The ability to see and hear a presenter and their material with the least amount of difficulty is critical to the successful design of a presentation environment. Often while there’s nothing wrong with the presentation equipment or the quality of the installation, something is still not quite right — and it detracts from the presenter’s ability to communicate his/her message.
Usually what is at fault is the size of the image on the projection screen or monitor, declares Steven J. Thorburn of Thorburn Associates, an acoustic and audiovisual consultant. “The most common problem here is that the image is too small.” But there are other common causes of physically poor presentations, he adds, including the viewing angles of the display and its location in the space.
Additional problems arise from not choosing the right screen for the job at hand, which today can range from intimate sales presentations to classrooms or boardrooms to large-scale meetings in auditoriums. To complicate matters, any particular situation might involve displaying one or more of a variety of sources including film or video, digital media, documents, overhead or slide projection, etc.
In terms of image size on the screen, Thorburn says, “the standard rule of thumb for sizing images is the four six eight (4-6-8) rule. This means the furthest viewer should be no more than 4, 6, or 8 times the image height away, depending on the material being viewed. Conversely, if you know the furthest viewer is 60 feet from the image, then the screen should be 15, 10 or 7.5 feet high, respectively, based on the 4-6-8 rule.
“The 4 factor is used when we need to inspect the image, such as a CAD drawing or a fine detailed map that requires close inspection,” he continues. “The 6 factor is used for reading or detailed viewing, such as spreadsheets or text with images. This is the most common size for presentation environments. The 8 factor is used for general viewing, watching a movie or images with few words.
“The 4-6-8 rule is derived from studies showing the minimum symbol height (i.e. text) the human eye can resolve along with a factor to account for viewers of varying visual acuity and varying viewing angles. No matter what the material, if you are doing a presentation, the 4-6-8 rule can be applied. Once the image height is known, the aspect ratio determines the screen width.
“For traditional video displays, the aspect ratio is 4:3 (i.e., 4 units wide by 3 units high or 1.33); for HDTV displays, the aspect ratio is 16:9 (i.e., 16 units wide by 9 units high or 1.78). So the 10-foot-high image mentioned earlier would be 13.3 feet wide for a traditional video display and 17.8 feet wide for an HDTV display.”
In terms of viewing angle, Thorburn recommends a “preferred” angle of “no more than 45 degrees to each side of the center of the image, or within a 90-degree ‘viewing cone’ centered on the image. This cone can be expanded to 45 degrees off the edge of the image for acceptable viewing areas. Using the aspect ratio above, you now know the width of the image and can determine the acceptable viewing area.
“So between the 4-6-8 rule and the viewing cone we have established that the acceptable and preferred viewing areas in any room have a direct relationship to the size of the image.
“We know how large the image should be but how big should the screen be? How high should it be off the floor? How far below the ceiling? Well, all of this depends on what or who may be blocking your view. If a short viewer can be seated behind a tall person, we have to either raise the screen, staggering the seating, or if in an auditorium, slope the floor.
“When you arrange the seats to create a greater distance between the viewer and the head directly in front of the viewer, you can lower the minimum height of the projection screen. Using a sectional drawing with tops of heads and eye locations drawn in (average top of head at four feet and eye at 3 feet 6 inches), you can quickly determine how high the projection screen needs to be off the floor to allow everyone a good view of the screen.
“In the end, the solution wil l be a compromise,” Thorburn notes, “but we have found for most spaces the bottom of the screen is at 42 inches above the floor, and the top of the screen is no closer than 6 inches to the ceiling.”
For front-projection displays, the type of presentation application has a major impact on the quality and viewability of a projection installation. Three basic types of media applications present quite distinct challenges, according to the Da-Lite Screen Co., which offers an online tutorial on projection technology:
Classroom/Business Training. This “classic application” is described as a setting in which a “medium-size group of people is seated around an instructor” with “presentation media as the focal point.” Typically, participants take notes and refer to desk materials, and “interaction and eye contact are vital in teaching and maintaining audience interest.”
Important factors in this presentation setting often include:
- Stray natural lighting through windows in multi-purpose rooms may not be fully controllable.
- Higher ambient light may be required for note-taking or reading at one’s desk.
- Variety of projection methods such as video, data, overhead or slides may be used on the same screen.
- Business/Boardroom Meetings. This common application is where “the main emphasis is imparting information and not for the express purpose of interacting with the audience during the presentation.” Typically, the rooms are “designed or earmarked for presenting,” with controllable lighting and seating arrangements.
Important factors in this presentation setting often include:
- Viewing angles for some attendees may be poor in small rooms or where there are many participants, sitting too close to the screen.
- Image brightness may suffer depending on the projection method and room size.
- Variety of projection methods may be used on the same screen, alone or in combination.
- Auditorium/Large Group Gatherings. This standard means of communicating to a large group may involve members of the audience standing or seated at a distance from the screen. Rooms used for large-group presentations can range in size and capability from those with built-in projection to reliance on portable methods, including areas commandeered for “impromptu or infrequent presentations.”
Important factors in these settings often include:
- Image quality may suffer due to wide viewing angles or distance from the screen for some audience members.
- Projector method/source must provide “substantial lumen output” in order to project an image of sufficient size.
Da-Lite identifies seven criteria for selecting the right screen surface, taking into account the factors noted above peculiar to each type of media presentation application. These criteria are:
- Projection Method. Standard methods include video (e.g., CRT, LCD, DLP, D-ILA, LCoS, 35-mm slide), overhead projection, and 3-D projection. (Due to polarization of images, the latter requires a much higher gain screen than other forms of projection.)
- Ambient Light Conditions. One must determine if light conditions can be controlled to minimize ambient light, as desired, and also if uncontrolled light (e.g., from ceiling fixtures) is directed at the screen causing the projected image to wash out.
- Display Brightness. To calculate the brightness of a display in foot-lamberts (luminance), divide the ANSI lumens of the projector by the square footage of the screen.
- Room Configuration/Size. In rooms of greater width than depth, some people may be forced to view the image at an angle greater than 30 degrees, requiring use of a special a screen fabric.
- Projected Image Makeup. Standard types of projection include: conventional video (not very fine detail), continuous tone images (used in all slide film projection), and digital or pixelized data (as found in LCD and DLP projectors) with fine detail.
- Projector Placement. When the projector (ceiling mount) is located higher than perpendicular to the screen center, a reflective screen is recommended. Screens with a projected angle below perpendicular demand a higher gain (e.g., from a beaded or retro-reflective screen).
- Screen Maintenance. Soiling or damage to the screen can occur from inadvertent or constant handling, such as in high-traffic areas or where there is a problem with airborne debris and dust.
Traditional beaded screens were valued for their ability to provide high gain and thus a visible image in situations where ambient light was present. These retro-reflective screens are still widely available, but they function best when mounted directly in front of a projector: the more gain, the narrower the viewing angle.
High-gain screens were desirable “in the very early days of video projection,” notes technology consultant Michael Heiss, “to compensate for the very low light output of the CRT projectors and optical systems in use at the time.” Today’s high-brightness projectors, however, have reversed the situation. “Some of the more popular screens in use today actually have a slightly negative gain, in the 0.8 to 0.95 range.”
The reason is to improve the perceived contrast, which “makes the difference in viewable images as much or more than brute-force brightness,” Heiss explains. “With the high light output available from today’s fixed pixel video projectors and less use of film-based projectors, gain is no longer as key a decision point as it once was.”
Today’s projectors have become bright enough to satisfy the human eye, even in relatively bright environments. However, modern single-lens projectors offer relatively poor contrast and black levels. Thus, “contrast” has become a buzzword in the past few years, replacing brightness as the most important image parameter.
Image contrast is defined as the difference in brightness between whites and blacks in an observed image. It can be expressed quantitatively in a formula that relates three factors: image brightness, screen reflection of ambient light, and projector contrast ratio. Image brightness (B) depends on the screen gain and size, plus the brightness of the projector. The screen’s reflection of ambient light ® depends on the ambient light level and the screen’s reflective index. And the projector contrast ratio (C) is listed in the spec’ sheets for each projector model.
The formula demonstrates that the most important parameters for the image contrast level (at typical values) are the screen surface reflectance of ambient light ® and the image brightness (B). Thus, the contrast level of most projectors is less critical than screen reflectance and brightness, and, paradoxically, image contrast will not be perceptively increased from a projector with a higher contrast level.
In short, the image contrast is mainly dependent on the choice of screen. Screen manufacturers, responding to the situation, have developed a range of new products with different screen gain levels and surface textures. “Regardless of the brand, you will typically find two models, each with a slightly different gray shade,” Heiss relates, which “allows the screens to deliver enhanced contrast and black levels, and to even counter the impact of some of the room’s ambient light.”
“The negative gain units are for use with ultra-high output units and installations where light control is good, while screens with a 1.1 or 1.35 gain sacrifice a bit of viewing angle in exchange for the ability to combat a bit of ambient light or deal with a slightly less powerful projector.”
Sources for this article include enewsletter (January 2004) from Thorburn Associates, Inc. ([email protected]), da-lite academy of da-lite screen co., inc. (www.da-lite.com/education), “screen plays” by michael heiss from systems contractor news (November 2003), and backstage (necember 2003) from dnp denmark as (www.dnp.dk).
