After several years of rapid development of LED lighting, the optical efficiency of the whole lamp (the ratio of output lumens to input power) has been able to meet basic lighting needs and energy efficiency. With the continuous improvement of heat dissipation and power supply technology in recent years, the reliability of LED lamps has been greatly improved, and the LED lighting industry is developing towards intelligent control and high lighting quality. Among them, the lighting quality mainly refers to the light quality output by the lamps, and the evaluation indexes mainly include parameters such as color difference, color temperature and color rendering index. High-quality lighting with LED flexible secondary light distribution technology gives LED lamps a variety of lighting effects (including color and light shape). Secondary light has been widely used lights, tunnel lights, and spot lights and other lighting fixtures. Secondary light distribution technology mainly uses the refraction and reflection of the lens to control the light. However, the refraction will produce dispersion in the complex color light (such as white light), and the dispersion may cause yellow spots, blue spots or uneven color of the spot, which will greatly reduce the light quality of the lamp.
Based on the principle of refraction of light in a homogeneous medium, this paper analyzes the causes of dispersion in LED lamps and proposes measures to reduce dispersion in terms of dispersion. Finally, taking the RGB floodlight as an example, a total internal reflection lens with a microlens array was designed to improve the light distribution effect of the RGB floodlight.
Cause of dispersion
Dispersion refers to the phenomenon that complex color light is decomposed into a plurality of monochromatic lights to form a spectrum. The dispersion phenomenon has been around for a long time. As early as the 17th century, the scientist Newton used the prism to separate the color spectrum from white light, and concluded that white light is composed of light with different refractive properties. Dispersion phenomena are widespread in light transmission, and most have a negative effect. For LED luminaires, dispersion causes chromatic aberration problems. Dispersion is generated because the complex color light composed of different wavelengths is separated when the refraction angle is different. The refraction of light can be described by the law of refraction:
Wherein, N represents the sequence number of the light to distinguish different rays; niN and ntN respectively have refractive indices in the incident medium and the exit medium; θiN and θtN are the incident angle and the refraction angle of the light, respectively. Figure 1 shows the transmission behavior of light when the light is dimmed from the photonic medium niN (niN < ntN). According to the formula (1), for the complex color light having the same incident angle, the magnitude of the refraction angle depends on the refractive index corresponding to each wavelength, and the greater the refractive index difference between the two media, the greater the degree of deflection of the light.
The degree of dispersion is usually measured as the degree of separation (angle) of refracted light in the case of incident at the same angle, ie
It can be seen from equation (2) that in the case where the spectral components are determined, the larger the incident angle θi, the more severe the dispersion; the refractive index difference The bigger the dispersion, the more serious the dispersion. Therefore, in practical applications, the dispersion can be reduced by reducing the incident angle and the refractive index difference between the two media. In addition, by using reflection instead of refraction to change the direction of light propagation, the dispersion can be fundamentally eliminated.
Dispersion problem of white LED
There are two main methods for realizing white light in existing LED lamps: one is to activate yellow phosphor by blue LED chip ; the other is to achieve proportional mixing by three kinds of monochromatic LEDs of R, G and B. However, in either method, when light of different colors is incident on the same lens non-perpendicularly, the angle of refraction will be different, and dispersion will occur to cause chromatic aberration. In the spatial distribution, the chromatic aberration of the luminaire can be divided into the chromatic aberration inside the spot and the chromatic aberration at the edge of the spot. The chromatic aberration inside the spot is mainly caused by the mixing ratio of various wavelength components fluctuating with the spatial position, resulting in color inconsistency; the external chromatic aberration of the spot is mainly affected by the prism-like dispersion, so that the light with a large wavelength appears in the outer ring, such as a street lamp lens. The phenomenon of macular in the middle. Therefore, the optical design of the luminaire must take into account the effects of chromatic aberration caused by dispersion. For internal dispersion, a microlens array can be used to homogenize the spot and reduce the undulation of the mixing ratio; for external dispersion, the angle of incidence of the light should be minimized, and reflection should be used instead of refraction to change the direction of light propagation.
Optimized dispersion optical lens design and results
RGB floodlights use color light of three primary colors of red, green and blue, and are superimposed in different proportions by electronic control technology to produce a variety of color lights. Since the illumination of the LED lamp bead itself is about 120 degrees (Lambertian distribution), and the illumination angle required for the optical design of the projector is generally less than 60 degrees, the lens used for the projector must be able to face the LED at a large angle. The glazing can achieve a large change in direction (such as the reflection or refraction of the lens). The lens used for the projection lamp to achieve a narrow illumination angle is generally a Total Internal Reflection (TIR) ​​lens, which is characterized in that it has a surface that enables total internal reflection of light to change the direction of light propagation (such as the light in FIG. 2). 1), its design can be achieved by means of LED energy grid division. As previously analyzed, this total internal reflection surface reduces the dispersion of light. Further, within the range allowed by the angle, the incident angle of the light is reduced by reducing the curvature of the inner surface to reduce the dispersion (light 2 in FIG. 2). In practical applications, due to the influence of dispersion and lens processing accuracy, the R, G, and B lamp beads may have large differences in their illumination angle or light intensity distribution after using the same lens.
Figure 3 shows the results of a simulation of a RGB lamp bead from a large lamp bead manufacturer using the same internal reflection lens with an illumination angle of 30 degrees (optimized for green light). The results show that there are great differences in the illumination angles and light intensity distributions of the three lamp beads, with the red light having the largest illumination angle and the blue light bead being the smallest. The reason for the difference is related to the consistency and stability of the lamp bead packaging technology. Although the manufacturer will divide Bin according to the color temperature, luminous intensity and voltage of the lamp after the production of the lamp of the same color, the illumination angle and intensity distribution of the lamp are not treated accordingly. In addition, since the refractive indices of different wavelengths are different, various light rays have different deflection directions, and finally the lens has different illumination angles.
The difference between the illuminating angle and the intensity distribution causes a phenomenon in which the color distribution of the R, G, and B lights is uneven after mixing, that is, chromatic aberration. In order to reduce the influence of chromatic aberration, in the design of the total internal reflection lens, a microlens array can be fabricated on the light exit surface of the lens. The microlens array uses a microlens with a very short focal length to diffuse a small range of light into a large area of ​​space, similar to a scatterer, and its working principle is shown in FIG. Suppose a beam of uneven intensity passes through the microlens array and separates several small beams. Since the aperture of the microlens is small, the light entering each microlens can be approximated as being uniform. Each small beam diverges after being focused first, and finally has a distribution of approximately Gaussian functions in the target plane. Let the distance of the target plane from the light source be
d, the angle of illumination of the lens is θ, then the effective diameter of the spot on the target plane is If the illumination angle of the floodlight is θ=600 and the projection distance d=7m, the effective diameter of the spot is about D≈8m. For an LED floodlight, the radius d0 of the illumination is about 16 cm. Since d0îD, the floodlight can be approximated as a point source. After the light of different colors passes through the microlens, the light spot spreads to the target plane with an approximate Gaussian distribution and is linearly superimposed. According to the above design idea, in the optical design, the light can be made as perpendicular as possible to the surface of the lens to reduce the initial dispersion, as shown in Fig. 5(a).
When no microlens array is added, the lens illumination angle can be controlled within 10 degrees. Then, the focal length of the microlens is designed with the illumination angle of the luminaire as the target, and the shorter the focal length, the larger the lens illumination angle. The regular hexagonal microlens used in this example has a side length of 0.5 mm and a radius of curvature of 1 mm, and the three-dimensional model of the lens is as shown in Fig. 5(b). After applying the total internal reflection lens with microlens array, the simulation results of R, G and B three kinds of lamp beads are shown in Fig. 6. The light distribution angle and light intensity distribution of the light distribution curve are very similar. In addition, experimentally, the distribution curve of the floodlight was measured by a remote distribution photometer (as shown in Fig. 7), and the illumination angles of the three kinds of lamp beads of R, G and B were 29.1 degrees, 29.3 degrees and respectively. 29.7 degrees. The measured results show that the illuminating angle and intensity distribution of the measured light distribution curve are also very close. Although the microlens array increases the roughness of the light exit surface of the lens, it has a limited influence on the transmittance of light. In this example, the reduction in light transmittance with the microlens array is only 1%.
In order to verify the optimization of the dispersion of the microlens array, Figure 8 compares the CIExy chromaticity diagram of the lens strip with and without the microlens array (this article is only theoretically analyzed due to the lack of experimental measurement conditions). Any color on the CIExy chromaticity diagram can be represented by a pair of (x, y) coordinates. The greater the distance between the two color points, the more obvious the difference in color. Figure 8(a) is a CIE chromaticity-space angle distribution map of a spot using a lens without a microlens array. It can be seen from the figure that x and y are distributed in the [0.20.5] interval, which is basically in the white light region on the CIE1931xy chromaticity diagram. However, the spatial distribution of x and y is not uniform. From the distribution of x and y values ​​in Fig. 8(a), the large center of the numerical center is small, which indicates that the color temperature center of the light has a low edge and a high color difference. After the lens is attached to the microlens array, as shown in Fig. 8(b), the numerical range of x, y is reduced, and x, y are concentrated in the interval of [0.20.4] and [0.250.35], respectively. Therefore, the microlens array functions as a coloring dispersion to make the chromatic aberration of the luminaire spot small.
in conclusion
Dispersion is a common physical phenomenon in life, and is also ubiquitous in the optical lens of LED lamps. Dispersion separates the complex light (mainly red, green, and blue) in the luminaire, causing chromatic aberration in the luminaire. In general, the effect of dispersion can be reduced by combining two glass (冕 brand glass and vermiculite glass) lenses with different dispersion abilities. However, due to the material and cost constraints of the lens, this practice cannot be introduced into the lens of the LED fixture for the time being. Based on the structural characteristics of the LED lens itself, some measures for improving the dispersion including total internal reflection instead of refraction, reducing the incident angle of light and designing the surface microlens array are proposed. The simulation and experimental results show that the total internal reflection lens with surface microlens array can maintain good illumination angle and uniformity of light intensity distribution for the same series of R, B and G lamp beads, and mixes with RGB floodlights. The light effect plays a positive role. In addition, these methods are also applicable to lighting fixtures such as stage lights and spotlights.
Based on the principle of refraction of light in a homogeneous medium, this paper analyzes the causes of dispersion in LED lamps and proposes measures to reduce dispersion in terms of dispersion. Finally, taking the RGB floodlight as an example, a total internal reflection lens with a microlens array was designed to improve the light distribution effect of the RGB floodlight.
Cause of dispersion
Dispersion refers to the phenomenon that complex color light is decomposed into a plurality of monochromatic lights to form a spectrum. The dispersion phenomenon has been around for a long time. As early as the 17th century, the scientist Newton used the prism to separate the color spectrum from white light, and concluded that white light is composed of light with different refractive properties. Dispersion phenomena are widespread in light transmission, and most have a negative effect. For LED luminaires, dispersion causes chromatic aberration problems. Dispersion is generated because the complex color light composed of different wavelengths is separated when the refraction angle is different. The refraction of light can be described by the law of refraction:
Wherein, N represents the sequence number of the light to distinguish different rays; niN and ntN respectively have refractive indices in the incident medium and the exit medium; θiN and θtN are the incident angle and the refraction angle of the light, respectively. Figure 1 shows the transmission behavior of light when the light is dimmed from the photonic medium niN (niN < ntN). According to the formula (1), for the complex color light having the same incident angle, the magnitude of the refraction angle depends on the refractive index corresponding to each wavelength, and the greater the refractive index difference between the two media, the greater the degree of deflection of the light.
The degree of dispersion is usually measured as the degree of separation (angle) of refracted light in the case of incident at the same angle, ie
It can be seen from equation (2) that in the case where the spectral components are determined, the larger the incident angle θi, the more severe the dispersion; the refractive index difference
The bigger the dispersion, the more serious the dispersion. Therefore, in practical applications, the dispersion can be reduced by reducing the incident angle and the refractive index difference between the two media. In addition, by using reflection instead of refraction to change the direction of light propagation, the dispersion can be fundamentally eliminated. Dispersion problem of white LED
There are two main methods for realizing white light in existing LED lamps: one is to activate yellow phosphor by blue LED chip ; the other is to achieve proportional mixing by three kinds of monochromatic LEDs of R, G and B. However, in either method, when light of different colors is incident on the same lens non-perpendicularly, the angle of refraction will be different, and dispersion will occur to cause chromatic aberration. In the spatial distribution, the chromatic aberration of the luminaire can be divided into the chromatic aberration inside the spot and the chromatic aberration at the edge of the spot. The chromatic aberration inside the spot is mainly caused by the mixing ratio of various wavelength components fluctuating with the spatial position, resulting in color inconsistency; the external chromatic aberration of the spot is mainly affected by the prism-like dispersion, so that the light with a large wavelength appears in the outer ring, such as a street lamp lens. The phenomenon of macular in the middle. Therefore, the optical design of the luminaire must take into account the effects of chromatic aberration caused by dispersion. For internal dispersion, a microlens array can be used to homogenize the spot and reduce the undulation of the mixing ratio; for external dispersion, the angle of incidence of the light should be minimized, and reflection should be used instead of refraction to change the direction of light propagation.
Optimized dispersion optical lens design and results
RGB floodlights use color light of three primary colors of red, green and blue, and are superimposed in different proportions by electronic control technology to produce a variety of color lights. Since the illumination of the LED lamp bead itself is about 120 degrees (Lambertian distribution), and the illumination angle required for the optical design of the projector is generally less than 60 degrees, the lens used for the projector must be able to face the LED at a large angle. The glazing can achieve a large change in direction (such as the reflection or refraction of the lens). The lens used for the projection lamp to achieve a narrow illumination angle is generally a Total Internal Reflection (TIR) ​​lens, which is characterized in that it has a surface that enables total internal reflection of light to change the direction of light propagation (such as the light in FIG. 2). 1), its design can be achieved by means of LED energy grid division. As previously analyzed, this total internal reflection surface reduces the dispersion of light. Further, within the range allowed by the angle, the incident angle of the light is reduced by reducing the curvature of the inner surface to reduce the dispersion (light 2 in FIG. 2). In practical applications, due to the influence of dispersion and lens processing accuracy, the R, G, and B lamp beads may have large differences in their illumination angle or light intensity distribution after using the same lens.
Figure 3 shows the results of a simulation of a RGB lamp bead from a large lamp bead manufacturer using the same internal reflection lens with an illumination angle of 30 degrees (optimized for green light). The results show that there are great differences in the illumination angles and light intensity distributions of the three lamp beads, with the red light having the largest illumination angle and the blue light bead being the smallest. The reason for the difference is related to the consistency and stability of the lamp bead packaging technology. Although the manufacturer will divide Bin according to the color temperature, luminous intensity and voltage of the lamp after the production of the lamp of the same color, the illumination angle and intensity distribution of the lamp are not treated accordingly. In addition, since the refractive indices of different wavelengths are different, various light rays have different deflection directions, and finally the lens has different illumination angles.
The difference between the illuminating angle and the intensity distribution causes a phenomenon in which the color distribution of the R, G, and B lights is uneven after mixing, that is, chromatic aberration. In order to reduce the influence of chromatic aberration, in the design of the total internal reflection lens, a microlens array can be fabricated on the light exit surface of the lens. The microlens array uses a microlens with a very short focal length to diffuse a small range of light into a large area of ​​space, similar to a scatterer, and its working principle is shown in FIG. Suppose a beam of uneven intensity passes through the microlens array and separates several small beams. Since the aperture of the microlens is small, the light entering each microlens can be approximated as being uniform. Each small beam diverges after being focused first, and finally has a distribution of approximately Gaussian functions in the target plane. Let the distance of the target plane from the light source be
d, the angle of illumination of the lens is θ, then the effective diameter of the spot on the target plane is
If the illumination angle of the floodlight is θ=600 and the projection distance d=7m, the effective diameter of the spot is about D≈8m. For an LED floodlight, the radius d0 of the illumination is about 16 cm. Since d0îD, the floodlight can be approximated as a point source. After the light of different colors passes through the microlens, the light spot spreads to the target plane with an approximate Gaussian distribution and is linearly superimposed. According to the above design idea, in the optical design, the light can be made as perpendicular as possible to the surface of the lens to reduce the initial dispersion, as shown in Fig. 5(a). When no microlens array is added, the lens illumination angle can be controlled within 10 degrees. Then, the focal length of the microlens is designed with the illumination angle of the luminaire as the target, and the shorter the focal length, the larger the lens illumination angle. The regular hexagonal microlens used in this example has a side length of 0.5 mm and a radius of curvature of 1 mm, and the three-dimensional model of the lens is as shown in Fig. 5(b). After applying the total internal reflection lens with microlens array, the simulation results of R, G and B three kinds of lamp beads are shown in Fig. 6. The light distribution angle and light intensity distribution of the light distribution curve are very similar. In addition, experimentally, the distribution curve of the floodlight was measured by a remote distribution photometer (as shown in Fig. 7), and the illumination angles of the three kinds of lamp beads of R, G and B were 29.1 degrees, 29.3 degrees and respectively. 29.7 degrees. The measured results show that the illuminating angle and intensity distribution of the measured light distribution curve are also very close. Although the microlens array increases the roughness of the light exit surface of the lens, it has a limited influence on the transmittance of light. In this example, the reduction in light transmittance with the microlens array is only 1%.
In order to verify the optimization of the dispersion of the microlens array, Figure 8 compares the CIExy chromaticity diagram of the lens strip with and without the microlens array (this article is only theoretically analyzed due to the lack of experimental measurement conditions). Any color on the CIExy chromaticity diagram can be represented by a pair of (x, y) coordinates. The greater the distance between the two color points, the more obvious the difference in color. Figure 8(a) is a CIE chromaticity-space angle distribution map of a spot using a lens without a microlens array. It can be seen from the figure that x and y are distributed in the [0.20.5] interval, which is basically in the white light region on the CIE1931xy chromaticity diagram. However, the spatial distribution of x and y is not uniform. From the distribution of x and y values ​​in Fig. 8(a), the large center of the numerical center is small, which indicates that the color temperature center of the light has a low edge and a high color difference. After the lens is attached to the microlens array, as shown in Fig. 8(b), the numerical range of x, y is reduced, and x, y are concentrated in the interval of [0.20.4] and [0.250.35], respectively. Therefore, the microlens array functions as a coloring dispersion to make the chromatic aberration of the luminaire spot small.
in conclusion
Dispersion is a common physical phenomenon in life, and is also ubiquitous in the optical lens of LED lamps. Dispersion separates the complex light (mainly red, green, and blue) in the luminaire, causing chromatic aberration in the luminaire. In general, the effect of dispersion can be reduced by combining two glass (冕 brand glass and vermiculite glass) lenses with different dispersion abilities. However, due to the material and cost constraints of the lens, this practice cannot be introduced into the lens of the LED fixture for the time being. Based on the structural characteristics of the LED lens itself, some measures for improving the dispersion including total internal reflection instead of refraction, reducing the incident angle of light and designing the surface microlens array are proposed. The simulation and experimental results show that the total internal reflection lens with surface microlens array can maintain good illumination angle and uniformity of light intensity distribution for the same series of R, B and G lamp beads, and mixes with RGB floodlights. The light effect plays a positive role. In addition, these methods are also applicable to lighting fixtures such as stage lights and spotlights.
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