The idea of replacing traditional TVs with living room 4K UST laser projectors became a trend last year. Today many industry leaders including Optoma, Hisense, Epson and Samsung are actively pursuing this idea. But for all its attractiveness, this solution has a significant nuance. Traditional home theater projectors are mainly used in well-darkened rooms with no ambient light. Accordingly, the requirements for its reflective properties are much lower. But using a projector as a day-to-day bigscreen TV in high daylight conditions dramatically increases the demands on the screen properties.
A white wall is poorly suited for projection for a simple reason reason. Usually, they are not perfectly flat and have a non-uniform texture what clearly appears under the light from the projector. As a result, the image contains hot spots, dull areas, and unpredictable performance at different viewing angles. Finally, most walls are not perfectly white, which disrupts color reproduction by distorting colors.
Of course, these cons can be ignored when using a budget LED projector for watching cartoons. But that decision hardly makes sense for an expensive 4K HDR laser projector, which can provide amazing quality on a 100″ screen and beyond.
Therefore, a high-quality projection screen is becoming one of the mandatory attributes of expensive modern models.
A matte-white screen is a good choice for a home theater with dark walls and controlled light. Gray screen deepens black gradations, but reduces the image brightness.
These diffusive screens reflect light in all directions and are ideal for dark-walled home theaters. But in a room with light walls and ambient light from windows or lamps, the image on such a screen will be pale and washed out. Ambient-light rejecting (ALR) screens significantly reduce this problem.
In addition, the image quality depends on the angle at which the light hits the screen. As known, ultra-short-throw (UST) projectors for living rooms sit just a few inches from the wall and shoot their light onto the screen up at a steep angle. But the screen should reflect this light into the room, and not bounce it up (the angle of incidence is equal to the angle of reflection). Therefore, special UST ALR screens have been developed for UST models.
As known, diffused daylight from a window or lamp light significantly reduces the contrast and image clarity. Of course, dimming effectively solves this problem, but this method is not always convenient. Ambient-light rejecting screen is an alternative solution.
Strictly speaking, ALR screen absorb the ambient light or redirect it away from the viewing area depending on the angle of incidence. But, the ALR term is shorter and simpler.
Modern ALR screens are divided into angular (specular) reflective and retro reflective.
Angle reflective ALR screens use a fundamental law of optics: the angle of incidence equals the angle of reflection. It reflects projector’s light into the viewing area and scatters light from sources at a large angle.
Such screens are available from, for example, Da-Lite Parallax Stratos, Elite Screens CineGrey 3D and CineGrey 5D, Screen Innovations Black Diamond and Slate, and Stewart Filmscreen Phantom HALR.
A retro-reflective ALR screen is made from a material whose top layer contains microscopic triangular ridges (serriform or lenticular microstructure). It reflects light back toward its source and absorbs light from sources at an angle.
In fact, a light is only reflected from gaps between the microscopic ridges. Wherein, their black sides absorb light coming from above and below.
For example, Da-Lite Parallax Pure, dnp Supernova and Supernova Infinity STD, and EVP / Elite DarkStar are available on the market today.
Typically, the ridges are positioned horizontally across the screen, effectively absorbing overhead lighting. But they are less effective against side light sources. Therefore, manufacturers sometimes make the angular-reflective base layer to improve side-light absorption. Unfortunately, ALR screens are not very effective against light sources on the projector optical axis.
Some key features
All modern ALR screens typically have a gray base layer to increase the image contrast. But more expensive screens often use multiple layers, including a diffusive layer to increase the viewing angle and the layers of microscopic optical beads. In this case, black beads absorb stray light, and yellow beads reflect the projector’s light into the viewing area.
Additionally, the ratio between the number of yellow and black optical beads provides regulation of the screen gain. In particular, increasing the number of yellow beads increases the reflected light output.
Unfortunately, this technology can create a speckling effect (“hot spots”) that significantly reduce the picture uniformity.
Retro-reflective vs angular-reflective screens
– black levels (contrast) are significantly better in retro-reflective design, but their narrow viewing angle limits the choice of the optimal viewing point, which is located opposite the screen center. Also, screens with microscopic optical beads can create “hot spots” and are not very effective for ceiling-mounted projectors. In addition, reflective screens are limited in size, and their multi-layer microstructure is very sensitive to mechanical stress. However, DNP Supernova Infinity screens from seamlessly assembled modular tiles are not limited in size;
– angular-reflective screens have less contrast, but are cheaper, unlimited in size, and can be used with a retractable mechanism.
UST ALR screens
UST (ultra-short throw) projectors have radically eliminated the problem of complex projector installation. Indeed, a ceiling mount requires a rather complicated installation, and the wires are not very convenient for projector placement on a coffee table in the middle of the room. Plus, the ceiling mount virtually eliminates the ability to move the projector to other rooms or outdoors.
The UST projector provides a large screen from a very short distance and can be installed close to a wall with a screen.
In fact, they are ideal as a living room projector that can replace the traditional large TV. But the lighting in the living room is significantly different from the well darkened Home Theater. In addition, the optics of these models also differ from mid-focus projectors, providing a steep angle onto the screen. Accordingly, a screen of such a projection system has some features. It should divert and absorb the ambient light, effectively reflecting the projector’s light into the viewing area.
Taking these factors into account, the developers make UST screens with lenticular ridges without retro-reflective. Otherwise, the screen will reflect the light back down to the projector. Therefore, the ridges take the form of right triangles with the angled side facing downward.
As you can see in the diagram, their angled side directs light from the projector into the viewing area.
Today the market offers a fairly wide range of such screens, including the Da-Lite Parallax Pure UST 0.45, dnp Supernova STW and Supernova Infinity UST, Elite Aeon / StarBright CLR and CLR 2, Screen Innovations Short Throw, and Stewart Filmscreen Balón Edge UST.
Of course, the developers are actively experimenting, optimizing the structure and materials of the screen coating. For example, XY Screens offers two straight-lenticular UST ALR screens, including PET Crystal screen with horizontal lenticular ridges and the PET Grid with crossed ridges.
Companies also often use lenticular ridges with a Fresnel microstructure. As known, the French physicist Augustin-Jean Fresnel invented the lens of concentric glass discs. Later, this idea became the basis for projection screens.
The downward-facing sides of the lenticular ridges reflect projector’s light to the viewer, absorbing or scattering ambient light. Today the list of Fresnel-based UST ALR screens includes, for example, the dnp Supernova STS and XY Screens INP-FNE.
They have a relatively narrow viewing angle and gray color to improve contrast. But some companies use a substrate with the ridges, which have a white reflective bottom surface and a black upper absorptive surface. Moreover, sometimes companies use additional layers, including angular-reflective layers and absorptive microscopic beads.
Unfortunately, the lenticular, layered structure of most UST screens excludes their frequent folding due to the risk of mechanical deformation. But the developers are quite successful in trying to solve this problem. In particular, Screen Innovations currently offers its ST material in a rollable motorized casing, and Elite Screens announced a rollable CLR material this year.
Today market offers a very wide range of screens. The competition has already lowered prices for ALR screens for mid-focus projectors. But screens for UST projectors are still very expensive. For example, the cost of a 100-inch UST ALR screen today reaches $ 1,000. Of course, buyers should consider this factor.
This aspect puts companies in a difficult position also. Really, their UST laser projectors deliver 4K HDR quality on the screen from 100 inches and up. But low efficiency of inexpensive screen significantly reduces the claimed image quality, threatening the company reputation. Epson is trying to solve this problem by offering a high quality UST ALR screen, for example, in the EpiqVision LS500 projector kit. Unfortunately, the 100″ and 120″ screen increases its price by $ 1,000 and $ 2,000, respectively. But in the long term, a decrease in the price can dramatically increase the competitiveness of living room UST projectors vs TV.
Of course, buying a very expensive UST ALR screen with a complex microstructure coating would be a waste of money for a mid-focus projector that is installed at right angles to the screen. In turn, a traditional ALR screen will be ineffective for a UST projector due to the reflection of projector’s light outside the viewing area.
An expensive ALR screen is unlikely to be justified for a budget portable LED projector under $ 200. For viewing, for example, cartoons will be quite adequate a flat white surface or a budget screen.
In fact, the screen helps to realize the potential of an expensive projector by shielding its high quality projection from the effects of ambient light.
The video compares different screens from XY Screens.