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Anyone using 'bias' lighting with their CRT front projector?

Discussion in 'Projectors, Screens & Video Processors' started by Gary Lightfoot, Sep 6, 2005.

  1. Gary Lightfoot

    Gary Lightfoot
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    I saw a picture of a projection screen with bias lighting behind, and although it's a good idea for direct view TVs, it's probably the best way to destroy any advantages CRT has over other displays, especially the black level and contrast. :rolleyes:

    Please tell me no-one here has that?

    Gary.
     
  2. Welwynnick

    Welwynnick
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    I've never heard of anyone using bias lighting with CRTs, but if you can keep the light off the screen, mightn't there be some perceived benefit. The actual contrast obviously can't be improved, but it might look as if it had.

    Nick
     
  3. Jeff

    Jeff
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    I had it for a while, it was installed while I had my old Plus DLP. It's not as effective as with DLP but the effect is still quite pleasant. It's good in that the room is no longer completely dark but has little effect on contrast. If I'm watching Sky rather than a film I prefer some light in the room.
     
  4. Gary Lightfoot

    Gary Lightfoot
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    Hi Nick,

    I think the idea with bias lighting for TVs is because in a dark room with a small screen it can cause eye strain because the dark room causes the iris to open due to the low average picture level and with so much light from the tv going in as a pinpoint of bright light it can be unpleasant.

    With lower contrast displays and larger screens it works by again closing down the iris and makes the dark grey look black as you suggested. With a CRT it will hide a shed load of shadow detail (due to the iris) if it isn't allready bouncing off the walls and washing out dark area detail. CRTs don't need any help in the black level department, but they do need to improve their ANSI and bias lighting will have th eopposite effect.

    A perfect example of this is using the Avia needle pulse test when setting black levels - if you use the test image that has both half white and half black with the moving bars, the bright white half causes the eyes iris to shut down and you can't see the moving black bars. You can shield your eyes from the bright white with your hand and that will allow you to see the moving black bars better as the iris opens up a little to let in more light. Using the all black test image with the black bars will allow you to set the black level more accurately but with the possibility of not seeing shadow detail if it's present in brighter scenes.

    Jeff,

    It does improve the apparent black level but even with DLP now days it can still prevent you from seeing detail. Not a problem with Sky etc but serious movie watching will still be compromised.

    Gary.
     
  5. kalW

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    Actually, the biggest reason for not being able to see the two moving bars when half the screen is white on most CRT projectors is because most are air coupled which reduces ANSI contrast. Light from the bright area is bouncing around in the lenses and between the tube faces and first lens element, effectively reducing the ANSI contrast. Liquid coupled units fare better of course. Digitals are much better at this, but of course, have other shortcomings. :)

    Kal
     
  6. NicolasB

    NicolasB
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    Ambient light is a Good Thing (tm) so long as it doesn't reflect off the front of the screen.

    The two principal reasons are:

    1) The eye is slightly less sensitive, and thus the darkest parts of the picture look darker.

    2) The eye's perception of colour is strongly dependent on the intensity and spectrum of ambient light. (In fact, if you're in almost total darkness then the part of the eye that perceives colour shuts down altogether).

    Try the effect of displaying a grey rectangle in the middle of your monitor screen with your Windows desktop set to uniform black. Then change the background to white. The rectangle appears to change colour with it. It's only by getting the correct colour and brightness around the picture that the picture itself can look the way it is supposed to look.
     
  7. Gary Lightfoot

    Gary Lightfoot
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    Hi Kal,

    Indeed. :)

    Nic,

    Any light in the room will severely reduce your contrast even if it isn't falling directly on the screen - it just needs to reflect off any object (including the walls - are they black?) onto the screen to raise the black level, and unless you have completely covered your room in black felt I think that refected light will effect all of us. Turn off your projector and if you can see your screen that is your best possible black level - probably a dull white. Ambient lighting is not ideal for a front projection set-up unless you have a huge amount of lumens to give you decnt CR.

    You just need to double the black level to halve your CR: If you have a CR of 9000:1 and 12ft lamberts at white you have a black level of 0.00133 ft lamberts. If you double that to 0.00266 then you have just 4500:1 CR. If you have a CRT you've just destroyed one of your biggest attributes and your black level with it. The amount of light from your LED displays can be greater than 0.00133 ft lamberts, and is enough to put light onto the screen from 17ft away (I tried it recently). If you have your CRT black level set to total fade to black you now have total fade to DLP grey/black. :)

    Bias lighting will fool the eye into thinking the blacks are blacker but the reality is different as is the loss of visible shadow detail. You may literally be only getting half the picture (assuming the pj is set for shadow detail).

    Bias lighting may work and be preffered for viewing a monitor (I believe it's common practice in the video proffession), but it's far from ideal with a front projector, and don't forget it will effect the colour balance too, which won't be at D65 any more (if it was to start with). Some (Tungsten?) lamps can produce a light at around 2300k which is quite a warm yellow.

    Here's a link to the effect of ambient light on your CR:

    http://krasmuzik.home.comcast.net/AMBONOFF.pdf

    Gary.
     
  8. kalW

    kalW
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    I agree with Gary: Ambient lighting has no place with CRT front projectors in any circumstance.

    Kal
     
  9. Jeff

    Jeff
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    Sorry Gary but this simply isn't true, I small amount of light behind the screen has very little effect on contrast.
     
  10. Godfather

    Godfather
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    Gary I can't disagree with the theory, but when we went to see Yas's G90 I noticed there was some light coming from the garden behind the screen (as well as coming from the equipment though neither were falling directly on the screen), yet you thought the quality of the image was the best you had seen. So may be a little back light doesn't do that much harm.
     
  11. crteaman

    crteaman
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    Gary, you are correct with your assumption regarding a bias light, basically it's an ISF thing really and as you noted is intended to reduce eye strain more than anything else.

    KAL now you're showing off cos you have LC :rotfl: but as you said it really does help.

    Hi Henry, hope Gary and your good self can make along to see the ultra in it's new clothes ;) soon.

    I have recently received a mail from someone up north who was instrumental in arranging the mini-event, he wants to know if I have considered doing another one, perhaps even a winter event hmm... "Build it and they will come" eh?

    Henry, we can now talk Proceed eh? ;) and perhaps :thumbsup:


    James
     
  12. crteaman

    crteaman
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    Nick forgot to say Hi mate...sorry

    James
     
  13. Gary Lightfoot

    Gary Lightfoot
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    Jeff,

    You have to agree though, if you can see the screen with the projector off then you have ambinet light interfering with the black level and therefore reduced contrast. The simple maths also bears it out.

    Henry,

    There's more to image quality than contrast and black level, and if you remember I did suggest that I couldn't see more detail in the shadows than I could on my own display - I've even posted a thread over on avs tryong to find out more on the subject. I did also say that I saw more detail in the 3 chip DLP in that particular scene of the Osiris clip so maybe the small amount of light did make a difference? That still doesn't alter my opinion of the G90 being the best image I've seen so far *overall* though. I guess it depends on how much you like your absolute black level as to whether or not you can live with some ambient light in the room, but I would have thought a CRT owner wouldn't want to deliberately compromise the image, especialy in an area where it has a unique advantage over other displays.

    James,

    Nice to see you. :)

    Hope to be able to make it down some time... Any more mods you want to tell us about?

    Gary.
     
  14. crteaman

    crteaman
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    Anyone recieved the latest issue of 'Widescreen review' the special 100th issue edition?

    I was reading a friends copy a couple of day's ago and among the many sections was an interview with Sam Runco conducted by Gary Reber, Gary is basically giving his opinion to Sam saying his prefered display device by far is still the nine inch crt.

    Sam responds by saying (from memory) wow those crt's, those devices have almost no light output, we used to have one at home in order to see it you had to view it with the lights off, trouble is you know what happens when you do that? "you fall asleep"

    Wow Sam, that was really technical and scientific mate, maybe i'm just an insomniac but I have never fallen asleep whilst watching a movie with the lights off, how about you guy's on this thread?

    My god lets get all those seats at the cinemas turned into beds :rotfl:
    Since most 9" units (and many 8") easily achieve 8-12 FL about the same as the average cinema.

    Imagine it, "bill what did you think of war of the worlds at the flicks last night?" "sheesh tom just got past the opening credits and I went out like a light, best eight hours sleep I ever had" :rotfl:

    Doh! this has to be a candidate for the most ridiculous home cinema statement this year.

    Just thought I would share it with you.

    James
     
  15. Welwynnick

    Welwynnick
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    Fall asleep while watching a film?
    Someone has been watching the wrong films!
    I have to admit that I have fallen asleep in the cinema while watching a couple of musicals that my wife has insisted we watch. But then she is almost guaranteed to fall asleep at home watching my choice of film - even with the lights on!

    Nick :)
     
  16. NicolasB

    NicolasB
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    I said "so long as it doesn't reflect off the front of the screen." I didn't say "directly", did I? Obviously indirect reflections will have the same effect.

    Well of course they are. Why wouldn't they be?

    It's not just a question of blacking out the walls and furniture, though, it's also a question of being very careful about the direction the light is coming from. If you set it up correctly you can get a significant amount of light behind the screen, angled in such a way that a negligible amount ends up being reflected off the front of the screen.

    It would never have occurred to me that anyone could be silly enough to use anything other than proper 6500K lighting in this context, but yes.
     
  17. Gary Lightfoot

    Gary Lightfoot
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    James,

    I have WR 100 but don't remember seeing that! I'll have another look. Quite funny but I hope they weren't serious. :)

    Nic,

    I see your point, and black non reflective walls will help (black velour or felt is probably better than paint), but can you see the screen with the bias lighting on and the projector off? If so then it's compromising your CR and black level in real terms.

    It will also still bias your eye enough to make shadow detail invisible if you have a dark scene, so I still don't see any advantages for bias lighting with a CRT front projection system (or any fp system to be honest). Why do you use it though - you've gone to the trouble of installing it so I'm curious. Are the lights 6500k or D65k?

    I guess you can get away with it provided the amount of light that makes it to the screen is less than the black level of the projectors output when projecting black, but that will be such a small amount of light I would have thought it hardly worth having. If you have your black level set for shadow detail then you may not have compromised the on/off CR as much but if you have it set for total fade to black then you will.

    Just thought I'd add that the set-up I saw allowed you to see the entire room, not just the screen so it was certainly impacting on the image in a big way. A high lumen digital would have a higher CR in that room than a CRT.

    Gary.
     
  18. Nic Rhodes

    Nic Rhodes
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    I keep falling asleep :)
     
  19. crteaman

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    Nic I think the timing is right for us to move into the medical world, we now have a worldwide fix for all those insomniac's, black room and a crt = bang out for the count :D

    Yes Gary it was a serious article,like I said turkey of the year award winner, its got to be.

    James
     
  20. Gary Lightfoot

    Gary Lightfoot
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    Well, I'm going down the black felt route for the walls near the screen in an attempt to improve my ANSI contrast which is affected by the close proximity of my walls to the screen. If you don't hear from me for a while, give me a ring cos I've probably fallen asleep in a pile of black felt... :)

    Must dig out my WR 100 in the mean time....

    Gary.
     
  21. Welwynnick

    Welwynnick
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    Seeing James’ Marquee must have made quite an impression on my wife, because to my great surprise and delight, she has been subtly steering me towards a projector ever since. However, although the wall paper is getting on a bit, I’m not sure she would be too pleased if I re-decorated the living room in black felt.

    The loft has been suggested as a more suitable location. I haven’t measured things up, yet, but did you say it wasn’t feasible to get a CRT up there, Gary? That would be a shame. I had difficulty just getting all those amps, speakers, monitors up there (most of which would have to come back out). I can think of a few other problems, too. Has anyone ever installed a CRT in a loft?

    Cheers, Nick

    PS. It’s been a bit quiet round here, lately. Are we missing someone?
     
  22. crteaman

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    Nick, I have done a loft or two in my time, I also have a spare low hour Marquee 9500............ ;)


    James
     
  23. Gary Lightfoot

    Gary Lightfoot
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    Hi Nick,

    It depends on how you do the conversion. In my case it was on a budget so still has a loft hatch, and the 2ft wide (roughly) ceiling wouldn't take the weight, so even if I could get it up there without killing me or the CRT, it would have to go on the floor, and take up the seating area. If I did put it on the floor it would be over a load bearing wall so that would have been OK weight wise, I just don't know where we would have sat - I still have the purlin supports there too.

    If you had some structural work done for a proper staircase and ceiling, then you might be OK for one though. It depends on how much you want to spend. You could make up a CRT using cardboard just to see where it would go etc in case you decided to get one up there at any cost, and then you could see where you could sit etc. and if you needed to do a proper conversion or a cheapo one like mine. :)

    Gary.
     

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