Initial Prototyping - Day 4

Thursday came with a great deal of frustration. With two scheduled days to go, one completed lens and only half a housing constructed was not enough. I wanted to investigate corrected surface profiles, and with time running out and no time after work to do any research, I was beginning to struggle. In design I am convinced you could create the best designs in the world, if you had the time. But more often than not, time is just something that isn't available, so overnight I took on the challenge of becoming an expert in basic Aspherical lens design. This type of lens design reads like the holy grail of design methods. Just typing it into google brings up article after article explaining why aspherical elements are so good. They supposedly give the following advantages over un-optimized simple lenses like the one I have already manufactured:

• The main advantage is that due to the unusual profile of the aspherical surface, light rays from both the centre and the extremities of the lens element should converge on the focal plane at the same point, resulting in even focus across the image plane. 
• Less image distortion
• Less spherical aberrations
• Less chromatic aberrations thanks to thinner lenses

 Finding a generic equation to define the surface of the most used type of aspherical lens is easy, even Wikipedia has it. However.... why doesn't anyone explain what all the variables in the equation actually relate too?? It took me far to long on Google to find anything that explained how these lens surfaces are actually designed, but in the end I stumbled across a Korean university paper about defining aspherical lens surfaces through numerical analysis. Nice bit of bed time reading I think!

Now maths isn't my strong point. I got A*A at GCSE and then continued on to fail my AS........... But anyway, these lenses are formula driven, so I have no choice but to get on top of them and work them out. It turns out maths is actually not that bad when you know what the hell the numbers relate too. At school I struggled in core maths because it was all theoretical with numbers that related to bugger all, where as I was brilliant at mechanical maths and decision maths which were based on real things and logical constructs which I can easily get my head around thanks to the way I think! 

With the knowledge I have about lenses and how the numbers associated with them all inter-connect, working through the equations wasn't too difficult. I was able to use one of the many equations to figure out a surface profile for a plano-convex ASPH lens element, based on the refractive index of PMMA which I (thought.....) was 1.5025. Armed with numbers churned out from the equation, I plugged them into a SolidWorks model, exported the profile as a DXF and ran it on the CNC on a fresh billet.

It turned nice and quickly, looked good in the machine, and having let the machine finish I took it out and quickly polished it. I then proceeded to part the lens off (preserving the rest of the billet, no waste, thanks to the back face being Plano). I took the lens and sanded the plano side off using P1000 and buffed it up, as seen below:

So that was that, an ASPH lens element designed and made within the first 2 hours of the morning. The results were........ well I'm going to have to dedicate an entire post to that actually so I'll save that for later.

Moving on, I took the lens body which I had started yesterday and continued to work on it. I bored the centre out to a size that will allow a central column to slide in and out of it, about 52mm diam'. This was all done manually, something I was a bit unsure about doing because using a boring tool means you can't see where or what you're cutting. Also, when boring out the very central bit at the start, the tool ran very hot. Spraying lubricant onto the tool as it ran helped, but it often boiled and evaporated as soon as it hit the cutter.

 In the end the barrel was finished, and gently buffed up to a dull finish. You can see here how it mates nicely with the mounting ring: