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short throw projection problems

#1 User is offline   Impact Pete 

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Posted 10 April 2012 - 10:44 AM

Right any help and advice would be great, Iím trying to solve a projection issue without spending silly money, currently installed is a Hitachi cp x1250 with a short throw lense that projects onto a 3m wide rear projector screen.

The main problem with the short throw is that there is a reasonably bright image in the centre and around the edges of the screen the image is dull. On top of this the lamp is running at at least 2/3rds of its lamp life.



Options are replacing lamp at around £400 and not gaining much, or buying a long throw lense if still available at at least £1000 and projecting from the front which would mean buying a front projection cover for the screen frame another big expense.



So ideally buying a new rear projector with a lot more lumens would be best with a short throw lense.



If this was the option and I had a new decent projector with better spec would I still have the bright spot in the middle of the image?



What would be a good projector for this and a rough cost?



I think behind the screen there is approx 3m only.



Thank you



#2 User is offline   timsabre 

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Posted 10 April 2012 - 11:14 AM

Is the screen a proper rear-projection surface? You shouldn't get a hotspot with a proper RP screen.


I've got a CPX1250 and we've had lots of trouble with the lamp going dim... and Hitachi said it was a common problem with that type of lamp.

This post has been edited by timsabre: 10 April 2012 - 11:17 AM

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#3 User is offline   ferretwrangler 

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Posted 10 April 2012 - 11:15 AM

Hi Pete,

There are two issues here I think.

Hotspots are common with most rear projection scenarios they can be mitigated by the type of screen surface - there are solutions out there which have differing Q/gain values which will reduce hotspots - generally they will also result in reduced apparent brightness. Also the use of wide angle lens' and their close proximity and required central positioning will also make the hotspots worse. Ultimately theres no cheap solution that I can see. Front projection with a decent surface will not exhibit hotspots but would require a complete overhaul. A different RP surface may help but its hard to tell without knowing what you have now and trying alternatives. Possibly suggest hiring a 12x9 fast fold screen from an AV company and seeing if that surface has better results. I don't see any advantage re-lamping the projector with this issue.
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#4 User is offline   Impact Pete 

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Posted 10 April 2012 - 11:39 AM

View Postferretwrangler, on 10 April 2012 - 11:15 AM, said:

Hi Pete,

There are two issues here I think.

Hotspots are common with most rear projection scenarios they can be mitigated by the type of screen surface - there are solutions out there which have differing Q/gain values which will reduce hotspots - generally they will also result in reduced apparent brightness. Also the use of wide angle lens' and their close proximity and required central positioning will also make the hotspots worse. Ultimately theres no cheap solution that I can see. Front projection with a decent surface will not exhibit hotspots but would require a complete overhaul. A different RP surface may help but its hard to tell without knowing what you have now and trying alternatives. Possibly suggest hiring a 12x9 fast fold screen from an AV company and seeing if that surface has better results. I don't see any advantage re-lamping the projector with this issue.



Hi the screen I have is a very good quality screen, aluminium frame with black velour edging with the proper rear projection fabric stretched over, this was not done in anyway cheaply can't remember the exact make but it is very pro.

#5 User is offline   ferretwrangler 

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Posted 10 April 2012 - 12:48 PM

have a read of this to explain different types of RP surface and their relative advantages.LINKY

High gain surface = bright image but potential hotspot.

Since most WA lens' require to be at the centre of the screen it often results in hotspots as the light source is directly behind the surface and scattering is not occurring. On a longer throw it is possible to have the projector higher/lower and as a result increase the scattering effect which reduces the hotspots.

Have a read what Da-lite have to say about hot spots LINKY

Great article explains that all RP surfaces are not equal
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#6 User is offline   AHYoung 

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Posted 10 April 2012 - 05:46 PM

We own 4 cpx1250s and I see no reason for the uneven brightness other than the fact that its a inappropriate choice of BP. Generally what you need is a Low gain surface, generally about 0.6 is a good compromise between brightness, viewing angles and hotspotting. You say that the unit is at 2/3rd its lamp life- Replace it as these units often will be at about half brightness by this point, also with regard to pricing, dont know where you get 400 quid from, but we can supply an original lamp for not much more than half that {plus vat} which would be closer to the going rate.
The Cpx1250 is a decent if long in the tooth unit and if its panels are unburnt { no blue or yellow tinge} and its fairly clean - focus to the extremes to check for dust, its worth persisting with. You wont get a better short throw solution for under 2- 3K, it sounds like your problems are down to and aging lamp and wrong bp surface rather than the projector. Im assuming the unit is on axis ie half way up the screeen without any lens shift, basic stuff but if its not that would explain a lot.

#7 User is offline   Impact Pete 

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Posted 13 April 2012 - 12:31 PM

View PostAHYoung, on 10 April 2012 - 05:46 PM, said:

We own 4 cpx1250s and I see no reason for the uneven brightness other than the fact that its a inappropriate choice of BP. Generally what you need is a Low gain surface, generally about 0.6 is a good compromise between brightness, viewing angles and hotspotting. You say that the unit is at 2/3rd its lamp life- Replace it as these units often will be at about half brightness by this point, also with regard to pricing, dont know where you get 400 quid from, but we can supply an original lamp for not much more than half that {plus vat} which would be closer to the going rate.
The Cpx1250 is a decent if long in the tooth unit and if its panels are unburnt { no blue or yellow tinge} and its fairly clean - focus to the extremes to check for dust, its worth persisting with. You wont get a better short throw solution for under 2- 3K, it sounds like your problems are down to and aging lamp and wrong bp surface rather than the projector. Im assuming the unit is on axis ie half way up the screeen without any lens shift, basic stuff but if its not that would explain a lot.



Hi thanks for input, we have had it for 10 years only had hot spot since using short throw lense.

It's a fairly good projector but we have had the usual filter needs cleaning sign pop up, and even via the service menu was unable to switch off, it is cleaned every two months and all boards are as should be

We are going to place a laptop nearer the projector to eliminate cables and switcher boxes, then test with a light meter, I was thinking of a fairly simple ppt
slide that has a white spot in the middle and R, G and B spots around it. We can then take a measurement in the middle of each spot with the sensor
touching the screen and the normal stage and house lights on (to reduce the variation due to natural light brightness) , then we will replace the lamp with a Genuine makers lamp which we purchased for £200 inc vat and delivery and try again with the light meter etc.

#8 User is offline   AHYoung 

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Posted 13 April 2012 - 07:32 PM

Sounds to me that the screens at fault ? im assuming that you replaced the screen when switching to short throw RP, If you use a decent low gain Bp screen with the projector on axis, you shouldnt get a bad hotspot.

Ive got a cpx1250 that has the exact same fault with the filter warning. As a result it has a bit of tape over the 2 warning lights, and its probably done less work than any other unit as you cant hire it out as when you plug it in it give a dire warning on screen and the flashing lights never stop, which worries the paying customer somewhat ever though it works fine. It never got fixed any of the times I sent it back under warranty, so I doubt there is a simple fix...

Also im highly sceptical that a lamp purchased for 200 inc vat delivered is original . we sell a lot of lamps, and thats below cost of on original hitachi DT00601 lamp module. The world is awash with fake lamps, and I get to see a fair few that customers bring in that simply dont fit . An original Hitachi lamp module includes a few sheets of bumf and a spare filter module in the box, the fakes generally dont.

#9 User is offline   Impact Pete 

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Posted 15 April 2012 - 04:18 PM

View PostImpact Pete, on 13 April 2012 - 12:31 PM, said:

View PostAHYoung, on 10 April 2012 - 05:46 PM, said:

We own 4 cpx1250s and I see no reason for the uneven brightness other than the fact that its a inappropriate choice of BP. Generally what you need is a Low gain surface, generally about 0.6 is a good compromise between brightness, viewing angles and hotspotting. You say that the unit is at 2/3rd its lamp life- Replace it as these units often will be at about half brightness by this point, also with regard to pricing, dont know where you get 400 quid from, but we can supply an original lamp for not much more than half that {plus vat} which would be closer to the going rate.
The Cpx1250 is a decent if long in the tooth unit and if its panels are unburnt { no blue or yellow tinge} and its fairly clean - focus to the extremes to check for dust, its worth persisting with. You wont get a better short throw solution for under 2- 3K, it sounds like your problems are down to and aging lamp and wrong bp surface rather than the projector. Im assuming the unit is on axis ie half way up the screeen without any lens shift, basic stuff but if its not that would explain a lot.



Hi thanks for input, we have had it for 10 years only had hot spot since using short throw lense.

It's a fairly good projector but we have had the usual filter needs cleaning sign pop up, and even via the service menu was unable to switch off, it is cleaned every two months and all boards are as should be

We are going to place a laptop nearer the projector to eliminate cables and switcher boxes, then test with a light meter, I was thinking of a fairly simple ppt
slide that has a white spot in the middle and R, G and B spots around it. We can then take a measurement in the middle of each spot with the sensor
touching the screen and the normal stage and house lights on (to reduce the variation due to natural light brightness) , then we will replace the lamp with a Genuine makers lamp which we purchased for £200 inc vat and delivery and try again with the light meter etc.


Sorry but it is a genuine lamp not a fake or a copy, the alternative non branded was £169 inc vat.

Due to the age of the projector these lamps are easily available I don't think the company would be very happy if they were been accussed of selling fakes !

#10 User is offline   david.elsbury 

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Posted 15 April 2012 - 07:44 PM

View PostImpact Pete, on 15 April 2012 - 04:18 PM, said:

Sorry but it is a genuine lamp not a fake or a copy, the alternative non branded was £169 inc vat.

Due to the age of the projector these lamps are easily available I don't think the company would be very happy if they were been accussed of selling fakes !

Wasn't it you that said that you were paying "around £400" for a lamp? I'm confused.

View PostImpact Pete, on 10 April 2012 - 10:44 AM, said:

Options are replacing lamp at around £400 and not gaining much

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#11 User is offline   Impact Pete 

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Posted 16 April 2012 - 07:22 AM

View Postdavid.elsbury, on 15 April 2012 - 07:44 PM, said:

View PostImpact Pete, on 15 April 2012 - 04:18 PM, said:

Sorry but it is a genuine lamp not a fake or a copy, the alternative non branded was £169 inc vat.

Due to the age of the projector these lamps are easily available I don't think the company would be very happy if they were been accussed of selling fakes !

Wasn't it you that said that you were paying "around £400" for a lamp? I'm confused.

View PostImpact Pete, on 10 April 2012 - 10:44 AM, said:

Options are replacing lamp at around £400 and not gaining much




If you read it properly this was posted days ago when we were looking at options, price was a guess at that time.
Why are people wanting an argument!

#12 User is offline   AHYoung 

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Posted 24 April 2012 - 12:28 PM

I think this Article in AV Magazine possibly answers the reason why you can buy "genuine" lamps below cost

http://www.avinterac...,2XXWRU,2AAI9,1

If its too good to be true...

#13 User is offline   jamwa 

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Posted 02 May 2012 - 12:06 PM

okay....



its not your projector all though it probably could do with the optics having a clean.



I do a lot of rear pro work - seamless blending etc....
your issue is a short throw lens and stand rear pro screen will have a hot spot - basically you are seeing the light source from the other side of the screen.

to overcome this you need to look at screens like opera blue or cream - very expensive as these are high density low gain so these diffuse the the light more evenly. but you will need a projector that outputs 30% more...

so if a client says I want a 10x7 rear pro screen with an 8000 ansi lumen projector on a short lens - either they live with the hot spot or you allow for a high density low gain screen and a 10k plus lumen projector to overcome the light loss........



on the plus side these screens can be used for both from and rear- but you do need to have these screens the right way around.....
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#14 User is offline   pumphouse 

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Posted 02 May 2012 - 12:34 PM

I very much doubt the projector is the issue here. Replacing the lamp will give you a brighter hotspot and brighter corners, but the ratio between them will probably be the same.

As others have said, it is the screen. If this is a permanent or semi-permanent install then I would recommend looking at an optical screen that includes a fresnel lens on the back. They are available in various focal loangths including variants designed for short throw use. DNP are my favourite manufacturer of these - UK Distibutor here. They are not cheap however but do give very good results.You'll be in for much more than the cost of a new lamp or lens!

Most 'fabric' rearpro screens are simply a diffusion material as others have suggested with varying gain characteristics, and not ideal for short throw usage. You really need a spot photometer to do the measurements from the viewing side of a rearpro acreen to get anything meaningful in terms of brightness evaualtion. A standard incdent light meter is not designed for this job. There is an InfoComm standard for the process of doing this which is based around contrastm but details suitable measurement techniques.

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#15 User is offline   Impact Pete 

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Posted 03 May 2012 - 03:24 PM

To finish this thread off;

1. Lamp checks out and is genuine Hitachi lamp.

2. After testing image with RGB and White spots projected on to the screen and a light meter we got 20lux on white, 3 on red, 13 on green and 3 on blue spots, we then removed all cable runs and placed laptop direct into projector having no effect on results but when we put the genuine hitachi new lamp in it was far better to the point that the hotspot was hardly noticable, White spot was 128lux, red 25 etc...

So as far as we are concerned screen is good, projector with short throw not ideal but works, lamp change more often recommended.

Thank you for those who put positive help and ideas into this thread some made good reading





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