100 % Brighter 7" with 230mm Objective?

[Guy Grotke]

If you were not using a field fresnel (second fresnel) to direct the light into the lens, then the inverse square law would apply.

Inverse square applies to a point source with light radiating off in all directions. But the light does not radiate off in all directions from each LCD pixel. We use the condensor fresnel to make the rays mostly parallel. Then we use the field fresnel to make them converge into the projection lens. So we get MUCH MUCH more light on the screen than we would without the fresnels.

OTOH: Inverse square does apply for the lamp-to-condensor-fresnel system. A 200 mm fl condensor fresnel will get more light through it than a 220. Of course, the difference between the brightness at the center and the brightness at the corners will be that much more pronounced with the 200 mm fl fresnel. This is why some people us a pre-condensor lens to send the light cone to a 330 mm fl condensor fresnel instead: More even lighting

[dj_holmes]

so what is the outcome?? you get twice the image size but its brighter?? that seems like a good trade to me!!

i need some non technical straight answer to the equation. i'm in the mood for experimentation. i got 2 pj's on the way.

[Guy Grotke]

The outcome of what?

"twice the image size but its brighter" is very unlikely, but I don't understand what you meant.

If you were referring to the use of fresnels, then you are mistaken. They do increase the brightness, but they don't make the screen image larger to a noticable extent. Even with a split fresnel design, the fresnel past the LCD is so close to it that it has very little effect on the image size. (Unless you tilt it to do keystone correction, which is another kettle of rotting fish!)

Generally, increasing your image diagonal size by 2 times will result in a screen image that is 4 times dimmer, if everything else is held constant. The most common mistake DIY builders make is to try for a "super-sized" image that they can barely see. Smaller but bright is much better. Also, screen images that are too big for your viewing area give you a sore neck and eye strain, from moving your head so much.

[dj_holmes]

so basically IT IS 100% BRIGHTER??? just like the heading suggests?

[dracul]

Ok, now that i have a little more experience i can answer this question myself. Assuming an unsplit design the answer is NO.
Using a 260mm objective will not make your image any brighter.

All you are doing is focusing the arc of the bulb at the back of the objective. As long as you get the entire arc to fit within the glass at the back of the objective you are maximizing the brightness.
So if your fresnel or fresnels are set to focus this arc at 320mm or at 5 miles the brightness on your screen will be the same as long as you get the entire arc filling the objective and all other things being equal.

Now if you had say a 220mm back fresnel and suddenly you switched to a 160mm back fresnel. As long as you can still focus the arc perfectly at the objective you will have doubled the brighteness according to the inverse square law of light. And here this law applies because it is before it passes through any optics.

Does anyone know where i can find 160mm back FL fresnel in a OHP doublet design? That is 160mm and 320mm unsplit doublet.

[dj_holmes]

dracul....mmmmm interesting keep me posted or else

[Guy Grotke]

I know there are plenty of sources for fresnel lenses, but I suspect you are looking for cheap fresnels. I have seen some cheap 200 mm fl. (I think it was at 3dlens.com) That should give you 21% more light than a 220.

Also, I have seen projector designs where they used a stack of fresnels together. Two 330 mm fl fresnels together would give you a 165 mm fl condensor.

[dracul]

Also, I have seen projector designs where they used a stack of fresnels together. Two 330 mm fl fresnels together would give you a 165 mm fl condensor.

But i want to use unsplit design behind the lcd. So would i join all three 330mm FL Fresnels?

[dj_holmes]

this guy got excellent results using 3 fresenles sandwich first one 220 second 330 and third 330...his images was sooooo bright.. cant remember the link but i defo saw it

[dracul]

The 3 fresnel system will force me to place one fresnel in the image path. Can I use this double fresnel at 160mm from the arc unsplit?

http://www.exclusiv-online.com/shop/...duct&art=OSET1