In an experiment I am trying to image a phosphor surface on a camera using a 3x zoom lens. The distance of phosphor surface to lens input surface is ~100 mm. The effective focal length of the zoom lens is ~20 mm and the magnification is ~1/5x. This is a C mount lens and have 6 elements. 3 element for condensor lens (front lens) and 3 elements for focusing lens (back lens). All the lenses have anti reflection coatings. In order to decrease the working distance have inserted a spacer (5 mm, to increase image distance) between the lens and camera.

Schematic of the setup

Now the problem is that the collection efficiency of this setup is very poor. I have searched the internet but I only found this article. The article points out the problem description but do not provide any solution.

The problem here is that the condenser lens is of large focal length and do not divert all the incident light to the focusing lens. Only a part of incident light is intercepted by the focusing lens due to its small size. I have put the focusing lens at minimum distance from condenser lens, to increase the collection efficiency.

I have also tried to put a Short focal length plano convex Macro lens in front of condenser lens but It increases the distortions.

The question is how can I increase the collection efficiency of the system, by adding some additional optics without compromising with sharp focusing. If someone can suggest any other technique or any other lens arrangement or any other product which has large focusing lens that would be really helpful.

There are loads of lens combinations available in the market but I do not have clue how to select the right product.

I am searching for the answer from quite some time. Please help. This may not be a basic physics problem but related with experimental aspects.

One of the side queries is how can I estimate the effective collection efficiency of this lens (either experimentally or theoretically).

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    $\begingroup$ A macro lens with high resolution, large aperture and large f-stop is very hard to design (I naively tried that for over a month using an optics design program before giving up, for a very similar application to what you are describing here - it's one of my ultimate design failures). If you have the money (and we are talking about a potentially expensive item), I would suggest looking for a suitable high end photographic lens. If not, you definitely have to use aspheric components and... before you get started - get a textbook on lens design and articles which describe the design procedure. $\endgroup$
    – CuriousOne
    Commented Jul 5, 2016 at 7:58
  • $\begingroup$ @CuriousOne Thanks for your comment. Can you suggest any good book, which also has simple examples using Zemax. $\endgroup$
    – hsinghal
    Commented Jul 5, 2016 at 8:13
  • $\begingroup$ I found some very good papers on the topic, back then, but I basically had to abandon the project and I left all the materials behind... so, sorry, I can't give you the references. I think WetSavannaAnimalAkaRodvance is deeply into optics. Maybe he can give you the right pointers? What I do remember about the design procedures are two essential lessons: 1) You have to place the lenses in such a way that the angle of refraction for rays are as small as possible. Look at condensor lenses and microscope objectives for guidance. 2) I got much better results with aspherical elements. $\endgroup$
    – CuriousOne
    Commented Jul 5, 2016 at 8:21


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