Inkie is because of PacMan you goof!
No, while Stagecraft once discussed in length the development of the "Birdie
", I don't remember the advent of the Inkie but it probably has something to do with the movie industry.
If of help I do remember single contact versions of the Inkie still in constant use. Great help in the dark in a mixed inventory. Twist a dual contact into a single contact base
and it would not work. Screw a single contact lamp into a dual contact newer base
and you had an explosion as it were.
Useful light? if in need to ask this question....
The about max spread of a say PAR
20 can would be about 40 degrees I expect. Go to TMB or Thomas's webstiets and see if they have specs listed.
As for useful light, lots of it over that of a PAR
by way of soft light and was.