Nice visualization! But the optimizer could use some work. It wasn't really able to handle the data I had lying around for an 0402 hyperbright white LED (EAST10

Nice visualization! But the optimizer could use some work. It wasn't really able to handle the data I had lying around for an 0402 hyperbright white LED (EAST1005WA0) even though this should have been in its ballpark. (Though this LED is particularly "soft" which might have been troublesome for the basic 3-parameter model?) If you want to improve things the change that will probably get the biggest benefit for the least work is to have the optimizer work in log Is instead of Is directly. (At least, that helped me a lot when I did the super-basic 2-parameter version of this in Excel for somewhat cruder purposes.) If you're up for heavier lifting, because this is only a 3-parameter model, you can probably write out an explicit or approximate explicit solution for 3 input data points, then expand that into a least-squares solution for N data points, then use the actual heavy machinery optimizer as the final correction step. It will run very quickly if you give it a good initial starting parameter set. That parameter set will also be a decent starting position for the more detailed models used by LTspice and friends, should you like to go there.