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In the theater and dance world, Fresnels are most often used for top or backlighting, at medium throw distances. In small venues, they are occasionally used for front light, although the relative lack of control, when compared to an ellipsoidal, is a major disadvantage. The distinctive soft beam of a fresnel can make it very useful as a special in certain situations.

However, water has an absorption minimum in the red region of visible light as well as in the adjacent infrared region of the spectrum (short-wavelength IR-A). This explains the relatively large penetration depth of IR-A radiation (780–1,400 nanometres), which can penetrate up to some 5 millimetres into the skin, allowing it to reach the hypodermis and act on it directly. In general, the shorter the wavelength of infrared radiation, the greater the penetration depth. IR-C (3,000 nanometres – 1 millimetre) and IR-B (1,400–3,000 nanometres) are absorbed in the upper layer of skin, the epidermis, which is the only layer on which they have a direct effect. The thermal effects of IR-A radiation are spread over a larger volume than that of IR-B and IR-C radiation, but indirect heat conduction allows the temperature increase to affect deeper layers as well.

Fresnel Light

The degree to which the lamp may be focused is limited by the length of the housing. To reduce the width of the beam, the lamp and reflector are moved further back from the lens (spot focus). However, the farther back in the housing the lamp is placed, the more light is wasted in the housing.[2]

Heat and pain receptors are primarily located in the upper layer of the dermis, just beneath the epidermis, and act as the body's warning and protection system. They are less sensitive to IR-A radiation than to IR-C and IR-B radiation, which acts near the surface and on a smaller volume. This might well be the therapeutic intention in medical applications that are designed to produce greater warming of deeper regions. However, this factor must be taken into account in any application of infrared radiation.

A Fresnel lantern (pronounced frəˈnɛl or fruh-nel) is a common lantern used in theatre that employs a Fresnel lens to wash light over an area of the stage. The lens produces a wider, soft-edged beam than a spotlight or key light, and is commonly used for back light and top light.

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The eyes are particularly sensitive to thermal effects. Suitable protective goggles can protect the eyes against excessive exposure to infrared radiation.

How does aFresnel lenswork

When infrared radiation strikes biological tissue, it causes molecules to vibrate, producing heat and causing the temperature to rise. As human tissue is largely made up of water, the absorption capacity of water for the various wavelengths of incident infrared radiation plays a key role in determining the penetration depth and effects of the radiation.

While the focus can alter the size of the beam, the distinctive scatter of light that the Fresnel lens emits also requires a way of controlling its shape. Since Fresnels cannot use internal shutters, such as those found in an ellipsoidal spotlight, they are often fitted with distinctive barn doors to control the spill and shape the beam of light. These are large metal flaps that may be mounted just beyond the color slot at the front of the lantern. They are colloquially known as "Harris Flaps" in the United Kingdom theatre industry.[3] [4]

What areFresnellights used for

Adverse effects are particularly likely if the temperature increase and exposure time exceed critical limits. Excessive exposure can result in damage or even burns. In general, thermal burden can lead to disturbances in the heat balance of the entire organism.

Theatrical Fresnel lanterns are typically made in three-, six-, or eight-inch lens diameters, with lamps ranging in power from 150 W to 2,000 W. The three-inch variety is referred to as an inkie. Fresnel lenses can be placed close to the light source and are inexpensive to produce.

Because the reflector of a Fresnel lantern cannot be larger than its lens aperture, the lamps are not very efficient: only the light emitted straight forward or backward (and redirected forward by the reflector) is utilized, with the rest absorbed by the casing as waste heat.

Fresnels use a spherical reflector, with the filament of the lamp at its focus point. This effectively doubles the light delivered by the fixture, as all that is emitted backwards into the reflector is reflected back out the front. As with most lighting fixtures, the lamp and reflector cannot move independently, and are moved together as a unit to focus the beam. This is done by a slider on the bottom of the lantern, or a worm track and crank in the back of the unit. The lamps work best with their bases facing up, with bulb life shortened significantly when mounted upside down.

Infrared radiation can promote local blood circulation and reduce muscle tension. Examples of traditional medical applications of infrared radiation include the relief of muscle pain and tension, as well as the treatment of autoimmune diseases or wound-healing disorders. However, the question of whether it is sensible to treat an illness or complaint with heat, or whether it may in fact be harmful to do so, must always be assessed by a doctor on a case-by-case basis.

The distinctive lens used in a Fresnel lantern is named after its inventor, Augustin-Jean Fresnel, who developed it for use in lighthouses. It has a 'stepped' appearance instead of the 'full' or 'smooth' face of other lenses. This allows the lens to focus the light by tilting each successive ring of glass slightly more steeply as its distance increases from the center. If the glass were completely flat, this would cause a corresponding pattern of circles of light, so Fresnel lenses are usually stippled on the flat side. This pattern of small bumps helps to break up the light passing through the lens and gives Fresnels their characteristic soft beam. This means that the intensity of the light is consistent across the spread of the beam of light, as opposed to being less intense around the edges as in an ellipsoidal reflector spotlight (ERS).

The most powerful natural source of infrared radiation is the sun. Even in antiquity, the sun's "thermal radiation" was used to relieve a variety of complaints. By virtue of its beneficial effects, artificially generated infrared radiation therefore enjoys widespread applications in medicine and the wellness sector.