Hi everyone,

I'm gathering some info on the effect of the focal lenght of the field fresnel. The only thing i've come up with is the "You should use a field frenel with a Focal Lenght thats is a bit longer then your triplet's FL" rule. What I would like to know, is what would happen if I was to use a 330mm field fresnel with the 500mm pro lens ? Would I simply be using less of the lens surface ? Would it affect the ability to focus the image ?

Using the focal calculator (in split mode), you can see that it as only a minimal impact on the throw length, so I assume that it as a different impact on the results.

I'm asking because I will soon start testing a possible project using a luxeon leds network (actualy, i'll be using the prolight copy of the luxeon) with individual reflectors creating a 10 degree beam (10 degree to the normal, so it's in fact a 20 degree cone of light). If the technical paper are right, I calculated that 75% of the light is in the first 5 degree to the normal and I want to gather has much of it as possible in the triplet. Since the LED setups don't use a colimator fresnel (you assume that the light is already pretty much collimated), the field fresnel becomes very important. I think that using a 550mm or 650mm field fresnel, alot of light won't get in the triplet. Am I wrong ?

Can someone point me to some info regarding the effect of the focal length of the field fresnel ? Searching and browsing the forums didn't get me an answer in the last two hours.

PS. : I'll create a proper post for this project once I get enough info to know if it is doable

I kind of think the whole point to having the fresnel at a little longer FL is that
you don't want a point of light at the triplet, but rather a small image of the LCD.
If it's too short then the image is too big and doesn't fit in the triplet...

Someone else please chime in because I'm really not certain about that.

I don't understand your logic behind not wanting to use a long FL fresnel. Why do you think a 330mm fresnel will send more light to the triplet?

I believe a 330mm wouldn't work. It will not direct the light far enough to even hit the triplet before the image is flipped. The goal is to fill the triplet with the 'cone' of light from the field fresnel.

Doing some raytracing with Optlab, I could see that my array of 10 degree cones of light were focused on a smaller spot-circle using a 330mm field fresnel (~30mm radius) then with the 440mm fresnel (~40mm). That was the basis of my asumption.

Now, I have to say that i am somewhat confused by this subject. I didn't realise at first that the absolute minimal distance between your lens and your field fresnel is the lens' focal length. I had it the other way around (using the field fresnel FL has the minimal distance). The focal calc program doesn't give you an error when you use too short a FL on the field fresnel in unsplit mode.





That's another thing that wasn't clear. Some of the tutorials on www.allinbox.com (french site) clearly show that the images is fliped before it hits the triplet, but others put the field fresnel FL on the other side of the triplet. Some even say that the perfect field fresnel would have it's focal point exactly on the first lense of the triplet (yet again, without explaining why it would).

On another subject, anyone knows of a better raytracer then Optlab that is easy to use ? I can't simulate 550mm lenses with it because the rays aren't traced far enough to converge.

the distance between triplet lens and LCD is fixed. The fresnel is just used to focus light at the appropriate distance.



nope it sure doesn't



the image is flipped before it hits the triplet because the triplet will flip it and reverse it again. This is why be put the LCD in upside down and backwards.

We don't want the focal point of the field fresnel to focus on the first lens, or anywhere inside the triplet.. we want it on the otherside of the triplet, so that the cone fills the triplet.. like so (also pictured is what would happen with a shorter FL fresnel w/ the pro triplet.. albeit exaggerated)


and if link doesnt work http://img2.freeimagehosting.net/image.php?ea1bf36dea.gif

The fresnels create an image of the arc at the triplet. The triplet creates an image of the LCD at the screen. To get most light throughput the arc image should be created inside the triplet. If the arc image is focused too short to the triplet then the corners start to turn yellow/brown. To far forward and they turn blue.
I think I got the colors the correct way around, if not I blame it on the flue.

DJ

That explains the note in my 3M 9200 ohp manual about adjusting the corners.

Click to view attachment

As you rotate the adjuster, the bulb moves in or out closer or further from the bottom mirror which affects the distance from the light to the triplet by a few mm either way.

I wondered what effect that would have.

i was actually asking this same question in another post, as the focal calculator led me to beleive that you might be able to change your throw by changing the front fresnel. the focal calculator also decreases the distance from the lcd to the triplet so that the image hits the right spot on the triplet instead of in front of it like in your diagram. what would this do to the picture?