Brightness, uniformity and color coordinates of Led backlit

 > Brightness, uniformity and color coordinates of Led backlit

LED backlit is the main form of backlight used by tft display now. LED backlit has the characteristics of light and thin, power saving and long life. Backlight has three important parameters: brightness, color coordinates and uniformity. Brightness refers to the brightness of the led backlit, and the unit is cd/㎡. The color coordinates describe the color of the led backlit and need to be measured with a special instrument. Uniformity is a measure of the uniformity of light emitted by the led backlit.

Among them, brightness and color coordinates are more important, so what factors affect them, and how to measure them?

Factors that affect brightness

The led backlit consists of many parts, namely: shading film (PSA), upper prism sheet (TBEF), lower prism sheet (BBEF), lower diffusion sheet (DIF), light guide plate (LGP), reflective sheet (REF), LED Components (FPCA), iron parts (BEZEL), adhesive (TAPE), plastic frame (HG). during the production of backlight, due to many factors affected, it is inevitable that there will be differences in brightness. The main reasons are:

      1. Differences in the LED itself: the brightness range deviation is +/-3.5% (the selected chip itself has a difference in brightness, assembly process and other factors will cause brightness deviation).

      2. LED attenuation difference after reflow soldering: Due to the impact of high temperature impact on LED after reflow soldering, especially after lead-free soldering, the temperature is higher (170~245°C), resulting in different degrees of attenuation of LEDs, which generally cause attenuation. 0~5% of the initial value.

      3.  LED assembly error: When the light guide plate and the LED are close together, most of the light can be guided into the light guide plate, but in the actual assembly, there will be a certain gap between some LEDs and the light guide plate (about 0~0.2mm), resulting in light Part of it is reflected, and part of it is lost when it is conducted in the air. According to empirical values, the loss is about 0~8%.

led assemble

     4.  Influence of injection molding process on brightness: During the injection molding process of the light guide plate, the injection pressure, speed, temperature, stability, etc. will affect the surface gloss, flatness, thickness, etc. of the light guide plate, resulting in differences in brightness, as follows As shown in the figure, according to experience, it will lose about 0~10% of the brightness.

Light guide plate picture

Light guide plate picture

Microscopic photo of the light guide plate

Microscopic photo of the light guide plate

There are countless small lenses on the light guide plate, which can also be called reflectors. These reflectors can refract horizontal light into vertical light. The quality of the reflector directly affects the brightness.

     5. Deviation of transmittance of other materials: In the mass production of backlight materials, there are also deviations of reflectance and transmittance, such as: deviation of reflectance of reflective film, deviation of light enhancement rate of light-enhancing film, and transmittance deviation of diffusion film, the light guide rate deviation of the guide plate, these deviations probably affect the brightness value of 0~2%.

      6. Difference in flatness of film lamination: In the assembly of materials such as reflective films, diffusion films, and enhancement films, there will be differences in flatness. When the gap between optical films exceeds the design value, the light transmittance will decrease. Because there is always some unevenness in the lamination of the film, the loss of brightness is about 0~3%.

Factors Affecting Color Coordinates

1. LED color shift after reflow soldering: Same as brightness attenuation difference screen, LED color will also have a certain shift after reflow soldering. Variety.

2. Color differences caused by LED selection: LEDs will be separated into different color blocks. In actual LED sorting, brightness, voltage, and color need to be sorted at the same time. A single type of LED lamp beads will be sorted into more than 200 types. In actual mass production, the same type of backlight will use a combination of several groups of lights with different color blocks. to ensure the stability of production, which will cause certain differences in color coordinate.

3. Material color difference:

    3.1 Light guide plate: Affected by fluctuations in injection molding conditions, there will be certain differences in the transparency of the light guide plate. For example, temperature will affect the color of the light guide plate. In addition, one injection mold can produce 2 or even 4 light guide plates at the same time, and there will be differences between them.

Evenly test the brightness and color coordinate data of 12 points of the light guide plate and obtain the effect diagram of the light guide plate through statistics. The more uniform the effect diagram, the better the effect.

Bad light guide effect

Bad light guide effect

Good light guide effect

    3.2 Reflective film: Different reflective films have a great influence on the color shift, such as silver-plated reflective film and white reflective film, the color coordinates between them even differ by 0.03~0.04, even for the same type of reflective film, due to different batches Secondly, the reflection effect also has a subtle effect on the color coordinate.

    3.3 Diffusion film, enhancement film: the transparency of different batches of materials is affected, and there is also color deviation when light passes through these materials.

led backlit structure

Methods for Measuring Color Coordinates, Brightness and Uniformity

Measuring instrument: Top Colorimeter Kang BM-7, Konica Minolta CA310 or CA410

Measuring distance: 35cm (definition testing distance: the vertical distance from the colorimeter lens to the backlight emitting surface).

Test points: According to the size of the backlight, choose 5 points, 9 points, 16 points, 25 points, etc., generally use 9 points for testing.

led backlit test point
Color coordinate = take the color coordinate data of the middle position of the backlight, the tolerance range of the color coordinate is ±0.02
 

Brightness = average brightness of N points = (L1+L2+…+LN)/N, the tolerance range of brightness is ±10%

Uniformity= (minimum brightness/maximum brightness) *100%, the uniformity of brightness generally requires ≥80%

More Resources:

CIE 1931 color space– Source: Wikipedia

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