A quantity measurement of light illumination from a light source. The original measurement was made using a "standard candle" placed at the center of a 1-foot radius hollow sphere. The light spread over a 1-square foot area of the sphere was 1 lumen. The surface area of the sphere is 12.57 sq. ft., so the candle is said to produce 12.57 lumens. One-foot lumen is equal to 1-footcandle (fc).
Lux
A metric quantity measurement of light illumination from a light source.
- 1 lux = .093 lumens
- 1 footcandle = 10.76 lux
Inverse Square Law
Mathematically, illumination from a light source varies inversely with the square of the distance from the measuring point. As an example, a light source produces 6000 lumens. At a distance of 10 feet, the light density would be 60 footcandles, and at 20 feet the light density would be 15 footcandles.
CALCULATION
- a) @ 10 feet = 6000 ÷ (10)(10) = 60 footcandles
- b) @ 20 feet = 6000 ÷ (20)(20) = 15 footcandles
Footcandle (fc)
A measure of lumens per square foot. As an example, a 6' x 8' screen receives 800 lumens from a projection lamp/lens. The average light density on the screen is 20 foodcandles.
CALCULATION
- a) 6' x 8' = 48 sq. feet screen size
- b) 800 lumens ÷ 48 sq. feet = 20fc
Color Temperature
Measured in degrees Kelvin (ºK). Lamps rated at 3200ºK produce clear white light. Lower temperatures will tend to give yellowish light. Color temperatures above 3200ºK with prolonged exposure may cause ultraviolet irritation to skin and eyes.
Electrical Formulas
Basic electrical calculations can be made for
VOLTS (E), E = W÷I: Amperes (I), I = W÷E: Watts (W), W = E•I. As an example, 10 fixtures with 500 watt, 120 volt lamps will require a 41.6 ampere current service.
CALCULATION: 10 fixtures @ 500 watts = 5000 watts
Using Amperes formula: I = W÷E, I = 500÷120, I = 41.6
Sufficient Brightness
"Sufficiently bright" has been defined by Society of Motion Picture and Television Engineers (SMPTE) in standard 196M as 12-22 footlamberts (41 - 75 cd/m2), though often 16 footlamberts is taken as the nominal goal.
However, this standard was developed for movie theaters with full light control. In a room with ambient light (i.e. light "leakage" from windows or adjacent areas), this level of brightness may be insufficient. As a comparison, a CRT TV measures approximately 50 footlamberts (200 cd/m2) [and peak luminance can be much higher], a LCD TV approximately 117 footlamberts (400 cd/m2), and many Plasma TVs approximately 175 footlamberts (600 cd/m2). A cloudy day outdoors is about 100 - 300 footlamberts.
It should also be noted that the eye's sensitivity to colors is strongly correleated to brightness, and a dark image is experienced as being washed out ("grayish"). This is because the eye's color receptors are less sensitive to light than the luminance receptors. Hence, increasing the brightness of the image gives it a more vibrant look, thanks to the better perceived color saturation.
Curved Screens
The purpose of a curved screen is to direct all the light that is projected to the screen back to the viewer. With a flat screen you will get light that bounces off the screen and bounces around the room. With a curved screen the vast majority of the light is bounced back to the source which results in a very bright image. Curved screens tend to have a very high gain value, i.e. a gain of 13 is common. A curved screen can get away with such a high gain because it essentially turns the entire screen into a giant hot spot so there is no visible hot spot. Curved screens are extremely bright and work very well.
Foot Lamberts
Foot lamberts relates to how bright the screen actually is. The ideal measurement is 11 fL with 10-11 fL good. For reference a direct view TV measures between 25-35 fL. You can get a good idea of the foot lamberts of a projector/screen combo using some simple math. Take the number of ANSI Lumens of your projector and divide it by the screen size in square feet (area), then multiply that by the screen's gain. For example a projector with an output of 400 ANSI lumens matched with a 100" screen (60" by 80" which is 33.34 square feet) with a gain of 1.3 will produce an image with a brightness of 15.6 fL.
REAR-PROJECTION SCREEN SURFACES
The fundamental difference between front- and rear-projection screens is that the front-projection screen reflects as much of the light shined on its surface as possible, whereas a rear-projection screen allows light to pass through its surface while reflecting as little light as possible in either direction. There is no reference standard for rear projection as there is in front projection with the matte white surface, so a little more research is required to determine the best surface type for your application. The following information will give you a good understanding of the basic elements and principles involved to aid you in making the right selection.
Common Formats and Their Aspect Ratios
Format
Aspect Ratio (Width/Height)
NTSC video
1.33
PAL video
1.33
HDTV video
1.78
Letterbox video
1.85
Cinemascope
2.35
35 mm filmstrip
1.32
2×2 standard 35 mm double-frame slides
1.50
SXGA
1.25
The most common method of reflecting light through a rear-projection screen is by applying a Fresnel lens surface to the back of the screen. The Fresnel lens was invented by Augustine Fresnel (pronounced fray-nell but most often mispronounced as frez-nel) in France in 1822. The lens is basically a prism with thousands of reflective surfaces that serve to focus and redirect the projected image. This lens was first used in lighthouse towers to increase the strength of the light shining out to sea. In the video projection application, the Fresnel lens technology consists of thousands of horizontal grooves, or angles, usually with a dot pitch of around 0.5 mm. This surface amplifies the light source (video) by redirecting the light rays and transmits the image forward through the actual screen material.
Transmission should not be confused with gain. Gain is controlled by the diffusion and governs the degree to which light from the projector is scattered. Transmission is reduced by the quantity of darkening pigment in the screen material and governs the total amount of light that gets through the screen. Obviously, the balance between diffusion and pigmentation is a delicate one, which is why such a wide selection of diffusions screens is available.
In addition to diffusion coatings, there are also “profiled” screens that are composed of lenticulations (geometric embossed patterns) or Fresnels. Lenticulations have no particular influence on uniformity of light distribution as Fresnels do. Although they are lenses, their only function is to scatter light about its angle of incidence. The difference, of course, is that lenticulations restrict their dispersion to the horizontal axis only. That results in excellent horizontal viewing angles but does not result in reducing brightness discrepancies between an image's center and its corners. The Fresnel lens is the only screen element that can improve uniformity.
Of the billions of light rays that come out of a projector at any instant, you can illustrate the function of a Fresnel lens by examining the path of just three rays. First, there is the on-axis ray, the one that is going to pass exactly through the middle of a screen. Then there are the outermost light rays on the left and right.
The angular direction of left and right rays are aimed far away from the direction of the on-axis ray. Therefore, as you sit in front of this projection beam, it will be particularly difficult to detect much brightness at all from these rays because they are not aimed anywhere near your eyes. The angles through which those outer rays would have to be bent in order to reach your eyes are called bend angles.
A Fresnel lens reduces these bend angles so that each light ray emitted by the projector is bent back just enough to be parallel with the on-axis ray. At the center of the projection beam, the Fresnel is not doing much work. But by the time you move out to the edges of the beam, the Fresnel is bending the rays through ever larger angles until you get right out to the “edge” rays where the bend angle is maximum. Notice that a Fresnel has its greatest effect at the very places you need it most: at the extremities of the image.
The original purpose of a Fresnel lens was to increase screen gain. Although it still does that, it's no longer the major consideration, because higher-brightness projectors are now available. The real value in a Fresnel lens today is its ability to make the corners and edges of an image less dim, which significantly reduces the brightness falloff from the center and thereby serves to increase overall uniformity. The process by which divergent light rays from the projector are bent so that they are all parallel is called collimation. No other rear screen property is more important to the critical question of image uniformity.
SOLID SCREEN OR PERFORATED?
All of these screen types, both front and rear projection, can be perforated to allow sound to pass through them. In movie-theater applications, it is common to have the majority of sound reinforcement coming from behind the screen. In smaller surround-sound applications and in some rear-projection applications, it is also common to locate the center-channel loudspeaker behind the screen. Many of the manufacturers mentioned in this article already have “acoustically transparent” versions of their screens available, and some of them offer custom perforating to suit your needs. Bear in mind that acoustically transparent is a vaguely defined term. Any solid matter placed in front of a loudspeaker will attenuate the signal to some degree, at certain frequencies more than others. It's important to find out what the attenuation level is for the screen you are using so that your audio system can be balanced accordingly, although these specifications are sometimes difficult to come by. Note also that any less screen material will affect reflectivity to some degree; that is why some cinemas prefer solid (nonperforated) screens, placing speakers outside the screen perimeter.
The selection of the most appropriate projection screen requires that you know many (if not all) details about the installation or application beforehand. Visit the Web sites of the manufacturers who produce video projection screens; you'll find much useful information that goes beyond the scope of what is provided here. Just as is the case with sound systems, consider the whole system in your decision making — remember that the room, the projector, the program material, and your customer's preferences and budget all play a role in these important decisions.
Screen Size Conversion Charts
Use the following charts to convert an existing NTSC video format screen size to either an HDTV or a letterbox format size.
Calculated using existing height
NTSC (1.33)
VIEWING AREA
HDTV (1.78)
VIEWING AREA
LETTERBOX (1.85)
VIEWING AREA
Height × Width
Height × Width
Height × Width
43" × 57"
43" × 77"
43" × 80"
50" × 67"
50" × 89"
50" × 92.5"
57" × 77"
57" × 102"
57" × 105"
60" × 80"
60" × 107"
60" × 111"
69" × 92"
69" × 123"
69" × 128"
87" × 116"
87" × 155"
87" × 161"
105" × 140"
105" × 187"
105" × 194"
Calculated using existing width
NTSC (1.33)
VIEWING AREA
HDTV (1.78)
VIEWING AREA
LETTERBOX (1.85)
VIEWING AREA
Height × Width
Height × Width
Height × Width
43" × 57"
32" × 57"
31" × 57"
50" × 67"
38" × 67"
36" × 67"
57" × 77"
43" × 77"
42" × 77"
60" × 80"
45" × 80"
43" × 80"
69" × 92"
52" × 92"
50" × 92"
87" × 116"
65" × 116"
63" × 116"
105" × 140"
79" × 140"
76" × 140"
A Screen With a View-Through: Transparent Screens
One of the most unusual and futuristic types of video screens are transparent screens — so-called holo-screens because they appear to produce holographic images. If you saw the Steven Spielberg film Minority Report, you saw Tom Cruise directing and selecting his precrime images on them. Certain attractions at the Disney theme parks also use this type of screen. They can be used in either front- or rear-projection applications, using special coatings to diffuse and reflect light from the video source. Glass or Plexiglas screens can be mounted, hung, or freestanding and are becoming a common sight at trade shows and in retail displays. Some manufacturers even offer a transparent screen material that can be applied to an existing glass surface, such as a storefront window.
One such manufacturer is the German company G+B pronova GmbH, which makes the HoloPro Holographic projection screen, which is a transparent projection surface for rear projection that can be used in any environment, regardless of ambient light conditions. In the absence of a projected image, the HoloPro appears to be just a pane of clear glass. The projection is directed onto the screen from a specially calculated angle and directed toward the observer by “holographic optical” elements. The company also offers the HoloPro Holographic Mirror screen, which allows front projection. Both are available in screens in sizes up to 67 inches diagonal.
How to Calculate a Custom Screen Size
Use the following formulas to calculate a custom size. The formulas will assist you in finding the viewing area only.
NTSC (1.33)
Video Format
HDTV (1.78)
Video Format
Letterbox (1.85)
Video Format
SXGA (1.25)
Video Format
Diagonal ÷ 1.667 = Height
Diagonal × 0.49091 = Height
Diagonal × 0.4762 = Height
Diagonal × 0.625 = Height
Height × 1.33 = Width
Diagonal × 0.87247 = Width
Diagonal × 0.88 = Width
Diagonal × 0.78125 = Width
Width ÷ 1.33 = Height
Height × 2.0395 = Diagonal
Height × 2.10 = Diagonal
Height × 1.60 = Diagonal
Height × 1.667 = Diagonal
Width × 1.14585 = Diagonal
Width × 1.135 = Diagonal
Width × 1.28 = Diagonal
Height × 1.78 = Width
Height × 1.85 = Width
Height × 1.25 = Width
Width × 0.561837 = Height
Width × 0.5405 = Height
Width × 0.80 = Height
Example 1
Calculate screen brightness when a 1000 lumens projector is used to project on a 6ft wide, 16:9 screen with a gain of 1.
- The screen height is 9/16 * 6 = 3.375ft.
- The area of the screen is 3.375 * 6 = 20.25 sq. ft.
- The brightness can be estimated to be 1000 / 20.25 = 49.4 footlamberts.
Is 49.4 footlamberts sufficiently bright? To see how "sufficient brightness" can be estimated, click here.
Example 2
Assuming the 16:9 screen with a gain of 1, what is the screen size limit for a 1000 lumens projector?
We want to achieve 16 footlamberts, i.e. 16 = 1000 / screen area. This implies that screen area = 1000/16 = 62.5 square foot.
The width of a 16:9 screen = 1.33 * square root of area = 1.33 * 7.9 = approximately 10.5 feet.
In other words, to achieve 16 footlamberts with a 1000 lumens projector the screen should be no wider than 10.5 feet (corresponding to a diagonal of approximately 12 feet = 124 inch).
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