This species was the first Emperor species I came into contact with, near Bergerac in France in 2005. That trip produced the butterfly photograph on the left and below that, of the clytie form. 

I then had to wait some years to see the nominate form, pictured further down, which was very different, seen in woodlands near Geneva. The four below show the iridescence variability of the species, also taken near Geneva, in 2017.