Brightness of projector

[Matthew Robinson]

So, as you may remember, a little while ago I was asked to do some projection for a production. I've not heard much since, but I have been slowly plodding away at sorting it all out in free time (sadly lacking ATM). I haven't discussed with the director, but I think the best way to do this is with a single screen, back projected. Because of the layout of the performance space, I think I have a 9ft gap to put the screen in. Working with the traditional 4:3 aspect ratio, this means that I have a screen 9ft x 6ft 9in and it will have a large space to be projected from. What I need to now work out is the brightness of the projector, which I have no idea how to do. Can anyone offer any advice for working out this kind of thing?

[ramdram]

You might be getting a tad too technical here MR. Find a projector and try it. Then get the Director to see the result and let the Dir make the call.

 

It may be you have to change the angles, distance, height, all sorts of things. Never ever forget that you cannot predict results with anything like 100% accuracy.

 

However, you, having set up the kit will have gained very valuable experience and will be able to recall the effect whenever. Don't forget to carry a notebook so you can make a note of exactly what you did, which plan worked and of course, what didn't.

[badweasel]

Hi Matthew,

 

Screen area: 9' x 9'6 = 60.75 sq ft

Screen gain: 0.5 (estimated)

Luminance: The observed brightness of a projected image measured in foot lamberts

Luminous Flux: Light output measured in lumens

 

SMTPE state 12-22 foot lamberts of luminance for a cinema theatre, where you don't have to compete against other lighting. On a stage, depending on light levels, I would consider 20-30 foot lamberts to be a good starting point.

 

Luminance = Luminous Flux * Screen Gain / Screen area

 

So you'll be looking for a projector of between 2400 and 3600 lumens. Remember that a lamp's output drops to 50% by end of life, so consider re-lamping costs when setting a budget for the projector if the production is on for a while.

 

Hope that helps,

 

Dave

 

PS as ramdram suggests, nothing beats actually trying it out and seeing how it looks.

[mac.calder]

It is very much a how long is a piece of string question. There are a number of factors, including the type of surface - large viewing angle requirements generally mean a lower gain surface to get best effect, if the angle is narrow, a high gain surface can be used. If you are using a bed sheet instead, then you are looking at a fairly shocking screen gain. Age of the lamp in the projector, how well you focus it etc all affect the end result. And in the end "Bright Enough" is subjective.

 

You will probably find 9ft wide screens fairly hard to find - they are not an off the shelf size. 10'x7.5' and 8'x6' are quite common though - I would suggest getting a screen like an AVStumpfl fast fold screen if your budget allows for it.

[ramdram]

Should have mentioned I am touring at the moment, we use a projector which is used simply to inform the audience of the locale in which the scenes are set, so to speak.

 

To use the old saying, so beloved by the football pundits, "at the end of the day" (literally of course), the Dir is just happy to get ANY image, ANYwhere on "a" screen, ANYwhere on stage which is not occluded by any business. Added to that of course is that some venues don't do DBO owing to the curtains across the windows (in a VH, say) being for privacy and not total light exclusion. Bright midsummer evenings are a pain, especially if the long side of the VH faces west.

 

Brightness per se is sacrificed for the audience being able to see the captions. Sometimes we use our "proper" screen and sometimes on a small stage it may be a sheet placed where it will fit. Believe me when I say whoever invented "keystoning" is a genius who must in all conscience ascend directly to heaven:

 

http://www.projectorpeople.com/resources/keystone-correction.asp

 

It may be you could site your projector nearer to the screen and rely on the keystone function. It may be you have to compromise as said earlier and use a smaller screen perhaps?

 

You will take especial note that the final decision is for the Dir. Saves faffing around when you may have the lanterns to focus and specials to be cobbled in as best as the venue allows. Probably the best way to think of the whole deal is compromise.

[mac.calder]

 

Brightness per se is sacrificed for the audience being able to see the captions. Sometimes we use our "proper" screen and sometimes on a small stage it may be a sheet placed where it will fit. Believe me when I say whoever invented "keystoning" is a genius who must in all conscience ascend directly to heaven:

 

 

 

Without wanting to jump too far on the puritan band wagon - whoever invented keystoning on projectors surely has a special place reserved for him in hell - next to child molesters and those who talk at the theatre. The only thing worse than keystone is when people use sideshot. There are very few excuses for the use of sideshot (aka "Horizontal Keystoning")

[ramdram]

You have not done a lot of touring, then, MC? We are grateful the facility exists at all believe me. In fact I have now decided the keystoning inventor should be on the short list for archangels.

[mac.calder]

I do corporate - so yes, I have toured shows and set up more projectors than I would care to count in horrendously designed venues with shocking rigging and difficult conditions. Decent projectors will give you about 50% lens shift vertically and about 30% horizontal, and if that does not work, I generally try to move hell or high water to avoid having to keystone an image. Sometimes it is unavoidable. But largely, I have found it just tends to make a lot of technicians lazy.

[ramdram]

Ah, well, now MC, touring the VH circuits in deepest Cornwall is what I was banging on about. Not enough room to swing a Manx cat on some stages, let alone build a set, shunt some actors in their positions...and find space for a screen of some description. Plus being offered anything from a beer crate to a card table to the apron for the projector position. We could not do the caption thing without the keystone function.

 

It certainly does not make for laziness...more like desperation to settle for any compromise as 7PM arrives and the doors open. Still that is half the buzz anyway...but there always space for the projector and screen in the van during the get out, ** laughs out loud **.

[AHYoung]

With regard to brightness, id aim for at least 3000 ~ 4000 lumens and you should be ok, but there are so many variables all of which interact that the only way is to try and see.

 

With regard to Keystoning - using any keystoning when its perfectly possible to simple put the unit in the correct place is the mark of the lazy tech, and as such less than ideal and could be described as the devils work

 

However in the real world of theatre touring , Its going to get used. what you really want is corner keystoning A la Sanyo Xu106 etc. That way if the light out of the projector can hit the screen, the image can be square and that includes projecting onto the centre from the wings etc. Its going to be less than ideal optically , but if you cant put the unit anywhere else and have another 100 things to do, you have a show. Id never use it unless I have too, but where its needed Its invaluable.

[Matthew Robinson]

Ok, thanks for the replies. I was planning on using something like this. As you can see, it is 2.8M wide, so just over 9ft wide, and can be cut down to the necessary height. My problem is that with it being a university thing, and having to hire the projector, I only have one shot at getting the right one. Otherwise, I would just try it and see, and swap out the projector if necessary.

[AHYoung]

In my experience Rosco Twin white is equally bad as either a front or rear projection surface, and its only saving grace is you can use it for either, but the same could be said for a bedsheet.

If you want rear, A Front and a rear surface are 2 very differnent things and I thing that does both is going to be a massive compromise

 

The standard Rosco Grey is pretty good and if you only need 2.8m wide, its a pretty cheap way of making a screen. The black is very good as a high contrast screen and will pick up much less of the stage lighting, Personally Id buy Harkness stuff but its not available off the roll in the same way rosco is.

 

 

I once made a load of 1.8 / 1.4m black screens by attaching the material to lengths of 20mm timber batton with drawing pins and they must of cost about 20 quid each and looked pretty decent. If you get a good screen material the DIY route can be very effective.

 

 

[Matthew Robinson]

Ok, thanks for your reply. I will bear that in mind. I'll ask the director what colour is better for him, and choose whichever he says he prefers. Have you any experience with the sky blue and other similar colours?

[AHYoung]

Ok, thanks for your reply. I will bear that in mind. I'll ask the director what colour is better for him, and choose whichever he says he prefers. Have you any experience with the sky blue and other similar colours?

 

The point of the screen is that it shows the projected image without any artifacts, ive never projected onto a colour through choice and when I have, ive needed to alter the colour balance to get it looking respectable. Screens arent coloured and rangle in the geys from white to black, white/ light grey generally for front and darker grey - black for rear. The point of a screen is you dont look at the screen, you look at the image. Then again if the projections only used a few times in a show, the look of the bare screen is probably more important. Basically outside cinema there are no real rules, just vaque best guesses that will become more accurate the more experience you get, the only way to know is to test it...