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Discussion Starter · #1 ·
Last night I was watching Stargate SG-1 (TV series made to DVD--actually pretty good: 16:9, DD sounds, and nudity :)). I was playing with LT150 focus ring like an anal fool and my finger accidentally covered the top part of image. I then noticed that my ceiling arc of light (dim sliver of light leakage but noticeable on dark scenes) disappeared for a fleeting moment. I thought, hey there is a possibility here !!! So I turned on light and tape an one of the kid's flash card covering the top of the lens adjusting it so that it protrudes just enough to eliminate the arc of light and not enough to affect the actual image. This improved contrast and even the 4" halo wrapping the image dimmed around L and R edges (some still there but much less). The top halo of course disappeared completely as was the arc of light. Then I thought hey if it works well for top why not the bottom. So I tape a piece onto my coffee table from bottom again just enough to cover the bottom "black bars" just to the edge of image. Now I have masked all around the image "virtually" without ugly black felt or fabric on walls. Wife was much happier than letting me tape black stuff all over the walls. I'll probably replace the white index cards with black poster board (cheap, cheap at Wally World--Walmart) eventually. I'm tickled that I can mask and rid of light leakage on ceiling without major effort or ugly masking. You can actually adjust the cards to vary with differing aspect ratios. Duct tape peels and resticks pretty easy without any residual glue on my table :) These cards also make a protection to my lens from prying fingers of my kids. Oh yeah, to focus you can just lift up the top card to uncover the focusing ring. To clean lens you can do the same or even move the projector back away from bottom card to access the lens easier. You can also use velcro strips to make moving the cards easier and more professional instead of duct tape. Thought I'd share with you guys. I'd imagine that this method of virtual masking can work with other projectors too.


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Huey ;-]
 

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You got any pictures of this magic solution? ^_^


I'm not quite sure how you're using the cards and a picture would say more than a thousand words.


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/frode
 

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Masking at the projector has been discussed before. I've always thought it had merit, but here are the concerns I remember being put forth:


1. The light at the lens is not focused, therefore you can't get a clean mask like you can at the screen where it *is* focused. You'll still have some light spill, and the "projected" mask line showing on the screen will not be razor sharp.

2. Worry about heat from the projector (light) affecting the masking material - maybe even a fire hazard.


Even with these concerns, I've often thought it would be worthwhile, to cut down on ambient light in the room.


Huey - I'd love to see some pictures, too.
 

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rick.. I have some comments to your comments http://www.avsforum.com/ubb/smile.gif


1. While you are indeed correct that you won't get a clean mask that should in most cases reduce the halo so much it will be masked by the small black mask included on most screens. Position of projector and screen is crucial to get this to work though. As for light spill - I can't see why you should see any at all as you're effectively blocking off all light except the 4:3 (or 16:9) image.


2. Think about it. Why is there a novel called "Farenheit 451"? Unless you soak the blocking cards in alcohol I don't think you have to worry about a fire hazard. The lens used is also a dispersion lens - not a focusing lens so the amount of light shouldn't be a factor either. Even a 10000 lumens projector wouldn't light a piece paper.


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/frode


[This message has been edited by Frode (edited 09-30-2001).]
 

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Discussion Starter · #5 ·
True that the mask is not sharp but if you plan for this it'll still be effective without image degradation. If you worry about fire hazard, you can always use sheet metal, cut to 2"X2" size, and paint it with flat black, barbecue-grill, heat resistant paint. I personally don't see how this could catch fire as the light beam itself is not hot at all. Card was slightly warm after 2 hours of movie and not hot. This was just a temporary solution but more permanent, nicer solution should be attainable along the same line. I think because the LT150 has such a high vertical shift that the top card can mask the light leakage without affecting the image. I'll take some digital photos tonight as I'm at work. You can try it for yourself. The test is FREE. If it doesn't work don't use it and all you're out of is some tape and index cards. But I think it does work and you'd be pleansantly surprise. Of course most of you HT fanatics probably already have masking, dark walls in place :) I'm not saying this is better than true masking but it beats the heck out of no masking at all.


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Huey ;-]
 

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I did the same thing with a "stand-alone" mask. Took a piece of cardboard from a cereal box, about 4" by 5". Cut a hole in the middle at 4:3 ratio. Traced the cardboard on a piece of black velvet and cut it out. Glued the fabric to the cardboard. Since I use the projector on a coffee table, and set it up for each viewing, I have this mask mounted to a small metal base that I can set about 2" in front of the lens, and the velvet absorbs the light so as not to be a distraction to the viewers. It seems to me this is a better solution than masking at the lens as light isn't reflected back into the projector, reducing contrast.


When I build a hushbox, I plan to recreate the mask on the optical glass.


Note that this eliminates the stray light above and to the right of the screen, but doesn't do anything for the halo.
 

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Discussion Starter · #8 ·
RideTi, how did you have the height to be perfect to rid of top arc of light. L and R can be adjusted by moving your metal stand, but top and bottom will be tough to get exactly right. I guess trial and error will do (tracing light image on card first before cutting out hole?). You're probably right about having less reflection and sharper shadow mask if the virtual masking system is further from lens. I just did mine makeshift to see if it'll work and it did. I did not notice a significant drop in contrast or brightness. I'll post pictures once I get home tonight (if wife will let me play with HT :))


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Huey ;-]
 

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Discussion Starter · #9 ·
Hey guys, I've got picture of the virtual masking system (VMS) :)

This show set up in front using black poster board (that I had laying around kids room):
http://www.avsforum.com/ubb/image_uploads/lt150_6.jpg

This show set up from L and R side:
http://www.avsforum.com/ubb/image_uploads/lt150_5.jpg
http://www.avsforum.com/ubb/image_uploads/lt150_7.jpg

This show set up with top card flipped back so can be focused (card is taped to LT150 with clear tape):
http://www.avsforum.com/ubb/image_uploads/lt150_1.jpg

This show how it looks with lights out to show extra light blockage:
http://www.avsforum.com/ubb/image_uploads/lt150_2.jpg

This show MI2 with system off showing arc of light leakage and halo (labeled):
http://www.avsforum.com/ubb/image_uploads/lt150_3.jpg

This show MI2 with VMS on eliminating arc and halo:
http://www.avsforum.com/ubb/image_uploads/lt150_4.jpg


As you can see this free VMS works quite well for me :) Note that this experiment was not meant to show off image quality of LT150 so camera was not on tripod (image blurry due to shaky hands and ISO 400 :))




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Huey ;-]
 

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Thanks for sharing the idea.


I know the LT150 weighs only 3.3 lbs, but is it really necessary to use a bungy cord to keep it from blowing away? http://www.avsforum.com/ubb/smile.gif
 

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Signal,

I'm pretty sure the bungie is only necessary when then fan is on! http://www.avsforum.com/ubb/wink.gif


Huey,

Thanks for the pictures. The before and after images really show the difference. Thanks for sharing.


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Todd

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-scan-lines-annoy-me-

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[This message has been edited by sonogr4 (edited 09-29-2001).]
 

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Discussion Starter · #12 ·
Bungie is for kids, kids, kids. They can knock anything over :) Plus it prevents LT150 from scooting around if I focus or push any buttons on top.


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Huey ;-]
 

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Huey,


I'm always interested in other 150 setups, thanks for the pictures!


Looks like you have the projector behind the seating, just above the level of the chair with book at the back to shift the image down.


Does that force you to use keystone corrections, also what about the noise, isn't it right at ear level, why the additional fan?

 

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Discussion Starter · #14 ·
Noise is very mild and not distracting at all. My room is short (13 feet) so I don't have much room to put LT150 on coffee table like most. I didn't want to ceiling mount for cost, holes in ceiling, heat, falling LT150, etc. Extra fan is because I had one occasion when power went out during movie and fan and everything shut off quickly without cool down cycle. I smelled some thing electrical (not quite burnt smell). When power came back a few seconds later, no buttons would respond (it locked up !!) I had to unplug and hard reset unit before it'll work. It's worked fine since ( 2wks so far. This worried me so I called tech support. They reassure me and promised new replacement (unit was less than 30 days old at that time) if it breaks. Extra fan is there for emergency use in case of this happening to cool unit down for these rare emergencies. Normally it's not on at all. Plus I now have unit and fan on UPS to cover these brown or black outs. It was always on nice surge protector (IBM 1800 joules) but this did not protect unit from crashes when power goes out. Once I get less paranoid I will remove fan as it's ugly. Heat can shorten life of unit and bulb so that's why fan is there.


As far as keystoning, I have my screen tilted outward at bottom using bungie cords (I love bungies :)) to correct keystone. Image appears perfectly square this way without using digital keystoning which can degrade image.


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Huey ;-]
 

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Discussion Starter · #16 ·
Cool, but that looks like a lot of work :) I'm kinda lazy :)


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Huey ;-]
 

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Hey guys, I was thinking of ordering an extra lens cap or two, cutting out a rectangle in the appropriate position, and then attaching it to the LT150. I thought this would be a more esthetically pleasing solution and more solid than taping pieces of paper in front of the LT150. The inside of the lens cover could be painted black or lined with felt. Would this work? Would there be a heat issue?
 

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Tim - funny you should mention the lens cap. I was at a high end dealer for a demo of the $13k Dwin Transview DLP, which also suffers from the "halo". I ask the sales guy about this and he said the suggested fix from Dwin is to take a metal lens cap and cut a rectangle in it.


I wonder if they give you an extra lens cap for your thirteen thousand dollars http://www.avsforum.com/ubb/smile.gif Reed.
 

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Discussion Starter · #19 ·
It should work. You'd start with a small 4:3 window and keep enlarging it until the image shadow mask is perfect. It won't be a sharp mask but it'll be enough to block stray lights. If the shadow impact edge of image, just widen 4:3 square by 1 mm. The only problem is if you screw up, your lens cap is history. Maybe you should practice with a cheap lens cap from binoculars or something similar before you go drilling into LT150 lens cap. Rotozip or other high speed rotary tools with a small bit may work well. Another is to melt plastic with soldering tip or heated jewel (tiny) screw driver tip to melt and cut plastic. I personally think the stand alone method mentioned by RideTi above will be cheaper, safer, easier, less light reflection (improves contrast), and sharper mask (the further from lens the sharper the shadow edges). My method was just very simple and easy to do (black poster board and clear tape) :) If you want the cards to blend better you can paint the exposed sides with gray paint. :)


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Huey ;-]
 
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