[today]

I was previously considering an Infocus LP350. But now I am interested in checking out the NEC LT150 since it has gotten some good press here. First, I need to figure out if it is going to work in my house without zoom control.


Since I will be installing this unit in a room with a cathedral ceiling, the mounting distance is pretty fixed. It has to be between 19' and 20' away from the screen. I used the throw calculator at ProjectorCentral. Typing in 20' gave me a 147" screen! Argh! At most, I can do a 9' (108") screen, 8.5' (102") would probably be the biggest tensioned pull-down screen I could do.


The PDF on this product claims a projection angle of 17 degrees. I don't know exactly what that means, but I think that it means the bottom of the picture shoots directly straight out of the lens and the top of the picture shoots up at a 17deg angle from the lens.


Going back to high-school trig, we can figure out the height of the image at a 20' distance by multiplying that distance by the sine of 17 degrees. That gives us 70". Now, the NEC is XGA, meaning 1024x768, or an aspect ratio of 4:3. Applying this aspect ratio to the height, I get 93.5". This is *QUITE* a bit different than the 147" claimed by the calculator and well within my requirements.


So, who is right? How is this calculation supposed to be made? Backcalculating the ProjectorCentral number of 147", I would get a projection angle of over 27 degrees.


Anyone who owns one of these... could you please post the distance to the screen you are using and the resulting height and width of the image?


If it doesn't look like the LT150 will work at 20' out of the box, are there any fixes I could apply to the lenses to make it work?


Thanks for any help!

-todd-

 

[Halcyon]

I have mine about 10 feet away projecting a 100" diagonal (80" x 60").

The spec sheet in my manual indicates that at 130.7 inches, you get a 100" diagonal screen. (so i guess that's how far mine is).


As for 20 feet, that's 240", and that would make a 180" diagonal screen (actual throw distance is 235 inches).


The full spec sheets on throw are:

(Throw distance in inches / screen size diagonally in inches)

47.2 / 36

52.8 / 40

78.7 / 60

104.7 / 80

130.7 / 100

157.1 / 120

196.1 / 150

235 / 180

261 / 200

 

[today]

Halcyon -

Quote:

As for 20 feet, that's 240", and that would make a 180" diagonal screen (actual throw distance is 235 inches).

Thanks for the information. Looks like the calculator at ProjectorCentral was right. So I guess the LT150 won't work for me, then. Too bad.


I'd still be interested in knowing what "Projection angle" means, though...


-todd-

 

[today]

Okay, I now know where the confusion on my part lies. This document
http://www.nectech.com/presentationp..._throw_v16.pdf

clears it all up. Check out page 2.


Basically, the projection angle is the angle between the center of the lens and the center of the screen. This angle actually only tells part of the story of screen height and cannot be used by itself to calculate this dimension.


-todd-

 

[rlsmith]

A bit more about the projection angle.


Projection angle refers to the angle between a horizontal line drawn from the projector to the screen and a line from the projector to the center of the screen.


If the projector has direct access, as it is sometimes called, then this angle would be zero. 0 would mean that you aim the projector directly to the center of the lens.


Many business projectors assume that you are sitting the projector on a table aimed at the screen (or, alternatively, hanging the projector from the ceiling). In such a case you need to have a non-zero angle built into the projector, or else you will get the "keystone effect".


A projection angle of 17 degrees is fairly aggressive and indicates a fairly wide-angle lens, which the LT150 does in fact have. It has a projecton ratio of about .76, which means the ratio between the screen size and the distance from the screen. In general, the bigger the projection ratio, the bigger the projection angle will also be, because manufacturers will configure the projector to be able to sit on that table. (The LT-150 has a fixed focal length lens (i.e., no zoom) so there is only one projection ratio).


You can compute the distance from the screen for a given screen size with the projection ratio. You can also use the projection angle, with a bit of trigonometry, to figure out what the offset must be to avoid a keystone effect.


Some projectors have a digital keystone correction BTW. You will not want to use it with video because it looks absolutely hideous.


Hope that this helps.