U.S. patent application number 11/693600 was filed with the patent office on 2008-10-02 for color sensor with infrared correction having a filter layer blocking a portion of light of visible spectrum(as amnended).
Invention is credited to Farn Hin Chen, Gim Eng Chew, Boon Keat Tan.
Application Number | 20080237453 11/693600 |
Document ID | / |
Family ID | 39736418 |
Filed Date | 2008-10-02 |
United States Patent
Application |
20080237453 |
Kind Code |
A1 |
Chen; Farn Hin ; et
al. |
October 2, 2008 |
COLOR SENSOR WITH INFRARED CORRECTION HAVING A FILTER LAYER
BLOCKING A PORTION OF LIGHT OF VISIBLE SPECTRUM(AS AMNENDED)
Abstract
A light sensor that generates a first output signal indicative
of an intensity of light received from a predetermined direction in
a first band of wavelengths is disclosed. The light sensor includes
a substrate having first and second photodetectors, a first filter
layer, and a controller. The photodetectors are sensitive to light
in the infrared portion of the optical spectrum as well as to light
in the first band of wavelengths, and generate first and second
photodetector signals. The first filter layer transmits light in
the first band of wavelengths and light in the infrared portion of
the optical spectrum while blocking light in a portion of the
visible spectrum outside of the first band of wavelengths, without
altering light received by the first photodetector. The controller
processes the first and second photodetector signals to produce the
first output signal that is corrected for infrared in the input
light.
Inventors: |
Chen; Farn Hin; (Ipoh,
MY) ; Chew; Gim Eng; (Bayan Lepas, MY) ; Tan;
Boon Keat; (Bayan Lepas, MY) |
Correspondence
Address: |
Kathy Manke;Avago Technologies Limited
4380 Ziegler Road
Fort Collins
CO
80525
US
|
Family ID: |
39736418 |
Appl. No.: |
11/693600 |
Filed: |
March 29, 2007 |
Current U.S.
Class: |
250/226 ;
250/214.1; 257/E27.128; 257/E31.1; 257/E31.121 |
Current CPC
Class: |
G01J 3/51 20130101; G01J
3/52 20130101; G01J 3/513 20130101; G01J 3/524 20130101; H01L
31/02162 20130101; H01L 31/145 20130101; H01L 27/1443 20130101 |
Class at
Publication: |
250/226 ;
250/214.1 |
International
Class: |
H01L 31/00 20060101
H01L031/00; G01J 3/50 20060101 G01J003/50 |
Claims
1. A light sensor that generates a first output signal indicative
of an intensity of light received from a predetermined direction in
a first band of wavelengths, said light sensor comprising: a
substrate having first and second photodetectors that receive light
from said predetermined direction, said first and second
photodetectors being sensitive to light in the infrared portion of
the optical spectrum as well as to light in said first band of
wavelengths, said first and second photodetectors generating first
and second photodetector signals, indicative of an intensity of
light received by each of said first and second photodetectors,
respectively; a first filter layer that transmits light in said
first band of wavelengths and light in the infrared portion of the
optical spectrum, said first filter layer intercepting light from
said predetermined direction before said light reaches said second
photodetector and blocking light in a portion of the visible
spectrum outside of said first band of wavelengths, said first
filter layer not altering light received by said first
photodetector; and a controller that processes said first and
second photodetector signals to produce said first output signal,
said first output signal depending less on light in said infrared
portion of the optical spectrum than said second photodetector
signal.
2. The light sensor of claim 1 further comprising a third
photodetector that generates a third photodetector signal and a
light blocking layer that prevents light from entering said third
photodetector, said controller utilizing said third photodetector
in generating said first output signal.
3. The light sensor of claim 1 further comprising a fourth
photodetector that generates a fourth photodetector signal and a
second filter layer that transmits light in a second band of
wavelengths that is different from said first band of wavelengths,
said second filter layer intercepting light from said predetermined
direction before said light reaches said fourth photodetector and
blocking light in a portion of the visible spectrum outside of said
second band of wavelengths, said second filter layer not altering
light received by said first photodetector, wherein said controller
generates a second output signal that depends less on light in said
infrared portion of the optical spectrum than said fourth
photodetector signal.
4. The light sensor of claim 1 wherein said photodetectors generate
dark currents having substantially the same magnitude.
5. The light sensor of claim 1 wherein said photodetectors are
substantially identical to one another.
6. The light sensor of claim 1 wherein said photodetectors comprise
photodiodes.
7. The light sensor of claim 1 wherein said photodetectors comprise
phototransistors.
8. A method for generating a first estimate of an intensity of
light received from a predetermined direction in a first band of
wavelengths, said method comprising: providing a substrate having
first and second photodetectors that receives light from said
predetermined direction, said first and second photodetectors being
sensitive to light in the infrared portion of the optical spectrum
as well as to light in said first band of wavelengths, said first
and second photodetectors generating first and second photodetector
signals, indicative of an intensity of light received by each of
said first and second photodetectors, respectively; providing a
first filter layer that transmits light in said first band of
wavelengths and light in the infrared portion of the optical
spectrum, said first filter layer intercepting light from said
predetermined direction before said light reaches said second
photodetector and blocking light in a portion of the visible
spectrum outside of said first band of wavelengths, said first
filter layer not altering light received by said first
photodetector; and processing said first and second photodetector
signals to produce said first estimate, said first estimate
depending less on light in said infrared portion of the optical
spectrum than said second photodetector signal.
9. The method of claim 8 further comprising providing a third
photodetector that generates a third photodetector signal and a
light blocking layer that prevents light from entering said third
photodetector, and using said third photodetector signal in
generating said first estimate.
10. The method of claim 8 further comprising providing a fourth
photodetector that generates a fourth photodetector signal and a
second filter layer that transmits light in a second band of
wavelengths that is different from said first band of wavelengths,
said second filter layer intercepting light from said predetermined
direction before said light reaches said fourth photodetector and
blocking light in a portion of the visible spectrum outside of said
second band of wavelengths, said second filter layer not altering
light received by said first photodetector, and generating a second
estimate of a light intensity in a second band of wavelengths, said
second estimate depending less on light in said infrared portion of
the optical spectrum than said fourth photodetector signal.
11. The method of claim 8 wherein said photodetectors generate dark
currents having substantially the same magnitude.
12. The method of claim 8 wherein said photodetectors are
substantially identical to one another.
13. The method of claim 8 wherein said photodetectors comprise
photodiodes.
14. The method of claim 13 wherein said photodiodes have
substantially the same size.
15. The method of claim 8 wherein said photodetectors comprise
phototransistors.
16. A method for fabricating a light sensor, said method comprising
the steps of providing a substrate having first and second
photodetectors that receive light from said predetermined
direction, said first and second photodetectors being sensitive to
light in the infrared portion of the optical spectrum as well as to
light in a first band of wavelengths, said first and second
photodetectors generating first and second photodetector signals,
indicative of an intensity of light received by each of said first
and second photodetectors, respectively; and depositing a first
filter layer that transmits light in said first band of wavelengths
and light in the infrared portion of the optical spectrum, said
first filter layer intercepting light from said predetermined
direction before said light reaches said second photodetector and
blocking light in a portion of the visible spectrum outside of said
first band of wavelengths, said first filter layer not altering
light received by said first photodetector.
17. The method of claim 16 wherein said substrate further comprises
a third photodetector that generates a third photodetector signal
and wherein a light blocking layer that prevents light from
entering said third photodetector is deposited over said third
photodetector.
18. The method of claim 16 wherein said substrate further comprises
a fourth photodetector that generates a fourth photodetector signal
and wherein a second filter layer that transmits light in a second
band of wavelengths that is different from said first band of
wavelengths is deposited over said fourth photodetector, said
second filter layer intercepting light from said predetermined
direction before said light reaches said fourth photodetector and
blocking light in a portion of the visible spectrum outside of said
second band of wavelengths, said second filter layer not altering
light received by said first photodetector.
19. The method of claim 16 wherein said photodetectors generate
dark currents having substantially the same magnitude.
20. The method of claim 19 wherein said photodetectors comprise
photodiodes having substantially the same size.
Description
BACKGROUND OF THE INVENTION
[0001] Inexpensive photodetectors that measure the intensity of
light in a number of wavelength bands are required in a number of
devices. For example, light sources that utilize red, blue, and
green LEDs to generate light that is perceived as being a
particular color often utilize photodetectors in a servo loop that
maintains the output of the LEDs at predetermined levels to
compensate for aging. The photodetectors are used to measure the
output of each LED by measuring the light generated by the LEDs in
each of three spectral bands. A controller varies the average
current to each LED such that the measured outputs are maintained
at target values determined by the perceived color of light that is
to be generated.
[0002] Each photodetector typically consists of a photodiode that
is covered with a pigment filter that limits the photodiodes
response to light in a corresponding band of wavelengths. The
signal from the photodiode is determined by the incident light, the
bandpass filter characteristics of the pigment and various
background signals that are present independent of the intensity
level of the light reaching the photodiode. The light-independent
signals are often referred to as the "dark current". The errors
generated by the dark current can be removed by measuring the
output of the photodiode when no light is present and then
subtracting the measured signal value from the signals generated by
the photodiode in the presence of light. For example, an additional
photodiode that is covered by an opaque layer that blocks all light
can be included in the photodetector. The signal from this
photodiode is then subtracted from that generated by the
photodiodes that are covered with the various pigment filters.
[0003] Unfortunately, the pigment filters that are available for
use in inexpensive photodetectors have significant transmission
bands in the infrared portion of the optical spectrum as well as
the bands in the desired visual region of the spectrum. In many
cases, the light sources of interest also include light in the
infrared region of the spectrum; hence, the signal generated by a
photodiode that utilizes one of these pigment filters can include
an unwanted infrared background signal if the light source includes
a significant amount of infrared light. The infrared light is
either generated by the light source being controlled or by
background ambient light sources that introduce light into the
input to the light sensor.
[0004] In some prior art systems, an infrared blocking filter is
provided over the various pigment filters to block the unwanted
infrared signal. However, the additional filter increases the costs
of the light sensor. In addition, the infrared filters are less
than 100 percent transparent in the visible region of the spectrum,
and hence, the added filter over the pigment filters attenuates a
portion of the light of interest, and hence, reduces the
sensitivity of the light sensor.
SUMMARY OF THE INVENTION
[0005] The present invention includes a light sensor that generates
a first output signal indicative of an intensity of light received
from a predetermined direction in a first band of wavelengths. The
light sensor includes a substrate, a first filter layer, and a
controller. The substrate has first and second photodetectors that
receive light from the predetermined direction. The first and
second photodetectors are sensitive to light in the infrared
portion of the optical spectrum as well as to light in the first
band of wavelengths, and generate first and second photodetector
signals that are indicative of an intensity of light received by
each of the first and second photodetectors. The first filter layer
transmits light in the first band of wavelengths and light in the
infrared portion of the optical spectrum, the first filter layer
intercepting the light before that light reaches the second
photodetector. The first filter layer blocks light in a portion of
the visible spectrum outside of the first band of wavelengths, but
does not alter the light received by the first photodetector. The
controller processes the first and second photodetector signals to
produce the first output signal, the first output signal depending
less on light in the infrared portion of the optical spectrum than
the second photodetector signal.
[0006] In another aspect of the invention, the sensor could also
include a third photodetector that generates a third photodetector
signal and a light blocking layer that prevents light from entering
the third photodetector. The controller could also use the third
photodetector in generating the first output signal to correct for
dark current.
[0007] In another aspect of the invention, additional filter layers
and corresponding photodetectors could be incorporated to provide
measurements of the light intensity in other spectral bands.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a cross-sectional view of a prior art
photodetector that utilizes pigment filters.
[0009] FIG. 2 is a cross-sectional view of one embodiment of a
light sensor according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE
INVENTION
[0010] The manner in which the present invention provides its
advantages can be more easily understood with reference to FIG. 1,
which is a cross-sectional view of a prior art photodetector that
utilizes pigment filters. Photodetector 20 is typically constructed
from a die 21 having 4 photodiodes fabricated thereon. Photodiodes
22-24 are used to measure the intensity of light in three
wavelength bands that are determined by pigment filters 25-27,
respectively. Photodiode 28 is covered by an opaque layer 29 that
blocks all light from reaching photodiode 28 and is utilized to
measure the dark current. The pigment filters are applied by
photolithographic steps that require a number of masking and
deposition steps. A similar photolithographic step is needed for
applying layer 29. As noted above, the pigment filters do not block
infrared light from reaching the photodiodes. In principle, an
additional filter layer 35 can be provided over photodiodes 22-24
that blocks the infrared light from reaching the photodiodes.
However, the fabrication of such a filter significantly increases
the cost of the light sensor. Furthermore, each additional filter
layer reduces the amount of light that reaches the photodiodes, and
hence, decreases the sensitivity of the photodetector.
[0011] The present invention overcomes these problems by including
an additional photodiode that measures all of the incident light
including the infrared component. The additional photodiode does
not interfere with the operation of the remaining photodiodes or
require additional fabrication steps. The signals from the
photodiodes are then combined to provide signals that measure the
intensity in each of the desired wavelength bands corrected for
both the dark current and the infrared radiation that was present
in the incident light.
[0012] Refer now to FIG. 2, which is a cross-sectional view of one
embodiment of a light sensor according to the present invention. In
this embodiment of the present invention a photodiode 31 that
measures all of the light reaching the light sensor is included on
substrate 36. For the purposes of this discussion, it will be
assumed that the pigment layer 25 is a band pass filter that passes
light in the red portion of the spectrum, pigment layer 26 is a
band pass filter that passes light in the green portion of the
spectrum, and pigment layer 27 is a band pass filter that passes
light in the blue portion of the spectrum. As noted above, each of
these pigment layers is transparent to light in the infrared
portion of the spectrum. Denote the signals from photodiodes 22-24,
28, and 31 by I.sub.22, I.sub.23, I.sub.24, I.sub.28, and I.sub.31,
respectively. Then
I.sub.23=f.sub.r*I.sub.red+I.sub.d+f.sub.26*I.sub.r (1)
I.sub.24=f.sub.g*I.sub.green+I.sub.d+f.sub.27*I.sub.r (2)
I.sub.22=f.sub.b*I.sub.blue+I.sub.d+f.sub.25*I.sub.r (3)
I.sub.31=I.sub.red+I.sub.green+I.sub.blue+I.sub.d+I.sub.r (4)
I.sub.28=I.sub.d (5)
Here, f.sub.r, f.sub.g, and f.sub.b are the fractions of the light
in the red, green, and blue wavelength bands, respectively, that
are transmitted by filters 25-27, respectively. The coefficients
f.sub.25-f.sub.27 are the fractions of the infrared light in the
input light that is transmitted by filters 25-27, respectively.
And, I.sub.red, I.sub.green, I.sub.blue, and I.sub.r are the
intensities of the input light in the red, green, blue, and
infrared regions of the spectrum. Finally, I.sub.d is the dark
current. For the purposes of this example, it is assumed that all
of the photodiodes are substantially identical in size and
construction, and hence, the dark current is substantially the same
for all of the photodiodes. For the purposes of the present
discussion, two photodiodes will be said to have substantially the
same construction and dark current if the differences between the
individual photodiodes is within the statistical variation observed
for photodiodes constructed from the same processes and mask set on
a conventional integrated circuit fabrication line.
[0013] The various coefficients representing the fraction of the
light that is transmitted through each filter can be measured for
any light sensor. For example, the signals generated by each
photodiode could be measured when the photodiodes are exposed to
monochromatic light sources of various wavelengths. Hence, the
system of equations shown above can be solved for I.sub.red,
I.sub.green, and I.sub.blue by controller 37 to provide the
corrected output, i.e., I.sub.red, etc.
[0014] In one embodiment, the pigment filter layers 25-27 are
constructed from materials that are essentially transparent to
infrared radiation. That is, f.sub.25=f.sub.26=f.sub.27=1. It
should be noted that, in this case, one can replace I.sub.d+I.sub.r
by I', and the system of 5 equations can be reduced to a system of
4 equations, namely,
I.sub.23=f.sub.r*I.sub.red+I' (6)
I.sub.24=f.sub.g*I.sub.green+I' (7)
I.sub.22=f.sub.b*I.sub.blue+I' (8)
I.sub.31=I.sub.red+I.sub.green+I.sub.blue+I' (9)
Since, the quantities of interest are I.sub.red, I.sub.green, and
I.sub.blue, the controller need only solve this simpler system of
equations. As a result, the dark current measuring photodiode 28 is
no longer required. Hence, this embodiment of the present invention
requires no more photodiodes than the conventional light sensor. In
addition, this embodiment does not require the opaque layer
associated with the dark current measuring photodiode, and hence,
the number of fabrication steps is substantially reduced.
[0015] The above-described embodiments of the present invention are
based on an additional approximation that will now be discussed in
more detail. In general, the output of the k.sup.th photodiode can
be written in the form
I.sub.k=.intg.T.sub.k(x)R(x)I(x)dx (10)
where k=red, blue, or green, T.sub.k(x) is the transmission of the
k.sup.th filter layer as a function of wavelength, x, R(x) is the
signal generated by the k.sup.th photodiode when illuminated with
light of unit intensity as a function of wavelength, and I(x) is
the intensity of the input light as a function of wavelength. The
integration is performed over the entire optical spectrum, or at
least the region in which all of the functions T.sub.k(x), R(x),
and I(x) are non-zero. This equation can be re-written in the
form:
I k = .intg. visible T k ( x ) R ( x ) I ( x ) x + .intg. infrared
T k ( x ) R ( x ) I ( x ) x ( 11 ) ##EQU00001##
By comparing eq. (11) to eqs (1)-(5), it can be seen that eqs
(1)-(3) assume that
.intg. visible T k ( x ) R ( x ) I ( x ) x = f k I k = f k .intg. k
R ( x ) I ( x ) x ( 12 ) ##EQU00002##
independent of the shape of the intensity function I(x) and the
shape of the filter functions T.sub.k(x). This assumption is not
exactly satisfied for all light sources and pigment filters.
However, the present invention is based on the observation that it
is sufficiently correct to allow an approximate correction for any
infrared light present in the input signal without requiring an
additional infrared filter.
[0016] The above-described embodiments utilize three color pigment
filters that provide band pass filters in the red, green, and blue
regions of the visible spectrum. Such pigment color filters are
known to the art, and hence, will not be discussed in detail here.
For the purposes of the present discussion, it is sufficient to
note that such filters are used in integrated circuits for cameras.
However, embodiments that utilize filters in different spectral
regions could also be constructed without departing from the
teachings of the present invention.
[0017] Furthermore, the number of band pass filters can be varied
from the three discussed above. For example, color filter systems
based on four colors are utilized in a number of applications to
provide an extended gamut for the reproduction of colors. Such
systems often require light sensors that also track the intensity
of the 4 component light sources. Similarly, the number of band
pass filters and corresponding photodiodes could be less than 3.
Light sources having a single photodiode and utilizing a feedback
system to correct for aging effects are also known to the art and
require a light sensor that tracks the intensity of that light
source in environments that have infrared background light.
[0018] The above-described embodiments of the present invention
utilize photodiodes as the photodetector. However, other forms of
photodetector such as phototransistors could be utilized.
[0019] The embodiments of the present invention described above
utilize photodetectors that are substantially identical to one
another. However, embodiments in which the photodetectors differ
substantially from one another could also be constructed provided
the relationship between the light received by the photodetector
and the output signal generated by the photodetector is known. For
example, the photodetector used to measure the total light received
by the light sensor could be a different size than the other
photodetectors. In this case, the constants shown in Eqs. (1)-(9)
would need to be altered to take into account the differences in
construction.
[0020] Various modifications to the present invention will become
apparent to those skilled in the art from the foregoing description
and accompanying drawings. Accordingly, the present invention is to
be limited solely by the scope of the following claims.
* * * * *