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Discussion Starter · #1 ·
I want to get some minds working on ideas for what can be done to improve our ANSI CR.


For those that do not know what good CR can do for you, I will tell you what I have learned.


Really the only time I have been able to see any problems with ANSI CR with my 10PG is when there is a bright object on a black background. What you get is a smearing of the black around that bright object. Now this was lesser of a problem before I had my theater done, because the room made for a poor CR to begin with. Now as the room gets blacker with each new thing I learn about making it black, the ANSI CR becomes more important. Do not get me wrong thinking this is a big problem, because it is not. It is just that as I slay the last dragon and the image gets better I start looking for the next conquest.


Most people would look at me and then at the image on the screen and then look at me again and then at the screen, "Are you out of your mind, there is absolutely nothing wrong with what I'm seeing here".


OK on to more of what I know is going on.


Start at the source, the camera that is used for capturing the image has a lens on it, lenses add to the problem because of light bouncing around inside of them.


Then I can imagine there are some problems (processing form film or video to DVD) with getting that image to us without lowering the CR any farther.


Then we have our HTPC or DVD player that can add noise reducing the CR more. The same could be said about the PJ's adding noise and reducing the CR.


Then we have our lenses reducing the CR.


After all that, it's pretty impressive the CR we do have.


Now what can be done about it?


What I would like to explore is modifications to the lenses.


Guy mentioned that he masked the CRT so as to block some of the reflected light from getting back to the face of the CRT. Has anyone else tried this and did you see a improvement?


What about masking the front of the CRT to reduce light from bouncing off the screen and coming back through the lense to be focused back on to the face of the CRT? Or how about a one way reflective coating applied to the face of the last lense? Not sure if any of this would help, but try this little experiment. Point a flashlight at the screen and then point a lens at the light, hold something up about where the face of the CRT would be in relation to the lens and guess what, you now have the light from the screen focused onto that object. That would add up to lower CR maybe even mess with the focus some.


I have even though about lining the insides of the lenses with velvet to reduce any light scatter in there. Delta seems to have already paid alot of attention to this area so I do not know if there would be any improvement there.


What about you guys? Done any experimentation with trying to improve the CR.


Deron.
 

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My room is not black, but is sufficiently dark that I know exactly what you're talking about. I was watching blackhawk down last night and it is amazing how the overall black level is affected by the brightest object on the screen. The inky blacks that are present when there is not a bright object go away when there is one, similar to the washout when someone turns on a light, except it only affects really dark objects.


When you say Delta paid attention to this in the lenses, what features of the lenses were you referring to? The NEC seem to be regarded as having more than their share of the 'halo' effect, is it true for instance that the NEC is more halo-succeptable than a marquis 8000, or is it an issue on all non-lc pjs to the same degree? Is it a function of overall brightness? - I would think so.


It strikes me that the first reflection must be the worst since that is where the c-element is on lc projectors, kind of makes sense that the first would be the worst, and would be the most detrimental to CR since it reflects directly back on the tube face.
 

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Guys,
http://cms.3m.com/cms/US/en/0-78/cei...iewimage.jhtml


pages 23-25 have some info on some of this. It does seem as though there could be some improvement in painting/matting the insides of the lenses. The "dramatic" difference in LC vs non LC is the reduction of 4% of the light coming from the crts bouncing back off of the glass back to the phosphor surface, where it is redistributed as a point source.

Note that we are dealing with the lens bouncing light effect x3 in our machines, and all are focusing relatively bright sources.

Vic
 

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Discussion Starter · #4 ·
A couple of observations.


This is what I think is happening having no LC versus LC. With no LC the light coming from the phosper hits the first "flat" reflective surface and is reflected back at a certain angle hitting the phosper, causing the halo effect. On a LC the light coming from the phosper hits the first "curved" reflective surface and is reflected back in all directions causing a washing out of the image and the area around the image. This may have a effect of making the image not as sharp as a non LC PJ. That is just a guess, but I have never heard of anyone explain why a LC PJ would be less sharp.


Now I'm begining to think the only way to correct this is to get a fluid that has better qualities for contrast in our cooling chambers. Hitachi has a page about this, here is what they say.


"Hitachi High Technologies partners with the leader in Chemical Industry to provide the Global PTV industry Optical Coolant. Optical coolant is a critical component in the CRT RPTV lens assembly.


Optical Coolant is a combination of various chemicals that permits high optical transmission optimum refractivity. It also functions to keep the CRT lens assembly cool with no invasive or corrosive effects on the lens components.


Custom manufactured and blended to meet customer specifications.


This customer specific formula is blended to provide the optimal refractive index and cooling properties necessary for CRT lens assemblies".


Another observation. I our eyes could be contributing to a lower contrast ratio. Looking at a LED or a similar bright object in a dark room, I believe that there is a little washing out around that bright object.


Also looking at a CRT monitor It looks like I see some smearing around the image.


Vince


They blacked out the inside of the lense barrel and if they went that far they probably painted the edges of the lenses black.


As far as different PJ's being different in the way of halo's, I would think it would have to do with what type of glass and fluid they used, something to do with the reflective index of them being as close as possible. Also it may have to do with the design of the CRT, not sure how much light makes its way back towards the electron gun, but any light that goes back that way can come back out towards the phosper. One thing I noticed is it looks like they have some sort of mirror type surface that is on the gun side of the phosper. This might do two things, provide the path for the HV and control unwanted light.


Overall brightness, that is a good question. With a brighter object the halo would be brighter, but the object would be brighter too, so there would probably be some sort of ratio that would be maintained between the object and the halo it was causing. I not sure what would happen to the black area in the image, I would think that it would lose some of it's low level blackness as the object and halo got brighter.


Vic


Is it really times the number of lenses you have?


Think of it this way, you have one lens and there is a 99 lumens going through it to get to the screen. Now if you have three lenses and you want 99 lumens on the screen you would send one third of the lumens through each lens. So you have one third as much light bounce in each lens to deal with. The PJ with a single lens has to deal with three times as much light going through it as any one of the three lenses. So it can be said it has to deal three times as much light bounce.


Deron.
 

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Deron, you may be correct on the light bounce being the same with one lens. I hadn't thought about it that way.


The LC vs non LC is partially explained in the 3M white paper I linked to above, on the pages I referenced. You could be correct about the fact that the C-element allowing for the light being sent in its direction not ever reaching the critcal angle needed for the transition glycol --> c-element to cause light to be reflected back to the phosphor.

The 4% light bounce from the glass back to the white phosphor, which acts to reflect the returning light in all directs (point source) is also explained in that white paper.

You could be correct about the glycol and c-elements/glass having an effect as well.

However, the lenses are quite reflective inside on the black surfaces, and could be improved by painting them/covering with black felt.

The sides of the lenses are indeed painted black, at least in HD-8s. However, the lens facing the phosphor surface has a shiny second layer that I feel needs to be masked, as it causes quite a lot of relfection when light hits it directly face on.

Vic
 

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Quote:
With no LC the light coming from the phosper hits the first "flat" reflective surface and is reflected back at a certain angle hitting the phosper, causing the halo effect.
I'm not sure which flat surface you mean here, but according to the *very interesting* document linked above, the refraction is most significant on the outside tube face, and the reflection actually happens inside the tube face....

Quote:
In

the case of an air coupled lens, light emitted by the phosphor travels through the glass faceplate

and reaches the air/glass interface. Fresnel reflection takes place at this interface, with about

four percent of the light reflecting from the interface and traveling back toward the phosphor and

ninety-six percent of the light transmitting through the interface. The reflected four percent will

strike the phosphor, which is white and an excellent scatterer of light. Now the whole face of the phosphor, light and dark areas alike, will have an increased brightness due to the light

reflected at the interface
Quote:
In an optically coupled lens, the air/glass interface is replaced by a liquid/glass

interface. Since the amount of Fresnel reflection is proportional to the difference in index of

refraction on the two sides of the interface, it is dramatically reduced in this case.
So this is saying it's not the reflection off the first lens surface at all. It's the reflection from the outside surface of the tube face glass (i guess this would be the outside edge of the piece of glass that contains the glycol) and that the liquid coupling works by changing the difference in the index of refraction.
 

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Vince, thats what I was trying to say - thanks for clarifying. The document is indeed very interesting, and also states that there is a very small amount of light bounce needed to dramatically lower ansi contrast. Therefore, the lenses, which all do reflect light inside (even though they ARE black, they are fairly shiny) will have a large effect on this as well.

Vic
 

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Discussion Starter · #8 ·
I tried to link to that article but could not.


I will have to look into it further, because I still do not understand how the C-element works. Both the LC and non LC have a glass air interface, the only differance is the LC is a curved lens. I would think that the non LC's glass would still act like a lens in certain ways, except that it would not have any focusing effect on the image.


Deron.
 

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Also, light reflection from things like the lens mounts and light coming from the adjacent tubes could also contribute to the light pollution causing further reduction in ansi contrast. A lot of this, due to the agle from which the light is coming, would likely be reduced with darkening the insides of the lenses. However, there would still be a reduction just doing this. The amount might be slight, though.

I have a feeling that at least a bit of the improvement in contrast that B. Hegelstad found uopn switching to HD144s was the result of the fact that he used black for the lens mount adapters.

Vic
 

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Deron, true that the light will eventuall hit an air interface, but it may have to do with the fact that in an air coupled machine the reflective surface is parallel to the tube face and all the reflected light is coming right back to the phosphor surface where it has a direct path back out the lens to the screen, wheras the lc machine reflective surface is the concave face of the c-element and would not be parallel to the tube face, so the light would be dispersed away from the projection axis (to the sides) instead of back at the tube face and thus not have a direct path back to the phosphor and ultimately the screen.
 

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What also kills ANSI contrast:

Look at the back of your tube bell, and you see it.

It is lit up because of the backward emission of the phosphor.

It is covered with an aluminum reflection layer so the backward emitted light is reflected directly and increases light output, but as this aluminum layer is so thin, part of the light passes it, is partly reflected at the tube bell and spread over the whole phosphor face again through the aluminum.

The tube manufacturer could increase the aluminum thickness but then the electron beam would be attenuated and the projector would be too dim.

I wonder if there is a difference in ANSI cr of OEM and rebuilt tubes...

Roland
 

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So this is saying it's not the reflection off the first lens surface at all. It's the reflection from the outside surface of the tube face glass (i guess this would be the outside edge of the piece of glass that contains the glycol) and that the liquid coupling works by changing the difference in the index of refraction. [/b][/quote]


here's what i read:


the refractive index of air (in air coupled sustem) is much farther from the refractive index of glass (front of tube) than is the refractive index of the fluid in the c-element.
 

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Quote:
Originally posted by RoBro
I wonder if there is a difference in ANSI cr of OEM and rebuilt tubes...
I think that is a very good question.


--Darin
 

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Quote:
I wonder if there is a difference in ANSI cr of OEM and rebuilt tubes...
I'm not seeing why it would be any appreciable difference.


I'll get back into this thread in a bit to link some past threads on this issue.


A number of things, such as masking the tube face, painting the inside of your lenses, etc, have been used as benefit.


Another thing to mention is that the flat plate glass that hold the coolant on non-LC units causes the very well-defined circle halo that you see (that I see too on my marquee 8500 and barco 808). The curved elements in the LC lenses I suppose help eliminate this significantly, as well as the whole LC part.
 

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Discussion Starter · #16 ·
Vince


If you have not looked at it yet that article that is linked to here shows how what you are talking about works. It goes on to describe how a small-area contrast is reduced by halation. The worse of it comes from what is described as the critical angle causing the halo.


Now what you say is true, the curve of the C-element will spread out that critical angle causing the light to be spread out, but I doubt if it is all pushed away to the sides of the tube where it will not cause a problem. I think that instead of causing a halo like a flat reflective surface would it spreads the halo around so it is more of a washing out of the area.


The article goes on to say how a LC can reduce this effect. One of the principal things that it says helps is a index-matched fluid. That is why I mentioned Hitachi and changing of the fluid as a possible modification.


Vic


Yeah addressing the inside of the lense should help a little or alot depending on what lense you have. Velvet for light absorbtion is way better then almost anything that we have availible to us.


Robro


I wonder how great of a effect it is. Like you say some of the light does go backward hitting the bell and bouncing around, but thank that little layer of aluminum because it probably stops most of it from getting back at the phosper.


Do not rebuilt tubes need the aluminum layer too to work properly?


Chris


I wonder what has been used to mask the tube face and how much and type of improvement was seen?


Will masking the face cause problems too, because the masking material will cause reflections back towards the phosper.


Deron.
 

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Discussion Starter · #17 ·
One other thing I forgot to mention, it does look like we get light coming off the screen and back through the lense to hit the phosper. I will try masking the front of the lense with some velvet to see if that does anything.


Deron.
 

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Deron: Guy has discussed masking the unused portions of the tube face previously. The idea is to use some kind of dark material cut-outs placed in there. I would guess that improvements here would be fairly small, but it certainly can't hurt (unless you are carelss and break something or scratch something). As for painting the insides of the lens barrels, I recall someone doing this on lenses on RPTV in the past, I don't know if anyone's ever done this for any FP CRTs. It would involve taking the lenses completely apart, which is certainly a risky endeavour. I also recall them painting the outside rings of the actual lens elements, which is extra risky.


I am curious about the flat glass problems in front of the tube. I wonder if that glass uses any anti-reflective coatins or anything, I'll bet that might help out. I don't know anything about anti-reflective coatings, or if they come with negatives too, but it would seem that using a plate of glass that limits reflections at this point before the lenses would help reduce the defined halos that are very visible on many a CRT, as this halo is easily visible looking into the tubes even with the lens completely off.


Perhaps someone more knowledgeable than I might want to mess around with this?
 

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I think these glasses are AR treated. Once there was a post where someone showed a pic of the front of a pj where you see all 3 tubes and see that the red and blue show only very little reflection and the green shows very much reflection and I think it was stated that the front glass of the green tube had been changed because it had been broken and they might have taken standard not AR treated glass...

Roland


P.S.:

The rebuilt tubes would need the aluminum, and I think they also have it, but it may have a different thickness od different properties than the layer on the OEM tubes...
 

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Discussion Starter · #20 ·
Chris


I wonder what guy used to mask the tube face. Most things that are black reflect alot more light then what most of us would think. The reason I mention this is, I wonder if you could go backwards with masking the tube face.


A muti coated lense will only reflect less then 2% of the light. How much light is going to be reflected back by the mask?


Anti reflective coatings will look purple or green when viewed at the correct angle. I have never seen it on the glass face plate of tubes.


Deron.
 
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