COLOR IMAGE INPUT UNIT HAVING A RARE GAS CATHODE DISCHARGE TUBE

Abstract

 A rare gas cold cathode discharge tube (1) is provided with a conductive member (15) nearby a main electrode (13-a), allowing to stabilize the quantity of emitted light even when an intermittent lighting with a period of several mil­liseconds is carried out. The use of such rare gas cold cathode discharge tube as lighting apparatus allows an ima­ge input unit to be realized which is moderate in cost and less space-consuming, still ensuring a color correction function to enhance the color reproducibility of a printing output of the read image and a light adjusting function of the lighting apparatus to enhance the S/N ratio of the read image.

Description

[0001] The present invention relates to a rare gas cold cathode discharge tube and to an image input unit using it.

[0002] Image input units are used for reading images such as pho­tographs and the like and generating corresponding data which can be input into a computer. Known image input units may be classified into mainly three types depending on their reading system.

[0003] The first type is a camera type image input unit using a two-dimensional array of photoelectric transducer elements onto which the image to be read is projected. An image is read during 20 ms or less. This camera type image input unit is mainly used for reading images changing with time. Since an optical path length for imaging must be secured in construction, a large space will be required for the unit. Furthermore, since all of a picture image to be read must be irradiated at once and since it is difficult to irra­diate a greater area by a uniform brightness, a high accu­racy in reading the density values of the picture is hardly achieved. Still further, since the photoelectric transducer elements are arrayed two-dimensionally, a high precision technique is required for manufacturing, leading to high manufacturing costs.

[0004] The second type of image input unit is a drum scanner using an imaging system and a photoelectric transducer element for reading an image point by point. While a picture image is rotating on a drum, the photoelectric transducer element is shifted into the axial direction of the drum. By con­trolling the rotational speed of the drum and the moving rate of the photoelectric transducer, the reading resolu­tion may be readily selected, and a relatively high resolu­tion may be obtained. However, since the resolution depends on the mechanical precision of the component parts, high cost may result and a large-sized construction will be quite unavoidable with this type of image input unit.

[0005] The third type of image input unit uses a one-dimensional array of photoelectric transducer elements consisting of CCD (charge coupled devices) or the like. Synchronized with data reading, the photoelectric transducer elements are shifted relative to a picture image in a direction perpen­dicular to the extension of the array, thereby scanning the image line by line. This third type of image input unit may be regarded as a system intermediate of the aforementioned two types and enjoying the advantages of those two other types. The reading rate of this third type of image input unit is higher than that of the drum scanner and the space required for the unit is the minimum among the three types.

[0006] As described above, it can be said that the image scanner type image input unit using a one-dimensional array of pho­toelectric transducer elements is optimum as it is cheap, small-sized and allows a high resolution.

[0007] A lighting apparatus used in such an image scanner has to irradiate a picture image in the direction where the photo­electric transducer elements are arrayed. An LED array, a fluorescent lamp, a linear halogen lamp and the like are employed for this purpose. A sufficient tone representation capacity is required for the image input unit, and unless the density values of the picture image are quantized into for example 8 to 256 gradations, a picture image such as a photograph or the like with a fine change in intermediate density cannot be loaded accurately into a computer. It is necessary therefore that the picture image be irradiated uniformly by a constant brightness, and hence a lighting apparatus emitting a stabilized quantity of light is neces­sary.

[0008] With a rare gas cold cathode discharge tube filled with xenon or neon gas, the quantity of light is almost constant regardless of the working environmental temperature as com­pared to a general mercury discharge lamp, as to be taken from Fig. 16. Accordingly, when a conventional mercury discharge lamp is used, it must be warmed up by a heating apparatus like an electric heater. This requires a time of 1 to 2 minutes before such a lamp can actually be used. In­stead, the rare gas cold cathode discharge tube is ready for use as soon as the power is switched on. Since a hea­ting for the electrodes is not necessary, a rare gas cold cathode discharge tube can be made small in shape and mi­niaturized in overall size to, for example, a diameter of 1 to 6 mm. The power consumption is low, for example 4 to 10 watts. The luminous color can be arbitrarily selected de­pending on a fluorescent material applied to the inside of the tube wall. Therefore, a rare gas cold cathode discharge tube is appropriate not only for facsimile machines but also as a light source of an image scanner for reading co­lor picture images.

[0009] However, if a picture image read by the color image scanner is printed straightly by a color printer, the printed pic­ture image will be dark, different in hue and inferior in saturation with respect to the original picture image. The reason is that the reflective spectral characteristic of the existing color ink is not ideal. Therefore, a color data translation work will be necessary in order to correct an unbalanced color due to the spectral characteristic of the ink used in a printer. This is called color correction and is a usual practice for obtaining a printed matter ha­ving colors actually corresponding to those of the original picture image.

[0010] For the color correction, density values of the three pri­mary colors such as green, red and blue will be necessary per picture element, the number of picture elements depen­ding on the reading resolution. The volume of data of the three primary colors to be loaded by means of the color image scanner becomes extraordinarily high, an apparatus for realizing the color correction becomes expensive and the calculating time necessary for the color correction be­comes long. This is one of the stumbling blocks not al­lowing a printed matter bright in color to be realized on general computers.

[0011] Now, therefore, what is contrived as available for attai­ning correction swiftly and cheaply is a reading system in line sequence. In the conventional system which is called a page sequential system, the whole color picture is separa­tely read for each primary color by reciprocating three ti­mes in the order of green, red and blue. Instead, in the line sequential system, data of the three primary colors green, red and blue are loaded at every scanning line, fi­nishing the reading of the full picture image by scanning it only one time. In the line sequential reading system, a data volume necessary for the color correction may be mini­mized to one per several thousands (in the case of A4 size), which is advantageous as compared with the page se­quential system. Using a semiconductor memory (RAM) capable of writing and reading as a storage device for color cor­rection and providing an integrated circuit for exclusive use on color correction in the image scanner, the color correction can be carried out in real time in keeping pace with reading, and data after the color correction may be sent to a host computer.

[0012] As described above, the rare gas cold cathode discharge tube is advantageous with respect to the stability of the quantity of light against environmental temperature change, space occupation and so on as compared with a general mer­cury discharge lamp. It is defective, however, insofar as a light source for reading picture images is concerned. That is, the quantity of light in intermittent lighting which is necessary for a line sequential reading is not stabilized. The rare gas cold cathode discharge tube has been used so far under the condition that it was kept lighting conti­nuously for several seconds or longer each time it was energized. In the case of a rare gas cold cathode discharge tube, the gas pressure is 50 to 200 mmHg high, while it is a few 10 mmHg with general mercury discharge lamps. There­fore, a straight bright line called positive column is observed along the discharge tube at the time of lighting. For locating the positive column stably at a specified por­tion in the circumferential direction of the rare gas cold cathode discharge tube, an auxiliary electrode is provided along the wall of the tube.

[0013] However, in the aforementioned prior art, when repeating the intermittent lighting at a period of several millise­conds or so, the positive column is not stably drawn toward the auxiliary electrode. Therefore, the quantity of light of the rare gas cold cathode discharge tube is not fixed and the brightness of the read image changes. Specifically, while the positive column exists at all times, the light emitting position fluctuates within the discharge tube to come near or to go away from the picture image, and thus the quantity of light irradiating the picture image fluc­tuates by 1 to 10 percent. In a high performance image scanner for reading a picture image of fine density values at gradations of 32 to 256, even such several percent fluc­tuation of the quantity of light may exert an influence on the reproduced picture, and a stripe is produced even if a uniform density picture is read.

[0014] The present invention is intended to remedy the above men­tioned problems of the prior art and to provide a rare gas cold cathode discharge tube emitting a stabilized quantity of light even if it is intermittently lit at a period of several milliseconds or so. It is another object of the present invention to provide an image input unit using such a rare gas cold cathode discharge tube, which is moderate in cost and high in performance.

[0015] These objects are achieved with a rare gas cold cathode discharge tube and an image input unit, respectively, as claimed.

[0016] Specific embodiments of the invention will be described be­low with reference to the drawings, in which:

Fig. 1 shows a first embodiment of a rare gas discharge tube according to the present invention,

Fig. 2 is a perspective view for illustrating the discharge process nearby a main electrode 13-a of Fig. 1,

Fig. 3 is a perspective view of an image input unit embodying the present invention,

Fig. 4 is a sectional view taken along line A-A′ in Fig. 3,

Figs. 5 to 10 are enlarged views of a main electrode and its neighborhood of other embodiments of the rare gas cold cathode discharge tube according to the invention,

Figs. 11 to 13 are diagrammatic views of different embodiments of an image reading unit of an image input unit according to the invention,

Fig. 14 is a timing chart for explaining the line sequential reading,

Fig. 15 is an electric circuit diagram of a driver circuit and a discharge tube,

Fig. 16 is a graphic representation of the temperature dependency of the quantity of light emitted by a mercury discharge tube and a rare gas cold cathode discharge tube, respectively,

Fig. 17 is a simplified drawing showing the positional relation between a rare gas cold cathode discharge tube and a picture image, and

Fig. 18 is a drawing representing the photo­electric conversion characteristic of a storage type photoelectric transducer element.

[0017] Referring first to the reading process of the image input unit, Fig. 3 is a perspective view of an image input unit according to a first embodiment of the invention. In the image input unit an image reading unit 10 is shifted suc­cessively in the direction indicated by an arrow, by a dri­ving device such as a stepping motor or the like through a timing belt, a wire or the like which is not shown. Fig. 4 is a sectional view taken on line A-A′ in Fig. 3 of the image reading unit 10.

[0018] In Fig. 4, three cold cathode discharge tubes 1-R, 1-B, 1-G filled with rare gas, which constitute a lighting appara­tus, are used for reading the picture image 7 placed on a glass bed 6 and emit red, blue and green light, respecti­vely. The red tube 1-R is filled with a neon gas under a pressure of 1,3 to 6,7 kPa (10 to 50 Torr), preferably 2,7 kPa (20 Torr). A red color is obtained by the luminescence of the gas itself. However, an aperture formed inside or outside of the tube wall by a white film of titanium oxide powder or the like will be effective in directing the light efficiently for irradiation. The blue and green discharge tubes 1-B and 1-G are filled with xenon gas under a pres­sure of 8 to 20 kPa (60 to 150 Torr), preferably 10,7 kPa (80 Torr). The higher the charging pressure is, the more quantity of light may be expected. However, the tube vol­tage to be impressed on the discharge tube rises proportio­nally to the pressure. Since a high voltage requires an ex­pensive driver circuit, the pressure values specified above will be appropriate.

[0019] A color picture reading method will be described next with reference to Fig. 4 and the timing chart of Fig. 14. In the case of a line sequential reading, the image reading unit 10 shown in Fig. 4 is first shifted to an image reading start position by a driving signal shown under (f) in Fig. 14. Then, light of the three primary colors is successively irradiated onto the picture image 7 under the control of lighting signals shown as (a), (c) and (d), respectively, in Fig. 14. The reflected light is respectively imaged on a photoelectric transducer 3 by an imaging system 2. The out­put signal ((e) in Fig. 14) of the photoelectric transducer 3 is properly amplified, processed and sent to a host com­puter (not shown) as a one-dimensional image information of red, blue and green of the picture image. The above opera­tion is repeated to store on the host computer side one-di­mensional color image data shifted little by little in the direction perpendicular to the array of photoelectric transducer elements in the photoelectric transducer 3, thus reading a two-dimensional color image by means of the one-­dimensional photoelectric transducer. The rare gas cold cathode discharge tubes are lighted for 5 ms each, and the photoelectric transducer operates for 5 ms for reading, thus finishing the basic reading operation in about 30 ms. The lighting time and period are representative in value, and a signal precision and a read rate can be further en­hanced by properly selecting the disposition of the rare gas cold cathode discharge tubes and the sensitivity of the photoelectric transducer elements.

[0020] In the case of a green rare gas cold cathode discharge tube, an afterglow is produced due to the physical property of the fluorescent substance used to convert ultraviolet rays of the xenon gas into visible light. Consequently, when MOS type photoelectric transducer elements are used in which a light storage timing varies at every picture ele­ments, an afterglow removing time Trm, shown in Fig. 14, must be set. Unless it is set, the irradiation of red light following that of green light will be mixed with the green light due to the afterglow and, thus, the reproducibility of the red picture image is deteriorated. Trm should be as short as possible since it directly influences the read rate. Depending on the capacity of the used phosphor, a va­lue of 1 to 20 ms, preferably 5 ms, is appropriate.

[0021] In order to achieve a better lighting stability and durabi­lity characteristic of the cold cathode discharge tube, a preliminary lighting (intermittent or normally light-on) will be effected for 10 to 100 ms before the start of rea­ding, and the light is switched off when the image reading unit 10 is reset to a reference position after the reading has been completed. For correcting any disuniformity of the light along the longitudinal direction of the discharge tu­bes, a white reference picture image uniform in reflection factor is read to obtain reference data. Data read from a real picture image are then corrected by means of the refe­rence data, thereby obtaining data correctly representing the density distribution of the read image.

[0022] In case the excitation wave length of the currently used fluorescent substance is adjusted to 254 nm of the ultra­violet rays of mercury, xenon is the proper rare gas to be charged into the cold cathode discharge tube. However, rare gas other than xenon may be used if a fluorescent substance is developed having its excitation wave length adjusted to the wave length of the light emitted by said other rare gas. For example, if a fluorescent substance having an ex­citation wave length of 389 nm is used, helium will be ap­propriate as the rare gas. Rare gases such as argon, kryp­ton, radon and the like are usable likewise.

[0023] As shown in Fig. 4, the image reading unit 10 can be con­structed compactly by using a platelike or rodlike glass of a proper refractive index distribution as the imaging sy­stem 2. The light emitted by the discharge tubes 1-B, 1-G, 1-R and reflected by the picture image 7 is condensed by the imaging system 2 and irradiated on the photoelectric transducer 3. A reflector 4 consists of a white resin such as polycarbonate or the like and functions to condense and reflect the light from the lighting apparatus in the direc­tion of the picture image 7.

[0024] The lighting apparatus of the image input unit according to the invention will be described in detail next.

[0025] Designated as 1 in Fig. 1 is one of the three rare gas cold cathode discharge tubes 1-G, 1-B and 1-R in Fig. 4. Dis­charge tube 1 is controlled for lighting and fed with electric power by a driver circuit 12 to which two main electrodes 13-a and 13-b and an auxiliary electrode 14 of the discharge tube are electrically connected.

[0026] Fig. 15 is a circuit diagram representing one example of the driver circuit 12. A battery E is shown as a power source providing 12V or 24V DC power. Upon input of a ligh­ting signal S1, a transistor TR3 is switched on, energizing an inverter circuit including transistors TR1 and TR2 and a boosting transformer T. Upon self-oscillation of the inver­ter, a high-voltage, high-frequency alternating current of 500 to 2000V and 10 to 50 kHz is induced in the secondary winding of the boosting transformer T and applied to the discharge tube 1. Thereby, discharge tube 1 is controlled for lighting.

[0027] Capacitors C1, C2 and C3 function to limit the current flo­wing through the discharge tube and their capacitance is 50 to 200 pF, preferably 120 pF. Capacitors C4 and C5 function to stabilize the positive column or bright line formed by the current flowing through the discharge tube. Their capa­citance is 5 to 30 pF or so. The potential of the main electrodes 13-a, 13-b is stabilized by providing the capa­ citors, and a stable discharge state can be maintained even at the time of intermittent lighting in which the discharge tube is switched on and off repeatedly at milliseconds. It is preferable that one of the terminals of the capacitors C4 and C5 be grounded.

[0028] The auxiliary electrode 14 may be a strip of a hardened, conductive adhesive containing carbon for example, the width of the strip being 0,1 to 2 mm, preferably 0,8 mm and its resistance value 1 to 20 kΩ, preferably 3 to 6 kΩ per centimeter length.

[0029] Fig. 2 is a perspective enlarged partial view of the dis­charge tube of Fig. 1, for illustrating the discharge pro­cess at the main electrode 13-a and its neighborhood. A ring-shaped conductive member 15 of a conductive adhesive containing a powder such as for example copper, carbon or the like is formed on the discharge tube wall around the inner end of the main electrode 13-a (actually, the conduc­tive member 15 is displaced with respect to the inner axial end of the main electrode 13-a in the axial direction for reasons stated later). The electrode 13-a has the same electrical potential as that of the auxiliary electrode 14, whereas the electrode 13-b has the reverse electrical potential of that of the auxiliary electrode 14. The conductive member 15 is electrically connected with the auxiliary electrode 14.

[0030] When the high-voltage, high-frequency AC voltage is applied to the main electrodes 13-a, 13-b by the driver circuit 12, an electron beam is generated between the main electrodes 13-a and 13-b through the rare gas, for example xenon gas. The electron beam brings the rare gas into a plasma state, and excites it to emit ultraviolet rays, visible light and infrared rays, as is characteristic for the gas. This deve­ lops to a bright line which is observed as a positive co­lumn 17. The ultraviolet rays excite the fluorescent sub­stance applied to the inner wall of the discharge tube. A luminescence of arbitrary visible light as blue, green, red, white and so on may be obtained by selecting the kind of fluorescent substance.

[0031] A voltage is impressed on the auxiliary electrode 14 by the driver circuit 12 in order to establish a potential dif­ference between the main electrodes 13-a, 13-b and the auxiliary electrode 14. Due to this potential difference, the positive column 17 generated between the main electro­des is drawn to the discharge tube inner wall along the auxiliary electrode 14 and stabilized firmly.

[0032] However, if for example a piece of metal 16 is disposed nearby the main electrode 13-a of the discharge tube when the discharge tube is intermittently lightened, the positive column generated from the main electrode 13-a at the time of lighting start is drawn to the side of the piece of metal (indicated as 17-a in Fig. 2) and its path will not be stabilized along the auxiliary electrode 14 (the same effect will occur with any material other than a piece of metal, provided that it establishes a capacitance to the positive column of discharge an exerts an influence on the formation of the positive column. This corresponds for instance to the case where the discharge tubes and the image reading unit are fixed on a support member of the image scanner and are shifted along the surface of a picture image for reading.) The conductive member 15 functions to surpress a deflection of the positive column. The above mentioned problems occur when the piece of metal 16 is disposed in a region surrounding the nose or inner end of the main electrode 13-a. When the positive column is drawn thereto, since the conductive member 15 and the auxiliary electrode 14 are of the same potential, the positive column is caught on the conductive member 15 and a path 17-b is formed immediately along the auxiliary electrode 14. Once the positive column is caught on the auxiliary electrode 14, the position of the positive column is stabilized since it never comes outside of the position of the auxiliary electrode 14, even if a piece of metal 16 or any equivalent member is disposed nearby.

[0033] Accordingly, the conductive member 15 will preferably be positioned ahead (towards the opposite main electrode) of a discharge position of the main electrode. Concretely, it may preferably by displaced by 1 to 5 mm or so from the axial inner end of the main electrode.

[0034] Fig. 17 is a diagrammatic view showing the positional rela­tion between the discharge tube 1 and the picture image 7. The total length of the discharge tube must be long enough such that the length of a portion of the discharge tube emitting a substantially uniform quantity of light covers the width of the picture image 7 to be read. Unless the conductive member 15 is made of a transparent material like tin oxide, it is required to be positioned 3 to 10 mm out­side of the adjacent edge of the picture image in order to ensure a uniform quantity of light over the whole width of the picture image.

[0035] Figs. 5 to 10 are diagrammatic partial views of alternative embodiments of the rare gas cold cathode discharge tube ac­cording to the invention.

[0036] According to the embodiment shown in Fig. 5, the conductive member 15 consists of a spring piano wire which is fixed on the discharge tube wall by the spring force and which is electrically connected to the auxiliary electrode 14 formed of a conductive adhesive. Compared to the first embodiment of Fig.2, this embodiment is simplified not needing the application of the conductive adhesive for forming the conductive member 15, and thus is moderate in cost.

[0037] In the embodiment of Fig. 6, a lead wire for connecting the auxiliary electrode 14 to the driver circuit 12 (Fig. 1) is wound on the discharge tube wall, thus forming a conductive member 15 at the same time.

[0038] In the embodiment shown in Fig. 7, the conductive member 15 is formed by dipping the end of the discharge tube into a liquid conductive adhesive which is then hardened. In this case, care must be taken to ensure the necessary insulation between the conductive member 15 and the main electrode.

[0039] In the embodiment shown in Fig. 8, the conductive member 15 consists of a metallic box-like member or the like, serving as fixing means for the discharge tube at the same time and, hence simplifying the construction.

[0040] As shown in Fig. 9, the conductive member 15 does not ne­cessarily need to form a closed ring around the discharge tube wall and still may be effective enough to stabilize the positive column.

[0041] In the embodiment shown in Fig. 10, a sheet metal of phos­phor bronze having spring characteristics and a thickness of about 0.2 mm is curved cylindrically and fixed on the discharge tube wall. Provided with a projection 16, this conductive member 15 is capable of functioning as a posi­tioning member in the circumferential direction when the discharge tube is fixed.

[0042] In the image reading unit 10 described hereinabove with re­ference to Figs. 3 and 4, the color separation is carried out by switching a tricolor light source 1-G, 1-B, 1-R. Shown diagrammatically in Fig. 11 is an alternative embodi­ment of the image reading unit wherein the color separation is realized by a filter. In this case, a white rare gas cold cathode discharge tube 1 is used for irradiating the picture image 7. The white discharge tube, called a three-­wavelength type discharge tube, is obtainable by using as a fluorescent substance a mixture of green, red and blue emitting substances. The image is formed on a CCD photo­electric transducer 3 via a mirror 20 and an imaging system 2. More than one mirror may be used in order to minimize the general volume of the unit. For color separation, a tricolor filter 21 including green, red and blue filter portions is disposed in the imaging optical path. A driving device 22 is used to shift the green, red and blue filter portions successively into the imaging optical path. Since the filter portions of the filter 21 must be changed within a short time of several milliseconds, the filter 21 advan­tageously is formed as a circular plate segmented into three 120° portions corresponding to the green, red and blue filter portions. In this case, driving device 22 could be a motor switching from one filter portion to another by a rotation of 120° of the filter plate.

[0043] The three-wavelength type white cold cathode discharge tube 1 is intermittently driven chiefly for adjusting the quan­tity of light inputted to the photoelectric transducer (MOS type transducer, CCD transducer or the like). The reason for that will be described below.

[0044] Fig. 18 represents a conversion characteristic of a MOS type photoelectric transducer element consisting of amor­phous silicon (the situation with a CCD transducer element is similar). In the photoelectric transducer element, gene­rally called a storage type transducer, an output is ob­tainable in proportion to the integrated quantity of light irradiated within the reading period (domain B in Fig. 18). However, even without any light there exists a background output called noise (domain C in the drawing). This is cal­led a dark output. The S/N ratio indicating the precision of the read signals is expressed as the ratio of a signal output to the dark output. Of course, the larger the S/N ratio, the better it is. The integrated quantity of light to the photoelectric transducer element has a saturation threshold (domain A in the drawing), i.e. the output vol­tage of the transducer element becomes constant even if further light is applied. If saturation is reached, a den­sity change in the picture image can no longer be read cor­rectly. Accordingly, for better precision of an outgoing signal to extract, an irradiation lot if the light to use will come in the domain B in Fig. 18. For best results, the amount of light will be just below the saturation point.

[0045] Therefore, the lighting time of the rare gas cold cathode discharge tube should be set to an optimum value. However, the quantity of light emitted by the discharge tube varies from tube to tube in a mass production according to a sta­tistical dispersion. If the output signal is saturated (do­main A in Fig. 18), then the output becomes constant, not reflecting the image density. Therefore, the lighting time at the time of shipment is set somewhat short as compared with the time at which the output is saturated. Further, the quantity of light decreases gradually from a continuous service and thus the S/N ratio drops. Therefore, the capa­city inherent in the photoelectric transducer element can­not be made full use of. Thus, a precise signal output will be obtained if the lighting time of the light source is ad­justed to set an optimum quantity of light for the photo­ electric transducer elements whenever the image input unit is used. To make the lighting time variable, an intermit­tent lighting is carried out as mentioned before. In this case, the quantity of light may be stabilized by using the conductive member 15 on the tube wall due to the reasons mentioned above.

[0046] Fig. 12 diagrammatically shows a further embodiment of the image reading unit according to the invention. In this case, separate color signals are obtained at a time by pro­viding the transducer elements themselves with a filter at the state of manufacturing. Since mechanically operating parts are not required for color separation, problems like a vibration generated from such operating parts or the like will not occur. Chiefly for adjusting the quantity of light inputted to the photoelectric transducer elements, the three-wavelength type white cold cathode discharge tube 1 is intermittently lighted in this embodiment, too. Also, the conductive member 15 is preferably used with the dis­charge tube 1 in this embodiment.

[0047] Fig. 13 is a diagrammatic view of still another embodiment of the image reading unit according to the invention. While the embodiment of Fig. 4 uses a magnifying imaging optical system, the optical system 2 of the embodiment shown in Fig. 13 is of the reducing type, reducing to one-fifth to one-tenth or so. Like the embodiment shown in Fig. 4, the color separation is achieved by using three discharge tubes 1-R, 1-B and 1-G.

[0048] As described above, the intermittent lighting is a dis­charge tube lighting method required particularly for light adjustment and line sequential reading for color correc­tion. Further, a noise of a high frequency of 20 to 30 kHz produced at the time of discharge tube lighting during the normal reading is capable of affecting the output of the photoelectric transducer. It is therefore effective to carry out an intermittent lighting of the discharge tube including the three-wavelength type tube and also to gene­rate an output of the photoelectric transducer when putting out the light.

[0049] As described above, according to the invention, a rare gas cold cathode discharge tube is provided, allowing to stabi­lize the quantity of emitted light even when an intermit­tent lighting with a period of several milliseconds is car­ried out. The use of such rare gas cold cathode discharge tube as lighting apparatus allows an image input unit to be realized which is moderate in cost and less space-consu­ming, still ensuring a color correction function to enhance the color reproducibility of a printing output of the read image and a light adjusting function of the lighting appa­ratus to enhance the S/N ratio of the read image.

Claims

1. A cold cathode discharge tube having a pair of main electrodes (13-a, 13-b) in a linear tube charged with a rare gas and and auxiliary electrode (14) extending lengthwise of the tube, characterized in that a conductive member (15) for preventing deflections of a positive column formed between said main electrodes along said auxiliary electrode upon application of a voltage, is provided nearby one (13-a) of said main electrodes (13-a, 13-b).

2. The discharge tube according to claim 1, wherein said conductive member (15) is electrically connected to said auxiliary electrode (14).

3. The discharge tube as claimed in claim 1 or 2, wherein said conductive member (15) has substantially ring shape surrounding said tube.

4. The discharge tube according to claim 3, wherein said conductive member (15) is displaced from said main electrode (13-a) lengthwise of the tube towards the central portion thereof.

5. The discharge tube as claimed in claim 1 or 2, wherein said conductive member (15) is provided to surround said tube.

6. The discharge tube according to claim 5, wherein said conductive member (15) extends from said main electrode (13-a) lengthwise of the tube towards the central portion thereof.

7. An image input unit using a cold cathode discharge tube (1, 1-G, 1-B, 1-R) according to any of the preceding claims for irradiating a picture image, said irradiated picture image being read by a photoelectric transducer (3), wherein said discharge tube is subjected to an intermittent lighting.

8. The image input unit according to claim 7, wherein said discharge tube (1) emits light of two or more diffe­rent colors.

9. The image input unit according to claim 7, wherein a plurality of said discharge tubes (1-G, 1-B, 1-R) emit­ting light of different colors are provided, said picture image being successively irradiated by said different color discharge tubes.

10. The image input unit according to any of claims 7 to 9, wherein said transducer (3) includes storage-type photoelectric transducer elements and the rare gas cold cathode discharge tube is subjected to an intermittent lighting such that an output of the photoelectric transdu­cer elements has a value between a dark output when no light is irradiated on the photoelectric transducer element and a saturation output.

Drawing