LED Backlight Color Production Theory

If we want to know about LED backlight color production theory, we must learn about LED lamps first.  The colors of LED backlights are composited by the colors of LED lamps we are going to use.
First, how can we choose what kind of LED lamp we are going to use?
Basically, there are 3 main points to determine a LED lamp
1 The size and packing of the LED lamp. We can divide LED lamps into DIP LED and SMD LED according to the style of packing.
Please see the picture below, which is the blueprint of 5050 SMD.
5050SMD5050SMD LED front viewA variety of models of SMD
We usually name the SMD according to their length and width. For example, we call length 5.0 mm and width 5.0  mm SMD 5050 SMD. And length 3.5 mm and width 2.8 mm SMD 3528 SMD.
There are some pictures of DIP LED below.
PHW23450Z DIP LED wire frame view PHB23450Z DIP LED light view
SMD is a lot smaller than DIP if they adopt the same LED chips.

  DIP LED SMD LED
volume large small
weight light lighter
using time longer long
heat radiation better good
reliability high higher
anti-vibration ability strong stronger
soldering defect rate low lower

If we say that LED is the future of illumination, we can also say that SMD is the future packaging of LED.  Please see A brief introduction for SMD

2 The indexes of illumination. There are so many indexes of illumination, but we mainly use two indexes: Luminous flux and Luminous intensity.

Luminous flux is that light which is emitted by a light source in some amount of time. It is usually the total amount of the light which is transformed by 1 watt hour (electricity power). The greater luminous flux of the light source, the more the light is emitted.

If we compare the luminous flux and luminous intensity of illumination units to the force and pressure of the mechanical units, we can regard luminous flux as force and luminous intensity as pressure.

In order to make illuminated points much brighter, we not only should increase the luminous flux, but also should decrease the illuminating area, so we can get a better luminous intensity.

Look back at our last example. SMD LED has a wider beam angle, hypothetically, they are using the same LED chips, which means they have almost the same luminous flux, so DIP LED has a better luminous intensity.

The unit of luminous flux is LM. Generally, it is used to measure the brightness of the light. At present, the luminous flux of small power (0.06-0.08w) is 2-10LM; the luminous flux of middle power (0.2-0.3w) is 10-25LM; the luminous flux of high power (0.5-1w) is 50-200LM; the luminous flux of super high power can reach more than 200LM.

Human eyes have different feelings to different colors of light. For example: we are very sensitive to wavelength 555 nm yellow green light. 1W = 683 LM means that 1 W of electricity power transforms into 683 LM wavelength 555 nm yellow green light. Meanwhile, we can’t see infrared and ultraviolet and we are not sensitive to red light.  It is only 73 LM/W which means 1 W of electricity power transforms into 73 LM wavelengths 650 nm red lights.   It is very complicated for white light which has many different spectra.

Luminous intensity is that luminous flux per unit solid angle in a specific direction.

Please see formula below.

I=Φ/Ω

Φ: Luminous flux
Ω: solid angle.

Luminous intensity is a criterion which can describe how bright the light source is. The higher the luminous intensity, the brighter the light source and the objects which are illuminated by the light source in the same conditions are.

The unit of luminous intensity is mcd. For example: 1000 mcd =1 cd and 15000 mcd =15 cd. That is because the luminous intensity of LED lamps was very low in the primary stage.

By the way, I want to talk a little bit about the photoelectric conversion efficiency, which is a whole new other topic and my customers ask me all the time. We continue our example: 683 LM wavelength 555 nm yellow green light.  I am sorry to tell you that it is not going to happen because this is just a theory. In practicality, it is less than 200 LM per watt even in a lab. When we talk about the so called “high illumination LED lamp”, as a matter of fact, it has a relative higher photoelectric conversion efficiency, which means it can release more visible light energy when it consumes the same amount of electricity power because it has a better chip. For example: the most common used LED lamp is about 100 LM per watt; meanwhile, the high laminating LED lamp is about 140-150 LM per watt. How can we improve the photoelectric conversion efficiency?  This is an international technical problem. As far as I know, only time can solve this problem. It has been improving since the day LED was invented.

3 Wavelength and color temperature

Color

wavelength

luminous flux

Voltage

Current

RED

630-660 nm

50-150 LM

1.8-2.3 V

15-20 am

Orange

580-590 nm

50-150 LM

1.8-2.3 V

15-20 am

Yellow

560-570 nm

50-150 LM

1.8-2.3 V

15-20 am

Chartreuse

566-575 nm

50-150 LM

1.8-2.3 V

15-20 am

Green

500-535 nm

50-150 LM

2.6-3.4 V

15-20 am

Blue

450-480 nm

50-150 LM

2.6-3.4 V

15-20 am

purple

440-460 nm

50-150 LM

2.6-3.4 V

15-20 am

Each color corresponds to a certain range of wavelength. Please see the picture blew.
CIE
I am going to give a quick introduction to CIE chromaticity coordinate. The horizontal means blue and red line, the bigger the value, the redder it becomes; the smaller the value, the bluer it becomes.  The vertical means the blue and green line, the bigger the value, the greener it becomes; the smaller the value, the bluer it becomes. You can see the middle white part which has a wide range in the picture.  It is the white light which we can’t be described with wavelengths. But we can only describe it by color temperature or chromaticity coordinates.  You can post: color temperature

(C)=R (R) +G (G) +B (B

This is the color matching equation. Theoretically, any color can be made by Red, Green and Blue three colors.

4 Other Photoelectric Parameters

Absolute Maximum Ratings(Ta=25℃)

Items

Symbol

Absolute maximum Rating

Unit

Power Dissipation

PD

200

mW

Forward Current(DC)

IF

60

mA

Peak Forward Current

Ifp

150

mA

Operation Temperatur

Topr

-20~+80℃

Storage Temperature

Tstg

-40~+100℃

Soldering Temperature

Tsol

Max.260℃ for 3 sec Max

Typical Electrical & Optical Characteristics(Ta=25℃)

Symbol

Condition

Min.

Typ.

Max.

Unit

Luminous Flux

Lumens

If=60mA

10

12

lm

Forward Voltage

Vf

If=60mA

3.0

3.3

3.6

V

Luminous Intensity

Iv

If=60mA

20

22

LM

Reverse Current

Ir

VR=5V

   —

10

μA

Viewing Angle

2θ1/2

If=60mA

120

deg

Color temperature

TC

If=60mA

2800

3200

K

Please see the two tables above. These parameters are the using conditions of LED lamps.  Most of the parameters are mentioned before.  Only one parameter is special- Reverse Current.

How can LED lamps be driven by Reverse Voltage? Of course, it can’t. Normally, when LED lamps are driven by Forward Voltage, they can be lit and we can test the current. When LED lamps are driven by Revere Voltage, they can’t be lit and there will be no current.  Before the LED lamps are shipped, we will use very high Reverse voltage to test them. For example: 5V or 20 V.  If the reverse current is less than 10μA, they are qualified.

As I mentioned before, white LED lamp is special. How can we make it?  There are 4 different kinds of methods we can use.
the color formation principle of white LED lamp1
Blue LED lamps which are adding yellow phosphor imitate white light.
Advantage: it has a simple structure and a high luminous efficiency.
Disadvantage: it lacks red light and has a poor reduction which is only about 65%.
Ultraviolet LED lamps which are adding RGB phosphor emit white light.
Advantage: it has a better reduction.
Disadvantage: it accelerates the aging process of epoxy resin and phosphor.
the color formation principle of white LED lamp2
RGB triple LED chips are packaged in one white LED lamp.
Advantage: The structure of LED lamp is close and does not need external light.
Disadvantage:  It is affected a lot by the current and the heat dissipating .And it has some problems when packaging with the good performance chips.
RGB three individual LED lamps emit white light by mixing lights together
Advantage: It has an individual dissipating heat structure and high efficiency output of light
Disadvantage: It requires dedicated mixing light structure which will increase the cost.

The method 1 is the most commonly used because it is cheapest.

Finally, how can we make multicolor we want in LED backlight?

1 It is the simplest one we use a particular wavelength LED lamp.  Please look back at the wavelength and color temperature section.  It is easy, but we usually don’t use this method.  For example: if we want 7 colors (Red, Green, Blue, Yellow, Cyan, Pink and white) in one LED backlight, so we have 7 different kinds of colors of LED backlight.  It is too expensive.   Besides, we may not have enough space to install so many LED lamps.

2  Use RGB 3colors to make any kind we want .

Red + Green = Yellow
Green + Blue = Cyan
Blue + Red = Pink
Red + Green + Blue = White

See the blueprint below:
RGB 3colors make any color
If we manually make RBG 3 colors in permutation and combination, we can only make 7 colors.  And theoretically, we can make thousand of colors by RGB. But how? The answer is simple. We use an electronic controller which can regulate the input voltage to manipulate the luminous intensity of RGB LED lamps. So we can use RGB 3 LED lamps through an electronic controller to make almost any color we want. Life is full of possibility, isn’t it?

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