Dreams Unlimted ahjxxum: Feb 28, 2008

Projector screens are generally divided into two types base on their functionalities: reflection projector screen and transmission projector screen. It can be also divided into soft and hard screen base on the materials they are made from.

Home theater generally uses soft reflection screen. My brother-in-law originally wanted to buy a 1000 (~$150) so-called "import screen", but a friend of his who sells projector screens told him that it is hard nowadays (in China) to distinguish the genuineness of an import screen, it is hard even for himself. Some of them that are labeled with 'import' or 'joint capital' were actually manufactured somewhere in the south of China. He felt that he'd rather to buy a 300 domestically manufactured screen with good feelings than buy this kind of "import screen". What this friend said makes perfect sense. But after doing some research, my brother-in-law found that all screens on the local market are made from high gain Bolivian bead that is used for projecting newspaper clips, they are simply not suitable for video frequency.

Theoretically speaking, a white wall with one smooth side actually is the best "screen". Because its gain is 1, meaning that the light projected can be completely reflected out, which is an ideal state of being "no absorption, no gain". Unfortunately, for the purpose of absorbing and proliferating the sound wave, he already made the wall a background wall with sound-absorbing material and plywood installed. making it impossible to serve as a "projector screen', he had to find another solution.

You might be wondering at this point: why do people still bother purchasing expensive screens if we can all use white walls?

Well, there are always benefits and advantages of using a professional screen: convenient, artistically beautiful and dignified, good screen can also make up the insufficiency of a projector and improve visual effect. Among the expensive screens, one type is "gray screen" (cost about 15,000, roughly $2000). This kind of screen probably was originally designed for liquid crystal projectors. The biggest problem with liquid crystal projector is that the color appears dark and grey, insufficiently calm. This is its "congenital defect" that is caused by its liquid crystal board and path of rays.

Regarding gray screen, we all know that gray is merely a lighter black, and black absorbs all visible light. Gray can only partially absorb visible light, it is like brightness of the picture is reduced. If you have used any picture processing software's "brightness / contrast gradient" option, you should certainly have noticed such phenomenon that reducing brightness is equivalent to increasing contrast gradient? Same concept, since the brightness has been reduced, it in turn increased its contrast gradient. The black effect gets improved due to the bigger contrast. We can also experience the same effect when we look out through the sunshade glass of our car. In fact, there are many ways to just reduce the brightness, you don't have to use gray screen. There are magazines recommending putting the light gray filter of a photographic camera to the projection lens, the principle is the same. You can even use more simpler method, namely you need to adjust the projector's output brightness or increase the contrast gradient. No need to spend a cent, you may achieve the similar effect, but the premise is that showroom must be dark enough.

Back to the bottom line, if a gray projector screen cost you $2000, definitely it is not just because the screen color is changed from white to gray. Speaking from the optical principle, I'm afraid there's a lot more behind. I'm guessing probably certain chemical compositions have been added to the material of the screen that changed the reflection or absorption intensity of different wavelength of light, thus changed the luster and the contrast gradient of the entire image, that, makes up the inborn flaw of liquid crystal board after all. In addition to this, what other tricks do you think they can play? It doesn't seem to be possible with the meager knowledge of physics that I have.

It sounds more like it to throw in a 150,000 screen if your projector cost you 15,000. But adding a 15,000 screen to a 15,000 projector doesn't make much sense at all. If I have to buy a 15,000 screen, then it would simply work better if I put the money together and buy a 30,000 higher level projector to achieve better effect without any extra effort. A 15,000 screen is a crazy price to my brother-in-law (imagine his monthly income is merely 3000). Also if he buys a name brand Japanese gray screen, then he actually spend most of the money to pay for the labor which he personally doesn't feel comfortable.

The ideal screen for the DLP projector that my brother-in-law purchased should be like a white wall, just let the project light reflected completely without any "reservation". He figured that he really didn't need such costly screen. So he finally decided to make one on his own.

Exactly how did he do it? You may not believe how simple and inexpensive it really was! He spent a bit over 10 (about $1.50) in a home decorating store on a self-adhesive pure white matted formica PVC panel with dim grains, cut the right size, pasted to his original background wall, that is it, flat and smooth! With such PVC screen, he doesn't need to worry about the 'curl-up' phenomenon that may occur to a regular projector screen after around 12 years of use, he also doesn't need to worry that it would turn yellow one day due to natural oxidation. But remember it requires some pasting techniques to make it work perfectly for you. The result? Great!

Here are couple of self-made projector screen photos from my brother-in-law as 'evidence':
www.news-blogs.com/_images/entertainment/diy_screen.jpg
www.news-blogs.com/_images/entertainment/diy_screen2.jpg

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Dreams Unlimted ahjxxum

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Thursday, February 28, 2008

Homemade Projector Screen - The Principle & How to DIY