Since a previous query on this forum indicated that currently-available lamphouses are at least f/2.0, the lenses are the element that's most likely to become a bottleneck, if not selected wisely (given proper optical alignment and components, of course).

The f-number is a mathematical statement, relating to the angle of light that a given component will pass. Just like in an SLR camera.. the higher the f-number, the smaller the aperture; the slower the lens, i.e.- less light will pass. However, with slower lenses comes greater depth of field and vice-versa. With very fast projection lenses (on the order of f/1.6 or less), it can be difficult to keep a projected image in focus.

The best way to illustrate the importance of matching the various elements in an optical train, is to think about looking down the centers of several, identical donuts, stacked one on top of the other. Now place a donut with a smaller hole anywhere in stack, and you will only see the small opening. This is how a mismatch can limit the performance of the whole system.

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Better Projection Pays!

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Better Donuts Pay!

I forget the screen width/recommended bulb size rules of thumb, but I wouldn't chance 2kW past 35-40 ft wide in a drive-in. And 3kW should only be good for drive-in screens up to about 48-50 ft. Any wider than that, and count on going with 4kW.

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Better Projection Pays!

I will just add that a very rough rule-of-thumb was to take the height of the screen squared times 12 to find suggested wattage.

15 foot high screen

(15x15) x 12 = 2700 watts

In actual practice, I find this to give wattages that are a little low, especially with focal lengths below 80mm or so. Remember, though, it's not a formula for actually buying equipment by.

Your "rule of thumb" pretty much agrees with mine of "5 watts per square foot of scope format screen area":

15 ft high x 36 ft wide = 540 square feet

540 square feet x 5 watts = 2700 watts

This "rule of thumb" is based on the Strong slide rule, and assumes a two blade 90-degree shutter, normal (f/2.0?) lens, matte screen with gain = 1.0, and normal efficiency. A properly curved gain screen requires less power, depending on the gain.

Glad our thumbs are the same size.

JP: So, this Strong sliderule isn't just a lens computer? That's pretty good about the 5 watts... I'm going to have to remember that.

I checked a few manufacturer's web site, but did not see "projector efficiency" or "speed of lamphouse," or other numbers that would be required.

The "rules of thumb" or tools like the Strong "Lens Selector and Picture Size Computer" slide rule are good starting points. I usually err on the side of specifying the next lamp size up, because it's easier to defocus the lamp and get better uniformity, than to struggle to get 16 footlamberts with an undersized lamp.

Working with an equipment dealer, the design specifications should state that the system will consistently deliver 16 footlamberts. They can then work with you to achieve it on reasonable sized screens. (For a 30 x 72 foot screen, don't expect to meet SMPTE standard with 35mm prints).

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