A Fresnel lens is a lens whose optical surface is divided into narrow rings. This allows the lens to be much thinner and lighter than conventional lenses.

The power of a lens is a measure of its ability to converge or diverge the incident beam of light. It is given by the reciprocal of focal length.

Opticallens

When an object is imaged off the optical axis of the lens, coma aberration takes place. Coma aberration can be eliminated by taking the curvature of the two lens surfaces that matches the application. Best form lenses can be used for it.

The first National Lighthouse Lens Survey was released in 2001 at the Sixth Maritime Heritage Conference in Wilmington, N.C. It listed more than 400 classical Fresnel lighthouse lenses in the United States, and two pre-Fresnel, Winslow Lewis lenses. The inventory was a compilation of databases researched by Mike Vogel of the Buffalo Lighthouse Association and the American Lighthouse Coordinating Committee, and by Al and Helen Gademsky of Ohio.

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A concave lens is a type of lens with at least one side curved inwards. A concave lens with both sides curved inward is known as a biconcave lens. Concave lenses are diverging lenses, that is, they spread out light rays that have been refracted through them. They have the ability to diverge a parallel beam of light. For a concave lens, the edges are wider than the centre, or the centre is thinner than the edges. Concave lenses are used in spectacles in order to overcome myopia or short-sightedness.

Lenticular lenses are a group of microlenses that are used in lenticular printing. These lenses produce images that have an illusion of depth.

The English and Scottish lighthouse authorities wanted to produce all Fresnel lenses and other lighthouse equipment in their own country and persuaded the Cookson Glass Company to begin production of Fresnel lenses in 1834. Cookson brought in Leonor Fresnel, Augustin’s brother, as a consultant and produced Fresnel lenses from individual pieces and prisms as done by the French lens companies.  The first of these better designed Fresnel lenses, built by the Cookson Co., was installed in Scotland in 1835, in the Inchkeith lighthouse.

A simple lens is a single piece of magnifying material, while a compound lens consists of a number of simple lenses arranged along a common axis. Optical aberrations are sometimes found in simple lenses, while this property is eliminated in a compound lens. Another advantage of the compound lens is that the magnification of the lens can be adjusted as per the user’s requirements.

Some years ago Chad Kaiser of the US Lighthouse Society ran across a photograph of what was supposed to be the most powerful lighthouse in the world at 1 billion candlepower.  Yes, that is billion not million.  I became intrigued and started researching this light and its history.  This story will describe this very unusual lighthouse in France and how it came to be.  It is actually not a true lighthouse at all.  Instead it is an example of one of the earliest and most powerful Airway Beacons ever built.  But, first we need to understand a little more about air

A concave lens always generates a virtual image. It can never form a real image. The image is always formed on the same side of the lens as the object, and thus can be seen in the lens only. It cannot be formed in a screen, and as the distance of the object from the optical centre increases, the size of the image decreases.

Lenticularlens

An aspheric lens is often known as a non-spherical lens. An aspheric lens is a lens whose surface is not part of a sphere or a cylinder. The complex surface of an aspheric lens reduces or eliminates optical aberrations as compared to a simple lens. A single aspheric lens can replace a combination of simple lenses resulting in a system with a much-reduced size.

Fresnel lenses represent a unique aspect of U.S. Coast Guard history.  These artifacts are highly sought by a wide variety of museums and associations.  Due to their historic significance, fragility, high value and the U.S. Coast Guard’s policy to protect and preserve these artifacts, an additional set of conditions is placed upon prospective borrowing organizations.  Requirements for the care, security and display of Fresnel lenses are as follows:

The area of the lens suitable for refraction is called aperture. The aperture of the lens is the effective diameter of its light-transmitting area.

A lens has two spherical surfaces; these two spherical surfaces form a part of a sphere. The centre of these spheres is known as the centre of curvature.

A convex lens with one side flat is called a plano-convex lens. The lens found in the human eye is a prime example of a convex lens. Another common example of a convex lens is the magnifying glass that is used to correct hypermetropia or long-sightedness. Convex lenses are used in cameras as they focus the light and produce clear images. Convex lenses are also used in compound lenses which are employed in magnifying devices such as microscopes and telescopes.

Optical aberration is a property of the lens that causes distortion or blurriness during image formation. When it occurs, the light is dispersed or spread out rather than being focused on a certain fixed point. Optical aberration is an undesirable property of lenses and can be eliminated by using a combination of lenses rather than using a single piece of lens.

Most lighthouse enthusiasts think that the Fresnel lens was the first lens used in lighthouses.  However, that assumption is incorrect in that a number of lenses were proposed and put into use in the years before Augustin Fresnel designed his famous lens.  This story will give you information about these early attempts to use lenses to augment the power of lighthouse optics.

Lenses that have a curvature along only one axis are classified as cylindrical lenses. Their main purpose is to convert laser diode elliptical light into a round beam or to focus light into a line. Motion picture anamorphic lenses are an example of such lenses.

Cylindricallens

Fresnel lens

The French Commission for Lighthouses was established on April 29, 1811.  In 1819, Arago who had become a member of the French Commission for Lighthouses in 1813, offered to make a systematic review of possible improvements in lighthouse illumination.  He requested that Claude Mathieu and Augustin Fresnel be assigned as his coworkers on the project.  His request was granted on June 21, 1819, and Augustin agreed to serve on the committee.

A concave lens produces a smaller image for the viewer. The focal point of a concave lens is the point from which the light rays parallel to the axis seem to diverge after passing through the lens. The distance from the optical centre of the lens to the focal point is called the focal length of the lens.

The German government etched symbols onto the prisms of their Fresnel Lenses. This is the story of those symbols. The German government marked most of their lenses with a government certification symbol. Before 1933 and the Nazi Reich, the symbols used were the German Imperial Eagle and a symbol that looked like a castle, which represented the German Seamarks Service. These marks were etched onto the lens glass to certify that the lens met the requirements for government usage.

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Chromatic aberration occurs due to dispersion. When it occurs, a lens fails to focus all colours on the same point. Chromatic aberration can be observed as colourful fringes around an image. It can be fixed using an achromatic doublet (or achromat).

Lenses are basically magnifying glasses with curved sides. A lens is a piece of transparent glass which concentrates or disperses light rays when passes through them by refraction. Due to the magnifying property, lenses are used in telescopes and other magnifying devices. They are employed in cameras for gathering light rays.

In cameras, not one but a group of lenses are used for the gathering of light. Magnification of a lens is the relation between the size of the image formed and the size of the object. Lenses can also be used in groups in order to avoid the blurriness or distortion caused to the image formed by the lens.

One of the primary reasons for the occurrence of spherical aberration is that the spherical surfaces of the lens are not the ideal shape, and as a result, the beams are parallel to but distant from the lens axis. This causes the blurring of the image. Spherical aberration can be corrected using a normal lens having the right surface curvatures for a particular application.

A convex lens is a lens with an outward curve. Unlike the concave lens, the thickness at the centre of a convex lens is more than the thickness at the edges of the lens. Convex lenses are converging lenses. They have the ability to converge a parallel beam of light into a point. This point is called the focal point of the convex lens, and the distance from the optical centre to the focal point is called the focal length. The focal point is on the opposite side of the lens from which the light rays originate.

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Today, there are different types of lenses available. Generally, they are categorised either as a simple lens or a compound lens.

Fresnelscreen

Below is a listing of the major changes in the Fresnel lens through time and in most cases who the creator of each change was.

The centre of the spherical refracting surface of the lens is called the pole. It is the point where the principal axis meets the surface of the lens.

There are several types of aberrations, such as spherical aberration, chromatic aberration and coma aberration, that can all affect image formation and quality.

When a linear object is placed perpendicular to the principal axis of the lens, a linear image is formed perpendicular to the principal axis due to the refraction of the lens. The position, size and nature of the image formed depend on the position of the object with respect to the lens.

Many companies were involved in the development and refinement of the Fresnel lens.  Some supplied only the glass for the lenses, either in rough form, or both cast and polished.  Other companies assembled the glass elements and performed finish work on the lens elements.  Still other companies produced both the glass and the finished lenses.  This story will be divided into several parts, each discussing one or more of these companies and their contribution to the development of the Fresnel lens.

There are other types of aberrations, and some of them include field curvature, astigmatism, and barrel and pincushion distortion.