FRESNEL PRISMS AND LENSES by optom. Jithin Johney

the presentation will offer a comprehensive understanding of Fresnel prisms and lenses. Fresnel prisms and lenses are optical components used in various applications, such as vision correction and light manipulation. The presentation likely covers their basic principles, construction, applications, and potential benefits. It aims to educate the audience on the functionality and significance of these optical devices.

1. FRESNEL PRISMS AND LENSES Jithin Johney , Assistant professor of Optometry

2. Normal lenses and prisms vary in thickness depending upon the power of the lens or prism and upon the size of the lens or prism. This is not the case with Fresnel lenses and prisms, since they are constructed differently. Though not a replacement for normal lenses, Fresnel lenses and prisms are highly versatile and are very useful in certain specific circumstances.

3. A, Fresnel prism is thin because it is really a series of prisms stacked one on top of the other. The concept of individual prisms attached to a thin piece of plastic is shown. B, In actuality, the Fresnel prism in molded from one piece of material.

4. WHAT IS A FRESNEL PRISM? A traditional prism has two flat, nonparallel surfaces. Parallel light entering the prism is bent toward the base of the prism and leaves the back surface at an angle. The larger the prism, the thicker the base of the prism will be. A Fresnel prism attempts to circumvent thickness by building a “tower” of small, wide prisms.

5. How a Fresnel prism works; imagine cutting off the tops of a large number of equally powered prisms and gluing them, one above the other, onto a thin piece of plastic. A fresnel prism is molded into one flexible piece, its construction duplicates this imaginary example. A Fresnel prism is only 1 mm thick.

6. A Fresnel lens has a series of slightly visible lines on the surface. These lines are really ledges that indicate the location of the base of the prism. The base direction is at right angles to the direction of the visible lines.

7. What Are the Advantages of a Fresnel Prism? It is very thin and extremely lightweight. It is flexible and can be applied to an existing spectacle lens, making it possible to apply the lens in-house, without an in-house optical laboratory. Because the lens is made from a soft, flexible material, it can be cut to any shape with scissors or a razor blade. This means that it can be cut and applied to one sector of a lens. Conventional prisms have a large increase in thickness from apex to base, a high-powered prism is troubled by magnification differences and changes in power across the lens. Fresnel lenses do not eliminate this problem, they do reduce magnification differences considerably.

8. What Are the Disadvantages of a Fresnel Prism? Fresnel prisms look different than conventional lenses. They are different enough that they may be noticed by others; Fresnels have a number of small ledges, they are harder to clean than conventional lenses. High-powered prisms will cause a slight decrease in visual acuity. Most of this is due to the chromatic aberration and distortion associated with prisms. This decreased acuity occurs in both conventional and Fresnel prisms. Fresnel prisms also cause a slight loss of visual acuity caused by reflections at the prism facets, especially under certain sources of illumination. The minimal acuity decrease through Fresnel prisms may be slightly less than a line on a Snellen chart at a 90% contrast level compared with acuity through conventional prisms.

9. WHEN ARE FRESNEL PRISMS USED? There are a variety of clinical applications for Fresnel prism. The following six sections discuss major applications. High Amounts of Prism Because of its thickness advantage, Fresnel prism is especially useful for high amounts of prism. Use and Reuse Fresnel prism lenses are easy to apply and remove. They may be used and reused. This is helpful when determining how a given prism amount will work long term or for use during visual training.

10. Sectorial Application A partially paralyzed extraocular muscle may result in a different amount of prism needed for different directions of gaze. A Fresnel lens can be cut to fit that particular lens area. Prism is present only where it is needed. Visual Field Defects With visual field defects, prism may be applied in one section of the lens with the base direction in the direction of the defect and the edge of the prism close to the central visual area. This way the eye travels only a short distance before it picks up the image through the prism. The image appears closer to the center and can be seen without moving the head.

11. These temporary 30 to 40 Δ base out Fresnel prism segment areas are used to create peripheral diplopia for those who have homonymous hemianopia visual field loss. They will later be replaced with prism segments constructed within the carrier spectacle lens.

12. Cosmetics of Nonseeing Eyes The use of prism to improve the appearance of a blind or prosthetic eye. Fresnel prisms can be used in such instances.

13. Slowing of Nystagmus Nystagmus is a condition characterized by a constant back and forth movement of the eyes. Such movement is involuntary and reduces vision. In some cases nystagmus may slow when the person looks to one side or the other. For example, if the examiner sees that movement slows when the person looks to the right, equal amounts of prism may be applied to both lenses. The correct base direction would be base left. Because the eyes turn toward the apex, prism base left will keep the head pointed straight while the eyes turn to the right. Since the eyes are turned to the right, nystagmus slows.

14. WHAT IS A FRESNEL LENS? The front and back surfaces at the optical center (OC) of a lens are flat. But as the distance from the OC increases, the lens surface becomes more angled. A Fresnel lens is similar to a series of concentric prisms, each with a slightly higher prismatic effect. When the concentric surfaces are angled correctly, a plus or minus sphere of any desired power may be created. Advantages and disadvantages of Fresnel lenses parallel those of Fresnel prisms.

15. TYPES There are two main types of Fresnel lens: Imaging and non-imaging. Imaging Fresnel lenses use segments with curved cross-sections and produce sharp images, while non-imaging lenses have segments with flat cross-sections, and do not produce sharp images. As the number of segments increases, the two types of lens become more similar to each other. Imaging lenses can be classified as:

16. 1. Spherical A spherical Fresnel lens is equivalent to a simple spherical lens, using ring-shaped segments that are each a portion of a sphere, that all focus light on a single point. This type of lens produces a sharp image, although not quite as clear as the equivalent simple spherical lens due to diffraction at the edges of the ridges. 2. Cylindrical A cylindrical Fresnel lens is equivalent to a simple cylindrical lens, using straight segments with circular cross-section, focusing light on a single line. This type produces a sharp image, although not quite as clear as the equivalent simple cylindrical lens due to diffraction at the edges of the ridges.

17. When Are Fresnel Lenses Used? Nonspectacle Uses Fresnel lenses are not just used for spectacles. A common application may be found when looking through the writing surface of an overhead projector. (Adjust the focus to be slightly off and see the concentric rings of the lens projected on the screen.) Large minus Fresnel lenses are sometimes applied to a window to create a wider field of view, or are used for the warning beams of seaside lighthouses so that the illumination projected from the source within the building is increased.

18. Short-Term Wear Clinically, Fresnel lenses are useful on a temporary basis, such as during vision training or frequent changes in refraction that may result from unstabilized diabetes or certain postsurgical situations. Creating Adds Fresnel lenses can also be applied to one portion of the spectacle lens. High add powers can be created for low-vision or occupational purposes.

19. How to Apply a Fresnel Lens or Prism to a Spectacle Lens Fresnel prisms and lenses are applied using the following steps: 1. For lenses, mark the desired position of the lens OC on the front of the carrier lens. For prisms determine correct base direction. 2. Take the carrier lens out of the spectacle frame. 3. Place the Fresnel lens or prism on the back of the carrier lens with its smooth side against the carrier. Be sure the OC or base direction is properly oriented. 4. With a razor blade, trim the Fresnel lens or prism flush with the beveled edge of the carrier lens. 5. Remove the Fresnel lens or prism and reinsert the carrier lens into the frame.

20. 6. Wash both carrier and Fresnel lens with a weak solution of lotion-free, liquid detergent. 7. In a bowl of warm water, or under a stream of warm water, apply the smooth side of the Fresnel to the carrier. Work out any air bubbles that may be trapped between the two surfaces. 8. Give the lenses to the wearer, but instruct the wearer to handle with care for 24 hours until drying is complete.

21. How to Clean Fresnel Lenses or Prisms The manufacturer’s recommended method of cleaning these lenses is to rinse under warm running water. If the lenses have dirt in the grooves, use a soft brush. Blot dry with a soft, lint-free cloth. Hard contact lens cleaning solutions have also been used to clean Fresnel optics.

22. Thankyou