Have you ever seen a projection screen where one area is much brighter than the rest of the image on the screen? This is called hotspotting, and it is especially difficult to overcome when the projector is located close to the screen. There are ways to avoid hotspotting; as with all successful projection, it simply takes proper planning.
What causes hotspotting?
A “hot spot” is an area of the screen that is much brighter than the rest of the screen. It will look like a ball of light on the screen. If the throw distance is too short for the gain of the selected screen surface, the light hits the outside of the screen at a shallow angle. This reflects light away from the viewers’ eyes, so the outside of the image appears less bright than the middle of the image.
The amount of screen gloss can also make a difference, because it contributes to the overall specular reflectivity.
How do I prevent hotspotting?
Ultra Short Throw (UST) projectors are located less than 2 feet from the screen, so they are more prone to hotspotting, but it can be an issue with any system where the projector is too close to the screen, or the screen gain is too high for the projector’s throw ratio.
There are two easy ways to avoid hot spots on your screen. One solution is to use a surface with a gain of 1.0 or lower. Alternatively, you can increase the throw distance by moving the projector further away.
A key to planning a successful projection system is to choose the viewing surface first. The surface will have the biggest impact on your image and should be chosen to fit with the conditions present in the room. Then you can select the projector that will appropriately light that surface.
Which viewing surface works best?
The answer to this question depends on room conditions. For the best results with UST, we recommend TecVision XT1000X White and TecVision XH700X Grey. These diffusive surfaces have no or negative gain and provide uniform performance across the screen surface.
Ambient light rejection screens are more specular reflective than diffusive white or grey screens, so they typically require longer throw distances. However, Draper does have a solution for ultra-short throw projection under ambient light. TecVision XH800X UST ALR delivers improved contrast and color saturation in moderate to high ambient light applications. It rejects 57% of off-axis ambient light.
Whatever kind of projection system you are designing, make sure to refer to the screen manufacturer’s minimum throw distance ratio for each surface.