Projector Throw Distance

[fordgt40]

I am just about to construct my screen. It is 2 metres (6ft 6in) in diameter and I am using three short throw Optoma projectors with a throw ratio of 0.5,  similar to a number of other members. In calculating the various parameters I find from this forum and others that there is conflicting advice about how to calculate the throw distance. Specifically:-

1) One member strongly advises that a common mistake is to only use the distance from the projector to the chord (see attached drawing). He advocates that the correct throw distance should be projector to the chord plus the sagitta (distance from the chord to the arc.

2) Other members simply ignore the sagitta and just use the distance to the chord

Simply, if I was to follow the advice in option one above, then despite a good throw ratio of 0.5  I would find it almost impossible to fit the projectors within space available.

I did ask advice from  the supplier of warping software, but he seemed more interested in getting me to spend 100 Euros on his screen planning software rather than simply giving advice on option 1 or option 2!!

I would be very grateful to be reassured from members who have adopted option 2 and successfully  managed the geometry  etc.

David

[Flying_Fox]

Hi David,

Option 1. 

I did not ignore the sagitta.
In fact it's very easy to actually measure the desired throw distance (screen radius) even before making the circular screen.
Project the image to the flat wall. Adjust the image size and focus to your liking. Measure the distance from the lens to the left or right edge of the screen. That would be your screen radius.



Nick

[MCrevot]

Hi David,
i am affraid but the right way is the first one ! the projectors are intended to be used on a flat screen, that is the reason why when you use a curved screen the image is distorted on the side, because on the extreme side the distance to screen is diminished by the sagita !

See my subject http://www.cockpitbuilders.com/community/index.php?topic=4935.msg36162#msg36162

Michel

[sagrada737]

Hi David,

It is a lot of fun to build up a projection system for one's Sim.  The technical terms to consider can be confusing.  Not to oversimplify things, but there are a couple of practical consideration you may want to think about...

Firstly, the Optoma HD projectors are a good value for the money.   The throw ratio of 0.5 is just about right for our Sim display application using multiple projectors.  From your description, your screen has a "diameter" of 2 meters (6.56 ft.).  Since you are using three projectors, it must be that the 2 meters represents the radius.   Is that correct?

In any case, the physical mounting configuration and distance from the screen is a difficult consideration to deal with.   It boils down to what size projected display you want, and how many degrees you want to cover.  As mentioned, you will need alignment/blending software to accomplish this.   

With projected displays, you will need reasonable overlap of the projected images in order to blend them well - typically a minimum of 15 degrees overlap.  However, this is somewhat subjective and can vary quite a bit, depending on what level of quality you are looking for in the blended images.  This usually means that a 120 degree display image with the three projectors yields a good result.

The projector to screen distance has practical considerations in that you can only get the projectors physically so close to each other without having mechanical interference.    In the straight projection mode (L_C_R), the projected screen image will be smaller.   In the cross-over mode (R_C_L) the screen image will be larger.   Each has its own projector mounting considerations, that is further complicated by what size of Sim obstructions you may have, eg. Nose Section.

As a real-world example...   For my Sim, I use three Optoma HD GT1080 projectors in a straight projection configuration.  My screen is 12' in diameter (3.66 meters).  My projector to lens distance is 62" with the center of the lens 76" above the flight deck, with all projectors mounted level.  This means that the Optoma projectors are about as close together as they can get.  In my setup, I have the projectors arranged to yield a 220 degree projected screen image.   This means that the blended overlap is fairly small (about 5%), which does not yield a beautifully blended screen image.   However, since I like the 220 degree wide view, I don't mind the minor blending issue.  You see...  This kind of thing is subjective to the individual taste for what one wants for their Sim display.  In my case, I find the 220 degree offers a greater sense of immersion flying the Sim.  In your case, you may want a higher quality of blended display image.   And so it goes...

Note:  There are cooling issues with the Optoma projectors.  Having them so close together means that the hot air exiting one projector gets sucked into the imput of the other projector.   It is advisable to make deflectors out of cardboard or some material to deflect the hot air upward, away from the input of its nearby projector.  This is not as much of an issue in a cross-over projector configuration.

Once you have decided what kind of display size you want or need, then you can work out the projector location.  You will not have a lot of flexibility in this area, as projector location will dictate the image size, and projector rotation will dictate final image overlap.   These considerations must be dealt with and finalized before you can set about aligning the projected images.

A projector calculator program is a valuable tool, as it can save you a lot of grief in build up the wrong setup.  That said, I have tried various projection image sized and projector orientations.   In my case, what I liked the best was the 220 degree aligned/blended display, which yielded a 48" image height.

I might point out that the physical mounting of the projectors must be fairly solid so the projectors don't move around.  Not all the projector mounting gizmos do a good job of this.  The more solid the projector mounting, the more stable your aligned and blended display will be.

Lots of varying opinions on projection displays...  Much of all this depends on what you want to accomplish for your Sim and what makes you happy with the final display results.

Mike

[fordgt40]

A quick thanks to all who have replied so far. I am off early am to a family wedding in Nice, so will get back to you all on Monday/Tuesday

Regards

David

[FredK]

One other very important consideration......

My strong recommendation is that you devise a mounting system for your projectors that will enable their physical movement (that is, fine position adjustment) back/forward and side/side after mounting.  There are a couple of reasons for this.  I am currently using 3 Optoma 1080 HDs.  I can tell you that the actual throw ratio differs slightly for each projector...the throw spec is just not that precise, particularly for these Optomas which are basically cheap projectors.  So forget about convulsing over what the exact distance should be. Use such only as a starting ballpark planning number.

Optimal screen projection fill can then be achieved by simply moving the projectors physically forward or backwards, etc.  You will also be able to play with the overlap area to your liking. Your warping software will take care of everything else of course once the projectors are positioned to your liking.

My basic message is do not constrain yourself to a calculated fixed location for your projectors.  For my setup I devised construction of a rail system that enables me to physically move my projectors forwards or backwards.

Fred K

[Flying_Fox]

I second that. My projectors are also rails-mounted as you can see here:
http://elephantair737.blogspot.ca/p/projecto.html

[sagrada737]

Nick - That's a great write-up on your projection system!  It is interesting to see the even if the "projector cluster" has projectors stacked over another projector, that it is possible to align the projected images.  This approach will certainly yield a slightly larger screen image.

I totally agree with the "rail" mounting approach - especially for experimenting with projector configurations.  However, once the projector cluster is finalized, it is possible to fix mount them in place, then conduct the final align and blend.

Everyone does it differently, so there is not exactly a right or wrong way to mount projectors.  In the end, it is what works that counts.

Mike

[fordgt40]

All

Many thanks for the advice. It is now finally clear to me that the projector throw distance is from the lens to the arc, as explained by Nick and Michel. Also, thanks Mike for your comprehensive advice - by reverse engineering your data it is clear that you used the same method as the others. Mike`s warning about heat is also very helpful.

Fred and Nick (again!) thanks for the suggestion to mount the projectors on rails, I can see the benefit in that.

I am just about to finalise my screen and start building, the timber arrived yesterday. So after years of work the finish line will be this November, time then for some photos!

Regards to all

David

[iwik]

David,
Will be very nice to see if what i saw from your O/H build.
Look forward to that day like im sure you must be.
Les

[RayS]

...I TOTALLY misunderstood this thread title........

[fordgt40]

Please share what you were thinking