Recently you talked about reflex sights – what are the differences/pros and cons of reflex vs red dot vs holo sights?
The short, slightly sarcastic, and not completely inaccurate answer would be, holo sights cost a lot more.
Reflex sights are optics that use a semi-reflective surface to provide targeting information. These will bounce a light off said surface, usually a red or green LED, which when viewed from the appropriate position, will tell you roughly where the object is pointed. I’m phrasing it like this because reflex sights are actually used for a lot of different things, including nautical navigation tools, telescopes, and even some disposable cameras.
The advantages for reflex sights are that they’re relatively cheap, and they usually have a fairly substantial battery life. The internals are just an LED and a reflective glass layer. (Technically, there are a few variations of the technology; for convenience I’m describing the one used with firearms.)
Holo sights are, as the name suggests, actually holographic. They use a laser diode to create the targeting reticle in three dimensional space. This opens up some additional functionality that’s either difficult or impossible to obtain with a reflex sight. The big feature is the ability to adjust for range and windage. Finally, unlike reflex sights, they don’t need to have a tinted layer to catch the reflected light, so the optics are slightly clearer.
The most common firearm reflex sights are red or green dot sights. These use a red or green LED to create the targeting point. That said, some holo sights use a dot aim point. In that sense a red dot sight is more descriptive than an identifier.
Picking a color comes down to user preference. In general, red light has the least affect on night vision. Also because of the prevalence of ruby lasers and red LEDs, some people do approach firearms with the feeling that red is the “right” color.
The human eye is more sensitive to subtle differences in green than any other color. This is part of why some night vision setups display their feedback in green monochrome. In theory, this makes green dot sights easier to see. For some people this honestly seems to be the case. Also why you’ll sometimes see green lasers used as targeters on firearms.
Red and green aren’t the only options. Common LEDs include blue, white, and yellow, so if someone wanted an amber dot sight, that is an option. (Though, you’re going to be paying extra.)
There are other factors. Red was used because red LEDs were very cheap to produce until relatively recently.
Blue LEDs only date back to the 1990s. There’s also the direct physiological factors. Historically red light has been believed to produce limited or no eye adaptation, and had the least effect on night vision. My understanding is, that’s not really true, and that green/blue light actually interferes less with night vision, but this is a discussion I’m not fully versed in.
The idea of a dot sight, as opposed to other reticles is purely preference. A dot has a cleaner profile, but provides less information to the user. Just a simple, “bullets go here (we think).” Ring sights, or lines can be useful for judging drift, and can help the user adjust their aim. Alternately, the reticle selected may simply be to speed up target acquisition. This one really is about personal preference. A ring sight isn’t better than a dot sight, it’s about which works for the user.
On more expensive reflex sights (and most holo sights) it’s fairly common to have the ability to switch out the reticle on the fly. So, picking the right one is sometimes about choosing what’s right for this moment, not just picking one and sticking with it.
I will say, video games tend to gloss over these things. I can’t remember the last time I played a game that actually tinted the window for a reflex sight (maybe Far Cry 4), and I don’t think I’ve ever played one that attempted to display a holo sight properly.
Incidentally, some stuff that you can, technically do, includes open reflex sights, where the glass layer exposed to the air. You probably wouldn’t want to do this, because of the potential for damage, but it is a real option, and (partially) open reflex sights do exist. Ultimately reflex sights do need a surface to bounce off of. The name “reflex,” is a shortening of, “reflective,” not a reference to the user’s ability to react quickly.
One thing you can’t do is have a free floating hologram over the weapon. Existing technology doesn’t really allow for this, so you can’t have those neat holographic heads up displays you’ll occasionally see in sci-fi. That said, it’s just not something we can do today, not something that’s impossible.