A projector with a lens projection ratio of less than or equal to 0.4 is called a reflective ultra-short focus projector; A projector with a projection ratio of 0.4-0.7 is called an ultra-short focus projector; A projector with a projection ratio between 0.65 and 1 is called an ordinary short-focus projector. A projector with a projection ratio between 0.65 and 1 is called an ordinary short-focus projector. KUPA P 1 and KUPA P2 of laser tv are both super projection computers and reflective ultra-short focus projection. In recent years, the short-focus projector has gradually become a hot spot, and it has quickly gained the love of users in its development. According to IDC's research data, the market share of "short coke" products increased significantly in 2009, with a market share of 2. 1% and a sales volume of about 1.8 million units. It is predicted that the short-focus projector market will grow substantially in 20 10, with a market share of 4.4% and sales exceeding 40,000 units.

Short-focus projectors are best at projecting images of about 80- 120 inches in a short distance (generally about 1- 1.5 meters). Because the projection distance is shortened, the projector does not need to be hung on the teacher's head as before. This can not only avoid the projector's direct light to the speaker's eyes, but also avoid the speaker's shadow. In addition, when the short-focus interactive projection is combined with the electronic version, teachers can write and mark on the projection screen conveniently, and at the same time control the computer to realize the interaction with the computer, which makes the classroom teaching more vivid and convenient for students to remember and learn. The advantages are obvious, but according to the dialectical principle that everything has two sides, the disadvantages of short-focus projection are inevitable. At present, short-focus projection mainly adopts two kinds of technologies, one is direct projection with wide-angle fisheye lens, and the other is reflection. Fisheye lenses were first used in wide-angle cameras. This kind of lens has a protruding front end, which is quite similar to fish's eyes, hence the name. Fisheye lens is a special super wide-angle lens. Although the projection distance is shortened and the picture becomes larger when used on a projector, linear distortion is easy to occur.