Application of Fresnel Lens In Rear Projection Display

Abstract: 

Plastic Fresnel lens applied in the rear projection system, diffused in front of the screen, can significantly improve the brightness around, improve the overall display brightness uniformity.  Fresnel lenses are also used in light engines to calibrate light passing through LCD panels so that it is focused through a projection lens.  The disadvantages are increased lens cost, double image, Fresnel ring, Moire fringe and so on.  In addition, birefringence control is also very important in polarization-sensitive applications. 

1. Introduction 

Fresnel lens is usually used when a larger lens is needed in an optical system.  When the diameter of the lens is larger than 3 or 4 inches (76mm or 100mm), the manufacturing cost of glass lenses can be considerable.  If the application is in the display system, another factor to consider, is the lens weight and space should not be too large.  Fresnel lenses, on the other hand, can be made very thin, out of plastic, light and cheap. 

Plastic Fresnel lenses are also not suitable for all large size applications.  Due to its specific structural surface and the limitation of thermo-mechanical properties of plastic materials, its accuracy cannot meet some technological requirements, such as the large size lens usually needed in astronomical equipment.  But Fresnel lenses are perfect for display applications. 

2.Application. 

A Fresnel lens can be installed in the projection system, thereby reducing the number, volume, size and cost of components.  Field lense and Condenser Lense are commonly used in single-panel LCD projectors to pass light through the LCD more efficiently.  These two lenses are widely used in screen systems in rear projection displays, where the light is more uniform. 

The Overhead projection System uses Fresnel lenses as Field lenses and Condenser Lens lenses. Fresnel lenses are important focusing components in the camera and can significantly improve brightness and resolution. 

Fresnel lens is widely used in the rear projection television system.  Both Field lense and Condenser Lenses allow light to pass through the LCD more efficiently and then use a large Fresnel lens in the screen system to make the output image more uniform. 

Other applications include Fresnel lens arrays, such as Passive Infra Red sensors in mobile detection systems, and large Fresnel lens arrays are also used as solar concentrators in high-concentration photovoltaic systems. 

3. The rear display 

Fresnel lenses have proved most useful in projection systems, where they collimate and focus light rays. 

The advantage of Fresnel lenses in projection systems is to increase the brightness of the augmented display by focusing or adjusting the collimation of light rays. The figure below shows the light coming out of the lighting system (bulb, mirror, synthesizer) towards the LCD display.  The lighting will be diverted near the LCD panel.  If the collimation mirror is canceled, a large amount of light will be lost through the panel, and there will be an obvious hot spot effect in the display, reducing the brightness around the display screen.  Similarly, on the other side of the LCD screen, we have to focus light from the panel into a projection lens.  See the light distribution in Figure 4 using the increased brightness of the Fresnel lens before viewing the screen.

4. Fresnel Lens

4.1 Background

Fresnel lenses are circular concentric prism refraction structures. The surface structure of these prisms is designed to refract light. By changing the surface of a conventional lens to almost collapse into a plane, the thickness of Fresnel lenses is greatly reduced. 

The concentric prism refraction surface structures are Slope (inclined plane) and Draft (interference plane).  The inclination plane is actually close to the surface of a conventional aspheric lens.  Theoretically, all reflections should occur on inclined surfaces.  In order to make the lens thinner, it is necessary to design the interference surfaces between adjacent working surfaces to be of unequal height, so that the surface can be reduced to a plane.  Light incident on the interference surface will scatter on the imaging surface, which not only reduces the efficiency, but also causes some other problems (such as stray light and ghosting). 

The loss of interferential light can be minimized by designing the interferential surface and Fresnel lens direction reasonably.When collimating rays, the Fresnel lens faces the infinite conjugate.  In this way, the interference angle can be designed to not affect the ray path.

4.2. Fresnel Lens Detriments

4.2.1. Double Image

A common problem of Fresnel lens application in display system (especially fast display system) is the existence of double image.  Double image is a very annoying image dissociation that looks like a copy of an image as it shifts on the screen.  It is especially obvious in high contrast images and is absolutely unacceptable in text display.

The double image is caused by internal reflection of Fresnel lens working surface.  Light is reflected from the working surface, occurs internal total reflection (TIR) as it passes through one side of the lens plane, and then comes out in the wrong direction through the working surface or interference surface,  Although the interference angle can be designed as a straight path that does not interfere with the light, the light reflected by the double shadow will inevitably be reflected on the interference surface.

There are many ways to reduce the visibility of ghosting images. A very effective way to do this is to dye the lens material. Because the shadow ray has a longer path than the image ray, the shadow ray is thinner according to Beer's law. The disadvantage is reduced light transmittance. This is not acceptable in back-projection applications where the highest principle is to retain maximum brightness.

Another method of eliminating ghosting has been tested successfully, which is to deepen the color of the interference surface.  Although difficult to do, it is possible to blacken only the part of the interference surface with a black absorbent material, since most of the intensity of the glowering light travels through the interference surface, thus effectively eliminating the glowering. 

In principle, the application of a broad band antireflection (AR) coating on the Fresnel lens plane increases the transmission efficiency and reduces the double image phenomenon.  However, the application of coating is not completely successful, because the Angle of the antireflective coating needs to change gradually with the Fresnel lens center outward, because the Angle of the structural plane changes. 

4.2.2 Moire
When different light intensities pass through the Fresnel lens structure, they overlap, resulting in distinct Moire fringes. When applied in the rear projection display, the screen is usually composed of Fresnel lens and cylindrical mirror plus diffuser. When the image element is projected onto the screen, it will produce a very serious false image.  Therefore, the Fresnel lens and diffuser screen design must be carefully considered to match the pitch of the system's imaging elements.

4.3 Metrology

In order to quantify the performance of Fresnel lens in display, a new measurement method is needed to measure the double shadow and Moire fringe phenomena.  Ghosting when measured using a CCD camera on Fresnel lens, high strength light source on the Fresnel lens center, covered a layer with holes in front of the opaque outer garment, measure the effective focal length is decided by Fresnel lens diameter of hole position and scale (in figure 9 F / # = F/D), the CCD camera can record spin diffusion spots on the surface of the intensity and the proportion of ghosting and strength.

Moire fringes are currently measured using a Fresnel lens illuminated with uniform white light and cylindrical diffusion.  The device is similar to the glow-measure device above, but without a cover, the mole stripe produced by continuous CCD capture, and the ratio of black light to white edge is contrast.  This measurement method can also be further extended to measure projected enlarged raster patterns to produce moire effects on the test screen.