视频会议室灯光设计简介英文版.docx

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Designing Room Lighting For Videoconferencing
The new Design Guide from IESNA is the closest thing pro AV has to an international standard for videoconferencing lighting design.
Source: PRO AV Magazine
Publication date: December 1, 2005
By Tim Cape, CTS-D
While the ultimate goal in videoconferencing is good interactivity between near- and far-end rooms, one common lighting design problem can be summed up in a question that's remained constant throughout the evolution of videoconferencing technology: How can you provide good, comfortable lighting to optimize the camera and video codec operation in an architectural environment rather than in a studio? Although it's a fairly simple question, the answer can be quite complicated.
There must be some light in the room of the space you're designing just for the camera to produce an image, but making that image look good while maintaining a good projected video image can be easier said than done. On top of just getting the camera to produce a decent image, you also have to be concerned about what the codec will do to that image (particularly at low bit rates), putting more constraints on not just the lighting design but also the interior design of the room itself — once again going beyond the tenets of a basic AV system.
Up until now, AV professionals haven't had much consistent guidance on videoconferencing room lighting design, but the new design guide, offered by the Illuminating Engineering Society of North America (IESNA), gives them an authoritative place to start.
The new design guide
In development since 2002, the “Videoconferencing Lighting Design Guide,” DG-17-05, is now available for purchase through the IESNA website (..iesna.org). It includes a wide range of recommended criteria and general recommendations for approaches to lighting for videoconferencing facilities. It's specifically oriented toward small- to mid-sized videoconferencing rooms with a single camera axis (from displays to seated participants). It doesn't specifically cover dual-axis rooms that include a presenter near the displays or larger facilities, though much of the criteria can be applied to this wider range of rooms.
The Guide is intended to some extent for designers both with and without a background in lighting design.
The old design problem
Before answering the question presented above about how to optimize the camera and video codec operation in an architectural environment, you must understand several basic and sometimes conflicting requirements. And it's not just about the luminaires that emit the light, it's about the room, too. The primary tasks in terms of lighting are:

Light the participants for good video. This involves limiting the variation in light level enough to allow the camera to reproduce the full range of light and shadow in the picture. At the same time, there needs to be enough variation in the picture to keep it from looking “flat” with no shadow or depth.
Light the room to complement the participants. This means paying attention to the lighting for the walls, floor, and ceiling areas that might be in the camera view.
Coordinate the room finishes from the standpoint of color, contrast, and pattern, and how they relate to the people in the picture. This also involves consideration of the type of finish, and how it may affect the image picked up by the camera and transmitted to the remote site. For example, reflective, or specular, finishes such as chrome can create problems for the camera by creating an excessively bright spot in the image.
Sometimes there are more particular constraints placed on the lighting system when special cameras or higher than normal color rendering are needed for specialized applications like retail, fashion, and some military environments. However, the basics above apply to any videoconference room and are discussed in the Guide
The fundamentals
To create an environment that works well for videoconferencing, designers must address some fundamental lighting parameters. For those in pro AV, this may mean becoming familiar with at least the basic lighting terms you'll encounter in a lighting design project. While the AV designer may be qualified to be the lighting designer in some cases, it's more likely that he or she will be providing criteria to a lighting or electrical consultant who will then design the lighting under a separate scope of work. In either case, the AV designer needs at least some basic lighting knowledge.
To begin working with lighting, some basic lighting terms should be understood. Some of the typical terms used in discussing and designing lighting systems are related to those for projection and display technologies.
Illuminance is the light level incident on a surface or plane from a source or sources. It's expressed in lux (lumens per square meter — the preferred unit of measure) or footcandles (fc, lumens per square foot — the more obsolete unit). One footcandle equals 10.76 lux. It's useful to note that exact conversions should be used when reporting measurements, but when both are given as criteria a simple conversion factor of 10 is often used instead of 10.76.
Luminance is the luminous intensity from a surface in a particular direction. Luminance may be thought of in this discussion as the light reflected from a surface. It's expressed as candelas per square meter (footlamberts in the more obsolete English units).
Luminaire refers to a complete lighting assembly including the housing, reflectors, and lamps. Only when it's installed is this correctly referred to as a light fixture.
Light Reflectance Value (LRV) is a measurement often applied to painted and other room finishes indicating the percentage of light reflected from a particular surface independent of the color. LRV ranges from 0 to 100, with higher numbers indicating lighter finishes and lower numbers indicating darker colors.
Luminance ratio and contrast ratio refer to the mathematical ratio of the lightest area to the darkest area of a visual field of view. Lighting designers will often use the term luminance ratio in discussing room environments and mostly non-electronic illuminated surfaces. AV designers will use the term contrast ratio mostly in describing video projectors and displays. Their fundamental definitions, however, are the same. Color Rendering Index (CRI) is a measure of the effect a light source has on the perceived color of objects relative to being illuminated by a reference light source. CRI is applied to light sources and is measured on a scale of 1 to 100, where 100 is the most accurate color rendering.
Color Temperature (or correlated color temperature, CCT) is a measure of the color appearance of a light source. Measured on the Kelvin scale (K), so-called “warm” light sources have a more yellowish appearance and have a lower CCT (2,000 to 3,000 K), while “cool” sources tend to appear more white or bluish and measure above 4,000 K. Though physics majors will recognize that the Kelvin scale is based on star temperature in degrees Kelvin, lighting designers and the “IESNA Lighting Handbook,” another essential industry lighting publication that explains concepts, techniques, applications, procedures and systems, omit the word “degrees” when using the term.
The design approach
To create an effective lighting design, the fundamentals above are used within an appropriate design approach. For videoconferencing, the design approach is based on the concepts of photography and videography. Many will be familiar with the traditional key, fill, and backlighting elements used in these fields. While the fill and key light concepts apply well to the architectural/videoconferencing environment, the traditional backlighting technique isn't as practical. Instead, the concept of background lighting is used as defined below:
Key light is lighting from the side or above at an angle of approximately 45 degrees that creates the primary light source for the participants. This light is normally the brightest source and creates shadows on the faces to help with definition of facial features.
Fill light is the light that is intended to fill in the shadows created by the key light. Without adequate fill light, shadows under the eyes, nose, and chin can appear as solid black on camera due to the camera's limited dynamic range.
Background light is the light that covers the background in the camera's field of view, most notably lighting the videoconference room's back and side walls.
Each of these three elements must be properly ddressed to create a good videoconferencing picture. The difficulty comes in the application of these concepts to an architectural environment such as a conference room where studio lighting isn't an acceptable option.
Fortunately, there are many solutions. Several lighting manufacturers offer luminaires designed specifically for videoconferencing, most typically for providing good key lighting while providing good directionality to keep light off of the display area at the front of the room. There are many luminaires that provide good, even coverage of a wall without throwing light out away from the wall (often called “wall washers”). Standard luminaires that are designed for typical office areas can be used if the key, fill, and background lighting elements are addressed, but sometimes this involves a more creative solution in coordination with the architecture and interior design.
The basic criteria
Starting the design approach, we need to apply some criteria to each element to create a complete starting point for producing the lighting design package. There are a host of criteria provided in the IESNA Design Guide, but the most important ones have to do with the key, fill, and background light levels.
In general, the key light need not be above 500 lux, but should be a minimum 300 lux for most cameras to operate within their optimum range, given typical videoconferencing applications. Because the key light needs to be directed at an angle to the participants, it's important to use larger surface luminaires such as 1- by 4-foot or 2- by 4-foot fixtures where possible to reduce the potential for glare that can accompany point source fixtures such as track lighting.
Though fill light can come from the ceiling, more often we depend on the table surface to provide fill light on facial features, so the table surface at the participant's positions needs to be a lighter, neutral color to reflect light onto faces that are on camera.
The third basic criterion is that the side and back walls that may be in view of the camera need to be lit so that the luminance of the wall surfaces on camera is in line with the luminance of the faces in the foreground. The wall luminance is created by the combination of the light level striking the wall and the color and light reflectance value of the wall finish.
While background lighting can be relatively simple to accomplish on the back and side walls of a videoconference room, the background lighting is trickier when there's a presenter who is to be on camera and is located adjacent to a projected display at the front of the room. Most luminaires for this purpose are good at keeping light on the wall and out of the middle of the room, but lack of side control is a problem because having more light side-to-side can mean fewer fixtures to provide good, even wall coverage. However, this is at odds with what you need on the wall behind a presenter and next to a projection screen.
Don't forget to consider the display
Along with all of the other parameters, it's important to consider the type of display being used in the videoconferencing environment. While a direct-view display such as a CRT, plasma, or LCD monitor is very tolerant of light, front and rear projected displays require more attention to luminaire selection and overall lighting design. Many lighting designers are using indirect lighting approaches that work well for typical conference rooms, classrooms, and open offices without projected displays. However, this can be problematic when projected displays are to be used in a videoconferencing environment because the relatively uncontrolled light in these designs may put too much light on the display when light levels are set for camera use
The solution here is to limit the use of indirect lighting when projected displays are involved, using more controlled, louvered fixtures instead. Point source downlighting and track lights should be a last resort for primary participant lighting.
Other important issues
There's a plethora of other design information and criteria as well as some lighting fundamentals included in the 25 pages that make up the new IESNA Design Guide. Besides the issues noted in this article, lighting control, commissioning, cameras, and codecs are discussed to create a comprehensive guide beyond what we have had available in the pro AV industry to date. In addition, an IESNA ecommended Practice will be developed that provides even more depth and background based on the information in the design guide that should be completed in the next couple of years.
While the design guide isn't offered as a standard, it does contain comprehensive criteria that can be used as pass/fail comparisons for videoconferencing lighting applications. IESNA hopes that the new guide will provide lighting, AV, architectural, and electrical designers with a common basis for videoconferencing room lighting design.
SEEING THE DIFFERENCE
In the early days of videoconferencing, users were often just happy to have a picture that was being transmitted hundreds or thousands of miles. It wasn't until they used the technology for a while that users began to appreciate the difference between a “good” and a “bad” picture, which frequently had to do with lighting.
The video captures above show a cool white fluorescent overhead lighting scheme in the top photo compared to a warm white fluorescent directional lighting scheme in the bottom photo.
Although facial features can be discerned in both cases, notice how much visual information is “missing” in the overhead lighting arrangement due to shadows.
LIGHT DISTRIBUTION IN VIDEOCONFERENCE ROOMS
Below is a short checklist of lighting design considerations for a videoconference system and space. Refer to the IESNA Design Guide for complete detailed criteria and background information.
· Consider both key and fill light sources. The key light should be in the 400 to 500 lux range when measured with a vertically oriented light meter.
· Lamps of consistent color temperature should be used throughout the room that are compatible with the camera. The typical choice would be fluorescent lamps rated 3,000 K to 3,500 K.
· Be sure that back and side walls are evenly lit at about the same level as the participants (this may need to vary depending on how light or dark the finish color is