https://youtu.be/I786pgkby_I
Aimpoint introduced its first tubular red dot sight in 1985, the Aimpoint 2000. They were still making direct rail-mounted optics like the Aimpoint Electronic, but recognized the customer demand for a sight that could fit into normal scope rings. The 2000 included a number of other innovations, like a light sensor to automatically adjust brightness.
In 1989 the Aimpoint 3000 came out, which streamlined the profile of the optic by using a smaller battery compartment mounted tight tot he tube, and abandoning the automatic light adjustment. This was followed in 1991 by the Aimpoint 5000, which was essentially the same optic in a 30mm tube instead of a 1″ tube. Larger diameter optics were gaining popularity for increased light transmission, and the 5000 followed that trend.
A number of options were offered, especially on the Aimpoint 5000. Different colors were made, a “Mag Dot” option for pistol competition offered up to a 15 MOA dot, and even a version with a fixed 2x magnification was made for hunters who thought that would be a good idea (it really wasn’t). The last in the line was the Aimpoint 5000 XD which introduced a new diode assembly with much longer battery life – this would go on to be the M68 CCO as adopted by the US military.
Unlike the objective diameter of a riflescope, the diameter of a red dot sight is not increasing light transmission. There is no lens to gather light. A larger diameter red dot is more forgiving to the head position, though.
I honestly doubt that there is really all that much difference in light transmission between 1″ and 30mm. You’re talking just another 4.6mm in size, and while I’m not going to go digging into the actual optical light transmission numbers at the moment, from what I remember the differences are actually pretty trivial.
The real difference here is the whole European vs. American ecosystem in terms of scope mounts, hardware, and all the rest. I’ve got two Kahles scopes of about the same vintage and roughly the same cost/quality range. One was made for the US market, in a 1″ tube, the other is a European 30mm. I can’t tell any difference between the two.
I suspect that if you were to step up to the bigger metric tube sizes, you’d likely see a difference. But, between 1″ and 30mm? I think it’s purely in the imagination of the shooter.
I suspect that the real reason that Aimpoint changed their tube size owes rather more to the availability of 30mm rings and hardware in Europe, along with the fact that 30mm was becoming more common in the US at around the same time.
Light transmission differences? I am just not seeing it. At. All.