LED film and television stage photography professional lighting design points

According to the characteristics of LED and the requirements of professional lighting for film and television stage lighting, this paper expounds the design points of LED film and television stage professional lighting from three aspects: optical system, heat dissipation system and drive control system.

Optical system

Commonly used professional lighting fixtures can be divided into the following categories:

*Soft light fixture

* Spotlights

* Imaging fixture

*After lighting

1.1 soft light fixture

Soft light fixtures are mainly used for studio lighting. Before the appearance of led lamps, they were mainly realized with three primary color fluorescent lamps. The spot angle is generally required to be around 60°, the light quality is soft and uniform, and there is no obvious shadow, which can effectively control the irradiation area. Since it is generally used at close range, the glare control is required to be high. Three-primary fluorescent lamps are easier to control due to the large scale of the light source. LEDs are prone to glare due to their small chip area and high power density. The most direct way to solve the glare problem is to disperse the light source and reduce the power density. The main purpose is to reduce or eliminate the effects of glare by using a large number of low-power LEDs to form a uniform surface light source. Current light sources for LED soft light fixtures range from 20mA in-line LEDs to 0.2W to 1W surface mount (SMD) LED chips . In-line LEDs can only be exported through the pins, so the heat dissipation performance is not good and the light attenuation is relatively large. The surface-mount LED has many advantages such as good heat dissipation performance, long life, good consistency, and advanced technology, and it has been widely used. However, Litepanel, the world's leading brand of LED flat-panel luminaires, has been using 5mm in-line low-power LEDs, as shown in Figure 1. Why not use surface-mount LED chips with better heat dissipation, longer life, and better technology? Mainly because the surface-mount LED chip does not have a condenser lens, the light-emitting angle is very large, and the required illumination is not achieved without the lens. The stray light can not be controlled. In addition, the chip generally has a large power and is prone to glare. The 5mm in-line LED power is only 0.06W, and the lens that comes with the head can control the light angle very well, which is very suitable for soft light fixtures.

Some products on the market use LEDs of 1W or more and 1W or more, which will produce glare. A diffuser with a large scattering angle can reduce glare, but it increases the spot angle, reduces illumination, and increases stray light. Some lamps have a good total light efficiency, but the illumination is very low, which is why. For example, the low-power LED multi-point illumination effectively controls the glare, and the soft plate is configured to make the light quality softer and more uniform, as shown in Fig. 2. In the control of stray light, a honeycomb barrier control zone with multiple impulses (30°, 45°, 60°, 90°) can be selected. By choosing different ratios of aperture and thickness, the honeycomb compartment can completely block the light larger than a certain angle, effectively control the stray light, and basically does not affect the central illumination. For luminaires with multiple light sources and surface light sources, the illumination area can only be controlled by means of grid occlusion, and the use of concealer is completely unachievable. As shown in Figure 3, the concealer can only block the light close to the concealer, and the far-end light can still be emitted, which not only does not play a shading effect, but also brings an unsightly ghost. Some people say that the three primary color lamps are also equipped with concealers, but a closer look reveals that the three primary color lamps use a bright plate, which is used as a reflector instead of a concealer, see Figure 4.

At present, the LED's light efficiency and color rendering are greatly improved. The LED light effect that meets the color temperature and color rendering index requirements of the film and television stage has reached 100lm/w. The LED soft light lamp has replaced the three primary color soft light lamps. Technical Support. Taking a 100W flat-panel luminaire as an example, the 4m far illuminance is 607lx, see Figure 5; and the 4x55W tri-color soft luminaire 4m far illuminance is only 301lx, see Figure 6. Although the luminous flux of LED flat-panel luminaires is lower than that of three-primary soft-light fixtures, the beam angle of 60°-70° is suitable for most professional applications, and the three-primary soft-light fixtures are difficult to achieve good convergence due to the large light source. Light, and thus the beam angle is too large, in practice, large angles of light are generally considered to be invalid stray light and removed by the grid. It can be seen that the 100W LED flat-panel soft light fixture can completely replace the 4x55W three-primary soft light fixture. By combining, LED soft light fixtures can also replace large three-primary soft light fixtures, such as large 6x96W fixtures.

The ideal flat-panel luminaire should have a uniform surface source, so that the glare is minimal and no shadows are generated. At the same time, the spot angle should be controlled at around 6°. The remote phosphor technology is very suitable for this application. This technique separates the LED from the phosphor and a yellow phosphor that is excited by a sapphire blue LED to produce a white light. Since the phosphor is far away from the LED chip, the color coordinates are not affected by high humidity, the light mixing is more uniform, the light effect is higher, and the life is longer. Using this technology, a very uniform surface light source can be obtained, and the microlens array technology can be used to achieve the ideal light distribution effect of the soft light fixture, which will be the development direction of the professional flat-panel soft light fixture for the film and television stage.

Figure 7 shows the optical analog of a remote fluorescent plate without a microlens array. The light pattern is a typical Lambertian distribution with an exit angle of 120°.

Figure 8 is an optical analog of a remote fluorescent plate with a microlens array. Due to the concentrating effect of the microlens array, the light extraction angle is reduced to 60°, and the ideal light distribution curve of the soft light fixture is obtained.

1.2 spotlights

The concentrating lamps are hard and strong, and have clear shadows when occluding the files. Commonly used are lenticular lens concentrating lamps and flat convex lens concentrating lamps. Because of the softer edge of the lenticular lens concentrating luminaire, it is also called the stage soft luminaire in stage applications. Spotlights require less stray light, variable spot angles, and sharp or sharper shadows when occluded. In order to achieve the purpose of illuminating the illuminating light, the concentrating luminaire generally requires that the illuminating area be controlled by concealing.

The optical system of a concentrating luminaire is relatively simple. Taking a threaded concentrating luminaire as an example, it is generally preferred to use a light source with a good point of view, 82 pieces of a threaded lens. Some products add a convex lens with a short focal length near the light source, which can effectively converge the light and improve the light efficiency during the concentrating, but at the same time affect the zoom range of the luminaire, resulting in a large concentrating angle and astigmatism. Defects with small degrees need to be used with anger. Controlling the light zone with concealer is a very important function in film and television stage lighting. Some led spotlights use a multi-lens design, but also equipped with concealer, which is completely addictive, see Figure 9.

Multi-lens LED fixtures can not only control the light area, but also bring unsightly shadows. The principle is the same as the soft light fixture and concealer. Figure 10 shows the real shot effect of the above-mentioned luminaire plus concealer.

Multi-lens LHD luminaires also have the disadvantage of producing multiple shadows. Figures 11 and 12 show the shadows of the lenticular lens concentrating luminaire and the multi-lens LED illusion, respectively. The shadows produced by the lenticular lens are relatively clear. The lens LED luminaires produce separate shadows from the sub-light sources, which superimpose each Other to produce unsightly ghosting and effects.

Since the film and television stage concentrating lamps require thousands or even tens of thousands of lumens of total light, and the source is required to be good, this requires the LED module to reach a very high power density. A LED photography lamp with a 250W LED module, the point is close to the tungsten light source, the luminous surface diameter is only 17mm, the luminous flux is greater than 150001m, the color rendering index is greater than 85, and the color coordinate can be controlled within the 3rd order MacAdam. Suitable for spotlights, see Figure 13.

Figure 14 shows an LED threaded concentrating luminaire designed with this light source. It has all the functions of the traditional bismuth tungsten concentrating luminaire. The spot angle range is l0-180°, and the spot uniformity is superior to the traditional halogen tungsten luminaire. Figure 15 and Figure 16 show the measured data of LED lamps and tantalum-tungsten lamps. The luminous flux of LED lamps is 76601m, and the 2kW tungsten-tungsten condensers are only 65561m.

It can be seen that the well-designed LED spotlights have no problem in replacing the 1kW-2kW xenon tungsten spotlights, and the functions and performances are comparable to those of traditional lamps.

1.3 imaging lamps

The imaging luminaire requires a very hard and strong light, can produce a very clear spot, uniform spot, few stray light, clear shadows during the control, precise control of the illuminated area with the visor, and projection of the pattern For text and graphics, LED imaging fixtures can also project slides with film because of the low temperature.

ETC's SourceFour imaging luminaire is the de facto standard for imaging luminaires in the film and television stage industry. Due to the cage-shaped filament structure and the efficient ellipsoidal reflective bowl design, the 750W SourceFour imaging luminaire has a light performance of about 101m/w. At present, some LED imaging luminaires on the market do not even reach the light effect of SourceFour 囟 tungsten imaging luminaires, and there is no energy saving effect at all. Therefore, how to improve the light efficiency of LED imaging lamps and achieve sufficient total light clearance has become a first consideration for designers. At present, the mainstream LED photography lamp adopts the aforementioned high power density LED module, designs an aspherical concentrating system, performs ray tracing and optimization through optical analog software, and adopts a multi-layer coating process to improve the concentrating system. effectiveness.

Using optimized design of 300W LED imaging luminaires, the luminous efficiency has reached 20lm/W, and the illumination has been very close to 750W ETC source four imaging luminaires, as shown in Figure 18. The measured data comparison is shown in Figure 9 and Figure 20.

It is worth noting that many LED imaging luminaires within 200W, including EDC's LEDSourceFour, claim to replace 750WETCSourceFour, there is still a big gap.

Figure 21 shows the comparison of the spot of the tungsten-tungsten imaging fixture and the LED imaging fixture. The left side is the tungsten halogen imaging fixture spot, and the right side is the LED imaging fixture spot. It can be seen that the spot uniformity and imaging definition of the LED imaging fixture can also be fully achieved. Even more than tantalum tungsten imaging fixtures.

1.4 sky screen / floor lighting

The sky screen/floor lighting is dedicated to the skylight lighting, which is a kind of special Zhu lighting. The lamps are generally about 2m away from the sky, and it is required to evenly illuminate the skylights of 8m-10m high. Since the illumination distances are very different, if the floodlights with symmetrical light distribution are used as the skylight/floor lamps, there will be a case where the upper and lower lights are dark. . Therefore, the general skylight/floor luminaires adopt asymmetric designing, that is, the light intensity near the screen is weak, and the light intensity away from the screen is strong, thereby achieving the purpose of uniformly illuminating the sky.

Traditional asymmetric designs are achieved with asymmetric reflectors. Since the skylight illumination requires a high degree of uniformity, a sandblasting or hammering reflector is generally used, which can eliminate the bright and dark light band, but at the same time, it also causes a great loss of light efficiency. In the design of LED canopy/ground luminaire, the free-form surface is fully reflective, and the ray tracing is performed by optical analog software. The reflective surface suitable for skylight illumination is optimized, and good results will appear. See Figure 22.

In order to reduce the loss of light efficiency, some LED lamps do not use traditional sandblasting or hammer-printing reflectors. Instead, an anisotropic mirror-reflective material is selected, and the horizontal reflection performance is the same as that of ordinary mirror reflectors. The direction can produce a certain degree of scattering. In this way, high reflection efficiency can be obtained, and unsightly bright and dark bands can be avoided, and a uniform package illumination effect can be obtained. The thin/ground luminaires designed with this technology. See Figure 23. With a power of 350W, the luminescence clock reaches more than 10,000 lm, which is 50% higher than the traditional 1250W 囟 tungsten canopy. Figure 24 shows the light distribution curve of the luminaire, which can clearly see the asymmetric light distribution effect.

Due to the anisotropic specular reflector, the asymmetric light distribution and light efficiency are greatly improved. Figure 25 shows a live photo of a 4.5m high background plate illuminated evenly with a skylight.

2. Cooling system

The design of the heat dissipation system of LED lamps is an important part of the success or failure of lamps. Especially for high-power, high-power density LED modules, the heat dissipation problem is particularly prominent. If the substrate humidity of the LED module cannot be controlled within a reasonable range, the performance and life of the lamp will be greatly reduced.

Figure 26 shows the thermal energy transfer path for LED luminaires. It can be seen that the hot fort generated by the LED chip (about 70% of the LED power) is transmitted to the atmospheric environment through the LED substrate, the heat conducting device and the heat sink. What the luminaire designer can do is to try to reduce the thermal resistance of each link to reduce the temperature of the LED chip.

It should be noted that the thermal resistance of each link is a series relationship, and the thermal resistance of each link must be reduced at the same time to reduce the thermal resistance of the system to a reasonable range. From the LED chip to the heat sink, there are good technical solutions, such as ceramic substrate, copper substrate, high-efficiency thermal grease and heat pipe technology, which can reduce the thermal resistance to a small extent. But in the last step, from the radiator to the atmospheric environment, there is no perfect solution. It can only be achieved by strengthening the convection and increasing the heat dissipation area, which will lead to the increase of the sound exposure of the lamps and the volume of the lamps. There is a misunderstanding here. Some manufacturers claim to adopt a new technology with high thermal conductivity, which can eliminate the fan and make high-power LED lamps in a small volume. According to the above analysis, this is a violation of physical principles. No matter what heat conduction technology is used, 70% of the power of the lamp should be dissipated from the lamp body. The most effective way to dissipate the heat is to eliminate the fan. The heat dissipation area must be greatly increased. The surface area is not enough to cause the humidity to be too high.

In fact, as long as you choose an excellent low-expansion fan and reduce the speed operation, you can completely control the noise within the industry standard. The noise of LED lamps is negligible compared to the noise of computer moving head lamps. Some news studios have high requirements for the sound control of luminaires. At this time, high-directional microphones can be used together. It has been proved that the production of such TV programs is almost unaffected.

Some manufacturers have added power control devices to the fanless LED lamps. When the LED light source temperature is too high, the LED power will be automatically reduced. At this time, the amount of light emitted by the lamps will be greatly reduced. This is not possible in professional studios. Accepted. The forthcoming cultural industry standard "General Technical Conditions for LED Stage Lighting" stipulates that: in normal use environment, when the light output is stable, the fluctuation should not exceed 5%.

The only way to fanless LED luminaires is to ensure a sufficient heat sink area. Taking a fanless 100W LED spotlight as an example, the size of the heat sink is 200mmx340mm, and the large heat column with a diameter of 40mm is used to ensure that the temperature of the LED module substrate is controlled at about 60 °C. Figure 27 shows an infrared test image of this luminaire heatsink.

Drive control system

In the professional application of the film and television stage, the stability of the color temperature of the lamps is very high. The color coordinates and color temperature of the LED light source will vary with the driving current, junction temperature and usage time. Therefore, LEDs usually require constant current driving and pulse width modulation (PWM) dimming to ensure that the color temperature (white) or wavelength (color) does not change when the LED is lit. Figure 28 is a comparison of the color mixing of the LED lamp and the tungsten halogen lamp during the dimming process. It can be seen that the color temperature of the LED lamp is very stable during the dimming process.

Due to the extremely high response speed of LED light sources, small current changes can cause significant brightness jumps. The traditional 256-level dimming will bring great flickering, and the LED is guaranteed by the TLD (Tungsten-LikeDimmer) and the brightness resolution of up to 65536 and the unique dimming curve. The luminaire has no flicker and jump phenomenon during the process from complete shutdown to 100% brightness (increasing and dimming), which achieves the dimming effect of the traditional heat source luminaire, see Figure 29.

For LED lighting for film and television stage lighting, in order to avoid the stroboscopic phenomenon of the camera shooting, the PWM frequency is required to reach 20 kHz or more. If the dimming resolution is required to reach 65,536, the PWM minimum pulse width is: 1/20000/65536s. =7.6E-10s, which is 0.76 ns, such a narrow pulse of general MCU and device is not possible. The perturbed PWM algorithm (SPWM) is adopted. The basic principle is to solve the PWM frequency by equivalently decomposing a single PWM pulse in one cycle into a narrow pulse of equal width, achieving 65536 resolution and frequency. Up to 29kHz PWM dimming algorithm guarantees no stroboscopic and smooth dimming control.

In order to facilitate the operator to quickly use the RGBW mixed light to call out the desired color, the HSIC (H-tone, S-saturation, I-brightness, C-color temperature) color grading system is also issued, and the adjustment principle is as shown in FIG. .

When adjusting H (hue), the color coordinates of the output light move along the triangle formed by the three colors of RGB, so that the desired color can be quickly found from the continuously changing red orange yellow green blue purple color; then the S is saturated Degree) Adjust the color depth, as shown in the line segment shown in Figure 30, from the continuous change of grass green to white. This makes it easy to call up colors of different shades and different saturations. When white light output is required, the saturation can be adjusted to zero, and then C (color temperature) can be adjusted to conveniently adjust the color wetness along the black body radiation. Figure 31 shows the PWM duty cycle variation of the RGBW LEDs when adjusting the color temperature.

Since the light efficiency and color coordinates of the LED will also change with temperature, as shown in Figure 32, and the thermal stability of the LEDs of different colors is different. Therefore, in order to improve the light mixing degree, temperature compensation needs to be added, according to the current substrate mixing. The measured value adjusts the calculation parameters in real time.

At present, LEDs are limited by the production process, and the consistency of color coordinates, light efficiency and other parameters is poor, which has adversely affected the precision of the light mixing control. Therefore, LED selection and screening is a problem. In order to improve the consistency of the luminaire and achieve higher quality requirements, it is necessary to calibrate the optical parameters of each luminaire, and correct the calculation parameters through the measured values ​​to ensure that each luminaire has consistent optical performance.

In summary, the professional lighting of film and television stage using LED light source must optimize the design of optical system, heat dissipation system and drive control system, give full play to the advantages of LED light source, overcome its shortcomings, and design High-quality professional lighting, pushing green lighting in the field of film and television stage development.

Conclusion

At the same time as the live video, the audio studio simultaneously exports the two-track audio file, and the production director broadcasts the sound effects and language dialogues. These ensure that TV viewers can enjoy high quality and true sound. The sound design of "China Good Voice" fully considers the environmental factors on the scene, and takes the auditory feeling as the first to be tired, so that the sound field cover is even and consistent, the sound is full, clear and natural. At the same time, the system security and reliability have also been fully designed to bring an audio-visual feast to the audience and the majority of TV viewers.