Light Diffusion Properties Analysis in LED Lamps

The glass tube as the shell of the lamp tube continues to use the powder coating process of the traditional lamp tube. In order to eliminate the damage to the human eye caused by the glare caused by the high-brightness LED lamp beads, the shell of the LED lamp tube (hereinafter referred to as the lamp tube) is designed for light diffusion effect (eliminating glare) whether using PC (polycarbonate) tube or glass tube. The lamp tube with PC tube as shell material has been added silicone diffuser during the processing of PC plastic, and the glass is coated with light diffusion coating on the inner wall of the glass tube. The absorption loss of LED light is also different with different outer tube materials or light diffusion materials used in the process, which finally leads to the difference of light transmittance, light efficiency and beam angle. These differences will have a certain impact on the design of lighting fixtures and indoor lighting environment.

Technical analysis of light diffusion effect of lamp tube.

1.1 PC lamp shade.

PC tube can work in a wide temperature range (- 60 ℃ to 120 °C). It has many advantages, such as impact resistance, simple processing, easy forming, no need to apply light diffusion layer, no need to bond light strip separately, and is not easy to be damaged during transportation and use, but its disadvantages are poor resistance to alkaline corrosion, easy to crack due to stress, and yellowing and brittleness when exposed to light for a long time. The maintenance rate of the lamp is difficult to meet the requirement of more than 30000 h of LED lamp.

1.2 Glass lamp.

The coating process of light diffusion coating for glass tube is roughly the same as that of traditional fluorescent lamp, but the raw materials used for coating powder have nothing in common. At present, the raw materials used for light diffusion coating can be divided into three types.

The first category is solvent-based paint (also known as oily paint), which is the most mature process evolved from the coating industry, this kind of coating has paint-like performance after drying and has the most reliable firmness. The most commonly used coating is a combination of solvent-based acrylic resin and silicone diffusion powder, and its light transmittance is generally up to 92%. Although the application process of solvent-based coating is simple, due to the use of butyl esters or benzene diluents, it will produce a great irritating smell and benzene pollution in production, and will be gradually eliminated.

The second category is water-soluble coatings. Water-soluble coatings use water as diluent and water-soluble acrylic acid as binder. The process is pollution-free. In the past two years, it has become an important research and development topic in LED lamp industry, and important results have been achieved. At present, all known such adhesives are solidified into films by the cross-linking reaction produced after dehydration and drying. the biggest advantage is that the coating film after the cross-linking reaction is no longer soluble in water, and its firmness is second only to the solvent-based coating, but it is enough for the lamp. Although the water-soluble coating has many advantages, it is precisely because of the nature of the cross-linking reaction that determines the problem in the process-the difficulty of machine cleaning caused by the powder pulp left on the machine equipment and sprinkler after drying.

The third type is evolved from the water coating process of fluorescent lamp, using light diffuser instead of fluorescent powder for luminescence, and the binder is PEO (poly (ethylene oxide)). For manufacturers who manufacture traditional fluorescent lamps, this technology is relatively mature, the utilization rate of powder is high, the production cost is low, the high-temperature resistance of the coating is good, and the light transmittance of good silicone diffusion powder can also reach 92%. Its disadvantage is that the firmness of the coating is slightly worse than that of the water-based paint, but because the binder PEO adhesive has not gone through the roasting process like a fluorescent lamp, there is no high-temperature decomposition, and its firmness is much higher than the powder layer of the fluorescent lamp, as long as there are no scratches on the coating in the production process, it can be guaranteed that the powder layer will not fall off during the life period of the lamp.

1.3Glass-PET composite lamp.

The process of wrapping a layer of diffusion film on the outside of the glass tube has also made some progress. The technology of making light diffusion coating is to wrap a layer of PET (polyethylene terephthalate) film on the outside of the glass tube. This plastic film produces shrinkage effect at a certain high temperature and is tightly wrapped on the outer surface of the glass tube. Because the shrinking film is mixed with light diffuser, the purpose of scattering light can be achieved. This kind of composite lamp tube combines some advantages of PC tube and glass tube. It has the advantages of high light transmittance, simple processing and good heat resistance. It can be used for a long time at 120 °C. it has strong toughness and tensile strength, and is not easy to break. it shows the superiority of convenient processing for high-power lamp tubes with length > 1500 mm. The disadvantage is that the corrosion resistance of alkaline environment is poor, and it is not resistant to hot water immersion, so it is easy to crack and fall off in the process of long-life light, and lose the effect of light diffusion.

2 Review of light diffusion materials.

At present, the diffusion materials used in PC tubes and PET films are mainly organosilicon, while the light diffusion materials used in glass tube coatings are mainly organosilicon, calcium carbonate, talcum powder, phosphor, yttrium oxide, silicon oxide, barium sulfate and so on. A great deal of discussion has been made on the properties of these light diffusers in this paper. Some properties and applications of organosilicon light diffusers are supplemented in this paper.

Among all the light diffusers, silicone light diffuser has the highest light transmittance. The silicone transparent sphere material particles are added in the form of micron particle size, and the micrographs are shown in figure 1, which can be uniformly dispersed in the resin when used in PC, PET and filling glue. The light of the lamp can pass through the transparent spheres in these film-forming coatings, so that after many times of refraction and reflection, the strong light emitted by the LED lamp beads is scattered and evenly emitted from the surface of the PC tube or PET film, turning the point light source into a surface light source, eliminating the chord light and softening the light. Because the light directly transmits the light diffuser sphere, the multiple reflection and absorption are avoided, the light loss is reduced, and the light transmittance is improved.

Silicone light diffuser is convenient to use in PC tube and PET film, but it needs special treatment in water-soluble coating process, otherwise it is difficult to disperse evenly and form suspension, and the coating quality is difficult to control. At the same time, it is precisely because it is transparent, and the haze will be slightly reduced while the light transmittance is high, so it is generally not used alone, and the effect of mixing with inorganic light diffuser is better.

For inorganic light diffusers, light diffuses uniformly through the surface of tiny particles after multiple reflections, but it is difficult for light to directly penetrate inorganic powder particles, and inorganic light diffusion powder is as difficult to form spherical particles. Therefore, the light absorption of the diffusion powder itself increases, resulting in a decrease in light transmittance, and the light transmittance of good inorganic diffusion powder such as yttrium oxide can also reach about 91% (figure 2). At present, with the use of good diffusion material coating and high-quality PET film, the light transmittance of LED T8 lamp tube can reach 92%. The light transmittance of PC plastic tube can also reach 91.5%, which has much to do with the light absorption coefficient of plastic itself than glass. With the improvement of light transmittance, on the one hand, the light efficiency of the lamp can be improved, on the other hand, the temperature of the lamp tube can be reduced and the life of the power supply components can be prolonged. It is precisely because the light is difficult to directly pass through the powder particles of the inorganic light diffusion powder, while the reflection times are increased, the haze of the powder will also increase, so the haze of the inorganic light diffusion powder is higher.

3. Test and analysis of light transmittance.

3.1 effect of coating thickness on light transmittance.

Table 1 shows the measured values of light transmittance of different weight (thickness) coated with solvent-based binder. It can be seen from the table that with the increase of the amount of coating, the light absorption of the light diffuser increases, resulting in a gradual decrease in light transmittance, and it is also found that when the powder coating (that is, thickness) increases by 67%, the light transmittance only decreases by 1.7%.

The different segments of the water-soluble 1.2m long glass tube coated with water-soluble light diffusion layer were measured. The powder tube was divided into one zone per 300mm, and then the light transmittance of the four sections was detected respectively (see Table 2). From thin to thick, the light transmittance decreased from 92.7% to 90.8%, with a difference of 1.9%.

3.2 effect of wall thickness of glass tube on light transmittance.

Through the analysis of the light transmittance of Na-Ca-Si glass tube with different wall thicknesses, it is found that the light transmittance of the glass tube and powder tube decreases slightly with the increase of glass tube thickness, but the extent of the decrease is not obvious. The wall thickness increased from 0.65 mm to 0.90 mm, an increase of 38%, while the light transmittance decreased by only 1.0% (see Table 3).

4 Experimental analysis of 4 beam angle.

The design of the lamp depends on the parameters of the beam angle of the light source. When replacing the traditional T8 fluorescent lamp, the LED T8 lamp always strives for the larger the beam angle, the better (figure 3). The traditional T8 fluorescent lamp is a non-directional lighting source, and the lamp emits light at the 180 °cross-section of the C 0 ram, so when used for indoor lighting, the whole space has a transparent feeling, giving people a comfortable visual enjoyment. As for the LED T8 tube, the lamp beads in the light source strip are at the top of the lamp tube and glow from 180 °to the tube. After diverging light through different light diffusion paths, the directional beam angle formed in the C 0 ram 180 °cross section is also different.

4.1 The effect of particle size on beam angle.

The large particle size of light diffusion agent can produce high haze, while the small particle size can produce good light transmittance. Due to the limitation of the experimental conditions, the haze in the experiment can not be measured, and the haze can only be evaluated by observing whether the lamp is transparent or not. Three kinds of inorganic coating lamps with average particle sizes of 1.1 μ m, 4.6 μ m and 8.0 μ m were used to test the beam angle (see Table 4). It can be seen from the experimental data that with the same kind of lamp bead strip, the lamp bead can not be seen at the thin end of the powder thickness. with the decrease of the particle size and powder weight of the diffused powder, the beam angle of the lamp tube decreases gradually, and it is found that the maximum difference of the beam angle is 50 ° .

4.2 effect of different materials on beam angle.

Using the same LED light strip, using different diffusion methods and materials, the beam angle is different. From the experimental results, the beam angles of PC tube, oily paint, PET film and waterborne paint are basically the same, while the beam angle of the light diffusion coating made by the traditional fluorescent lamp water coating process can reach about 320 °(see Table 5).

This is due to the large particle gap of the traditional fluorescent lamp water-coated diffusion powder coating compared with the above coating processes (figure 4). The inorganic light diffuser material powder particles are non-spherical, irregular polygons, and the particle size consistency is poor, coupled with the inorganic light diffusion powder opaque, light can not directly pass through the powder particles, all these lead to light must be reflected many times irregularly in the powder layer before it can penetrate the powder layer, and the emission angle after light scattering is obviously larger than that after passing through the dense translucent uniform sphere coating.

5 concluding remarks.

The light diffusion material plays a decisive role on the light transmittance of the lamp, in which organic silicon is the most ideal light diffusion material. The traditional PEO– inorganic light diffusion powder process has obvious advantages in increasing the beam angle of LED T8 tube. Looking for light diffusion materials with higher light transmittance, better haze and higher performance-to-price ratio for water-soluble light diffusion coatings is the focus of research and development for a long time in the future.