltage was 7 kV, the frequency f was I KHz, and the electrostatic contrast was approximately 500 V. The development was carried out using liquid development and reversal development. From these experiments, it was possible to remove the unevenness of the image under the condition that the frequency f of AC static elimination was satisfied. That is, it means that the pitch according to the frequency of AC corona discharge on the photosensitive drum is Q, 3 mm. Therefore, the effect of the above equation 06) is achieved under more general conditions.

P is a constant determined by the capacitance of the photoreceptor, the width of the static elimination area, the development conditions, etc., and was 0.03 in the above example.

As still another example, the present applicant's Japanese Patent Publication No. 42-19
An electrophotographic electrostatic image forming process such as that described in Japanese Patent No. 748 is applied. That is, a photosensitive plate having a conductive support, a photoconductive layer, and an insulating layer as basic components is used, and the surface of the insulating layer is uniformly positively or negatively charged in advance by a first corona discharge, and the photoconductive layer and the surface of the insulating layer, or
A charge having a polarity opposite to the charged polarity is captured inside the photoconductive layer, and an alternating current corona discharge is applied to the charged surface to attenuate the charge on the surface of the insulating layer. The second embodiment is the same as the first embodiment except for the steps of irradiating light to form an electrostatic image on the surface of the insulating layer according to the brightness of the laser beam, and then developing the electrostatic image.

The photoreceptor and laser oscillator used in the first and second embodiments were as follows.

Combination A a) Laser oscillator: He-Ne gas laser, wavelength: 632.8 mμ) Photoreceptor: Add 90 g of cadmium oxide activated by copper, KIQg of vinyl chloride, and then add a small amount of thinner and mix. It is applied by spraying to a thickness of about 70μ on aluminum foil of about 100μ. Next, a Mylar film having a thickness of about 25 μm was closely laminated on the surface of this photoconductive coating using an adhesive to obtain a photosensitive plate, and the photosensitive member was further wound around a drum made of aluminum. In the case of this photoreceptor, the charge polarity of the first charge is positive.

Combination B a) Laser oscillator He Cd laser wavelength 441.6 m 8 mouths) A Te layer with a thickness of about 1μ is vacuum deposited on the photoreceptor aluminum base, and a layer of Se containing Te15 is further vacuum deposited with a thickness of about 90μ. In the case of this photoreceptor, the first charging polarity is negative.

Furthermore, various laser light sources currently announced or to be announced in the future may also be applied to the first and second latent image forming processes.

It is important to devise a combination of photoreceptors whose spectral sensitivity characteristics match the wavelength of each laser.

As a laser, an Ar gas laser, 1.〃 Ar + Kr // (visible) semiconductor laser, dye laser, double wavelength conversion of infrared laser light using a nonlinear crystal (YAG laser, semiconductor laser), etc. can be used.

In the present invention, such a copying apparatus is further provided with a beam detector as shown at 56 in FIG. Become. The beam detector 56 detects Gf of the swept laser beam 43.
The detection signal is used to determine the start timing of an input signal to the modulator 36 for outputting information from a computer or the like onto the photosensitive drum. As a result, it is possible to greatly reduce synchronization errors in the horizontal direction signals due to errors in the division accuracy of each reflecting surface of the polygonal rotating mirror 38 and uneven rotation, and it is possible to obtain high-quality images.
8 and the drive motor 39 are required to have a wider tolerance range of precision, and can be manufactured at a lower cost.

In a copying apparatus configured in this way, in order to record a data signal sent in the form of an electrical signal from an electronic computer, etc., on a single recording medium by superimposing it with information obtained from the material, an electronic computer is used. The driving mode of the optical modulator is changed according to the binary signal from the optical modulator.

To explain in more detail with reference to FIG. 9, (however, parts with the same numbers as in FIG. 2 are made of the same members), FIG. 9 has two levels like a binary signal, This shows a device for recording data signals sent in series in a superimposed manner with information obtained from document 18.

Reference numeral 60 indicates an application terminal for applying a binary data signal as described above, and the binary data signal applied from this terminal is applied to a switch 61.
It controls the

That is, this switch has a contact piece 62 and a contact point 63a.

53b, but when a signal indicating black, for example '1'' is applied from the terminal 6o, the contact piece 62 and the contact 6
3b is connected, and when a signal indicating white, for example 0"', is applied, the contact piece 62 and the contact 63a are connected.

A contact 63a of the switch 61 is connected to the output of the operational amplifier 24, a contact 63b is connected to a constant voltage source 64, and a contact piece 62 is connected to the optical modulator 13. Therefore, the constant voltage from the constant voltage source 64 is applied to the contact piece 62.
By making the voltage equal to the voltage derived from the output terminal 28 when the beam 16 is irradiated to the black part on the material 18 with the contact 63a connected, the voltage between the contact piece 62 and the contact 6
By connecting 3b, the beam 15 can perform the exposure necessary to draw black on a photosensitive drum (not shown).

In other words, when a signal corresponding to black arrives from the terminal 60, a constant voltage determined by the constant voltage source 64 is applied to the optical modulator 13 by forcibly connecting the contact piece 62 and the contact point 63b. However, in other cases, the contact piece 62 and the contact point 63a'
By connecting it to, control is performed as explained in FIG.

Note that in the recording device shown in Figure 2.9, although omitted, the beam emitted from the optical modulator 13 is repeatedly deflected in one direction (X direction), and The beam is configured to move in a direction (Y direction) perpendicular to the deflection of the one beam on the material, so that the entire area on the material is scanned by the beam.

The terminal 60 is used to apply a signal from a computer or the like as described above, and a brief description of the data signal forming device shown in FIG. 10 will be given here. In FIG. 9, information from an electronic computer applied to a terminal 60 is input in a predetermined format to an interface circuit 66 of the apparatus, either directly or via a storage medium such as a magnetic tape or a magnetic disk. Various instructions from the computer 77 are decoded by the instruction execution circuit 67,
and executed.

Data is stored in data memory 68 in fixed amounts.

In the case of character information, the data format is given in binary code, and in the case of graphic information, it is data in units of pixels that make up the figure, or data of lines that make up the figure (so-called vectors). data). These modes are specified in advance of the data, and the instruction execution circuit 67 controls the data memory 68 and line data generator 69 to process the data according to the specified mode. A line data generator 69 generates final data for one scan line.

That is, when data is given as a character code, the character number turns are read out from the character generator 70, and the character number turns for one line are lined up and bumped, or the character code for one line is bumped up and sequentially sent to the character generator. 70 and sequentially create data for modulating the laser beam for the scan line. Even when the data is graphic information, the data is converted into scan line data to create data for sequentially modulating the laser beam for one line scan. Data for one scan line is alternately inputted to a line buffer 71 and a line buffer 72, which are composed of a shift register or the like having a number of focus points equal to the number of pixels for one scan line, under the control of a buffer switch control circuit.

Furthermore, the line buffer 71 and the line buffer 7
The data No. 2 is sequentially read out one focus at a time for one scan line as a beam detection signal from the beam detector 73 as a trigger signal, and is applied to the switch 61 shown in FIG. While one reflective surface scans the photosensitive drum along a line perpendicular to the rotational direction, data for one scan line stored in the line buffer is applied to the switch 61. Data signals are read out alternately from the line buffers 71 and 72 under the control of the buffer switch control circuit 76. That is, when reading from one line buffer, writing is performed to the other line buffer.

This method allows all data to be applied to the modulator when the polyhedral rotating mirror sweeps over the photosensitive drum and the distance between one reflecting surface and the next reflecting surface is very short. Before scanning one scan line, the photosensitive drum continues to rotate at a constant speed and moves by an appropriate scan line interval.

As described above, according to the present invention, modulated light can be obtained with an extremely simple configuration.
The present invention is not limited to the embodiments shown in FIGS. 1 and 2, but can also be applied to the embodiment shown in FIG. 11 as is.

That is, as shown in FIG. 11(a), the beam transmitted through the material 18 may be detected by the photoelectric conversion element 20, or the condenser lens 17 may be placed between the beam splinter 14 and the material 18. Often, even more tedious condensing lenses can be omitted altogether.

Furthermore, the material may be irradiated with a beam that has passed through the beam splinter 14 as shown in FIG.
0, the light reflected from the material 18 may be detected, or the light transmitted through the material 18 may be detected.

Also in the embodiments (b) and (c), the condenser lens 17
may be placed between the optical modulator 13 and the beam splinter 14 or between the beam splinter 14 and the material 18, or may be omitted.

Furthermore, in the embodiments shown in FIGS. 1 and 11(a), (b), and (C), when a laser generator 11 whose generated beam intensity can be controlled by a control signal is used, in other words, modulation When a functional laser generator (for example, a current modulated laser) is used, the optical modulator can be removed by applying the output of the modulation signal forming circuit 21 directly to the laser generator.

As described above, the present invention can obtain light modulated by information on an irradiated object such as a document with a simple configuration.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a modulated light forming device according to the present invention, FIG. 2 is a block diagram showing the modulated light forming device according to the present invention in more detail, and FIGS. 3(a), (b), and (C) are An explanatory diagram showing the operation of each part in Fig. 2, Figs. 4 and 5 are a transparent perspective view and a transparent side view of a copying machine to which the present invention is applied, and Fig. 6 Ca) and (b) are explanations of AC static elimination. FIGS. 7 and 8 are circuits and enlarged explanatory diagrams for performing AC charge removal on a photoreceptor, FIG. 9 is a block diagram showing the present invention, and FIG. 1O is a data signal forming device. , and FIGS. 11(a), 11(b), and 11(c) are block diagrams showing a recording apparatus to which the present invention can be applied. Here, 11 is a laser generator, 13 is an optical modulator, 14 is a beam splinter, 18 is a document, 20 is a photoelectric conversion element, 2
1 is an optical modulation signal forming circuit, 22 is a photodiode, 2
4 is an operational amplifier, 26.degree. 64 is a constant voltage power supply, 61 is a switch, and 64 is a reference voltage device. Patent applicant Canon Co., Ltd. No. 70 Figure 8 0″-6/ Procedural amendment (method) Commissioner of the Japan Patent Office Kazuo Wakasugi 1 Indication of the case 1982 Patent application No. 184145 No. 2
Name of the invention Modulated light forming device 3 Relationship with the person making the correction Patent applicant Address: 3-30-2 Shimomaruko, Ota-ku, Tokyo Address: m146 Tokyo-tojinm
Ward Maruko a-3o-25, date of amendment order January 61, 1980 (shipment date) 6. Drawing subject to amendment Contents of amendment to drawing Z Engraving of drawing (no change in content)

Claims (1)

[Claims] A modulated light forming means whose output light amount is controlled in response to the application of a modulation signal, a scanning means which scans an irradiated object with the output light from the modulated light forming means, a reflection of the luminous flux from the irradiated object, or A modulated light forming device comprising: a detection means for detecting transmitted light; and a control means for controlling the modulation means so as to bring the detection output of the detection means to a reference level.