A solution for projectors with 17" LCDs!

[superdaveumo]

So all those who want to use a 17" LCD, this is for you. After searching hi and low and trying out different lenses, I have found a lens that works! The source? Opaque projectors. These projectors (aka artographs) are used to take "small" pictures and magnify the image onto a wall. It just so happens that some of these projectors accept 10" x 8" and even 14" x 11" pictures. Hey, sounds close to the size of a 17" LCD (13.3" x 10.6" approx). The great thing is, these can be found on ebay for cheap!

The particular model I will talk about here is the Beseler Vu Lyte III, although others will work too. When searching for a projector, look at the specs for it. First, it needs to have the focal length you are looking for. 18" (457mm) is very popular. The other spec to pay attention to is the size of the paper (often called the plate) the original opaque projector accepts (10x8, 14x11). Some accept smaller sizes like 6x6 or 4x6 pictures. Probably won't work. The Beseler Vu Lyte III has a focal length of 18" (457mm) and it accepted 10" x 8" pictures. So, on to the pictures!

This is the projector style you are looking for. It's big, ugly, and weighs a ton.



Here is the lens. It's about 137mm in diameter.



After a lot of unscrewing and pulling things apart, here is the lens.



This lens is around 157mm long. So about 20mm longer than wide. Not too bad.



There are other things you can get out of these projectors. I managed to get a front surface mirror, 2 sheets of glass (possibly IR filter glass?), the lens mount and adjustment, and a power cord.



Now, the test. This was a very crude but effective test. I haven't taken apart my LCD yet. I hooked the LCD up and set it facing the wall (about 8 feet away from the wall). Then I placed the projection lens in front of the LCD and projected the image onto the wall. I made a covering around the LCD and lens to get rid of a lot of light leakage. Here are the results.



And from Star Trek Enterprise.



Now these are upside-down and backwards because there is no image correction in my test setup. Considering the light source is the LCD backlight and I used a trash bag to get rid of light leakage and the lens is just set on a box and I didn't line anything up acuratly and the image is projected on a plain wall, the image shows very well. These images haven't been modified (minus cropping and resizing). The real point of these images is to show that the lens CAN "see" the whole LCD screen and project it well. There is also room to move the lens around. The edges are not darkened. In fact, I was able to see clearly the LED indicator lights (well below the actual screen) on the wall. You can see them in the upper right corner of the screenshots. Even though the lens was designed to "see" a 10" x 8" image (picture), it clearly "sees" my 17" LCD with room to spare! So with a proper setup, I think this lens will work great!

[CCMCornell]

Thanks for your findings. I think you touched on some of these issues, but I just wanted to ask to be clear:

is brightness uniform all the way to the corners? Usually, brightness falls off as you leave the center.

Is the entire image in good, uniform focus? As often discussed, large panels projected through the common projection lenses would result in corners that were slightly unfocused even if the center was in focus.

[superdaveumo]

Is the entire image in good, uniform focus? As often discussed, large panels projected through the common projection lenses would result in corners that were slightly unfocused even if the center was in focus.

You bring a good point too. These questions are a little hard to answer EXACTLY right now because the lighting was only the LCD backlight (which are known not to uniformly light up an LCD, let alone make a good projector light) and my lens was just sitting on a box. What I observed, as I moved the lens side to side (or up and down): The entire LCD can fit in the lens' image circle. In fact, the lens sees more than the LCD (it can see the bezel and LED power lights as well). So, darkening of the corners was not a problem.

As for the focus, I was a little scared when I first did the test setup because I was getting out of focus edges with the in focus center. BUT! The lens was not perpendicular to the LCD. So, after some adjustment (moving the lens more perpendicular) I was able to get the edges and the center in focus at the same time!

Again, I will have better reports when I get the actual setup built.

[Rox]

the main question here is;

what image size/throw did you have?

i think it is somewhere 70inch diagonal

[Guy Grotke]

I managed to buy JUST the lens instead of the whole 3 ton projector. (If you do buy an old opaque projector on eBay, you might consider asking the seller to ship you the lens and junk the rest!)

I use my Besseler 18" lens with a 15" LCD (250 Watt MH projector), and it works pretty well. The lens diameter is so large that I get a lot of light on the screen. I can see cracks in my LCD mounting frame that are well beyond the 15" LCD area in my projected image, so I think it could be used with a 17" LCD.

But it is not perfect: Instead of clean black screendoor around the pixels, I see grey screendoor. (That is a sign of somewhat lower quality of the image sharpness.) I also see quite a bit of fall-off in the Lux level from the center to the corners. It is not unpleasant to watch, but you can see it. Finally, I can't get the center and the edges to both focus perfectly at the same setting.

Even with those imperfections, I am still using it instead of my smaller process lens, because the screen image is so much brighter. And here is a key point: I can't see any of the imperfections when I am watching video from my normal viewing location! (Even with my vision corrected to better than 20:20.) Like I have written here in the past: I don't think you really need a perfect lens, because our pixels are so large (compared to film grain). You just need a lens that does flat-field and color correction with a wide-enough FOV, and these lenses do that very well.

[superdaveumo]

what image size/throw did you have?

Very good question. The screen in the picture was probably a 5 - 6 foot screen. I did test it at the location it will be in the room (about 10 feet away) and the image was just as good (a little dimmer of course). I did manage to get the image in focus at the edges and the center as I was messing around with the lens. Like I said, this is really just the tip of the iceberg for this lens. I still have to get proper lighting and mount it properly to get the full results. But, the initial tests have been VERY promising. I should do a circle test. I'll do that and post what I get.

[superdaveumo]

Ok, I did some image circle testing on this lens. The best setup I could do, for now, is to take an LED and move it vertically behind the lens until it disappeared on the wall. I started way above the lens and lowered the light until the led appeared on the wall (edge of circle) and measured its distance from the floor. Then I continued to move the led down until it disappeared from the wall (other edge of circle) and measured the led from the floor. The LED to lens distance was where the LED was in focus on the wall. I was able to move it: at 4ft around 75cm, at 8ft around 65cm, and at 14ft around 58cm. These measurements are the diameters of the viewing circle.

Now this doesn't say if it is razor sharp from corner to corner, but it does show how big of an image (LCD) you can use! With a viewing circle between 58cm and 75cm (22.8" and 29.5"), I feel QUITE confident that this lens will see a 17" LCD corner to corner sharply. I would even say this lens could see a 19" LCD. But that is just speculation.

[j3ff]

kewl lens! I love it's LF How much did you pay for the opaque projector? And for the field fresnel, what FL you think you'll be using? Eager to see your results

[Rox]

ok, there are two things;

1)what really the lens sees;

this is the light that enters the lens and gets out from the other side. The only limitation is the housing.

2)what the lens aberrations are corrected;

this is the lens FOV. The lens has been designed to work with image sources under this FOV. Inside the FOV the focus is under a critical value. I deally, the LCD should be inside this FOV.

This besseller opaque lens, is very close to the diyproyector one. I believe all of them are practically identical, and compatible if a remplacemnt is needed. Well, the only FOV angle spec from manufaturer i have seen, is the one from AWI optics. (135mm diameter, 450mm focal). And definitelly the FOV was really narrow.

Now this is up to you.

[superdaveumo]

And those two points are absolutely valid. What I measured was simply the viewing circle (how much can this lens see). I can say informally that the lens' FOV is probably greater than the AWI version. Only in the last few centimeters did the image (the LED) have less quality. Again, this is something that I will be able to test and report when I get more parts.

Now as I understand it, although this lens has similar physical sizes, the actual lens design can be EXTREMELY different verses other lenses of the same physical size and weight. So, although you may be right that it is similar to the AWI, it's unfair to conclude that this is so. The lenses could be curved differently and positioned relative to each other differently. Please correct me if I'm wrong about that.