double the distance increases area x 4
Sharp short throw lens
The new generation of presenters, educators and marketing gurus have adopted the now commonplace combination of tools: the laptop and multimedia projector. While the manufacturers of the technology have managed to make these presentation tools smaller, faster, cheaper and brighter, they couldn't avoid the laws of physics. Accordingly, we must be aware of a few important issues that affect our ability to optimize the technology we have and be as effective as possible in delivering your message.
An important consideration with setting up multimedia projectors is throw distance. Throw distance refers to the distance between a projector and the surface (a screen) that the image is being projected on. The laws of physics will dictate that throw distance is based on:
Image size can be measured by width, height or a diagonal measurement from two opposing corners (top left to bottom right, for example). Throw ratio is expressed as a number (0 or higher) and is dependent on the lens design. The ratio can be found on the manufacturer’s product sheet. A calculation for throw distance is expressed in the formula:
Throw distance (D) = Width (W) x Throw ratio (TR)
If we have a 5’ wide screen and a projector lens with a throw ratio of 1.5, the throw distance will be:
D = 5’(W) x 1.5 (TR) = 7.5’
The throw distance will remain the same whether we are using front projection (projecting from the front onto an opaque screen surface) or rear projection (projecting from behind the screen through a translucent surface).
Most projectors today come with a zoom lens; allowing some limited variation in the throw ratio. Correspondingly, this allows us to vary the throw distance somewhat. Many projectors have a zoom lens providing a throw ratio of 1.75-2.0. The throw distance of these projectors, on a 10’ wide screen, would be 17.5’-20’.
The relevance of a projector’s throw distance lies in the relationship between the throw distance itself and the size of the room being used. If rear projection is desired, the screen will be positioned in front of the projector, thus reducing the available amount of space in the room for audience seating.
In the case of front projection, this will also affect the amount of space in the room. The triangular area from the projector to each side of the screen will be reserved for the projection of the image and unusable for audience seating.
Where a shorter or longer throw distance is required, a projector that can be fitted with different lenses can be used. Projectors with this versatility are generally higher priced models.
Throw distance also has an effect on the brightness of the image on the screen. The farther the projector is from the screen, the more spread out the light will be. This is where physics dictates how light behaves.
Inverse square law applies to both sound and light. In the case of light, the law specifies that if you increase the distance of the light source from the surface, the intensity of the light on the surface decreases exponentially. This is expressed in the following formula where I is the resulting intensity and X is the multiplier of the original distance away from the screen:
To illustrate this, consider a projector that is 10 feet away from the screen. If we move the projector to a position 20 feet away from the screen, we have multiplied the distance by a factor of 2. Using the formula above, the effect on the intensity can be determined:
In this example, the resulting intensity at 20 feet is ¼ or 25% of the original intensity at 10 feet. For this reason, as image size increases, distance from the screen increases and projector brightness must also increase.
Successful presentations involve more than slick PowerPoint presentations and the best laptop. Thinking about your desired image size and required throw distance will help in the selection of your projector and an appropriately-sized room for your intended audience.
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