Chapter 2 Literature Review
2.10 Summary
Many temporal properties of human system are discovered in the past century. Both Broca-Sulzer effect and Brücke-Bartley effect can increase the perceived brightness.
Broca-Sulzer effect states that the flashing light at low duration (50-100 milliseconds) appears brighter than shorter or longer duration light. Brücke-Bartley effect illustrates that the light modulated (flickered on and off) at low frequency (5 to 20 Hz) appears brighter than lower or higher frequency light. For applying these properties to the low power LCD, psychophysical experiments are performed to model the relation between perceived brightness and flickering frequency. Base on the experimental results, a novel backlight driving scheme will be proposed.
Chapter 3
RGB LED Backlight Module
Currently, most of the backlight modules of LCD for laptop, PC and TV monitors are dominated by cold cathode fluorescent lamp (CCFL). However, the CCFL has three main problems including narrow color gamut, long response time, and mercury contained even through it is cost efficiency. To solve these problems, the RGB light emitting diode (LED) backlight module has been proposed for its features such as fast response, wide color gamut, and mercury-free. A LED backlight consists of light-bars with a number of RGB LEDs, which are semiconductor devices emitting light when electrical current flows through it forwardly.
Although LEDs can be applied to LCD backlight, it has disadvantages such as power efficiency, color light mixing distance, electro-optical characteristic variation among LEDs, temperature dependency of color and luminance and high cost. How to minimize these shortcomings must be considered in the design process. An RGB LED backlight module with dimming control was fabricated and its characteristics are compared with those of CCFL in this chapter.
3.1 Specification
This backlight module has the features as below:
Three primaries including red, green, and blue light
Red light has a peak wavelength at 631nm, green light has a peak wavelength at 535nm, and blue light has a peak wavelength at 460nm
Color gamut is larger than 120% of NTSC in the CIE x-y coordinate
Fits 19-inch screen (376 mm × 307 mm)
Maximum luminance is 5000 cd/m2
Dimming control by pulse width modulation signal
Edge lighting backlight module
3.2 Block Diagram
The aim of this RGB LED backlight module is to replace CCFL directly without changing any mechanical parts. In Figure 3-1, on the left is a traditional LCD with CCFL backlight, and on the right is the re-designed LCD with LED backlight. The backlight module employed the LED driver instead of the inverter used for CCFL backlight. Different from CCFL backlight, there are three dimming control signal for LED backlight. Therefore, the LED backlight is able to dim red, green, and blue separately.
Figure 3-1: The CCFL backlight and inverter (yellow block) are replaced by RGB LED back light (green block).
3.3 Backlight Module Design
In the backlight module design flow, there are three major considerations: optical design,
thermal issue, and electrical design [Anandan 2006].
First, a 19” LCD monitor (ViewSonic VX912) is reworked as a prototype for measuring the relation between luminance and electric current. Two lightbars, each with 24 LED chips, take place of the original CCFL backlights inside the LCD monitor. One was put on the top edge and the other at the bottom. Close-ups of a LED chip and lightbars are shown in Figure 3-2.
Figure 3-2: Close-up of LED chips with one red die, one blue die and two green dies (top) and LED light-bar with 24 LED chips (bottom).
This prototype worked as a traditional backlight after been powered by power supplies.
The electrical current flowing through the prototype and the corresponding luminance were measured and recorded. Based on the data, we calculated the power consumption of each LED.
Three-in-one (three primary colors in one package) RGB LEDs (Arima Optoelectronics Corporation 5WRGGB) were chosen to minimize the light mixing distance [Zwanenburg 2004]. The benefits of the three-in-one LED is that because the chips are all integrated in a single package, the light is essentially mixed within the package. Hence, there is no additional space required to achieve uniform color mixing. As shown in Figure 3-3, using discrete RGB LEDs with 10 mm pitch and three-in-one LEDs, the color-mixing distance is reduced from 28mm to 7mm.
Figure 3-3: The snapshot of color-mixing distance of the backlight module: comparison between three-in-one RGB LEDs (top) and discrete RGB LEDs (bottom) [Zwanenburg 2004].
Second, when more and more electrical current flows through LEDs, the LED lightbar becomes hotter and hotter because LEDs translated a lot of the electrical power into heat.
After the critical temperature point, the blue light declined firstly and dramatically. The amount of the electrical current was the upper limit for the LED driver. To prevent dissipate heat from the light-bar, the heat sink compound was applied to the gap between the thermal pads of LEDs and the lightbar. Furthermore, the lightbars must contact the metal frame tightly to improve heat dissipation.
Figure 3-4: The heat sink compound which was applied between the thermal pad of LEDs and the light-bar.
In the last step, the LED driver was the most important part in the LED backlight module.
A number of electrical circuits for driving LED are available. For efficiency and stability, current mirror was chosen as LED driver because it can supply stable constant electrical current and adapt to different forward voltages of each LED. It is very suitable for driving LED because the output luminance is a function of the forward current through LEDs.
Figure 3-5: The current mirror circuit with one hundred times current output provides a constant current for LED and is adjusted by changing IREF [SiTI].
Figure 3-5 shows a general current mirror. ILED is the constant LED forwarding current which is one hundred times the reference current IREF. The output light can be turned on by ascending the enable (EN) pin to high voltage level and turned off by descending the EN pin to low voltage level. The LEDs’ intensity can be controlled by adjusting the amount of IREF
via an external circuit.
In this backlight module, a high constant current mirror (Silicon Touch Technology DD311) was chosen as the LED driver because it can sustain up to one ampere forward current. It is a single-channel constant current LED driver incorporated current mirror and
current switch. The maximum sink current is 100 times the input current value set by an external resistor or bias voltage. The maximum output voltage of thirty-three volts can provide more series power LEDs in a string. The output enable (EN) pin allows dimming control or switching power applications [SiTI].
Based on the thermal and optical design mentioned above, the maximum forwarding current of LEDs can be determined so that the reference current can be derived by dividing one hundred. The LEDs’ voltage supply VLED can be determined by calculating each LED’s VTH in series connection.
3.4 Implementation
The implementation flow of LED backlight module consists of seven steps as illustrated in Figure 3-6.
Figure 3-6: From start to LED backlight module, the implementation flow consists of seven steps: schematic drawing, layout, PCB fabrication, soldering, electrical testing, heat sink assembling, and mechanical assembling.
The first step is schematic drawing, assisted by the software OrCAD® Capture, a design tool of Cadence®. The LED driver, LED lightbar, and other electrical components were designed by schematics. Two of the schematic designs are shown in Figure 3-7.
Figure 3-7: Two parts of the electrical schematics are consists of six LED drivers (left), and twenty-fore RGB LEDs (right).
The exported net-list file (*.NET) is a text file with information including the parts and the connections between blocks. Te layout step is to physically place the components and connections on a printed circuit board (PCB). In this stage, the computer-aided design (CAD) tool PROTEL® SE was employed to plot the physical outline of electrical components and the location on the PCB. Here the design rules must be observed including board size, layers, thickness, trace width, medium material, etc. The layout of LED driver and lightbar are plotted in Figure 3-8.
Figure 3-8: The PCB layout consists of eight strips of LED light-bar (upper), and two LED driver boards (bottom).
In the next step, layout drawing was translated into the GERBER file, a format of negatives for PCB fabricating. Bare PCBs were turned around after a week from the PCB plant. The sample bare PCB was shown on the top-left of Figure 3-9, and on the top of Figure 3-10.
The returned bare PCBs were tested for open-short, grounding and correct via. The PCB must be re-fabricated if any test failed. Before the soldering stage, each LED was tested for proper function. A lot of check items should be done because de-soldering will spend more effort than soldering. The emitting test of LEDs is shown as the top-right in Figure 3-9 and the soldering process in the bottom-left. At this stage, as the menu recommended, the hand soldering should be with a solder tip temperature of 230°C for less than ten seconds. Long soldering might cause the damage to the epoxy layer and short circuit in the array.
Figure 3-9: The bare PCBs of LED light-bar (top-left), LED testing (top-right), soldering process (bottom-left), and after assembling and emitting red light (bottom-right).
After the soldering stage, the LED light-bar should be connected directly to DC power supply and checked whether each LED emitted well as shown in Figure 3-9 bottom-right.
Some LEDs could not emit because of “cold solder joint.” This poor property usually occurs when the base metal is not warm enough to melt the solder. When it occurs, re-soldering each metal pin of LEDs with higher temperature is better than soldering more tin on the metal pin, but it might cause lower light efficiency or de-saturation.
For thermal dissipation, the LED light-bar reserved dissipated hole for each LED. The heat sink compound mediated between the thermal pad of LEDs and the metal frame so that the LEDs dissipated heat from the thermal pad of LEDs through heat sink compound to the metal frame. However, more electrical current might cause thermal damage.
Figure 3-10: The bare PCB (upper), and the PCB with LED driver soldering (bottom).
In the LED backlight module, there are two light-bars, each one with twenty-four chips of RGB LEDs. One is on the top side; the other is on the bottom side. In LED driver board, six DD311 chips are used for two light-bars and each one with an independent enabled control.
These six chips would be divided into two groups. One group controls red, green, and blue for the top light-bar. The other controls the red, green, and blue light for the bottom light-bar.
Therefore, this backlight module has ability to be used for spatial backlight scaling when an image with different color or luminance on the top and bottom side.
In the final fabricating stage, the light-bars were assembled and connected to the LED driver board, and then the RGB LED backlight module was accomplished. As shown in Figure 3-11, the LED backlight module can illuminate white, red, green, and blue with dimming control. The white point can be adjusted by the reference current of LED driver or the duty cycle from the external pulsed width modulation (PWM).
Figure 3-11: The snapshots of RGB LED backlight module (top) and emitting white, red, green, and blue light (bottom).
3.5 Calibration
Before the LED backlight module is used as an illuminator of LCD, the white point must be calibrated. The aim is to illuminate white with CIE 1931 x-y coordinate value at (0.33, 0.33) which is also named equi-energy stimulus SE, just as X = Y = Z. There are two approaches to adjust the white point: the first one is dynamically modifying the duty cycle of
each color, and the other one is statically modifying the reference current by high precision resistor. The second approach is more desired because it provides better stability and precision.
However, the first one is also useful in dynamic compensation for thermal problem. In the backlight module, it employed a bread board because exchanging external resistors is more convenient. After calibration progress, the value of external resistors for each color channel should be certain.
3.6 Performance of Backlight Module
We measured both the power consumption of LED and CCFL backlight, as shown in Table 3-1. The power consumption of backlight module includes driving circuits like the current-mirror circuits for LEDs or the inverter for CCFL. Both power and luminance were measured simultaneously and listed in the second and third column. In the last column, the quotient is luminance divided by power; the LED backlight illuminates more light than CCFL per watt. It is because the LEDs have higher light efficiency [Arima].
Table 3-1: Power efficiency of LEDs and CCFL
Backlight Luminance (nit) Power (W) L/P (nit/W)
LEDs 4739.24 21.20 224
CCFL 4384.50 23.17 189
The spectral emission characteristics of the LED backlight module are measured with ConoScope [ConoScope] and the waveforms are shown in Figure 3-12. The red had a peak at 631nm, the green at 535nm and the blue at 460nm. The equal-energy white point (0.33, 0.33) is shown in Figure 3-12 (d), of which consists the waveform of red, green, and blue.
(a) (b)
(c) (d) Figure 3-12: The spectra of RGB LED backlight: (a) Red LEDs had peak at 631nm. (b)
Green LEDs had peak at 535nm. (C) Blue LEDs had peak at 460nm. (d) White light at x-y coordinate (0.33, 0.33).
Furthermore, the color gamut can be calculated by the triangular area formula as below:
3
Where S is the area of a triangle which three apices are coordinates of red, green, and blue respectively. The area of each color triangle is calculated and listed at fifth column in Table 3-2. The results of LEDs comparing with CCFL, NTSC and EBU specification are
listed in Table 3-2 and plotted triangles on CIE 1976 uniform coordinate u’-v’ as Figure 3-13.
Obviously, the color gamut of LED backlight is up to twenty percent higher than NTSC, even higher than CCFL backlight for around forty percent, because the LED backlight improves greatly in red and some in blue from CCFL triangle, as shown in Figure 3-13.
Table 3-2: Color gamut comparison
u’ v’ Area NTSC
Figure 3-13: CIE 1976 u’-v’ coordinate comparing color gamut of LED backlight with the CCFL backlight, NTSC specification, and EBU specification. The blue and the green triangles indicate NTSC and EBU standard, respectively. The deep red triangle indicates LED color gamut that is larger than the triangle of all the others.
3.7 Summary
A RGB LED backlight with dimming control has been fabricated through the optical, mechanical, and electrical design. The problems like color mixing, thermal dissipation, and dimming control were conquered by modern techniques. After chromatic calibration, the characteristics of LED backlight are measured and compared with CCFL backlight. Based on the measured results on the prototype, the RGB LED backlight has better light efficiency, larger color gamut, and better dimming controlled by PWM then CCFL backlight. Its characteristics were conformed to the specification defined in previous section. Therefore, this was suitable to be the backlight module for the experiments.
NTSC EBU
CCFL LED
Chapter 4
Principle
4.1 Terminologies
Luminance is a physical measure defined as cd/m2. The luminance of an object can be measured by a luminance meter.
Brightness is the attribute of a visual sensation according to which an area appears to emit more or less light.
Lightness is the brightness of an area judged related to the brightness of a similarly illuminated area that appears to be white or highly transmitting.
Brightness and lightness are psychophysical terms and cannot be measured by instruments. Intuitively speaking, brightness represents the perceived luminance when there is only one single color in sight, while lightness represents the relative brightness of the color when the reference white is also present [Wyszecki 1982].
4.2 Backlight Luminance Scaling
The luminance of an LCD, L, is the product of the backlight luminous intensity b and the panel transmittance t. One can decrease the backlight luminous intensity to save the power consumption. The panel transmittance should be increased accordingly such that the luminance remains the same. In addition, for LCDs, higher transmittance can reduce the light leakage problem of liquid crystals and increase the image quality in terms of color saturation and viewing angles.
Consider a pixel consisting of red, green, and blue sub-pixel. Its color is determined by the product of the backlight luminous intensity (bw) and the transmittance of each sub-pixel (tR,tG,tB):
LR:LG:LB is the luminance ratio of red, green, and blue. For example, white is obtained at the ratio of 0.27:0.67:0.06. An LCD generates different colors by changing the transmittance ratio of sub-pixels tR:tG:tB. By increasing (tR,tG,tB), one can lower bW to save power and preserve (LR,LG,LB). This class of techniques is called backlight scaling. Note that (tR,tG,tB) are bounded by [0,1]. When (tR,tG,tB) need to be greater than 1, the original luminance can not be recovered and image distortion in terms of brightness/contrast occurs.
Figure 4-1: Backlight scaling concept: For minimizing the power consumption, the backlight scaling technique decreases the backlight duty cycle or intensity and increases the panel transmittance to keep the luminance.
4.3 Backlight Scaling Algorithms
Backlight scaling is by far the most effective technique for reducing power consumption in a transmissive display. To compensate for the visual quality loss due to reduced luminance,
proper image enhancement is necessary. Choi et al. proposed a technique that increases the pixel values (t) to recover the original luminance (L) [Choi 2004].
⎥⎥
Choi’s algorithm can preserve the luminance of the dark regions, but the bright regions will be over-saturated. In their study, the number of over-saturated pixels was chosen to evaluate the image quality loss.
Since preserving the original luminance is not always possible, finding a proper alternative transformation of luminance, L*=f(L), is the key of backlight scaling algorithms.
Cheng et al. proposed an algorithm to compensate for the luminance loss by increasing the contrast [Cheng 2004]. The following linear transformation was used:
⎪⎩
Where c, gl, and gu are constants generated by the optimization algorithm. Although Cheng’s algorithm is a compromise between preserving the brightness and preserving the contrast, it does preserve the original tonality, i.e., the proportional difference between bright and dark regions. The relationship between brightness and contrast, however, was employed without substantial support.
Iranli et al. proposed using histogram equalization, an image processing algorithm that balances the number of pixels on each gray-level, to perform the image enhancement [Iranli 2005]:
), (
* h' L
L = (16)
Where h’ is the derivative of the cumulative distribution function of the histogram.
Histogram equalization can reproduce each gray level distinctly without over-saturation or under-saturation. However, tonality will be distorted when the original histogram tends to be irregular.
(a) Original image
(b) Dim backlight to 50%
without compensation for saving power
(c) Enhance contrast to recover brightness but pixels over-saturated
(d) Optimal CBCS based on histogram
Figure 4-2: Concurrent Brightness and Contrast Scaling Concept.
4.4 Backlight Blinking
L(x)
x
L(x)
x
L(x)
x
L(x)
x
Conventional requirements of backlight design are spatial, temporal, and chromatic uniformity. Recently, the modern LCD technologies call for different backlight driving methods. For example, backlight blinking is adopted by LCD-TVs to deal with the motion blur problem. Unlike CRT monitors, because of the longer response time of liquid crystals, when steady backlight is used in an LCD, the fast moving edges appear to be blurred and degrade the sharpness of motion pictures. One solution is to pulse-drive (or “blink”) the backlight in order to generate CRT-like pulses. According to temporal vision study, a
Conventional requirements of backlight design are spatial, temporal, and chromatic uniformity. Recently, the modern LCD technologies call for different backlight driving methods. For example, backlight blinking is adopted by LCD-TVs to deal with the motion blur problem. Unlike CRT monitors, because of the longer response time of liquid crystals, when steady backlight is used in an LCD, the fast moving edges appear to be blurred and degrade the sharpness of motion pictures. One solution is to pulse-drive (or “blink”) the backlight in order to generate CRT-like pulses. According to temporal vision study, a