I took apart my PF1500 again today to get a detailed look at the possibility of upgrading the projection lens... and the news is not very good... in fact, it's very bad.
First... the DLP light path design on these products is rather "different" from other DLP projectors... in that the "off-pixel" light absorption area appears to be INSIDE the lens!! (ugh!) (see pictures)
Remember, in a DLP system, the DMD reflects the light source to either:
"target A" which is through the Lens (for an "ON" pixel), or
"target B" for an "OFF" pixel, in which case the pixel light needs to be absorbed and kept away from the desired
In other words, a "dark" or "OFF" pixel on the screen is being reflected to the light absorber... but that pixel is still a mirror and still reflects the light... since the light source floods the entire DMD chip, so that all pixels are being illuminated... it's just reflected to a location where you (hopefully) can't see it. The only time there's no light hitting the absorber is when you have a full-frame WHITE, where all RGB colors are being directed to the screen 100% duty-cycle (and that wouldn't be a pure-white, but rather 100% of each RBG colors, which may not create a true White.).
In older (larger) DLP projectors, this "OFF" pixel absorption area is as far away from the lens as possible, so that the "OFF" pixel light does not get scattered back into the active area of the lens.
(DLP makers should look into using "Vantablack 2.0" on the light absorber... it's amazing stuff!)
HOWEVER... this new style light engine is an attempt to keep everything as compact as possible, and it seems that the "OFF pixel" absorption area has been designed to be internal to the lens. (see images)
This results in a much higher possibility of "OFF" pixel light being scattered into the image path through the lens... resulting in background light that will never be as "black" as in larger DLP projectors.
LG (actually Young Optics
) used aluminum for the primary lens housing... this is the reason I've not seen any lens focus drift on my PF1500 (as I do on my Acer K132 and related LED 720p projectors)
I found something bad on my first lens (the first lens to get light - at the back of the lens assembly).
At first glance, I thought what we had was a plastic lens element that had been absorbing some of the light... and had begun to melt. (see pictures)
I discovered though that this first lens element is indeed glass (
), and it was some kind of clear-ish contaminant that either melted onto the lens, or it was simply left there during manufacture. (makes me wonder how many others are like this!)
OK... so here's the final story on trying to upgrade to an all-glass projection lens (in order to get a better full-image focus)... unless Young Optics
comes up with something, we're SOL... because, again, the OFF pixel absorption area is inside the lens... and that makes the lens 100% custom designed specifically for this light engine.
The first element of the projection lens is retained by a very thin stamped round steel retainer, held in with three tiny screws. This steel piece is black anodized to minimize reflections... but look at the picture and you will see that the black anodizing has been BURNT OFF where the light has been hitting it... adding to internal random light reflections.
I've been noticing a lot of light spillage across my ceiling (I have my projector ceiling mounted) and Left wall when operating this projector in total darkness, even with a full Black test image displayed. To me, this is a bit distracting when watching dark movies, so I experimented with making a "quick-n-dirty" (temporary) lens mask... see pictures for the results. While I did leave the factory chrome lens trim ring installed, I had previously "blacked out" the entire inside of it... all the red surface as well as the ID of the chrome ring... these are all surfaces that may be prone to causing reflections. (I used a black large permanent "sharpee" marker for this)
I'm going to design and 3D-print a lens ring clip-on image mask. I hope to push the mask opening right up almost against the front lens element for the best "effect". By making it "clip-on", I can make it easy to rotate separate from the lens focus ring... and align it after focusing.