Common sense for projection screen
文章内容:
Projection screen is not just a piece of cloth, actually there are many optical knowledge and materials science about it.
Projection screen classifications:
According to the projection way: Frontal projection screen and rear projection screen
According to the screen surface material: glass-beaded, matte white, metallic and high-quality pearl type
According to reflectivity: it can be categorized as high gain and low gain. The most important still is to classify by optical theory and material:reflection type, retroreflection type and diffuse reflection type.
According to screen ratio: 1:1 and 4:3 and 16:9
According to the function: manual screen, electric screen,pull-up screen, tripod screen, frame hard surface screen, etc.
Gain is one of the projection screen coefficients which indicates the reflective index of the screen. We take the normal matte white screen or the white wall as a standard, suppose their gain a 1,if other screen's reflectivity is higher than the matte white screen then the gain is more than 1.Instead, if a screen's reflectivity is lower than matte white then the gain is less than 1. If a screen's gain is 2, then its brightness of the reflected light is 2 times the brightness of the matte white; If the gain is 0.8, the the brightness of the reflected light is 20% less than the brightness of matte white.
viewing angle is the angle at which the audience watch the screen, generally speaking the gain is higher, the viewing angle is smaller.
Diffuse reflection has the feature that the reflection direction of light from every point on surface has nothing to do with the incidence angle of the light. Matte white screen is typical of the diffuse reflection type. Diffuse reflection screen has a large viewing angle but the gain will not be very high and the color looks natural and accurate on it.
The so-called reflection is that the Angle of the incident light and the angle of reflected light is symmetrical,but not completely symmetrical because it is not a mirror. Here the "symmetrical" means a great deal of light is reflected near the symmetric direction so from the point of reflection direction, it can achieve maximum brightness. The advantage of this kind of reflection screen type is that its brightness is higher than diffusion type and has a big viewing angle. But to obtain maximum brightness,the viewing angle should be paid attention to. Moreover it is better that the projector hangs to the ceiling,making the projection angle symmetrical about the screen surface's normal line to the audience's viewing angle.If the angle deviates, the image will dim.
Retroreflection type screen bear the feature that tha angle of the reflected light is same as the anlge of the incident light, basically the same way back. Retroreflection screen's surface is full of many tiny glass beads. The optical function of these beads is making the original incident light return in same angle.This kind of screen is also called glass-beaded screen. The advantage of this screen type is high brightness,good uniformity but the weakness is narrow viewing angle.But it has a high utilization of light.
Speaking of the reflection screen type, there is one thing we need to point out: when the screen has a bad uniformity,you will find that the middle is brighter but in the four corners it is less brighter and that is obvious when the screen has a poor uniformity.Even worse,sometimes there will be a very bright area in the middle of the screen,like a white ball which is called the hot spot.Of course,hot spot is not acceptable but when playing video, the screen will not be all white in a blur and hot spot phenomenon is unlikely to affect the effect of video playback. Sometimes audience feel nothing when watching movie on a screen which has a poor uniformity. So whether the hot spot is acceptable or not, it depends on audience's feelings when watching.
