How to improve the light color performance of LED stage lighting fixtures

The traditional stage lamps mainly use tungsten halogen lamps and xenon lamps. Taking a halogen lamp as an example, its characteristic as a stage lighting source is the spectral continuity of the heat radiation source, and the color rendering index ranges from 95 to 100, which is close to the ideal light source.

When the color temperature or light color conversion is required, a filter is required in front of the light source. The filter has the characteristic of selective absorption of light, and filters of different colors can be used to obtain new light colors.

Although halogen lamps can achieve satisfactory color rendering and meet the color requirements of artistic creation, their shortcomings are also obvious. During the luminous process of the halogen lamp, the luminous efficiency is low, and most of the electric energy will be lost in the form of thermal radiation, which will cause the temperature of the lamp body to rise significantly, and there will be hidden safety hazards in the application process.

At present, domestic and foreign lamp manufacturing companies directly use white LED light sources for stage lighting. White LEDs are commonly used in blue light-emitting phosphors. The color rendering, spectrum, and visual effects are the same as those of tungsten halogen lamps. There are significant differences.

Although the general color rendering index of some LED stage spotlights can be as high as 90 or more, due to the high blue light component and low red in its spectrum, it is difficult to feel the color reproduction, light color effect, artistic expression and other aspects during use. It meets the requirements of Wumei lighting personnel, which is also a serious shortcoming in the basic light lighting of LED stage lamps currently recognized in the performing arts industry.

This paper proposes a method to improve the light color performance of the LED stage basic lighting fixtures by using the RGBW four-color light source mixing technology.

Aiming at the characteristics and shortcomings of the white LED light source lighting technology, based on the research, an RGBW mixed color light source is proposed, supplemented by a PWM-based color mixing algorithm control system, heat dissipation design and secondary light distribution design, so that the performance of the LED basic light lighting fixture can reach requirements of Wumei Lighting staff.

Selection of LED light source

For the light color uniformity of multiple RGBW color mixing technologies, a simple optical combination cannot meet the light color uniformity. Breakthroughs in optical technology are needed.

In order to improve the performance of the lamp, we compared the RGBW light source modules on the market from the perspective of performance and cost, and finally decided to use a special light source, as shown in Figure 2.

The W (white light) part of the light source selects a color temperature of 3000K and a general color rendering index Ra≥85. The RGBW multi-color LED light sources with a total power close to 300W are evenly distributed in a circular area of ??less than 20mm, which solves the problem of light sources. Lay the foundation for improved light color performance.

Cooling and secondary light distribution design

"Thermal light decay" is one of the characteristics of LED devices. When the temperature rises above a certain value, its light efficiency drops sharply or severely, causing the entire LED to fail prematurely. Therefore, good heat dissipation design is a necessary condition to ensure the performance and life of high-power LED lamps.

In practical applications, a heat pipe design based on heat pipe technology should be adopted. The heat pipe technology can quickly transfer heat to the aluminum fin radiator, and cooperate with the silent shaft fan to quickly discharge the heat generated during the LED operation.

As the light output of high-power LED chips is mostly Lambertian distribution, the 50% light intensity angle is 120 degrees, which cannot meet the light distribution requirements of professional stage lamps.

Stage function lamps require control of light distribution, and different lamps have different requirements for beam angle, so the secondary optical design of LED is needed. The main need to use optical components to efficiently change the light intensity distribution of LEDs to meet the design requirements of lamps.

For long-range spotlights, the 10% light intensity angle is required to be as small as about 10 degrees.

For example, the surface light lamps used in large and medium-sized theaters generally have an irradiation distance of more than 30 meters. Usually, a telephoto plano-convex lens spotlight with very good light collection performance is used.

LED driving control based on PWM and RGBW color mixing algorithm

In stage applications, the stability of the color temperature of the lamp is very high. Therefore, according to the basic electrical characteristics of the LED light source, a precise and efficient constant current drive method should be used to make the light output stable and reliable.

The dimming method should adopt PWM (pulse width modulation) to ensure that the color temperature (white) or wavelength (color) of the LED dimming will not change.

The color mixing algorithm is the key link to control the dimming and color mixing of the light source and output the color light that meets the requirements.

According to the basic principle of PWM dimming, changing the duty cycle of the red, green, blue and white LED drive currents can change the intensity of each color light. By combining different duty ratios, light colors with different mixing ratios can be obtained. .

According to chromaticity calculations, if the determined three RGB LED spectra are used to mix white light with a certain color temperature in the range of 2300K ~ 3200K, there will only be a unique color mixing ratio.

However, only the three colors of RGB are used for color mixing, and the mixed spectrum is discontinuous. There are many visible light bands whose spectral energy is 0, and the color rendering will be poor. When using RGBW four colors to mix white light at a certain color temperature point, there will be an infinite number of mixed color ratios, and each ratio corresponds to a metachromatic light.

For lighting fixtures, because the color rendering index is a very important index, a failure of the color rendering index will cause a significant difference in the color of the object being illuminated, and it cannot be used. Therefore, it is necessary to select a set of ratios with the best color rendering index from a plurality of color mixing ratios for use.