A while ago, I acquired some microscope junk. In it I discovered some Swift bulbs that seemed to fit the Lomo Koehler illuminator. The voltage and the socket matched. I liked the flat surface of the filament. All the light entering the condenser would be on the same level. The bulb is a perfect fit and works well. The Koehler image is greatly improved. I even got to like this illuminator because it is so easy to set up when the illuminated field with these bulbs is acceptable.
Apparently these bulbs are still available. I Googled: Swift MA738 and many sites came up. I saw one that sells it for $9.-. This would be well worth the improvement.