# What Exactly Does a Condenser Lens Do?



## Ginseng

Can anyone explain what a condenser lens does to a light beam? Does it focus? Collimate? 

Thanks,
Wilkey


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## wasabe64

Ginseng, here is what I have found about it:

"A condenser lens is actually a pair of matched Convex lenses, (Convex/plano?) placed 'back to back' in such a way that the curved surfaces face each other in the middle, and the flats are on the outside. (The convex lenses are basiclly just magnifying lenses, but flat on one side) The idea behind the condenser lense is to allign the light rays into a more or less straight path..."

I found this in a photography forum here.


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## gwbaltzell

Wilkey

You might find this Edmund optics article interesting.

Or maybe the design of my ancient slide projector: spherical mirror -- quartz-*iodine* lamp -- condenser lens (2 lenses) -- slide -- projection lens -- screen

George


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## gwbaltzell

I had a call for further explaination of the condeser lens. I will undertake this but I'm hoping someone with more background in optics will field this. While we wait I'll be working on some drawings in case it comes down to me to explain it. /ubbthreads/images/graemlins/popcorn.gif


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## Ginseng

Wow, 
I certainly appreciate the effort. I've heard some apocryphal accounts of condenser lenses being used as collimators. I knew fresnels were also being used as collimators. When I tried fresnels, I found they focused the emitter to an image at a specific focal distance as opposed to creating a collimated beam of parallel rays. Must be my incomplete and imperfect understanding of optics that has resulted in this effect.

Wilkey


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## gwbaltzell

OK Wilkey, I'm going to try this. You may be disappointed because there is no real magic in a condenser lens. They are, of course, of value in some situations. I've also opted for the short (relative) explanation with one illustration.

A simple lens is one that's flat on one side and curved on the other and is thin. If you have rays of light coming from a theoretical source an infinite distance away, the rays are said to be collimated (parallel). A simple lens will take these rays and focus them to a theoretical point.





If we instead put a point of light at this focal point then the rays coming through the other side will be collimated. If there are no other optics, then this is *not* called a condenser. It's called a condenser when additional optics are used. Its purpose is to provide a large area of light on something else. On a microscope it lights the stage were the slide goes. On projectors the film. On theatrical gobos the pattern or shape that's to be shown on stage. On projection headlight the shape to be thrown on the road. All of these have additional optics to focus on the stage (microscope slide) or gate (as its called in the other uses). The amount of light the lens captures depends on the focal length (where that point is from the lens) divided by its diameter. This gives the _f_-number. The smaller the _f_-number the more light is gathered. However, we are only getting light from one side of our point. To get more of the rest we use a reflector. A spherical one could reflect light back to the point. A parabolic one could collimate the light by itself. However in combination with our condenser lens this is not what we really want. Note however many everyday light use a parabolic reflectors either by itself or as a condenser to feed fragmented (fresnel) lens in jobs like standard headlamps. The third reflector option is the most efficient but I suspect the hardest to design. A ellipsoidal reflector with the condenser lens focused at that second focal point of the ellipse.
Now some nagging points. It turns out for many jobs the collimated beam is not the most desired. In a projection system the light should spread to cover the rearmost element of the projection lens.
Why are most condenser lens two simple lens "belly to belly"? It turns out this is not anything special about condensers. A simple lens is subject to certain problems which are called "aberrations". Putting the lens together this way (a doublet) is the simplest way to correct for these aberrations to some extent. A good quality hand magnifier would also be made this way.
What is a fresnel lens? A basic fresnel lens is one that acts like a simple lens that would be much thicker and heavier. It does this by being divided into angled segments. A fresnel does not have to simulate one large lens. Different segments could be focused in different places like a car headlight.
If we were designing a spotlight /ubbthreads/images/graemlins/wink.gif and wanted sharp edges it would be designed as a projector with the edges of the gate defining our shape and size and could be adjustable (just like a gobo spotlight). If we don't need sharp edges then the projection lens and gate can go and the lens in front of the light is not called a condenser.

Did this help or make things worst?
Anyone see any errors or things that need further explaination?

edit: put in better illustration (not a simple lens though)
edit: A point I failed to mention. A mirror with the light source at its focal point produces collimated light from that side. At 2 times the focal point it is redirecting the rays back to the source. If we are using a lens then this is what we want. In reallity it needs to be just under 2 times to reduce the amount of light blocked by the very real light source.


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## gwbaltzell

I'm thinking I failed to cover a simple but important point. A beam is collimated when then focusing device (lens or reflector) is adjusted at a distance from the light source to produce an beam that is the same size as the lens or reflector (or combination) and will not vary in size with change of distance to were the beam is projected!


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## Ginseng

GW,

That is an excellent explanation and I understand it now. Basically, a condenser is a lens system with a specific lighting function, for example, collimation. So, if I inderstand this correctly, on ecould build a collimating light by using a Carley elliptical reflector and positioning a condenser lens at the "external" focal point of the reflector. The light issuing from the other side of the condenser would then be collimated to a beam the diameter of the condenser. Correct?

Wilkey


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## gwbaltzell

That's almost the way I understand it. I think the externally lens is positioned it's focal distance from the second focal point of the reflector. Most systems until recently usually tried to be calculated based on a point source or flat plane single sided emitter (Lambertian) which is what I believe a condenser models. Modern computers can do more complex ray tracing now in a reasonble amount of time.
Edit:
Just came across a .pdf document from Perkin-Elmer on their short-arc lamp. It has some raytraces with both parabolic and eliptical reflectors. Didn't see anywhere were they specifically talked about spotlight applications.


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