[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.
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.