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Patent No. 6141092
Method and apparatus for measuring a flicker level (Kim, Oct
31, 2000)
Abstract
Disclosed is a method for measuring a flicker level. The method includes the steps of applying a first voltage to a liquid crystal display panel, the LCD panel including liquid crystal material and varying in transmissivity of light according to voltage applied to the liquid crystal material, such that light passing through the liquid crystal material is emitted from the LCD panel; detecting brightness of the light emitted from the LCD panel; determining a maximum brightness value and a minimum brightness value; and introducing the maximum brightness values and the minimum brightness value into a retina responsiveness function according to the brightness to attain flicker level values. The retina responsiveness function is a function of a strength of light passing through the pupil of the human eye. An inventive apparatus for measuring a flicker level includes a brightness detector for detecting brightness of light emitted from a liquid crystal display panel; a max/min brightness measuring portion for receiving input of brightness values from the brightness detector and determining a maximum brightness value and a minimum brightness value; and a flicker level measuring portion for attaining flicker level values by introducing the maximum and minimum brightness values input from the max/min brightness measuring portion into a retina responsiveness function according to brightness.
Notes:
BACKGROUND
OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for measuring a flicker
level, and more particularly, to a method and apparatus for measuring a level
of screen flicker visible to the human eye.
2. Description of the Prior Art
Liquid crystal displays (LCSs) are increasingly being used for the display device
in televisions, personal computers, etc., and in many state-of-the-art equipment
such as automotive navigation systems and simulation devices. LCDs are significantly
lighter in weight and slimmer, consume far less energy and can reproduce a wider
range of colors than any competing technologies.
LCDs apply an electric field to liquid crystal material having an anisotropic
dielectricity and injected between two substrates, an array substrate and a
counter substrate, arranged substantially parallel to one another with a predetermined
gap therebetween, and control the amount of light permeating the substrates
by controlling an intensity of the electric field to obtain a desired image
signal.
Formed on the array substrate are a plurality of gate lines disposed parallel
to one another, and a plurality of data lines insulated from and crossing the
gate lines. A plurality of pixel electrodes are formed corresponding to respective
regions defined by the intersecting data lines and gate lines. Further, a thin
film transistor (TFT) is provided near each of the intersections of the gate
lines and the data lines. Each pixel electrode is connected to a data line via
a corresponding TFT, the TFT serving as a switching device therebetween.
Each TFT has a gate electrode, a drain electrode, and a source electrode, and
the pixel electrodes are connected to the drain electrodes. Here, common electrodes
are disposed on either the array substrate or the counter substrate.
The electric field applied to the liquid crystal material is generated by a
difference in levels of common voltage and data voltage applied respectively
to the common electrodes and the pixel electrodes provided in the LCD. An intensity
of the electric field is controlled by changing data voltage or common voltage
levels.
As the liquid crystal material degrades if the electric field is applied to
the liquid crystal material continuously in the same direction, the direction
in which the electric field is applied must be constantly changed. Namely, a
value of the data voltage minus the common voltage must be repeatedly alternated
from a positive value (hereinafter referred to as positive voltage) to a negative
value (hereinafter referred to as negative voltage).
Such a switching of electrode voltage values between positive and negative values
is referred to as inversion drive. Among the different types of inversion drive
methods are frame inversion, line inversion, dot inversion, and column inversion
methods.
In frame inversion, in which the polarity of data voltage is inverted to frame
cycles (typically 60 Hz), positive voltage is applied in odd frames, while negative
voltage is applied in even frames. Here, it is established such that a root
mean square (RMS) of the positive voltage is the same as a RMS of the negative
voltage.
However, in the actual performing of inversion drive in the LCD, kickback voltage
is generated by parasitic capacitance in the pixels such that the RMS of the
positive voltage comes to differ from the RMS of the negative voltage. Accordingly,
a brightness of light permeating the liquid crystal material in the odd frames
and that of light permeating the liquid crystal material in the even frames
become dissimilar. This results in screen flickers generating in units of one-half
of frame frequency of 60 Hz, or 30 Hz.
Such a screen flicker is measured using Formula 1 below introduced by the Apple
Corporation ##EQU1##
In the above Formula 1, F is the flicker level, and Po and Pf are amplitudes
respectively of DC elements and AC elements (flicker elements) of light emitted
from the LCD panel. Namely, according to the prior art flicker level measuring
method, the level of screen flicker is the ratio of an amplitude of flicker
elements to DC elements of light.
Referring to FIG. 1, shown is a graph illustrating Po and Pf of light emitted
from an LCD panel. In the drawing, a brightness of light is realized by a sine
function related to time, an average value (DC elements) of the sine function
being Po, and an amplitude of the sine function being Pf. In FIG. 1, as Po is
always larger than Pf, the flicker level attained using Formula 1 is always
a negative value.
According to Formula 1, the flicker level is determined with considerations
of merely the brightness of the light (DC elements and AC elements) emitted
from the LCD panel, but other factors besides the brightness of the light such
as screen size, distance between the screen and user, involuntary adjustment
of the size of the pupil, etc. also determine the amount of screen flicker visible
to the human eye. Accordingly, the flicker level attained using Formula 1 does
not take into account these other factors.
Reasons why the flicker level attained using Formula 1 and the flicker level
visible to the human eye are different will be explained hereinafter.
In Table 1 below, shown are DC elements Po and flicker elements Pf, and various
flicker levels attained using Formula 1 when the difference in common voltage
and data voltage is applied to 64 gray levels. A graph of the flicker levels
according to gray levels of Table 1 is shown in FIG. 2.
TABLE 1 ______________________________________ Flicker elements Gray level DC
elements (Po) (Pf) Flicker level (F) ______________________________________
1 0.83 0.01 -23.86 5 0.88 0.01 -22.03 9 1.01 0.02 -17.63 13 1.24 0.04 -14.87
17 1.83 0.05 -15.32 21 2.60 0.08 -15.35 25 3.88 0.11 -15.58 29 5.40 0.14 -15.85
33 7.52 0.19 -16.06 37 9.77 0.19 -17.02 41 12.55 0.17 -18.60 45 15.74 0.19 -19.18
49 19.35 0.16 -20.70 53 24.12 0.16 -21.85 57 28.80 0.06 -26.63 61 33.91 0.01
-38.02 64 34.84 0.01 -34.73 ______________________________________
According to Table 1 and FIG. 2, a flicker level value attained using Formula
1 at a gray level of 13 is largest. However, in actuality, a flicker level visible
to the human eye is largest at a more medium gray level of 32. Reasons for this
will be explained hereinafter with reference to FIG. 3.
FIG. 3 is a graph illustrating transmissivity of light with regard to voltage
Va applied to liquid crystal material of the LCD. In the drawing, light begins
to transmit through liquid crystal material when the voltage Va applied to the
same is above a threshold voltage Vth, with the transmissivity of light increasing
as the voltage Va is increased. However, when the voltage Va exceeds a saturation
voltage Vsat, the transmissivity of light no longer increases.
When the voltage Va applied to the liquid crystal material is at a point roughly
in the middle level between the threshold voltage Vth and the saturation voltage
Vsat, the transmissivity of light is greatly affected by even slight fluctuations
in the voltage Va. Namely, small changes in the voltage Va in the middle level
between the threshold voltage Vth and the saturation voltage Vsat produce large
differences in light transmissivity. Accordingly, flickering is most visible
to the human eye in this central gray voltage level.
Therefore, the prior method of calculating flicker levels is not accurate.
SUMMARY OF THE INVENTION
The present invention has been made in an effort to solve the above problem.
It is an object of the present invention to provide a method and apparatus for
accurately measuring a level of screen flicker visible to the human eye.
To achieve the above object, the present invention provides a method for measuring
a flicker level. The method includes the steps of applying a first voltage to
a liquid crystal display panel, the LCD panel including liquid crystal material
and varying in transmissivity of light according to the voltage applied to the
liquid crystal material, such that light passing through the liquid crystal
material is emitted from the LCD panel; detecting brightness of the light emitted
from the LCD panel; determining a maximum brightness value and a minimum brightness
value; and introducing the maximum brightness values and the minimum brightness
value into a
retina responsiveness function according to the brightness to attain flicker
level values. The retina responsiveness function is a function of a strength
of light passing through the pupil of the human eye.
An inventive apparatus for measuring a flicker level includes a brightness detector
for detecting brightness of light emitted from a liquid crystal display panel;
a max/min brightness measuring portion for receiving input of brightness values
from the brightness detector and determining a maximum brightness value and
a minimum brightness value; and a flicker level measuring portion for attaining
flicker level values by introducing the maximum and minimum brightness values
input from the max/min brightness measuring portion into a retina responsiveness
function according to brightness.
DESCRIPTION
OF THE PREFERRED EMBODIMENT
A preferred embodiment of the present invention will now be described in detail
with reference to the accompanying drawings.
Reaction to all external light is not direct with the human eye. That is, light
must first pass through the pupil before it is reacted to by the retina. Further,
as the size of the pupil varies according to different strengths of light, there
is a difference in the strength of light reaching the retina of the eye and
the actual strength of the external light.
The present invention takes into account this difference in the actual strength
of external light and the strength of light reaching the retina.
The size, or diameter, of the pupil with regard to brightness is determined
using Formula 2. The formula was developed by Degroot and Gebhard.
In Formula 2, L is brightness, which is in a unit of cd/m2, and D(L) is a diameter
of the pupil. As is obvious from Formula 2, the size of the pupil decreases
as brightness increases.
As mentioned above, only the light passing through the pupil reaches the retina.
The strength of the light (referred to as "trolands") reaching the retina can
be calculated using Formula 3 below. ##EQU2##
If D(L) of Formula 2 is introduced into Formula 3, the following Formula 4 results.
##EQU3##
In Formula 4, TD(L) is the strength of light reaching the retina and is a function
of brightness (L).
Referring to FIG. 4, shown is a graph illustrating an interrelation between
brightness and the strength of light reaching the retina. A horizontal axis
of the graph is a log value of brightness (L), while a vertical axis is a log
value of the strength of light reaching the retina TD(L). As shown in the graph,
the strength of light reaching the retina increases in a linear relation to
an increase in brightness but only to a point, after which the increase in the
amount of light reaching the retina tapers off, then eventually flattens such
that there is no further increase in the amount of light reaching the retina
with increases in brightness.
The light reaching the retina is converted into an electric signal through the
retina membrane, and the electric signal is transmitted to the brain through
optical nerves. The electric signal with respect to the strength of light passing
through the pupil (i.e. eye responsiveness) is modeled using Formula 5 below.
##EQU4##
In Formula 5, also known as the Michaelis-Menten Equation, R(L) represents retina
responsiveness. TD(L) is the strength of light reaching the retina, calculated
using Formulas 3 and 4, and A is a constant. A is assumed to be 1000 in the
preferred embodiment.
If TD(L) of Formula 4 is introduced into Formula 5, the following Formula 6
results. ##EQU5##
The strength of light reaching the retina TD(L), and responsiveness of the retina
with respect to brightness (L) attained using Formula 6 are illustrated in FIGS.
5 and 6, respectively.
In FIG. 5, a horizontal axis of the graph is a log value log[TD(L)] of the strength
of light reaching the retina TD(L), and a vertical axis is the responsiveness
of the retina R(L). As shown in this graph, when the log value log[TD(L)] of
the strength of light reaching the retina TD(L) is roughly 1 and under, the
responsiveness of the retina R(L) is at or slightly above 0, and when the log
value log[TD(L)] of the strength of light reaching the retina TD(L) is slightly
above 5, the responsiveness of the retina R(L) is at 1000.
In FIG. 6, a horizontal axis is a log value Log(L) of brightness L, and a vertical
axis is the responsiveness of the retina R(L). Here also, as in FIG. 5, the
responsiveness of the retina R(L) is at zero at a predetermined log value Log(L)
of brightness L, and at 1000 at a larger, predetermined log value Log(L) of
brightness L.
In Formula 6 and FIG. 6, retina responsiveness R(L) varies when brightness L
is at a maximum value and minimum value. In the present invention, these values
are determined as flicker levels. Formula 7 below is used in the present invention
to calculate flicker levels.
In the above Formula 7, Lmax indicates a maximum value of brightness, and Lmin
indicates a minimum value of brightness. The maximum value of brightness Lmax
is attained by adding a brightness DC element value Po to a flicker element
amplitude Pf, while the minimum value of brightness Lmin in attained by subtracting
the flicker element amplitude Pf from the brightness DC element value Po. The
following Formula 8 results by introducing Formula 6 into Formula 7. ##EQU6##
Results of flicker levels calculated using Formula 8 appear in Table 2 below.
In Table 2, as in Table 1 appearing in the Background of the Invention section
of this specification, shown are DC elements Po and flicker elements Pf, and
various flicker levels attained using Formula 1 when the difference in common
voltage and data voltage is applied to 64 gray levels, in addition to maximum
and minimum values of brightness Lmax and Lmin.
TABLE 2 ______________________________________ DC elements Flicker elements
Flicker Gray level (Po) (Pf) Lmax Lmin level (F) ______________________________________
1 0.83 0.01 0.84 0.82 0.12 5 0.88 0.01 0.89 0.87 0.19 9 1.01 0.02 1.03 0.99
0.55 13 1.24 0.04 1.28 1.20 1.18 17 1.83 0.05 1.68 1.78 1.30 21 2.60 0.08 2.68
2.52 1.56 25 3.88 0.11 3.99 3.77 1.81 29 5.40 0.14 5.54 5.26 1.99 33 7.52 0.19
7.71 7.33 2.22 37 9.77 0.19 9.96 9.58 2.00 41 12.55 0.17 12.72 12.38 1.55 45
15.74 0.19 15.93 12.55 1.49 49 19.35 0.16 19.51 19.19 1.14 53 24.12 0.16 24.28
23.96 0.95 57 28.80 0.06 28.86 28.74 0.34 61 33.91 0.01 33.92 33.90 0.03 64
34.84 0.01 34.85 34.83 0.06 ______________________________________
As shown in FIG. 7, illustrating a graph of flicker levels according to the
gray levels of Table 2, the flicker level values are the highest at a gray level
of roughly 32. That is, flicker levels visible to the human eye are the largest
at such medium gray levels.
In the above, the size (diameter) of the pupil D(L) according to brightness
L is determined using Formula 2. The same can also be calculated using Formula
9 below developed by Moon and Spenser.
The strength of light reaching the retina TD(L) and the responsiveness of the
retina R(L) are able to be attained by introducing Formula 9 into Formulas 3
and 5, respectively, and flicker levels can be determined using these formulas.
Flicker levels calculated in this manner come very to close to those actually
recognized by the human eye.
Referring now to FIG. 8, shown is a block diagram of an apparatus for measuring
flicker levels according to a preferred embodiment of the present invention,
as shown in the drawing, the inventive apparatus for measuring flicker levels
comprises an LCD panel driver 10, an LCD panel 20 driven by various drive voltages
output by the LCD panel driver 10, a brightness detector 30 for detecting brightness
of the LCD panel 20, a max/min brightness measuring portion 40 for receiving
input of brightness values from the brightness detector 30 and determining a
maximum brightness value Lmax and a minimum brightness value Lmin, and a flicker
level measuring portion 50 for determining flicker levels using Formula 8 and
the maximum and minimum brightness values Lmax and Lmin input from the max/min
brightness measuring portion 40.
The brightness detector 30 is vertically disposed to the LCD panel 20, and it
is preferable to provide at least one brightness detector 30 on the LCD panel
20. In the preferred embodiment of the present invention, brightness detectors
30 are provided at the center and at each corner of the LCD panel 20 for the
better measurement of the brightness of the LCD panel 20.
In the inventive apparatus for measuring flicker levels as shown in FIG. 8,
flicker levels are measured by changing gray levels of gray voltage, the gray
voltage being applied to the LCD panel 20 from the LCD panel driver 10. A maximum
value of measured flicker level values is attained, and this value is determined
to be the flicker level of the LCD panel 10.
An example of an apparatus for measuring flicker levels will be described hereinafter.
The BM-7 measuring apparatus developed by TOCON of Japan is an example of a
flicker measuring apparatus that is provided adjacent to the LCD panel to detect
brightness. The BM-7 device receives input of brightness from the LCD panel
and outputs the same as analog voltage Vout. The equation used for this process
is listed below in Formula 10.
As described above, the maximum value of brightness Lmax and the minimum value
of brightness Lmin must be defined to determine flicker levels, and the maximum
and minimum values of brightness are attained using a DC element value of brightness
Po and an amplitude value of AC elements (flicker elements) Pf. Here, brightness
DC elements Po is a frequency of 0 Hz, and brightness AC elements Pf is a frequency
of 30 Hz in the case of frame inversion.
However, because voltage output Vout from the BM-7 device is not output as voltage
of each brightness (frequency) but as a sum of all ranges of frequencies, it
is not possible to directly measure brightness values of the LCD panel from
the voltage output from the BM-7 device.
Accordingly, Hewlett Packard's dynamic signal analyzer (model No. HP35665) is
connected to the BM-7 device, and the output of voltage Vout from the same is
used as input by the dynamic signal analyzer (DSA). Here, the DSA output voltage
Vrms is converted into Formula 11 below. ##EQU7## In the above, Vref is a standard
voltage, which is: ##EQU8## As DSA output Vrms is output at each frequency,
voltage Vout of each brightness can be attained. Provided that a DSA output
of 0 Hz frequency is Vrms(0 Hz), and a DSA output of 30 Hz frequency is Vrms(30
Hz). Formula 11 is used to attain Vout(0 Hz) and Vout(30 Hz). ##EQU9##
By introducing Vout(0 Hz) and Vout(30 Hz) above in Formula 10, L(0 Hz) and L(30
Hz) can be attained. Here, L(0 Hz) and L(30 Hz) are respectively the DC element
value of brightness and the amplitude value of AC elements.
Accordingly, maximum and minimum values of brightness can be attained from L(0
Hz) and L(30 Hz), and by introducing the maximum and minimum values of brightness
into Formula 8, flicker values can be measured.
In the above inventive method and apparatus, as flicker levels are measured
taking into account pupil diameters and retina responsiveness, a more accurate
measurement of flicker levels that is recognized by the human eye is realized.
------------------------------
While this invention has been described
in connection with what is presently considered to be the most practical and
preferred embodiment, it is to be understood that the invention is not limited
to the disclosed embodiments, but, on the contrary, is intended to cover various
modifications and equivalent arrangements included with the spirit and scope
of the appended claims.