Basic light mixing methods of LED in landscape lighting

1 Introduction

Since the world's first commercial LED made of GaAsP material was successfully developed in 1965, with the development of new materials and the emergence of new device processes, LED has made several major progresses in the expansion of color, brightness and luminous efficiency. At present, LED is mainly used in display screens, lighting fixtures, lasers, multimedia imaging, backlight sources, instruments, traffic signals, automotive lighting, fiber-optic communications and toys. As a solid light source, LED has the advantages of energy saving, environmental protection, safety, long life, rich colors, white light, miniature, high brightness, easy dimming, etc. It is a light source that meets the requirements of green lighting. However, in order to replace traditional light sources in functional lighting fields such as road lighting and ordinary indoor lighting, it is currently limited by low luminous efficiency, low power, difficult optical system design and high price. However, in landscape lighting places such as parks, green spaces, squares, buildings and urban elevated roads, due to the rich colors of LEDs, in theory, only LED light sources can completely cover all saturated colors in the CIE chromaticity curve, and various colors of LEDs can produce almost any color without restriction through light mixing and organic integration with phosphors. At the same time, they can be powered by low-voltage DC and are easy to dim. Therefore, they have unparalleled advantages over other light sources in the field of landscape lighting. They have been widely used and will also be one of the areas with the greatest application potential for LEDs. This article mainly discusses the light mixing method of LEDs in landscape lighting.

2 Main characteristics of LED suitable for mixed light use

Not all light sources are suitable for light mixing and dimming, but LEDs have the characteristics of narrow spectrum, brightness approximately proportional to forward current within the operating voltage range, fast response speed and small size, making them very suitable for light mixing.

2.1 Spectral characteristics

Theoretical and practical evidence shows that the wavelength and frequency of light emission depend on the energy gap Eg of the selected semiconductor material, as shown in the following formula [1]:

Eg=hv/q=hc/(λq) (1)

λ=hc(qEg)=1240/Eg (nm) (2)

In the formula: λ is the emission wavelength, v is the electron movement speed, h is Planck's constant, q is the charge carried by the carrier, and c is the speed of light.

Since different materials have different energy gaps, LEDs made of different materials can emit light of different wavelengths. The light emitted by LEDs is not pure monochromatic light, but, except for lasers, its spectral line width is narrower than that of other light sources. For example, the spectral line width of gallium arsenide LEDs is only 25nm. Therefore, it can be roughly considered as monochromatic light, and several different colors of LEDs can be reasonably mixed to obtain relatively pure other colors of light.

2.2 Current/luminous brightness characteristics

Before forward conduction, almost no current flows through the LED. When the voltage exceeds the turn-on voltage, the current rises sharply. Therefore, the LED is a current-controlled semiconductor device, and its luminous brightness L (cd/m2) is approximately proportional to the forward operating current IF [2]:

L=KIFm (3)

Where: K is the proportionality coefficient.

In the small current range (IF = 1 ~ 10mA), m = 1.3 ~ 1.5, when IF> 10mA, m = 1, formula (3) can be simplified to:

L=HOW (4)

That is, the brightness of the LED is proportional to the current. When using, the appropriate IF value should be selected according to the required display brightness and specific LED parameters, so as to ensure moderate brightness and not damage the LED. If the current is too large, the PN junction of the LED will be burned. Therefore, the brightness of the LED can be adjusted by adjusting the current flowing through the LED within the operating current range of the LED.

2.3 Response time characteristics

The response time of LED is an important parameter that indicates its reaction speed, especially when it is pulse driven or voltage modulated.

The time response characteristic of LED refers to the delay characteristic of LED turning on and off with the change of electrical signal, which is described by response time. Response time includes rise time and fall time. Rise time is the time from when the power is turned on and the brightness of LED reaches 10% of the normal brightness to when the brightness of LED reaches 90% of the normal brightness. Fall time is the time when the brightness of LED drops from 90% of the normal value to 10% of the normal value after the power is turned off. LED response time mainly depends on the lifetime of carriers, junction capacitance of the device and circuit impedance, and has nothing to do with the current flowing through the tube. The response time of different types of LEDs is different, even the response time of the same type of LED is different, generally between 5 nanoseconds and 500 nanoseconds.

Therefore, LED can obtain modulated light or pulse light by using AC power supply or pulse power supply, the modulation frequency can be as high as tens of MHz, and the high-frequency on and off and the number of on and off times of LED will not affect its service life.

2.4 LED is small in size

In landscape lighting applications, to produce full-color dynamic change effects, it is necessary to combine red, green, and blue LEDs into light-emitting units in a certain proportion according to color matching requirements. In the same light-emitting unit, LEDs should be closely arranged so that the light spots of each LED overlap in the eyes of the viewer. The center distance between adjacent light-emitting units should be the same, and the center distance should meet the following requirements:

D≥2Ltan(θ/2) (5)

Where: D is the minimum center distance between adjacent light-emitting units, L is the vertical distance between the viewing point and the light source during normal use, and θ is the minimum resolution angle of the human eye.

The small size of LED also makes it more suitable for mixed light use.

3 Basic principles of LED three-primary color mixing

3.1 Basic contents of three primary colors

Color vision is a type of photopic vision of the human eye. The basic parameters of colored light are brightness, hue, and saturation. Although different wavelengths of colored light can cause different color sensations, the same color sensation can come from different combinations of spectral components. For example, a mixture of red and green light in appropriate proportions can produce the same color vision effect as monochromatic yellow light. In fact, all colors in nature can be mixed from three basic colors, which is the three-primary color principle [3].

The three primary colors are three colors that are independent of each other, and none of them can be produced by mixing the other two colors. They are also complete, that is, all other colors can be obtained by combining the three primary colors in different proportions. There are two primary color systems, one is the additive color system, whose primary colors are red, green, and blue; the other is the subtractive color system, whose three primary colors are yellow, cyan, and purple (or magenta). Adding the three primary colors in different proportions to obtain color is called additive color mixing, and the rule is: red + green = yellow; red + blue = purple; blue + green = cyan; red + blue + green = white.

According to the above-mentioned color vision characteristics of the human eye, three primary colors can be selected and combined in different proportions to cause various color visions. This is the main content of the three-primary color principle. The three primary colors are not unique. In principle, various three-color groups can be used. For the purpose of standardization, the International Commission on Illumination (CIE) has made a unified regulation, that is, the three-color light with a red wavelength of 700 nanometers, a green wavelength of 546.1 nanometers, and a blue wavelength of 435.8 nanometers is selected as the three primary colors. Experiments have found that the visual response of the human eye should depend on the algebraic sum of the three components of red, green, and blue, that is, their ratio determines the color vision, and its brightness is equal to the sum of the three primary colors in quantity.

3.2 Basic Principles of LED Three-primary Color Light Mixing

LED three-primary color mixing refers to the use of R-LED (red LED), G-LED (green LED) and B-LED (blue LED) to mix light to produce various lighting colors. According to the International Commission on Illumination (CIE) chromaticity diagram, the color of light is related to the ratio of the three primary colors R, G and B, r(λ), g(λ), b(λ), and satisfies the condition r(λ)+g(λ)+b(λ)=1. Therefore, this method can not only achieve brightness adjustment by changing the size of the LED current, but also change the ratio of the three primary color lumens according to the user's preference to obtain LED color adjustment.

Due to the visual inertia of the human eye, when the frequency of the periodic light signal is higher than the critical flicker frequency, the visual sensation of the eye to this periodic light signal is the same as constant light. According to Talbot's law, the visual brightness is:

L = 1/T∫T

D L(t)dt (6)

Where: L(t) is the actual brightness of the periodic light signal, called the brightness function, and T is the period of the light signal. Formula (6) shows that for periodic light signals greater than the critical flicker frequency, the visual brightness perceived by the eye is the average value of the actual brightness. When the brightness function L(t) is a constant L, the visual brightness of the periodic light signal is:

L average = t/(TL) = DL (7)

Where: t is the time of light stimulation in each cycle, and D=t/T.

Formula (7) shows that by periodically controlling the width of the light pulse, that is, controlling the duty cycle D, the brightness of the LED can be controlled. This control method is called PWM. The PWM method is flexible and easy to digitize, and is currently the main method for LED light mixing.

4 Basic ways of mixing three primary colors of LED

The main application methods of LED in landscape lighting are: 1. Use its original color or adjust its working current to obtain single-color brightness adjustment; 2. Use RGB three-primary color LEDs to arrange into light-emitting units and then control the switch combination to produce colorful changes; 3. Use RGB three-primary color LEDs to arrange in order and produce multi-color or even full-color dynamic changes through reasonable control; 4. Mix the ultra-bright RGB three-primary color LEDs to produce ultra-bright white light LEDs.

4.1 Mixing of monochrome and colorful lighting systems

For single-color LED lights, brightness adjustment is generally not required, and it is only necessary to properly control the on and off. However, due to its energy-saving and environmental protection, complete single-color varieties, and flexible use, it has been widely used in various non-functional lighting fields. Colorful LEDs are widely used in landscape lighting due to their simple control and good effects. The basic light mixing method is that red, green, and blue can be turned on separately to obtain three colors. After the light is mixed by the lamp, red and green are superimposed to obtain yellow, red and blue are superimposed to obtain purple, blue and green are superimposed to obtain cyan, and red, blue, and green are superimposed to obtain white. Then, through appropriate light mixing, combination, driving and control of lamps, landscape effects such as synchronous changes and colorful chasing can be achieved.

4.2 Mixed light methods for multi-color and full-color landscape lighting systems

Whether using LED to make a multi-color or full-color lighting system, in order to display various colors, the brightness of each LED that constitutes the light-emitting unit must be adjustable, and the degree of fineness of the adjustment is the grayscale level of the lighting system. The higher the grayscale level, the more delicate the lighting system display, the richer the color, and the more complex the corresponding control system. Generally, the color transition of a lighting system with 256 grayscale levels is very soft. Under the control of the control system, three colors can be made to have 256 grayscale levels and mixed arbitrarily to produce 256X256X256 (i.e. 16777216) colors, forming a combination of different light colors, which can achieve colorful dynamic change effects and various images. Regardless of the grayscale level, there are four main ways to mix light.

(1) Analog light mixing mode, that is, using variable resistor load dimming. Except for the red LED, whose brightness will saturate as the current increases, the brightness of other LEDs will generally increase as their operating current increases. Therefore, the brightness of the LED can be controlled within a large range by changing the variable resistor to adjust the operating current of the LED. For example, by applying 50% of the operating current to the LED, a brightness of about 50% can be achieved.

(2) Pulse Width Modulation (PWM). The response time of LED is generally only a few nanoseconds to a few hundred nanoseconds, which is suitable for frequent switching and high-frequency operation. At the same time, due to the specific current/luminous intensity characteristics of LED, the brightness of LED can be adjusted by periodic pulse width modulation, that is, controlling the output current duty cycle. For example, to halve the brightness, it is only necessary to provide current within a 50% duty cycle. To ensure that the human eye cannot perceive the PWM pulse, the frequency of the PWM signal must be higher than 100Hz. The maximum PWM frequency depends on the power startup and response time. To provide maximum flexibility and ease of integration, the LED driver should be able to accept a PWM frequency of up to 50KHz. Generally, we choose a switching frequency of 200 to 300Hz for PWM brightness adjustment. This is because the human eye cannot distinguish frequency changes exceeding 40Hz, and too high a frequency will cause white light color shift and brightness adjustment nonlinearity [4].

(3) Frequency modulation method: Keep the width of the rectangular pulse current (amplitude unchanged) applied to the LED unchanged. By changing the number of rectangular pulses applied to the LED per unit time, the average current obtained by the LED changes within a larger range, so that the LED brightness can be adjusted within a larger range.

(4) Angular modulation uses a series of binary pulses, and the width of each bit of the pulse sequence is extended in proportion to its bit value. By changing the width of the rectangular pulse current applied to the LED per unit time, the average current obtained on the LED can be changed within a larger range to adjust the brightness of the LED.

Comparing the above four mixed light brightness adjustment methods, it can be considered that the PWM method is more suitable for semiconductor lighting. The reasons are: (1) The analog mixed light method is not only inefficient, but also affects the service life of the LED. (2) Under a certain forward current, the LED can display the purest white light. As the working current deviates from this value, the light emitted by the white light LED may be blue or purple. Therefore, adjusting by changing the LED working current will cause the color of the light to shift, while using the PWM control method is not prone to such problems. (3) The high-power white light LEDs commonly used now all work under high currents, so they generate a lot of heat when working. In landscape lighting, the light sources of lamps used to achieve landscape effects require miniaturization and dense arrangement. Therefore, the heat dissipation problem of low-power LEDs is also very important. As the operating temperature increases, the performance of LED devices will decrease. Therefore, heat dissipation has a great impact on the working performance of LED devices. When the pulse average current and the DC current are equal when using the PWM control method, the LED device will have a lower temperature, so it can have a higher luminous brightness under the same PN junction temperature. At the same time, the main wavelength of the LED will drift as the junction temperature increases, so the PWM method is more suitable for the thermal characteristics of the LED. (4) Frequency modulation and angular modulation

Compared with the PWM method, the PWM driving and control circuit is easier to implement and is particularly suitable for digital control. Under the condition that intelligent controllers are widely used in current large-scale landscape lighting systems, the PWM method is the main light mixing method and development direction of multi-color and full-color LED landscape systems.

4.3 Technology Route of White Light LED

At present, the main way to obtain white light in landscape lighting is RGB mixing, while the methods for achieving ultra-bright white light LEDs are mainly concentrated in three technical routes [5]: one is to use the three-primary color principle to mix red, green and blue ultra-bright LEDs into white light; the second is to trigger phosphors and other fluorescent materials to emit white light through blue light photons; the third is to use ultraviolet LEDs to excite three-primary color phosphors or other phosphors to produce multi-color light mixed into white light. Practice has proved that all three technologies have been industrialized, and the latter two technologies have developed faster. This is because ultra-bright white light LEDs will be mainly used in functional lighting fields such as road lighting, which require single high-power LEDs. From the current point of view, the latter two methods are more suitable for producing high-power LEDs.

5 Conclusion

(1) In the field of landscape lighting applications, due to the characteristics and advantages of LED itself, the main method and development direction is to obtain colorful, multi-color and full-color landscape lighting systems through PWM RGB mixing. In particular, the colorful landscape lighting system has been widely used. Considering the complexity of control and the actual needs of landscape lighting systems, colorful and full-color lighting systems are only used in specific occasions. In LED display screens, colorful and full-color display systems using RGB mixing are currently widely used. In the field of ultra-bright white light LED applications, there is no special advantage in using RGB mixing to obtain white light LEDs.

(2) Light mixing is only one component of LED lighting. Effective light mixing and dynamic lighting control require a complex control system. The control system generally includes a controller part and an LED driver part, which requires dedicated hardware and software support. Currently, there is still a lack of unified standards, and each LED manufacturer develops its own. The unit volume of LED light sources is very small, which is obviously an advantage in many occasions, but sometimes it is a disadvantage. For example, on LED flat light sources, point-like spots and uneven color mixing usually appear. At present, the main problem with light mixing and color mixing is the contradiction between the uniformity of light mixing and color mixing and the transmittance. This problem is also a factor that hinders the development of LEDs and is of great significance to the practicality of LED landscape lamps. Therefore, it is necessary to develop new LED light mixing and color mixing technologies.