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Discussion Starter · #1 ·
A my quest continues, I have been riding on the fence for a long time regarding which type of projector to get.


I currently run a 120" diagnoal (8ft, 96" wide) screen running with a 600 lumen LCD projector.


First, I hear that anything but a 9" will not suffice at this size in terms of CRT projectors, and then I hear that this is all subjective.


However, the spec on the Barco 808 is 1200 lumen? How does this compare relative ot my 600 lumen LCD? It would appear that this would produce the same, if not brighter image than I have now.
 

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There are two CRT lumen ratings; peak (with 10% of the screen at max.brightness) and ANSI (with effectively the whole screen at max. brightness). A CRT's peak lumens reading is typically 6 or 7 times its ANSI rating. Older CRTs were usually ratedfor peak lumens, but now almost all are rated for ANSI.


Digital projectors have the same max. brightness regardless of how much of the screen is illuminated, so they have just one rating.


Since computer display images tend to have high background brightness with dark details, the CRT ANSI lumen spec is most appropriate for that application. Video and movies, however, run at an average brightness close to 30% with typically small areas of peak brightness, so the CRT peak lumens rating is not inappropriate. Even so, most people use the ANSI rating to calculate screen image brightness. That is a worst case situation.
 

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I was having the same problem, I got rid of my Sony VPL 400 QM LCD projector (450 Ansi Lumens) which I was running on a 140 inch diagnol wide screen, as I wanter a brighter projector as I do not have total light control, I saw the BG808 rated at 1250 Lumens and thought great, but it is rated at 210 ansi Lumens.


I am still thinking about it, or more so how to get total light control in the room.
 

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Discussion Starter · #5 ·
I guess lumen rating is pretty difficult, and with the differences in screen size and gain it makes it ieven more difficult to make a precise calculation or opinion for others.


I guess foot-lamberts would be more usefull, but the same problems arise.
 

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Quote:
Originally posted by joelco


Digital projectors have the same max. brightness regardless of how much of the screen is illuminated, so they have just one rating.
I would have thought so too, but my measurements have not agreed with this. Try taking readings with a Sharp 9000 DLP projector with a 100 IRE window and then see if you get the same measurement with a 100 IRE full field. I got lower readings... This was a one time experiment, and I was doing other things at the time, so I may have gotten it wrong, but I'd be interested to hear if anyone else has actually done measurements. Signal was from an Accupel, measurements with McMahan Lightspex.


William
 

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Discussion Starter · #7 ·
I would like to know how the measurement device takes into accoutn the fact that while each indivual spot may be the same brightness, the total screen will cause a greater brightness overall then just a small window.
 

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David, are you responding to my post? I don't know what you mean when you say that individual spots are the same brightness but the overall screen is brighter.


The measurement device I used, which is a fiber optic probe placed 1 foot from the screen surface, "looks at" a relatively small area of the screen. You can also do this measurement with an integrating sphere pointed at the projector; I intend to repeat the experiment in this way whan I have the opportunity.


I got lower readings with a full field, not higher. I do not yet have an explanation this. With some CRT projectors I would expect this, as a full field requires a lot more current than a window pattern.


William
 

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Discussion Starter · #9 ·
I was refering to this:


"Digital projectors have the same max. brightness regardless of how much of the screen is illuminated, so they have just one rating."


I don't know how this can be possible, and this is what I was asking..this would imply that the measuring device would read the same brigntess whether or not the whole screen, or just a small segment, is illuminated. Physically, this is not possible...more photons would be coming off the screen, correct?
 

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David,


The measurement of the screen luminance is in fL, which is a measure of brightness per area (one lumen illumination per square foot on a 1 gain screen equals one fL). As long as you are illuminating at least the field of view of the measuring device, usually a small spot, the total image size does not matter.


Peak lumens for a CRT projector are very misleading, since the phosphors will usually massively bloom at that light output.
 

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Thanks odyssey, you beat me to it...


OK, here's some new data:


Projector: Freshly calibrated JVC G15 (Dukane) with 3 hours on the lamp.


Screen: Stewart 1.3 gain microperf 80" wide


Signal: [email protected] (HTPC) 4x3 image

100 IRE Window pattern = 30.9 ft lamberts

100 IRE Full screen = 29.2 ft lamberts


This is 850 ANSI Lumens by the formula:


foot lamberts = (ANSI lumens) * (Screen Gain) / (screen area in square feet)


Signal: 720p (Accupel) 16x9 image

100 IRE Window pattern = 34 fl

100 IRE Full screen = 32 fl


So again, I am measuring lower light output with a full field white input signal than I am with a window pattern. And lower output with a 4x3 image than a 16x9 image.


For these measurements: McMahan Lightspex in "*LUM" mode, fiber optic probe on tripod at 1 foot from screen surface, projector positioned to produce 80" wide image.


I guess this should be over in the DILA (LCOS) section, but this thread started here...


William

(edited to correct my math error)
 

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Hi David,


Don't know about measurement. In my system, to my eyes, after proper contrast/brightness adjustment, the LT150 (white section OFF, 20 hours on the lamp, 600 lumen?) looks a bit brighter than my CRT projector NEC XG85 rated at 220 lumen with 1800 hours on the tubes.


Both projectors look VERY bright in my light control room.


regards,


Li On
 

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David,


Li On is right, a 200-250 ANSI lumen CRT is roughly equivalent to a 700-800 ANSI lumen (specified) digital projector for video signals. This takes into account the fact that most digital projectors output some 70-75% of their inflated specified output when adjusted for optimal image quality. joelco described the reason for CRT's providing more output for video signals earlier in this thread.


wm,


The very same phenomenon you are describing is reportedly also true for the Sony VW10. Unfortunately no one has provided an explanation for this.
 

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> A CRT's peak lumens reading is typically 6 or 7 times its ANSI

>rating


I don't understand how you can use the peak lumens on a CRT projector.


With a full white screen : above a picture/constrast value my projector is limiting the white level.


So how can you have a white whiter than that ? By going over this value with a not full white screen?
 

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Most crt projectors limit on total beam current. So if you project a full field white image, that's a lot of current, and you hit the limit earlier than with either a window pattern or a normal picture with average bright areas.


With most projectors if you exceed the limit they will compress the picture, clipping the brighter areas. An exception is the Sony G90 - it controls total beam current by adjusting the input signal gain down, so that there is little or no clipping. It just stops getting brighter.


William
 

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Hmmm,


I wonder if light from the "black" part of the screen may be reflecting back into the light path and bumping up the readings. This could happen in the projector or perhaps the light from the "grey" blacks may be influencing the white level. Although if that was true one would expect the full field reading to be higher. Interesting.


Wow... 750 lumens from a calibrated G15, is this typical?


Regards,


Kam Fung
 

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850 lumens (I corrected my earlier post) and yes, I think it's typical.


William
 

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The best rule of thumb to me in comparing CRT ANSI and Peak lumens and trying to compare that to digital is to average the ANSI and Peak and that is about how it would compare to a digital.


There are hundreds of other variables involved to do the comparison correctly but that is a good quick and dirty one.


-Mr. Wigggles
 
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