In trying to understand the Lux production of my pj I varied the distance from the pj to the screen, and I ran into the following problem.

I have a LCD image that is 720 pixels high (L), which at the Samsung 740N 0.264mm dot pitch is 7.48" (confirmed by direct measurement on the LCD). I have a LL Pro lens whose advertised fl (F) is 500mm.

The magnification (M) of the image is the height of the image on the screen (H) divided by L.

The relationship of the distance of the lens to the screen ( is B = (M+1) * F.

So ... if I measure the H I can determine M. If I then measure B I can determine the actual F of the lens.

So I did and got the following:

H = 59", M = 7.89, B = 3987.8mm, F = 448.57mm
H = 51", M = 6.82, B = 3454.4mm, F = 441.74mm
H = 42", M = 5.61, B = 2676.2mm, F = 395.79mm
H = 40", M = 5.35, B = 2463.8mm, F = 388.00mm

H and B were measured, M and F were computed; L remained constant.

I was expecting a 40" 16:9 image (given the Samsung L and the assumtion I was dealing with a 500mm fl) and 10.4' from the lens, I had to go to 8.08' to get it. FocalCalc predicted 9.86'.

Why does the fl appear to change? Why did FocalCalc predict the wrong B?

Thanks

What are the corresponding LCD to triplet distances for each of those screen heights?

I left the distance at 578mm since I wasn't interested in focus but size. I had split the fresnels and placed them 100mm apart so the EFL of the condenser magnified the 71mm light arc to just fill the 100mm diameter of the triplet, as per your reminder in my "Strange Dimness" thread. Moving the LCD sled 30mm toward the light (608 total distance to triplet) brought the 16:9 movie into focus but didn't change the screen height (30mm brought the sled to within 10mm of the rear fresnel).

I was in focus at 3,987.8mm (13'1") at 59" high on the screen -- but that only yields an effective 448mm fl for the triplet. When I move the screen toward the projector, I would have to move the LCD to maintain focus (since I held the triplet fixed to maximize the light throughput -- moving the triplet to focus now seems just plain wrong from a light level point of view). But the frustrum of light coming out of the triplet doesn't change, so the size of the image on the screen should just get smaller based on the focal length of the triplet and the distance to the screen.

Right?

I’m not sure what you are actually doing in you tests.
Where are you measuring the FL of the triplet?
Are you using a fresnel in front of the LCD?
Is the image focused when you measure these distances?

DJ

thats the key!

Hmm ... The test was this. I arranged the fresnels (330mm rear, 550mm front, 100mm apart) so the 71mm arc on my MH lamp is magnified to 100mm (clear diameter of the Pro triplet) at the screen edge of the triplet. This should put the maximum amount of light out the lens. The front fresnel is 560mm from the front of the triplet, the light arc is 397mm behind the rear fresnel (all distances calculated from the Edmunds Optics Handbook for Boys).

I place the LCD (with a 720 pixel high 16:9 movie (no letter-boxing) playing) 578mm behind the middle of the triplet (47mm behind the front frsnel and 53mm in front of the rear) and get a focused image 13'1" away 59" high. 720 pixels on a Samsung 740N is 7.48", so the image magnification is 7.89. My trusty Lux meter reads 9 Lux in the center of the screen.

Now I move the plane of the screen toward the projector, changing nothing in the projector. What should happen is the image on the screen gets smaller and brighter. It was the brighter I was testing for -- trying to see why 1/distance_squared wasn't working. I measured in the middle of the screen so there were no cosine effects.

At 13'1" (3.99m) I get (9 Lux * 3.99m * 3.99m) = 136.89 lm/m2 out the triplet. When I move the screen to 8'1" (2.46m) I measure 15 Lux -- what I expected was 136.89 / (2.46 * 2.46) = 22.6 Lux. The image height at 2.46m is 40" (5.35 magnification). The lens position at 5.35 M is (F / M) + F, where F is the fl of the triplet, = 593.5mm (15.5mm toward the rear fresnel). I had to move the LCD back 25mm to get focus. The image height was still 40".

I moved the LCD rather than the triplet because having aranged the condenser to fill the triplet with the lamp arc, it seemed a Bad Thing to move it to accomodate the focus. As long as the LCD stayed more than 20mm behind the front fresnel and 15mm in front of the rear fresnel, moving the LCD to focus this POS should be a Good Thing.

This is where I sat down and did my fl calcs given above.

One of the heuristics that I've found is that the front condenser lens fl should be GE to the LCD to triplet distance (not the triplet fl). I have a 550mm front fresnel from 3DLens (the rear 330mm is also from 3DLens). I will replace the 550 with a LL 650mm lens and rerun the measurements.

What I think I have to do is disassemble the proj and mount the components on an optical bench, where I can measure the light and image flows to see what is happening. I can also change the light from the current Venture 875W PS MH, to an Altman 360Q 4.5x6.5 575W light engine and precondenser-- and if I can see how to replace the G9.5 socket on the Altman with a G12, I could even try the 150W ceramic.

From what I can tell you have two issues here. First is that your arc is too long for this setup. About an 80mm arc image will be the most that you could hope for to fit through the triplet. This is going to throw out your lux measurements. Second issue is that you have another lens, the front fresnel, in-between the LCD at the triplet. This means that you need to allow for the extra magnification it adds to the system. The front fresnel is going to create a virtual image of the LCD that will be larger. So calculating the system magnification is not just a matter of image height / object height, it now becomes image height divided by virtual image height.
Click to view attachment
The focal calc will give you the virtual image details and total magnification as well as the fresnel’s independent magnification.
You should also note that even this gives a slight variation in the triplet FL. It is due to the thin lens equations used and the assumption that the principal plane is at the center of the triplet. If you want to be more accurate then you will need to measure where the first and second principal planes and their relative positions inside the triplet. It does start to become complicated doing this and why I left it out of the focal calc.

DJ

Thank you -- for your patience, your calculators, as well as your ongoing contributions to my optical education.

I varied the screen distance in focalcalc with the front fresnel distance set to 20mm until the triplet fl was about 500mm -- 10.01 feet. I'll rejig the proj to these numbers and try to ignore the pathetic lux. I have a Altman 360Q + 4.5x6.5 lens on order from a dealer for whom I wrote some software -- I thing the aperture is about 30mm, so with the precon I should get way more light through the triplet.

Thanks again.