Understanding Pre-Condenser lens

[Squeeky]

Ok I need some help, I'm looking to build a new projo using the 15.4 Proview. I would like to use a pre-condenser in order to increase brightness. Most lens I've seen are between 90-120mm dia. I plan on using a tubular bulb like the s400-dd, my concern is what is the optimal FL form the condenser lens. If I understand correctly I want to get a lens that matches the condenser fresnel.

So if I were to use a 220mm FL rear fresnel, could I use a 200mm FL condenser lens, mounted 20mm from the arc of the bulb?

Could someone please explain in stupid terms

Thanks,
Squeeky

forgot one last question PCX, DCX which is best to use or do they both increase brightness?

[Guy Grotke]

Pretty close, but you can't just subtract to get the answer. And you can't get anything within 20 mm of the center of a Ushio retrofit lamp's arc, because the T15 lamp (15/8") has a 23.8 mm radius! I have read posts that claim success with a 200 mm fl PCX with the flat side 1 to 2 mm from the lamp's outer envelope.

You have to do more than just move the lens toward the lamp until all of the fresnel's corner's are lit. The light has to enter the fresnel as if it was coming from the fresnel's focal length, if you want it to come out the other side parallel. (Of course, you may not want it parallel to adjust the fical point of the lamp arc image into the projection lens.)

If you do want it parallel, then you need to put the condensor lens at the position where it would perfectly intersect the cone of light going to the fresnel from a lamp at the focal distance. Then you need to move the lamp closer to the lens to compensate for the magnification. That should just light the corners of the fresnel.

Here is a graphical design you can use to make your own full-scale model. Substitute your own distances!

This example uses a 15" LCD, a 220 mm fl bottom fresnel, and a 100 mm diameter 200 mm fl condensor lens.
Start by drawing the central axis, line 1. (Make a full scale drawing.)
Draw the bottom fresnel, line 2. This is a 15 inch line, centered on the central axis.
We want the light to exit the right side of the fresnel toward the LCD perpendicular to the fresnel. (see arrow) For that to occur, draw line 3 from 7.5" up the fresnel, to a point on the axis that is 220 mm left of the fresnel. This is the path the light must follow to get refracted along the arrow at the top.
Now draw the 100 mm diameter condensor lens, line 4. Place it so it just intersects the required light path, line 3. At this point you can measure the distance between line 4 and line 2, the condensor-to-fresnel distance.
Now draw line 5, parallel to line 3, but through the center of the condensor lens. (Light that passes through the center of a lens is not refracted.)
Next draw line 6, 200 mm to the left of line 4. This is the focal plane of the condensor lens.
Since line 3 and line 5 are parallel on the right side of the lens, you can draw line 7 to the point where they focus on the left side of the lens. (This shows you how light traveling to the right along line 7 will be refracted to line 3.)
At point 8, line 7 intersects the axis. This is where you put a point-source light. You can measure the distance between point 8 and line 4, to get the light-to-condensor distance. You can also see the difference between such a projector with or without a condensor lens: Adding the condensor lets you move the light just a bit closer to capture a few more degrees of the light.
Without it, the light would be placed at the intersection of line 3 with the central axis. For a small-format LCD and a 220 mm fl fresnel, a condensor lens makes a much bigger difference.

[Squeeky]

Thank you soooo much, I'll be drawing this when I get home! Also wanted to ask does it matter if it's a PCX, DCX? The lens I found is a DCX will this cause an issue? If so where can I find a similar PCX. Do you recommend I get a condenser lens close to that of rear fresnel is FL? Thanks
Squeeky

[Guy Grotke]

The very best are Pyrex or similar heat-resistant glass with an aspherical curve on the LCD side and almost flat on the lamp side. Those are pretty expensive.

Next best are PCX: Flat on the lamp side. DCX have more spherical aberration, which translates into more light missing the projection lens.

The whole purpose of a condensor lens is to capture as much light as possible for the fresnels. You do this by getting a large diameter lens as close to the lamp as possible. You can try some drawings using different diameter and focal length lenses, to see how they fit. If you get one with too short a focal length, you can't get it close enough to the lamp arc to light the fresnel corners. If you get one with too long a focal length, it has to go farther away from the lamp and miss some light.

Lots of the DIY projection online stores have condensors. You can also look at edmundoptics.com, surplusShed.com, and rolyn.com. Find one with heat resistant glass.

[Inkog]

OK Guy,

If I'm getting this right, this means a shorter condenser fresnel FL will give you more usable light, and add to the benifit of a spherical reflector. Based on this, a shorter pre-condeser lens FL will also provide more usable light, at least up to a point.

See my images below for my reasoning.

Am I getting this right?

*edit for clarity

[Guy Grotke]

Right. A shorter focal length pre-condensor lens can be placed closer to the lamp arc and still send all the captured light to the fresnels. By putting it closer to the lamp, it captures more of the lamp's light output. But if you get one with too short a focal length, then you will put it right up to the lamp's outer envelope and it still will converge the light into a cone too small to light the corners of the fresnel. Then you are SOL!

Using a spherical reflector is independant of using a pre-condensor lens. It will increase the amount of light coming out the front of the lamp by reflecting the light coming out the back. If you do use both together, then the spherical reflector has to match the angular width of the light being captured by the pre-condensor lens. If it is too narrow then the image edges and corners will be much dimmer.

[Squeeky]

Could you please explain what occurs when two PCX lens are placed on top of each other. I read about people doing this in diylabs, but I would like to know how that is calculated as well please.

[Guy Grotke]

When you combine two thin lenses, in close contact, then the total power in Diopters is additive. Diopters is defined as:

Diopter = 1000 / focal length in mm

So Combined Diopters = D1 + D2

You can substitute one of these equations into the other and get:

1/Effective focal length = 1/f1 + 1/f2

For example, two 1000 mm fl lens together give you an EFL of 500 mm.

One interesting thing is that these equations still work for negative lenses. You just have to include the sign of the lense's focal length when you do the math.

So a +1 Diopter lens and a -0.25 Diopter lens gives you +0.75 D. Or in focal lengths:

1/EFL = 1/1000 - 1/4000 = 3/4000 = 1/1333

[Megaman]

I came up with these equations for the different distances:

Fresnel focal length, F.
Fresnel and lcd size, D.

Condensor diameter, d.
Condensor focal length, f.


Distance between condensor and fresnel, x : D/F = d/b, x = F-b.

Distance from condensor to light arc, a: 1/a = 1/f + 1/b

Using my expected setup:
F= 210mm
D= 264mm
f=112 mm
d=76mm

I get b = 60.5mm, x = 149.5mm and a = 39.3mm.

Am I correct here?
How would I set up the corresponding equations for the projection lens?

[Guy Grotke]

The most useful equation for a projector lens is:

1/EFL = 1/LCD-to-lens + 1/lens-to-screen

You usually know two of these, so you can solve for the third value. Once you know both distances, you can also calculate the image magnification:

M = (lens-to-screen) / (LCD-to-lens)

Image size is:

I = (LCD size) * M

Of course, you can also work backwards from a desired I value to get the projection lens EFL that you would need.