U.S. patent application number 14/151593 was filed with the patent office on 2014-05-08 for light sensing method.
This patent application is currently assigned to Himax Display, Inc.. The applicant listed for this patent is Himax Display, Inc.. Invention is credited to Ju-Tien Cheng, Wei-Ting Lan, Cheng-Chi Yen.
Application Number | 20140124656 14/151593 |
Document ID | / |
Family ID | 42825407 |
Filed Date | 2014-05-08 |
United States Patent
Application |
20140124656 |
Kind Code |
A1 |
Lan; Wei-Ting ; et
al. |
May 8, 2014 |
LIGHT SENSING METHOD
Abstract
A light detecting method is provided. The light detecting method
includes following steps. A light beam is sensed to generate a
photocurrent. A predetermined current is subtracted from the
photocurrent. The photocurrent is converted to a voltage.
Inventors: |
Lan; Wei-Ting; (Tainan
County, TW) ; Yen; Cheng-Chi; (Tainan County, TW)
; Cheng; Ju-Tien; (Tainan County, TW) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Himax Display, Inc. |
Tainan City |
|
TW |
|
|
Assignee: |
Himax Display, Inc.
Tainan City
TW
|
Family ID: |
42825407 |
Appl. No.: |
14/151593 |
Filed: |
January 9, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12688194 |
Jan 15, 2010 |
8658958 |
|
|
14151593 |
|
|
|
|
Current U.S.
Class: |
250/208.2 ;
250/206 |
Current CPC
Class: |
H03F 3/08 20130101; G01J
1/44 20130101; H01L 31/02019 20130101 |
Class at
Publication: |
250/208.2 ;
250/206 |
International
Class: |
G01J 1/44 20060101
G01J001/44 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 6, 2009 |
TW |
98111360 |
Claims
1. A light detecting method, comprising: sensing a light beam to
generate a photocurrent; subtracting a predetermined current from
the photocurrent; and converting the photocurrent to a voltage.
2. The light detecting method as claimed in claim 1, wherein the
light beam is sensed by a light sensor.
3. The light detecting method as claimed in claim 2, further
comprising a step of determining the amount of the predetermined
current and the amount of the photocurrent according to a color
enable signal and a register table.
4. The light detecting method as claimed in claim 3, wherein the
predetermined current is generated by a programmable current
source, and the light beam is sensed by a photodiode array.
5. The light detecting method as claimed in claim 2, wherein the
amount of the predetermined current is set as a threshold current
value such that the voltage is converted from the photocurrent when
the amount of the photocurrent is greater than the threshold
current value.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional application of and claims
the priority benefit of U.S. application Ser. No. 12/688,194, filed
on Jan. 15, 2010, now pending, which claims the priority benefit of
Taiwan application serial no. 98111360, filed on Apr. 6, 2009. The
entirety of each of the above-mentioned patent applications is
hereby incorporated by reference herein and made a part of
specification.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to a detecting method, and more
particularly to a light detecting method.
[0004] 2. Description of Related Art
[0005] In the current display market, either the cathode ray tube
(CRT) or the liquid crystal display (LCD) can not provide a large
frame yet due to the limitation of the process. In addition, for
the projector, the large frame is projected on the screen through a
light source, e.g. a lamp, and a plurality of lenses. Accordingly,
the large frame of 100.about.200 inches can be easily projected by
the projector in the inner space.
[0006] However, in the past, a dark inner space is necessary for
the projector because the brightness of the light source in the
projector is insufficient. With the progress in science and
technology, the light emitting diode (LED) is applied to the
projector to solve the issue. The LED has the advantages of high
brightness, low power consumption, small volume, long lifespan, and
so on. Accordingly, when the LED serves as the light source of the
projector, the projector can project a clear frame even if in a
bright inner space.
[0007] In order to stabilize the brightness of the light beam
emitted by the LED, a feedback control mechanism is used when the
LED serves as the light source. In the feedback control mechanism,
the brightness of the LED is detected, and the current flowing
through the LED is adjusted. FIG. 1 is a schematic diagram of a
conventional light detecting circuit. Referring to FIG. 1, the
light detecting circuit 100 is applied to the feedback control
mechanism of the projector. The light detecting circuit 100
includes a light sensor 101, a current mirror 103, and a sensing
resistor R.sub.SEN. The current mirror 103 is formed by PMOS
transistors Q.sub.1 and Q.sub.2. When being illuminated by the
light beam emitted by the LED, the light sensor 101 generates a
corresponding photocurrent according to the illumination of the
light beam. The current mirror 103 generates a mirror current I
flowing through the resistor R.sub.SEN according to the
photocurrent, so that a voltage drop V.sub.SEN across the resistor
R.sub.SEN is generated. Because the voltage drop V.sub.SEN is
proportional to the illumination of the light beam emitted by the
LED, the illumination thereof can be obtained by measuring the
voltage drop V.sub.SEN.
[0008] Accordingly, when the light sensor 101 senses a light beam
with the high illumination, the mirror current I generated by the
current mirror 103 and the voltage drop V.sub.SEN are both large.
Hence, according to the equation V.sub.SEN=I.times.R.sub.SEN, the
resistance of the resistor R.sub.SEN is small because the maximum
of the is the voltage V.sub.DD. FIG. 2A and FIG. 2B are
characteristic illumination-voltage curves of the light detecting
circuit in FIG. 1. Referring to FIG. 2A, the resistance of the
resistor R.sub.B is smaller than that of the resistor R.sub.A, so
that a larger range of the illumination can be detected while the
resistor R.sub.B serves as the resistor R.sub.SEN. However, if the
resistance of the resistor Rsen is small, the voltage interval
.DELTA.V corresponding to the illumination interval .DELTA.L is
also small, as shown in FIG. 2B. Accordingly, the resolution of the
voltage drop Vsen is reduced, and the illumination of the light
beam emitted by the LED can not be determined exactly.
SUMMARY OF THE INVENTION
[0009] An embodiment of the invention provides a light detecting
method capable of widening the detecting range and enhancing the
accuracy while detecting the high brightness light source.
[0010] An embodiment of the invention provides a light detecting
method including following steps. First of all, a light beam is
sensed to generate a photocurrent. Next, a predetermined current is
subtracted from the photocurrent. Finally, the photocurrent is
converted to a voltage.
[0011] In an embodiment of the invention, the light beam is sensed
by a light sensor.
[0012] In an embodiment of the invention, the light detecting
method further includes a step of determining the amount of the
predetermined current and the amount of the photocurrent according
to a color enable signal and a register table.
[0013] In an embodiment of the invention, the predetermined current
is generated by a programmable current source, and the light beam
is sensed by a photodiode array.
[0014] In an embodiment of the invention, the amount of the
predetermined current is set as a threshold current value such that
the voltage is converted from the photocurrent when the amount of
the photocurrent is greater than the threshold current value.
[0015] In view of the above, when the high brightness light beam
illuminates the light sensor, the light sensor generates a
corresponding photocurrent according to the illumination thereof.
By dividing the photocurrent corresponding to the low brightness
light beam from the total photocurrent through the current source,
the light detecting method can mainly detects the high brightness
light beam, so that the detecting accuracy is enhanced.
Accordingly, when being applied to detect the high brightness light
beam, the light detecting circuit utilizing the light detecting
method provided in the present application can provide a sensing
voltage in a wide enough range and a large enough sense scale, so
that the sensing voltage is easy to be distinguished by the rear
stage.
[0016] To make the aforementioned and other features and advantages
of the invention more comprehensible, several embodiments
accompanied with figures are described in detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The accompanying drawings are included to provide a further
understanding of the invention, and are incorporated in and
constitute a part of this specification. The drawings illustrate
embodiments of the invention and, together with the description,
serve to explain the principles of the invention.
[0018] FIG. 1 is a schematic diagram of a conventional light
detecting circuit.
[0019] FIG. 2A and FIG. 2B are characteristic illumination-voltage
curves of the light detecting circuit in FIG. 1.
[0020] FIG. 3 is a schematic diagram of a light detecting circuit
according to an embodiment of the invention.
[0021] FIG. 4 is a characteristic illumination-voltage curve of the
light detecting circuit in FIG. 3.
[0022] FIG. 5 is a schematic diagram of a light detecting circuit
according to another embodiment of the invention.
[0023] FIG. 6 is a schematic diagram of a light detecting circuit
according to an embodiment of the invention.
[0024] FIG. 7 is a schematic diagram of a light detecting circuit
according to an embodiment of the invention.
[0025] FIG. 8 is a schematic diagram of a light detecting circuit
according to an embodiment of the invention.
[0026] FIG. 9 is a circuit diagram of the light detecting circuit
in FIG. 8, in which the light sensor 801 is shown as an equipment
circuit of the photodiode.
[0027] FIG. 10 illustrates characteristic I-V curves of the light
detecting circuit in FIG. 9.
[0028] FIG. 11 is a schematic diagram of a light detecting circuit
according to another embodiment of the invention.
[0029] FIG. 12 is a block diagram of an LCOS color sequential
display system according to an embodiment of the invention.
[0030] FIG. 13 is a flowchart illustrating a light detecting method
according to an embodiment of the invention.
DESCRIPTION OF EMBODIMENTS
[0031] FIG. 3 is a schematic diagram of a light detecting circuit
according to an embodiment of the invention. Referring to FIG. 3,
the light detecting circuit 400 includes a light sensor 401, a
first current mirror 403, a first resistor R.sub.1, and a current
source 405. In the present embodiment, the light sensor 401 is a
photodiode, but the invention is not limited thereto. The first end
of the light sensor 401, e.g. the positive end, is coupled to the
master current end of the first current mirror 403. The second end
of the light sensor 401, e.g. the negative end, is coupled to a
first voltage. In this case, the voltage V.sub.SS is used to serve
as the first voltage. The first end of the first resistor R.sub.1
is coupled to the slave current end of the first current mirror
403, and the second end of the first resistor R.sub.1 is coupled to
the voltage V.sub.SS. The current source 405 is coupled to the
first end of the first resistor R.sub.1.
[0032] The first current mirror 403 includes a first transistor
T.sub.1 and a second transistor T.sub.2. In the present embodiment,
the transistors T.sub.1 and T.sub.2 may be p-channel
metal-oxide-semiconductor transistors (PMOS transistors). Herein,
the first end of the transistor T.sub.1 is coupled to a second
voltage, i.e. the voltage V.sub.DD, and the control end of the
transistor T.sub.1, e.g. the gate, is coupled to the control end
and the second end of the transistor T.sub.2, e.g. the gate and the
drain. Moreover, the second end of the transistor T.sub.1, e.g. the
drain, serves as the slave current end of the current mirror 403 to
be coupled to the first end of the first resistor R.sub.1. The
first end of the transistor T.sub.2 is coupled to the voltage
V.sub.DD, and the drain of the transistor T.sub.2 serves as the
master current end of the first current mirror 403 to be coupled to
the positive end of the light sensor 401.
[0033] In present embodiment of the invention, the current source
405 includes a second resistor R.sub.2, a second current mirror
407, and a third current mirror 409. The current mirror 407
includes a third transistor T.sub.3 and a fourth transistor
T.sub.4. In the present embodiment, the transistors T.sub.3 and
T.sub.4 may be PMOS transistors. Herein, the first end of the
transistor T.sub.3, e.g. the source, is coupled to the voltage
V.sub.DD, and the control end of the transistor T.sub.3, e.g. the
gate, is coupled to the second end of the transistor T.sub.3, e.g.
the drain. The control end of the transistor T.sub.4, e.g. the
gate, is coupled to the gate of the transistor T.sub.3, and the
first end of the transistor T.sub.4, e.g. the source, is coupled to
the voltage V.sub.DD. The drain of the transistor T.sub.3 is the
master current end of the current mirror 407, and the second end of
the transistor T.sub.4, e.g. the drain, is the slave current end of
the current mirror 407. The first end of the resistor R.sub.2 is
coupled to the drain of the transistor T.sub.3, and the second end
of the resistor R.sub.2 is coupled to is coupled to the voltage
V.sub.SS.
[0034] The current mirror 409 includes a fifth transistor T.sub.5
and a sixth transistor T.sub.6. In the present embodiment, the
transistors T.sub.5 and T.sub.6 may be NMOS transistors. Herein,
the second end of the transistor T.sub.5, e.g. the source, is
coupled to the voltage V.sub.SS, and the control end of the
transistor T.sub.5, e.g. the gate, is coupled to the first end of
the transistor T.sub.5, e.g. the drain. The control end of the
transistor T.sub.6, e.g. the gate, is coupled to the gate of the
transistor T.sub.5, and the first end of the transistor T.sub.6,
e.g. the drain, is coupled to the first end of the resistor
R.sub.1. Furthermore, the second end of the transistor T.sub.6,
e.g. the source, is coupled to the voltage V.sub.SS. The drain of
the transistor T.sub.5 is the master current end of the current
mirror 409, and the drain of the transistor T.sub.6 is the slave
current end of the current mirror 409.
[0035] The resistor R.sub.2 is used to limit the amount of the
current I.sub.3. The current mirror 407 generates the mirror
current I.sub.4 according to the current I.sub.3, and the current
mirror 409 generates the mirror current I.sub.2 according to the
current I.sub.4. That is, the resistor R.sub.2 can determines the
amount of the current I.sub.2. Accordingly, while being illuminated
by the light beam, the light sensor 401 generates the photocurrent,
so that the current mirror 403 correspondingly generates the mirror
current I. The minor current I is divided into the current I.sub.1
and I.sub.2. The current I.sub.1 flows through the resistor
R.sub.1, and the current I.sub.2 flows through the current source
405. In the present embodiment, while the light sensor 401 is
illuminated by the low brightness light beam, the amount of the
current I is smaller than the amount of the current I.sub.2. That
is, the current I is fully sank by the current source 405, so that
the amount of the current I.sub.1 flowing through the resistor
R.sub.1 is zero. Accordingly, the current I.sub.2 can be set as the
threshold current while the light sensor 401 is illuminated by the
low brightness light beam.
[0036] In the present, while the light sensor 401 is illuminated by
the high brightness light beam, the amount of the current I is
larger than the amount of the current I.sub.2. As a result, the
part of the current I is sank by the current source 405, and the
rest of the current I, i.e. the current I.sub.1 equal to I-I.sub.2,
flows through the resistor R.sub.1. Meanwhile, a voltage drop
across the resistor R.sub.1, i.e. the sensing voltage V.sub.sen in
FIG. 3, is generated by the current I.sub.1 flowing through the
resistor R.sub.1. FIG. 4 is a characteristic illumination-voltage
curve of the light detecting circuit in FIG. 3. As shown in FIG. 4,
the voltage interval .DELTA.V' corresponding to the illumination
interval .DELTA.L varies between the voltages 0 and V.sub.DD. In
other word, in the present embodiment, the light detecting circuit
400 can provide an exact accuracy and a high resolution for light
detecting while being illuminated by the high brightness light
beam. The brightness of the light emitting diode (LED) can be
obtained by measuring the sensing voltage V.sub.sen, so that the
sensing voltage V.sub.sen may be fed back to the control circuit
(not shown) to control the illumination of the light source (not
shown).
[0037] FIG. 5 is a schematic diagram of a light detecting circuit
according to another embodiment of the invention. Referring to FIG.
3 and FIG. 5, the light detecting circuit 500 is similar to the
light detecting circuit 400, and the difference between the light
detecting circuits 400 and 500 will be described as follows.
Referring to FIG. 5, the current source 405 includes a seventh
transistor T.sub.7 and a bias voltage generating circuit 411. In
the present embodiment, the transistor T.sub.7 may be an NMOS
transistor. Herein, the first end of the transistor T.sub.7, e.g.
the drain, is coupled to the first end of the resistor R.sub.1, and
the second end of the transistor T.sub.7, e.g. the source, is
coupled to the voltage V.sub.SS. Moreover, the bias voltage
generating circuit 411 may be a band gap voltage circuit. It should
be known to those ordinary skilled in the art that the band gap
voltage circuit is a voltage circuit with an adjustable temperature
coefficient, which can steady output a band gap voltage without
being affected due to the change of the temperature. In the present
embodiment, the bias voltage generating circuit 411 is coupled to
the gate of the transistor T.sub.7 to output the bias voltage
V.sub.B1. The bias voltage V.sub.B1 is used to drive the transistor
T.sub.7 to generate/sink the current I.sub.2.
[0038] FIG. 6 is a schematic diagram of a light detecting circuit
according to another embodiment of the invention. Referring to FIG.
6, the light detecting circuit 600 includes a first resistor
R.sub.3, an operational amplifier 603, a light sensor 601, and a
current source 605. In the present embodiment, the light sensor 601
is a photodiode, but the invention is not limited thereto. The
output end of the operational amplifier 603 is coupled to the
second end of the resistor R.sub.3, and outputs the sensing voltage
V.sub.sen to the control circuit (not shown) to control the
illumination of the light source (not shown). The first input end
of the operational amplifier 603 may be a non-inverting end,
denoted as "+", coupled to the voltage V.sub.SS. The first input
end of the operational amplifier 603 may be an inverting end,
denoted as "-", coupled to the first end of the resistor R.sub.3.
The first end of the light sensor 601, e.g. the positive end, is
coupled to the first end of the resistor R.sub.3, and the second
end of the light sensor 601, e.g. the negative end, is coupled to
the voltage V.sub.SS.
[0039] The current source 605 includes a second resistor R.sub.4, a
first current mirror 607, and a second current mirror 609. The
first end of the resistor R.sub.4 is coupled to the voltage
V.sub.DD. The current mirror 607 includes a first transistor
P.sub.1 and a second transistor P.sub.2. In the present embodiment,
the transistors P.sub.1 and P.sub.2 may be NMOS transistors.
Herein, the first end of the transistor P.sub.1, e.g. the drain, is
coupled to the second end of the resistor R.sub.4, and the second
end of the transistor P.sub.1, e.g. the source, is coupled to the
voltage V.sub.SS. Moreover, the control end of the transistor
P.sub.1, e.g. the gate, is coupled to the drain of the transistor
P.sub.1. The control end of the transistor P.sub.2, e.g. the gate,
is coupled to the gate of the transistor P.sub.1, and the second
end of the transistor P.sub.2, e.g. the source, is coupled to the
voltage V.sub.SS. The drain of the transistor P.sub.1 is the master
current end of the current mirror 607, and the first end of the
transistor P.sub.2, e.g. the drain, is the slave current end of the
current mirror 607.
[0040] The current mirror 609 includes a third transistor P.sub.3
and a fourth transistor P.sub.4. In the present embodiment, the
transistors P.sub.3 and P.sub.4 may be PMOS transistors. Herein,
the first end of the transistor P.sub.3, e.g. the source, is
coupled to the voltage V.sub.DD, and the control end of the
transistor P.sub.3, e.g. the gate, is coupled to the drain of the
transistor P.sub.3. Moreover, the second end of the transistor
P.sub.3, e.g. the drain, is coupled to the drain of the transistor
P.sub.2. The first end of the transistor P.sub.4, e.g. the source,
is coupled to the voltage V.sub.DD, and the control end of the
transistor P.sub.4, e.g. the gate, is coupled to the gate of the
transistor P.sub.3. Moreover, the second end of the transistor
P.sub.4, e.g. the drain, is coupled to the positive end of the
light sensor 601. The drain of the transistor P.sub.3 is the master
current end of the current mirror 609, and the drain of the
transistor P.sub.4 is the slave current end of the current mirror
609.
[0041] As shown in FIG. 6, the resistor R4 is used to limit the
amount of the current I.sub.7. The current mirror 607 generates the
mirror current I.sub.8 according to the current I.sub.7, and the
current mirror 409 generates the mirror current I.sub.6 according
to the current I.sub.8. In the present embodiment, the light
detecting circuit 600 is a circuit with an inverting close-loop
amplifier, but the invention is not limited thereto. Referring to
FIG. 3 and FIG. 6, the operation of the light detecting circuit 600
is similar to that of the light detecting circuit 400. Referring to
FIG. 6, while the light sensor 601 is illuminated by the high
brightness light beam, the amount of the current I is larger than
the amount of the current I.sub.6. The part of the current I is
provided by the current source 605, and the rest of the current I
is provided by the current I.sub.5. Meanwhile, a voltage drop
across the resistor R.sub.3 is generated by the current I.sub.5
flowing through the resistor R.sub.3, thereby generating a sensing
voltage V.sub.sen at the output end of the operational amplifier
603. As known from the characteristic illumination-voltage curve
illustrated in FIG. 4, the illumination of the light source can be
obtained from the corresponding sensing voltage V.sub.sen.
[0042] FIG. 7 is a schematic diagram of a light detecting circuit
according to another embodiment of the invention. Referring to FIG.
6 and FIG. 7, the light detecting circuit 700 is similar to the
light detecting circuit 600, and the difference between the light
detecting circuits 600 and 700 will be described as follows.
Referring to FIG. 7, the current source 605 includes a fifth
transistor P.sub.5 and a bias voltage generating circuit 611. In
the present embodiment, the transistor P.sub.5 may be a PMOS
transistor. Herein, the first end of the transistor P.sub.5, e.g.
the source, is coupled to the voltage V.sub.DD, and the second end
of the transistor P.sub.5, e.g. the drain, is coupled to the
positive end of the light sensor 601. Moreover, the bias voltage
generating circuit 611 may be a band gap voltage. The bias voltage
generating circuit 611 is coupled to the control end of the
transistor P.sub.5, e.g. the gate, and configured to output the
bias voltage V.sub.B2, i.e. the band gap voltage to drive the
transistor P.sub.5 to steady generate the current I.sub.6.
[0043] FIG. 8 is a schematic diagram of a light detecting circuit
according to another embodiment of the invention. Referring to FIG.
8, the light detecting circuit 800 includes a light sensor 801, an
operational amplifier 803, a current source 805, and a feedback
impedance unit 807. In the present embodiment, the feedback
impedance unit 807 includes a feedback resistor R and a feedback
capacitor C. Herein, the feedback resistor R determines the loop
gain of the operational amplifier 803 converting the photocurrent
I.sub.ph to the sensing voltage V.sub.SEN, and the feedback
capacitor C is used to prevent the circuit system from being
unstable and avoid the issue related to the circuit
oscillation.
[0044] The output end of the operational amplifier 803 is coupled
to the second end B of the feedback impedance unit 807, and outputs
the sensing voltage V.sub.SEN to the rear stage, e.g. an A/D
converter (not shown). The first input end of the operational
amplifier 803 may be a non-inverting end, denoted as "+", coupled
to the ground GND. The second input end of the operational
amplifier 803 may be an inverting end, denoted as "-", coupled to
the first end A of the feedback impedance unit 807.
[0045] In the present embodiment, the N+/P-Well photodiode is
exemplary for the light sensor 801. The first end of the light
sensor 801, e.g. the cathode end, is coupled to the first end A of
the feedback impedance unit 807, and the second end thereof, e.g.
the anode end, is coupled to the ground GND. Moreover, there is no
direct voltage drop between the two ends of the photodiode, and
thus, the dark current is not generated, so that the light
detecting circuit 800 has the higher signal-to-noise ratio and the
better sensitivity.
[0046] It should be noted that, the light detecting circuit 800 of
the present embodiment includes the current source 805 which is
coupled to the cathode end of the photodiode, and the threshold
current is set through the current source 805. It will be described
in detail later. In other embodiments, the light sensor 801 may be
a P+/N-Well photodiode, and the current source 805 is coupled to
the anode end of the photodiode. Furthermore, in the present
embodiment, the light detecting circuit 800 is a circuit with an
inverting close-loop amplifier, but the invention is not limited
thereto.
[0047] FIG. 9 is a circuit diagram of the light detecting circuit
in FIG. 8, in which the light sensor 801 is shown as an equipment
circuit of the photodiode. Herein, the equipment circuit of the
photodiode includes a photocurrent I.sub.ph, an equipment capacitor
C.sub.j, and an equipment resistor R.sub.sh. Referring to FIG. 9,
in the present embodiment, the current source 805 provides a bias
current I.sub.bias to set the threshold current. When the
brightness of the light beam reaches to a specific value, the light
sensor 801 can generate a photocurrent I.sub.ph larger than the
threshold current. In such a case, the photocurrent I.sub.ph can be
converted to the sensing voltage V.sub.SEN. On the contrary, if the
photocurrent I.sub.ph generated by the light sensor 801 is smaller
than the threshold current, the function of converting the
photocurrent I.sub.ph to the sensing voltage V.sub.SEN is cut off
to filter out the photocurrent corresponding to the low brightness
light beam. That is, the light sensor generates a corresponding
photocurrent according to the illumination while being illuminated
by the high brightness light beam. By dividing the photocurrent
corresponding to the low brightness light beam from the total
photocurrent through the current source, the light detecting
circuit can mainly detects the high brightness light beam, so that
the detecting accuracy can be enhanced.
[0048] Specifically, FIG. 10 illustrates characteristic I-V curves
of the light detecting circuit in FIG. 9. Referring to FIG. 9 and
FIG. 10, in the present embodiment, the N+/P-Well photodiode is
exemplary for the light sensor 801, and the threshold current is
set as the bias current I.sub.bias. When the photodiode is
illuminated by the light beam, if the photocurrent I.sub.ph
generated by the photodiode is smaller than the bias current
I.sub.bias, a forward current I.sub.F flowing through the feedback
resistor R is generated. At this time, the sensing voltage
V.sub.SEN does not change with the photocurrent I.sub.ph. On the
contrary, if the photocurrent I.sub.ph generated by the photodiode
is larger than the bias current I.sub.bias, a reverse current
I.sub.R flowing through the feedback resistor R is generated. At
this time, the sensing voltage V.sub.SEN change with the
photocurrent I.sub.ph in the linear manner.
[0049] As a result, the voltage interval .DELTA.V is increased to
the voltage interval .DELTA.V', so that the sensing voltage
V.sub.SEN changes between the voltages 0 and V.sub.DD, and the
slope .DELTA.V/.DELTA.I.sub.ph of the I-V curve in the photovoltaic
mode is changed to the slope .DELTA.V'/.DELTA.I.sub.ph of the I-V
curve while the bias current source is enabled. Accordingly, the
light detecting circuit of the embodiment can widen the detecting
range and enhance the accuracy while detecting the high brightness
light source. That is, the light sensor generates a corresponding
photocurrent according to the illumination while being illuminated
by the high brightness light beam. By dividing the photocurrent
corresponding to the low brightness light beam from the total
photocurrent through the current source, the light detecting
circuit can mainly detects the high brightness light beam, so that
the detecting accuracy can be enhanced. Accordingly, when being
applied to detect the high brightness light beam, the light
detecting circuit can provide a sensing voltage in a wide enough
range and a large enough sense scale, so that the sensing voltage
is easy to be distinguished by the rear stage.
[0050] It should be noted that, in the present embodiment, the bias
current I.sub.bias provided by the current source 805 can be set as
zero, so that the light detecting circuit operates in the
photovoltaic mode.
[0051] FIG. 11 is a schematic diagram of a light detecting circuit
according to another embodiment of the invention. FIG. 12 is a
block diagram of an LCOS color sequential display system according
to an embodiment of the invention. Herein, the light detecting
circuit 1100 of FIG. 11 serves as a color sensor of the LCOS color
sequential display system 1200. Referring to FIG. 11 and FIG. 12,
the light detecting circuit 1100 is similar to the light detecting
circuit 800, and the difference between the light detecting
circuits 1100 and 800 will be described as follows.
[0052] In the present embodiment, the current source 1105 is a
programmable current source. The programmable current source 1105
is coupled to the inverting end of the operational amplifier and
provides a programmable current as the bias current, so that the
threshold current can be freely set. The light sensor 1101 is a
photodiode array unit including a switch unit 1101S and a
photodiode array 1101D. The photodiode array 1101D is used to sense
the light beams with different colors and generate a plurality of
photocurrents I.sub.0-I.sub.4. The switch unit 1101S control the
summation of the photocurrents I.sub.0-I.sub.4 through the switches
d.sub.0-d.sub.4 which are coupled to the corresponding photodiode
strings in series.
[0053] As known from FIG. 12, the LCOS color sequential display
system 1200 includes an LCOS color sequential panel 1202, a
register table 1204, a selection unit 1206, and the light detecting
circuit 1100. Herein, the light detecting circuit 1100 serves as a
color sensor of the LCOS color sequential display system 1200. The
operation of the LCOS color sequential display system is well known
to those skilled in the art, and it will not be described
herein.
[0054] When the light detecting circuit 1100 serves as the color
sensor of the LCOS color sequential display system 1200, the
threshold current can be set corresponding to the different color
beams, and the photodiode strings can be turned on or off in the
light detecting circuit 1100. Accordingly, it is unnecessary to
design a plurality of corresponding color sensing circuits for the
different color beams, so that the cost of the circuit can be
reduced. Also, in this case, when being applied to detect the high
brightness light beam, the light detecting circuit can provide a
sensing voltage in a wide enough range and a large enough sense
scale, so that the sensing voltage is easy to be distinguished by
the rear stage.
[0055] Specifically, the sensitivity of the photodiodes for
different color beams is different, and it is usually the red beam,
the blue beam, and the green beam in turns. That is, when the
photodiodes are illuminated by different color beams with the same
brightness, the size of the photocurrents generated by different
color beams, from large to small, are the red beam, the blue beam,
and the green beam. Accordingly, when the light detecting circuit
1100 serves as the color sensor of the LCOS color sequential
display system 1200, it is required to set the threshold currents
corresponding to the different color beams, and turned on or off
the corresponding photodiode strings.
[0056] For example, when the light detecting circuit 1100 senses
the red beam, the programmable current source 1105 provides a bias
current I.sub.bias(R) as the threshold current for the red beam. At
this time, if the photocurrent I.sub.ph(R) generated by the
photodiodes for sensing the red beam is larger than the threshold
current I.sub.bias(R), the sensing voltage V.sub.SEN(R) change with
the photocurrent I.sub.ph(R) in the linear manner. Next, when the
light detecting circuit 1100 senses the green beam, the
programmable current source 1105 provides a bias current
I.sub.bias(G) as the threshold current for the green beam. At this
time, if the photocurrent I.sub.ph(R) generated by the photodiodes
for sensing the green beam is larger than the threshold current
I.sub.bias(G), the sensing voltage V.sub.SEN(G) change with the
photocurrent I.sub.ph(G) in the linear manner.
[0057] In the present embodiment, in order to provide the sensing
voltages in a wide enough range and a large enough sense scale for
all of the different color beams, it is required to set the
threshold currents I.sub.bias(G) and I.sub.bias(R) to be
approximate or to be equal while the light detecting circuit 1100
operates. However, the sensitivity of the photodiodes for the red
beam is greater than that of the photodiodes for the green beam.
Accordingly, by adjusting the switch unit 1101S in the photodiode
array unit 1101, all of the generated photocurrents can larger than
the corresponding threshold currents while the light detecting
circuit 1100 detects different color beams.
[0058] For example, in order to make the photocurrent I.sub.ph(G)
be larger than the threshold currents I.sub.bias(G), when the light
detecting circuit 1100 detects the green color beam, the switches
d.sub.0-d.sub.4 can be switched, so that the amount of the
photodiodes turned on for the green beam in the photodiode array
1101D is more than the amount of the photodiodes turned on for the
red beam when the light detecting circuit 1100 detects the red
color beam.
[0059] Similarly, in order to make the photocurrent I.sub.ph(B) be
larger than the threshold currents I.sub.bias(B), when the light
detecting circuit 1100 detects the blue beam, the switches
d.sub.0-d.sub.4 can be switched, so that the amount of the
photodiodes turned on for the blue beam in the photodiode array
1101D is more than the amount of the photodiodes turned on for the
red beam when the light detecting circuit 1100 detects the red
beam, but less than the amount of the photodiodes turned on for the
green beam when the light detecting circuit 1100 detects the green
beam. Accordingly, when being applied to detect the high brightness
light beams with different colors, the light detecting circuit can
provide the sensing voltages in a wide enough range and a large
enough sense scale, so that the sensing voltages are easy to be
distinguished by the rear stage.
[0060] In the present embodiment, the programmable current source
1105 receives a current selection signal b[4:0] from a selection
unit 1206 to determine the amount of the programmable current
I.sub.bias, i.e. the threshold current. The switch unit 1101S
receives a switch selection signal D[4:0] from the selection unit
1206 to determine the amount of the photodiodes to be turned on
when the light detecting circuit 1100 detects different color
beams, thereby controlling the total amount of the
photocurrents.
[0061] Moreover, the register table 1204 stores parameters of
driving the programmable current source 1105 and the switch unit
1101 corresponding to different color beams, such as the threshold
currents corresponding to different color beams, and the amount of
the photodiodes to be turned on when the light detecting circuit
1100 detects different color beams. When receiving the color enable
signal provided by the LCOS color sequential panel 1202, the
selection unit 1206 respectively outputs the current selection
signal b[4:0] and the switch selection signal D[4:0] to the
programmable current source 1105 and the switch unit 1101S.
Accordingly, when serving as the color sensor of the LCOS color
sequential display system 1200, the light detecting circuit 1100
can set the threshold currents corresponding to the different color
beams and turn on or off the corresponding photodiode strings
according to the color enable signal and the register table.
[0062] In the present embodiment, the light detecting circuit 1100
serving as the color sensor for the red, the green, the blue beams
is exemplary, and it does not limit the invention. In other
embodiments, the light detecting circuit 1100 may serves as the
color sensor for the white, the red, the green, the blue beams.
[0063] It should be noted that, in the present embodiment, the
light detecting circuit 1100 further includes a small
direct-current (DC) bias voltage source V.sub.photo coupled to the
non-inverting end of the operational amplifier 1103. Accordingly,
when the photodiode is the N+/P-Well diode, by the voltage source
V.sub.photo, it is can be avoided that the switches can not be
turned off because the negative voltage or other unexpected
voltages occur in the negative end of the photodiode.
[0064] According to the disclosure of the above embodiments, a
light detecting method is provided as follows. FIG. 13 is a
flowchart illustrating the light detecting method according to an
embodiment of the invention. Referring to FIG. 13, in the present
embodiment, the light detecting method includes following steps.
First of all, a light beam is sensed to generate a photocurrent. In
the present embodiment, the light sensor, e.g. a photodiode, is
adopted to sense the light beam and correspondingly generates the
photocurrent. Next, in step S1302, a predetermined current is
subtracted from the photocurrent to obtain a subtracted
photocurrent. Those ordinary skilled in the art can determine the
predetermined current in any method. For example, the photocurrent
is divided by using a current source to execute the step of
subtracting the predetermined current from the photocurrent. In
step S1303, the subtracted photocurrent is converted to a sensed
voltage V.sub.sen. According to the amount of the sensed voltage
V.sub.sen, the illumination of the sensing light beam is
obtained.
[0065] Obviously, in another embodiment, the amount of the
predetermined current and the amount of the photocurrent may be
determined according to a color enable signal and a register table
in the light detecting method. In the above-described embodiment,
the predetermined current, for example, is generated by a
programmable current source, and the light beam, for example, is
sensed by a photodiode array.
[0066] To sum up, in the embodiments of the invention, the light
sensor generates a corresponding photocurrent according to the
illumination while being illuminated by the high brightness light
beam. By dividing the photocurrent corresponding to the low
brightness light beam from the total photocurrent through the
current source, the light detecting method can mainly detects the
high brightness light beam, so that the detecting accuracy can be
enhanced. Accordingly, when being applied to detect the high
brightness light beam, the light detecting circuit utilizing the
light detecting method provided in the present application can
provide an output voltage in a wide enough range and a large enough
sense scale, so that the sensing voltage is easy to be
distinguished by the rear stage. Moreover, the light detecting
circuit can adjust the threshold currents for the photocurrents
corresponding to different color beams while being applied to a
color sensor. Accordingly, it is unnecessary to design a plurality
of corresponding color sensing circuits for the different color
beams, so that the cost of the circuit can be reduced.
[0067] Although the invention has been described with reference to
the above embodiments, it is apparent to one of the ordinary skill
in the art that modifications to the described embodiments may be
made without departing from the spirit of the invention.
Accordingly, the scope of the invention will be defined by the
attached claims not by the above detailed descriptions.
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