I am building a 7 inch lcd projector. At the moment i 'm planning on using the following gear.
220mm first fresnel
320mm top fresnel
320mm objective
What if I had the following equipment?
200mm first fresnel
200mm top fresnel
230mm objective
The projection lens will sit 90mm closer to the top fresnel which should give me almost double the light acording to the inverse square law of light.
Am I right or wrong?
220mm first fresnel
320mm top fresnel
320mm objective
What if I had the following equipment?
200mm first fresnel
200mm top fresnel
230mm objective
The projection lens will sit 90mm closer to the top fresnel which should give me almost double the light acording to the inverse square law of light.
Am I right or wrong?
The top fresnel simply directs all of the light into your objective/projection lens.
The only effect of changing the top fresnel and the objective lens to a shorter focal length will be to make your enclosure shorter and the image bigger at the same throw distance.
Tgreenwood
The only effect of changing the top fresnel and the objective lens to a shorter focal length will be to make your enclosure shorter and the image bigger at the same throw distance.
Tgreenwood
Ok lets say that you place the triplet at 50 feet and there is a fresnel set to direct all the light from the lamp into the triplet still.
So at 50 feet the back of the triplet should receive the same amount of light as at 20 inches away?
That would be true if there didn't exist something called the inverse square law of light. Every 25% increase in distance deminishes light intensity by half.
So at 50 feet the back of the triplet should receive the same amount of light as at 20 inches away?
That would be true if there didn't exist something called the inverse square law of light. Every 25% increase in distance deminishes light intensity by half.
on paper 100% yes. In the real world, it will look a whole ****load brighter, but I doubt you will truly get double the intensity. If it were me I would go ahead and do it, the only difference is going to be your image size. With the shortal focal length of the objective your image will be much larger at the same throw.
Inverse Square law only works for direct radiation, if you modify the light with lenses or reflectors it's no longer valid.
clarification.....
Inverse square law
Check out the link above.
Especially the part that says "The energy twice as far from the source is spread over four times the area, hence one-fourth the intensity."
Inverse square law
Check out the link above.
Especially the part that says "The energy twice as far from the source is spread over four times the area, hence one-fourth the intensity."
The distance they are refering to here is from your lens to the surface you are projecting onto.
The actual movement of the light through air does not decrease its brightness a noticeable amount. Think of a laser, it is just as bright no matter how far you shine it.
The actual movement of the light through air does not decrease its brightness a noticeable amount. Think of a laser, it is just as bright no matter how far you shine it.
You lost me. Through what medium do you suppose light travels between the lamp and the projection lens. Is that not air also.
Yes it is air and that why this distance doesn't matter. As long as you manage to focus your rays of light from bulb into the objective it doesn't matter how far away it is.
The Inverse square law really just says that if the objective lens is farther from the surface being projected onto it will be less bright. This is because the image will be larger. Not because it is travelling through more air.
I hope this helps
The Inverse square law really just says that if the objective lens is farther from the surface being projected onto it will be less bright. This is because the image will be larger. Not because it is travelling through more air.
I hope this helps
inverse square
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
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
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.
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.
what?
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.
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.
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.
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.
shorter fl fresnels
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.
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.
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
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/index.php?main=product&art=OSET1
http://www.exclusiv-online.com/shop/index.php?main=product&art=OSET1
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