I just published Part 1 of a multi-part article on building your own low-cost HMI type fresnel in the Articles section of the Blog.  In part 1 we cover a bit of the theory behind HMI, what it is and how we can find “generic”, less expensive versions of it to serve in our low cost fresnel.

Build Your Own Metal Halide (Low Cost “HMI”) Fresnel - Part I