Digital Camera Patent Abstract
A liquid crystal monitor (LC monitor) illuminating apparatus utilizes
a fluorescent lamp. The lamp is provided in a backlight portion
to illuminate a liquid crystal monitor from behind. The fluorescent
lamp is activated by a direct-current (DC) lighting circuit that
is provided with a switching circuit which reverses the polarity
of the DC lighting circuit. A digital camera with a liquid crystal
monitor including an LC monitor illuminating apparatus is also disclosed.
Digital Camera Patent Claims
What is claimed is:
1. A liquid crystal monitor illuminating apparatus in which a fluorescent
lamp is provided in a backlight portion to illuminate a liquid crystal
monitor from behind, comprising: a direct-current lighting circuit
configured to activate said fluorescent lamp by directly applying
direct-current generated from said direct-current lighting circuit
to said fluorescent lamp; and a switching circuit configured to
reverse the polarity of said direct-current directly applied to
said fluorescent lamp, every time said fluorescent lamp is activated.
2. A liquid crystal monitor illuminating apparatus according to
claim 1, wherein said switching circuit is configured to reverse
the polarity of said direct-current directly applied to said fluorescent
lamp, only when said fluorescent lamp is activated.
3. A liquid crystal monitor illuminating apparatus in which a fluorescent
lamp is provided in a backlight portion to illuminate a liquid crystal
monitor from behind, comprising: a direct-current lighting circuit
configured to activate said fluorescent lamp by directly applying
direct-current generated from said direct-current lighting circuit
to said fluorescent lamp; and a switching circuit configured to
reverse the polarity of said direct-current directly applied to
said fluorescent lamp, every time a main switch, provided on a main
body which sends indication data to said liquid crystal monitor,
is turned ON.
4. A liquid crystal monitor illuminating apparatus according to
claim 3 wherein said switching circuit is configured to reverse
the polarity of said direct-current directly applied to said fluorescent
lamp, only when the main switch is turned ON.
5. A liquid crystal monitor illuminating apparatus in which a fluorescent
lamp is provided in a backlight portion to illuminate a liquid crystal
monitor from behind, comprising: a direct-current lighting circuit
configured to activate said fluorescent lamp by directly applying
direct-current generated from said direct-current lighting circuit
to said fluorescent lamp; and a switching circuit configured to
reverse the polarity of said direct-current directly applied to
said fluorescent lamp, at a predetermined time interval.
6. A liquid crystal monitor illuminating apparatus according to
claim 5, wherein said predetermined time interval is determined
by a CPU, in accordance with a time which is measured by measuring
clock pulses which are generated by a clock generator which sends
indication data to the LC monitor.
7. A liquid crystal monitor illuminating apparatus according to
claim 5, wherein said switching circuit is configured to reverse
the polarity of said direct-current directly applied to said fluorescent
lamp, only at the predetermined time interval.
Digital Camera Patent Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an illumination apparatus for
illuminating a liquid crystal monitor and a digital camera having
a liquid crystal monitor using the LC illuminating apparatus.
2. Description of the Prior Art
In general, in known digital cameras, a liquid crystal monitor
(LC monitor) is used to view an object and an image of the object
taken by the camera. To enable a viewer to view the LC monitor even
in poor light, or in a dark place, a backlight is provided in the
camera.
The backlight is configured to allow light, emitted from a fluorescent
lamp incident upon the LC monitor through a light guide and a reflecting
plate, to illuminate the surface of the LC monitor. In case of a
fluorescent lamp that is a cold-cathode fluorescent lamp, an AC
lamp lighting circuit, whose service life is 10000 hours on an average,
is usually employed.
However, if the LC monitor is illuminated by a backlight having
a fluorescent lamp which is lit by the AC lighting circuit, the
image displayed on the LC monitor tends to be unclear due to noise
caused by an invertor of the AC lighting circuit, thus resulting
in a lower image quality than the image displayed on an LC monitor
using a DC lighting circuit. To prevent this, it is theoretically
possible to light the fluorescent lamp by a DC lighting circuit
instead of the AC lighting circuit, so that the noise caused by
the invertor can be reduced to thereby enhance the image quality.
However, in the fluorescent lamp activated by the DC lighting circuit,
a blackening phenomenon tends to occur within a shorter time span
than in the fluorescent lamp activated by the AC lighting circuit.
Consequently, the service life of the LC monitor is shortened.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a liquid crystal
monitor illuminating apparatus wherein if the fluorescent lamp is
activated by a DC lighting circuit to obtain a clear display of
the LC monitor in which a noise caused by an invertor is restricted,
a blackening phenomenon of the fluorescent lamp tends not to occur,
thus resulting in an prolonged service life of the LC monitor.
Another object of the present invention is to provide a digital
camera having a liquid crystal monitor using the LC monitor illuminating
apparatus.
In order to achieve the above-mentioned objects, there is provided
a liquid crystal monitor illuminating apparatus in which a fluorescent
lamp provided in a backlight portion to illuminate a liquid crystal
monitor from behind including: a direct-current lighting circuit
which activates the fluorescent lamp; and a switching circuit which
reverses the polarity of the direct-current lighting circuit.
Preferably, the switching circuit reverses the polarity of the
direct-current lighting circuit every time the fluorescent lamp
is activated.
Preferably, the switching circuit reverses the polarity of the
DC lighting circuit every time a main switch, provided on a main
body which sends indication data to the LC monitor, is turned ON.
Preferably, the switching circuit reverses the polarity of the
DC lighting circuit at a predetermined time interval.
Preferably, the predetermined time interval is determined by a
CPU, in accordance with a lapse time which is measured by measuring
clock pulses which are generated by a clock generator which sends
indication data to the LC monitor.
Preferably, the liquid crystal monitor illuminating apparatus is
provided in a digital camera.
The present disclosure relates to subject matter contained in Japanese
Patent Application No.10-230693 (filed on Aug. 17, 1998) which is
expressly incorporated herein by reference in its entirety.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be discussed below in detail with reference
to the accompanying drawings, in which:
FIG. 1 is a perspective view of a digital camera having a liquid
crystal monitor according to the present invention;
FIG. 2 is a perspective view of a backlight portion of an LC monitor
lighting apparatus according to the present invention;
FIG. 3 is a circuit diagram of a DC lighting circuit for an LC
monitor lighting apparatus and a switching circuit, according to
the present invention;
FIG. 4 is a block diagram of a digital camera having an LC monitor
and a system to reverse the polarity of a DC lighting circuit in
accordance with values of a main switch counter; and
FIG. 5 is a block diagram of a digital camera having an LC monitor
and a system to control the reversal of the polarity of a DC lighting
circuit, in accordance with clock pulses generated from a clock
generator, according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
As shown in FIG. 1, a digital camera having a liquid crystal (LC)
monitor includes a camera body 1, an LC monitor 2, a shutter button
3, a finder window 4, a mode selection button 5, an external display
portion 6, a main switch 7, a contrast adjusting knob 8, a memory
card insertion opening 9, and a battery compartment lid 10. An LC
monitor illuminating apparatus is provided behind the LC monitor
2 and includes a backlight portion 13, a DC (direct-current) lighting
circuit 17 and a switching circuit 23, as shown in FIGS. 2 and 3.
Each element of the LC monitor illuminating apparatus will be discussed
hereinafter.
The digital camera body 1 which sends indication data to the LC
monitor is provided therein with a CPU 12 shown in FIG. 4. Data
is sent to the CPU 12 from the main switch 7, from a photometering
switch/release switch 26, from the mode selection button 5 and from
an exposure control circuit 41.
The CPU 12 supplies signals to a strobe control circuit 27, an
AF drive mechanism 28, a diaphragm drive mechanism 29, and a clock
generator 32, based on the input data. Consequently, a strobe device
30, a diaphragm 31, and a CCD driver 33 are driven in accordance
with the respective signals supplied from the CPU 12, so that an
object image taken by a CCD 34 through a photographing lens 43 and
an optical low-pass filter 35 is recorded in a memory card 40 via
an amplifier circuit 36, an A/D converter circuit 37, a signal processing
circuit 38, and a compression circuit 39. The CPU 12 causes the
external display 6 to indicate photographing data.
A battery 24 is used as a power source for the above-mentioned
circuits. Power from the battery 24 is supplied through the DC-DC
converter 25.
The functions of these circuits, which are well known in the art,
are not the subject of the present invention, and hence, no detailed
explanation therefor will be given herein.
The CPU 12 is connected to a main switch counter 42, so that the
main switch counter 42 is alternately set to 0 or 1 each time the
power-on signal (which is issued when the main switch 7 is turned
ON) is input to the CPU 12. The CPU 12 supplies a polarity reversing
signal to the switching circuit 23 of the LC monitor illuminating
apparatus in accordance with the set value of the main switch counter
42, so that the switching circuit 23 reverses the polarity of the
DC lighting circuit 17 which is adapted to light the fluorescent
lamp 16 (i.e., to activate the backlight 13) in accordance with
the polarity reversing signal.
The backlight 13 includes reflecting plates 14a, 14b, a light guide
layer 15 and the fluorescent lamp 16. Namely, the light guide layer
15 is formed on the reflecting plate or sheet 14a. The LC monitor
2 is located above the backlight 13, as shown in FIG. 2. The fluorescent
lamp 16 which is covered by the reflecting plate 14b at the portion
that is not in contact with the light guide layer 15 is located
on the side of the light guide layer 15. Light emitted from the
fluorescent lamp 16 is repeatedly reflected by the upper surface
of the light guide layer 15 and the lower reflecting plate 14a,
and is diffused. The light which reaches the upper surface of the
light guide layer 15 partly passes therethrough and is emitted therefrom.
Thus, the upper surface of the light guide layer 15 forms a surface
light source from which the light is substantially uniformly emitted
to illuminate the LC monitor 2.
The fluorescent lamp 16 is activated by a DC power source 45. A
lamp lighting circuit to activate the fluorescent lamp 16 is made
of, for example, a kick-type DC lighting circuit 17, as shown in
FIG. 3.
The DC lighting circuit 17 includes a series circuit L1 in which
a resistor 18 and a secondary winding 19 are connected in series
to one of the electrodes, i.e., the electrode 16b of the fluorescent
lamp 16; and a series circuit L2 in which the primary winding 20
and a condenser 21 are connected in series. The series circuits
L1 and L2 are connected in parallel at contacts A and B.
The DC lighting circuit 17 is connected to a switching circuit
23 which is provided with movable switches 22a and 22b. The contact
A is connected to the movable switch 22a and the contact B is connected
to the movable switch 22b.
The DC power source 45 is provided with a power switch 44 and stationary
contacts C, D, and E. The power switch 44 is closed or opened in
response to a signal from the main switch 7.
The switching circuit 23 mechanically or electrically moves the
movable switch 22a or 22b to the stationary contact C or E, respectively,
on the positive terminal side of the DC power source 45, and simultaneously
moves the other respective movable switch 22b or 22a to the stationary
contact D on the negative side of the DC power source, according
to the polarity reversing signal supplied from the CPU 12.
If the main switch 7 is turned ON, the power switch 44 is closed
and the movable switches 22a and 22b are connected to the stationary
contacts C and D, respectively.
Since the voltage necessary to light the fluorescent lamp 16 cannot
be obtained from the DC power source 45, no electric current flows
in the series circuit L1 of the DC lighting circuit 17.
The transient electric current flows in the condenser 21 and the
winding 20 of the series circuit L2 in the direction F, so that
the electric charges are accumulated in the condenser 21.
Since the winding 19 of the transformer whose winding direction
is opposite to the winding 20 is boosted due to the transient current
flowing in the winding 20, a sufficient potential difference necessary
to light the fluorescent discharge lamp 16 is produced between the
poles (terminals) 16a and 16b of the fluorescent lamp 16. If a sufficient
amount of electric charges is accumulated in the condenser 21 so
that no current flows in the series circuit L2, the electric current
flows in the series circuit L1 due to the potential difference produced
in the winding 19, and thus the fluorescent lamp 16 is lit.
As an alternative, the main switch 7 is turned OFF, so that the
movable switches 22a and 22b are disconnected from the stationary
contacts C, D, and E. In this arrangement, the power switch is rendered
unnecessary.
Since the polarity of the DC lighting circuit 17 is switched every
time the main switch 7 is turned ON, if the main switch 7 is subsequently
turned ON, the movable switch 22a is connected to the stationary
contact D and the movable switch 22b is connected to the stationary
contact E.
As can be understood from the foregoing, since the polarity of
the fluorescent lamp 16 at both electrodes (terminals) thereof is
switched every time the main switch 7 is turned ON, it is possible
to prevent the polarity of each electrode of the fluorescent lamp
16 from always being identical. Consequently, blackening phenomenon
of the fluorescent lamp 16 in the vicinity of only one of the electrodes
thereof can be inhibited.
It is possible to periodically switch the polarity of the DC lighting
circuit 17 at predetermined intervals, instead of the switching
by each operation of the main switch 7.
In the block diagram shown in FIG. 5, instead of providing a main
switch counter 42, the clock pulses are input to the CPU 12 from
the clock generator 32. The CPU 12 detects the clock pulses and
measures the elapsed time, and outputs the polarity reversing signals
to the switching circuit 23 at a predetermined time interval, based
on the measured lapse time. The switching circuit 23 reverses the
polarity of the DC lighting circuit 17 in accordance with the polarity
reversing signals input thereto to light the fluorescent lamp 16
to thereby illuminate the LC monitor 2. The remaining structure
of FIG. 5 is the same as that shown in FIG. 4.
As may be understood from the above discussion, in an LC monitor
illuminating apparatus and a digital camera having an LC monitor
using the illuminating apparatus, if the fluorescent lamp of the
backlight portion is activated by the DC lighting circuit to obtain
a clear LC monitor display in which noise caused by the invertor
is restricted, since the polarity of the DC lighting circuit is
reversed in accordance with predetermined conditions, a blackening
phenomenon of the fluorescent lamp can be inhibited. Consequently,
the service life of the LC monitor can be prolonged.
Obvious changes may be made in the specific embodiments of the
present invention described herein, such modifications being within
the spirit and scope of the invention claimed. It is indicated that
all matter contained herein is illustrative and does not limit the
scope of the present invention. |