Most inexpensive red dot sights comes in two types: 1) the head-up version; and 2) the scope version. They both present a red dot to the shooter, in place of a traditional crosshair. But what is the real difference between these two types? We will attempt to answer the question in a theoretical and practical manner in this article.
Before we go any further, let's take a look at an example of each type so that we know exactly what we are talking about. The photo below shows a red dot scope on the left side and a red dot head-up sight on the right. The head-up sight resembles a head-up display (HUD) of a fighter jet.
Of course, this article discusses red dot reflex sights in general. There are always going to be extreme cases of each type that doesn't match the description of the information presented here.
In order to understand the differences, the commonality between the head-up and the scope should be identified. Although many of the commonalities are obvious and applies to non-red dots sight as well, it would be beneficial to be clear:
The head-up sight has a single reflective glass mirror at the very front of the sight. A LED toward the eye, below the mirror glass projects the red dot. The earlier photo shows clearly why the LED has to be below the mirror glass; there simply isn't enough structure to put the LED anywhere else. Because the LED has to be mounted below the mirror lens, the sight normally has to sit higher in order for the shooter to see the red dot.
The scope on the other hand still only needs one reflective glass. However, because of its tubular design, it usually has a frontal and a rear protective glass in addition to the reflective mirror. Because of its structure, the LED can be mounted anywhere--top, left, right, or bottom. When the LED is mounted anywhere, but bottom, the scope can be designed to sit lower than the head-up version.
With the simpler structure, the head-up sight is generally smaller than the scope version; take up less space on your gun. It's single lens design allows you to clean it easily, with no compartment to collect lint and dust. The head-up sight also obstructs less of your vision, allowing you to see everything around the sight picture. Because there is very little structure around the lens, it's generally hard to get the sight picture into your vision. Instead, you'll have to find other landmarks on your gun. But you should not use physical landmark to initiate the sight picture anyway as the alignment of your gun should be correct right off the bat based on motion discipline and muscle memory.
The scope is usually bigger than the head-up version. The adjustment knobs are generally bigger. It's also easy enough to clean, but if the lint/dust gets into the scope, then you'll have to service it. The scope generally obstructs your vision more, due to its size. The sight picture, on the other hand, is easier to initiate, due to the tubular structure. It's similar to lining up the front and rear sight. But as mentioned before, having the correct motion discipline and muscle memory is better than using landmarks to line up your gun.
From the photo shown earlier, you can see that the head-up version actually sit about half-an-inch higher than the scope version. That comes in handy for some sports, like airsoft and paintball where the sight needs to sit higher to clear the facemask. But in either case, the height of either type of reflex sights can be adjusted with rail risers.