Comparison of planar (top) and silicone dome (bottom) opto-isolator layout (both in DIP package for convenience). Based on drawings in:
planar: Forrest M. Mims (2000). Mims Circuit Scrapbook (volume 2). Newnes. ISBN 1878707493. p. 100.
dome: Christophe Basso (2009). Dealing with Low-Current Optocouplers. Energy Efficiency and Technology, September 1, 2009. Retrieved November 2, 2010.
Own drawing based on concepts patented back in the 1970s and (presumably) not protected now.
East of Borschov