Reflex sights, including red-dot optics, use an LED emitter diode to project a dot that’s reflected on the optic’s lens, which creates the dot you use to aim. That’s an oversimplification, but you get the idea. The difference between a closed emitter and an open emitter is pretty much what it sounds like: In a closed-emitter optic, the emitter that projects the light is enclosed between two pieces of glass, while in an open-emitter optic, the emitter sits behind the glass and is open in the back.
Both types give you an aiming dot. The difference is that in one, the device that emits the light is enclosed, and in the other, it’s open to the elements. Hence the names.
By nature, closed-emitter optics are larger, generally shaped like a box or a tube. Contrary to what you might think, the viewing window is usually smaller even though the optic itself is larger. These optics are typically more expensive as well, thanks to the additional material and slightly more complicated construction. On the plus side, because the emitter is enclosed, it’s protected from mud, rain, dust and anything else that might gunk it up. Nothing will get between the emitter and the front glass that could prevent the dot from appearing.
Examples of closed (left) and open (right) emitters. Image courtesy Leupold.
Open emitters, on the other hand, are smaller and lighter in weight, and generally cheaper. They have a base and a single piece of glass under a frame or hood, and that’s about it. They have a larger field of view than closed-emitter optics, which makes target acquisition fast and helps you maintain situational awareness in your periphery. That, combined with their small size, makes them an attractive option for concealed carriers who are trying to keep their carry gun lightweight and easy to conceal. However, the frame the glass sits in creates a sort of “cup” shape that can easily fill with water, snow, mud, pocket lint or some other sort of obstruction and block the emitter.
Because the emitter is unprotected in an open-emitter optic, it can get gunked up with debris and fail to project the dot. To get it running again, you’ll have to dig into the cup with your finger or your shirtsleeve and try to get it clean. That’s not too difficult, but it does take a second or two. With a closed emitter, you simply have a mostly flat surface to swipe off if it gets dirty so you can see through it again—the dot is always there, but the window you see it through might need to be wiped off. Think about removing a blob of mud from the bottom of a shot glass vs. swiping it off a flat plate. Which is faster?
Example of an optic with a closed emitter. Image courtesy Leupold.
Both styles are effective and efficient. Closed emitters tend to be more durable and rugged, and they’re often chosen for duty or hunting use, where there might be a lot of dust or mud or precipitation to deal with. Open emitters are smaller and lighter, with a wider field of view for fast target acquisition. Both are good choices for concealed carry, but open-emitter optics are a popular option because of their small size and lower price points. You won’t go wrong either way.
—Jo Deering
