I have both red dot Trijicon (MRO) "reflex" and Tritium/fiberoptic, prism, powered ACOG.
Per my above comment, I'm guessing the ACOG ?
Completely different mechanisms. The standard red dot is a standard LED (light emitting diode), reflex sight, that probably 99% of the red dot sights are designed after. Powered by a battery connected to the LED. The ACOG is a prism sight powered by the fiberoptic/Tritium. Actually they do have battery powered ACOG's, but they are in the minority, and expensive.
All prism sights are a great boon for us folks with "old" eyes. The dot (horse shoe / delta etc.) is always VERY crisp, with sharp edges all around. There's more brands that make the prism sight than just Trijicon. Type in "prism" onto the Optics Planet search... I've got a couple and all are VERY nice.
The LED powered reflex sights (like Holosun), you have to fiddle with the brightness to get a good sight picture and a somewhat clear dot (delta). It is somewhat doable, but no matter, the dot will never match the "prism" design.
Holosun, Trijicon (MRO), Leupold, (some Vortex) and maybe a couple others are pretty good, if you keep the light intensity low to keep the "star" pattern minimal. I have at least one copy of each reflex sight mentioned above. The Holosun and the MRO, Trijicon are my favorites.
I've found that the cheaper red dots (non-mentioned !), no matter how low the brightness is, it still a crappy sight picture.
The LED, reflex powered sights battery's, the battery's will last a LONG time, like Holosun (50,000 hours) pioneered (and now some have copied). While the prism, battery powered sights are more battery power hogs (2000 or 3000hours). Except for the ACOG, Trijicon, that is fiber optic / Tritium powered, which will last as long as the fiber optic isn't damaged.
Hope this helps.
Mike