ONLINE ASSIGNMENT

          PROJECTED AIDS - OHP , LCD & 

                     SLIDE PROJECTOR

SUBMITED TO

RANI S NAIR

ASST LECTURER

NATUARAL SCIENCE 

                  INTRODUCTION

                       Audio visual aids are instructional devices which are used to communicate messages more effectively through sound and visuals.Audio visual aids help in stimulating the sensory organs like ears and eyes and facilitate quick comprehension of the message by the audience. These may be use for literate as well as illiteratepeople.Audio visual aids are of two types. Projected aids & Non-projected aids. 

                        Projected visual aids are pictures shown upon a screen by the use of a certain type of machines. Non projected visuals are those aids which are used without any projection. So they translate abstract ideas into a more realistic format. OHP, LCD & SLIDE PROJECTOR are the important projected aids.

            PROJECTED AIDS

                  Audio instructional aids involve the use of sense of hearing. In other words, these teaching aids are used in the form of audio devices it impart knowledge on any subject and to develop understanding among the 

students in an easy way. 

                  A projected aid is one in which items to be observed and projected onscreen using mechanical devices. Slide Projector, Overhead Projector and LCD Projector are the typical examples. These aids are sound to be very effective because movement can be added. So sound and hence a realistic experience can provided. They are as follows.

         OVERHEAD PROJECTOR(OHP)

                      An overhead projector (OHP), like a film or slide projector, uses light to project an enlarged image on a screen. In the overhead projector, the source of the image is a page-sized sheet of transparent plastic film (also 

known as 'foils') with the image to be projected either printed or hand-written/drawn. These are placed on the glass surface of the projector, 

which has a light source below it and a projecting mirror and lens assembly above it (hence, 'overhead'). They were widely used in 

education and business before the advent of computer-based projection.

                 OPTICAL SYSTEM

                          An overhead projector works on the same principle as a slide projector, in which a focusing lens projects light from an illuminated slide onto a projection screen where a real image is formed. However some differences are necessitated by the much larger size of he transparencies used (generally the size of a printed page), and the requirement that the transparency be placed face up (and readable to he presenter). For the latter purpose, the projector includes a mirror just before or after the focusing lens to fold the optical system toward the horizontal. That mirror also accomplishes a reversal of the image in order that the image projected onto the screen corresponds to that of the slide as seen by the presenter looking down at it, rather than amirror image thereof. Therefore, the transparency is placed face up (toward the mirror and focusing lens), in contrast with a 35mm slide projector or film projector (which lack such a mirror) where the slide's image is non-reversed on the side opposite the focusing lens.The device has sometimes been called a "Belshazzar", after Belshazzar's feast ("In the same hour came forth fingers of a man's hand, and wrote over against the candlestick upon the plaister of the wall of the king's palace: and the king saw the part of the hand that wrote").

        CONDENSER

             Because the focusing lens (typically less than 10 cm [4 in] in diameter) is much smaller than the transparency, a crucial role is played by the optical condenser which illuminates the transparency. Since this requires a large optical lens (at least the size Because the focusing lens (typically less than 10 cm [4 in] in diameter) is much smaller than the transparency, a crucial role is played by the optical condenser which illuminates the transparency. Since this of the transparency) but may be of poor optical quality (since the sharpness of the image does not depend on it), a Fresnel lens is employed. The Fresnel lens is located at (or is part of) the glass plate on which the transparency is placed, and serves 

to redirect most of the light hitting it into a converging cone toward the focusing lens. Without such a condenser at that point, most of the light would miss the focusing lens (or it would have to be very large and prohibitively expensive). Additionally, mirrors or other condensing elements below the Fresnel lens serve to increase the portion of the light 

bulb's output which reaches the Fresnel lens in the first place. In order to provide sufficient light on the screen, a high intensity bulb is used which must be fan cooled.

        FOCUS ADJUSTMENT

                 Overhead projectors normally include a manual focusing mechanism which raises and lowers the position of the focusing lens (including the folding mirror) in order to adjust the object distance (optical distance between the slide and the lens) to focus at the chosen image distance (distance to the projection screen) given the fixed focal length of the 

focusing lens. This permits a range of projection distances.Increasing (or decreasing) the projection distance increases (or decreases) the focusing system'smagnification in order to fit the projection screen in use (or sometimes just to accommodate the room 

setup). Increasing the projection distance also means that the same amount of light is spread over a larger screen, resulting in a dimmer 

image. With a change in the projection distance, the focusing must be readjusted for a sharp image. However, the condensing optics (Fresnel lens) is optimized for one particular vertical position of the lens, corresponding to one projection distance. Therefore, when it is focused for a greatly different projection distance, part of the light cone projected 

by the Fresnel lens towards the focusing lens misses that lens. This has the greatest effect towards the outer edges of the projected image, so that one typically sees either blue or brown fringing at the edge of the screen when the focus is towards an extreme. Using the projector near its recommended projection distance allows a focusing position where 

this is avoided and the intensity across the screen is approximately uniform.

         HISTORY

               Some ancient projectors like the magic lantern can be regarded as predecessors of the overhead projector. The steganographic mirror possibly came closest to how the overhead projector was used.

German Jesuit scholar Athanasius Kircher's 1645 book Ars Magna Lucis et Umbrae included a description of his invention, the "Steganographic Mirror": a primitive projection system with a focusing lens and text or 

pictures painted on a concave mirror reflecting sunlight, mostly intended for long distance communication. In 1654 Belgian Jesuit mathematician André Tacquet used Kircher’s technique to show the journey from China 

to Belgium of Italian Jesuit missionary Martino Martini. It is unknown how exactly Tacquet used Kircher's system, but it is imaginable that he drew pictures on the projecting mirror while details of the journey were explained.

French physicist Edmond Becquerel developed the first known overhead projection apparatus in 1853. It was demonstrated by French instrument maker and inventor Jules Duboscq in 1866.

                An overhead projector designed by American scientist Henry Morton was 

marketed around 1880 as a "vertical lantern".

The use of transparent sheets for overhead projection, called viewfoils or viewgraphs, was largely developed in the United States. Overhead projectors were introduced into U.S. military training during World War II. After the war they were used at schools like the U.S. Military Academy.

                Overhead projectors were used early on for police work with a cellophane roll over a 9-inch stage, allowing facial characteristics to be rolled across the stage.As the demand for projectors grew, Buhl Industries was founded in 1953, and became the leading US contributor for several optical refinements 

for the overhead projector and its projection lens.

             Overhead projectors began to be widely used in schools and businesses 

in the late 1950s and early 1960s. In the early 60s, carousel slide projectors with a horizontally mounted tray was released by Kodak.In the late 1950s Roger Appeldorn was challenged by his boss at 3M to find a use for the transparencies that were the waste of their color copy process. Appeldorn developed a process for the projection of transparent 

sheets that led to 3M’s first marketable transparency film. The Strategic Air Command base in Omaha was one of the first big clients, using circa 20,000 sheets per month. 3M then decided to develop their own overhead projector instead of the one they had been selling until then, which was produced by an outside manufacturer. It took several prototypes before a cost-effective, small and foldable version could be An overhead projector designed by American scientist Henry Morton was marketed around 1880 as a "vertical lantern".The use of transparent sheets for overhead projection, called viewfoils 

or viewgraphs, was largely developed in the United States. Overhead projectors were introduced into U.S. military training during World War II. 

                After the war they were used at schools like the U.S. Military Academy.

Overhead projectors were used early on for police work with a cellophane roll over a 9-inch stage, allowing facial characteristics to be 

rolled across the stage.As the demand for projectors grew, Buhl Industries was founded in 1953, and became the leading US contributor for several optical refinements 

for the overhead projector and its projection lens.

            Overhead projectors began to be widely used in schools and businesses 

in the late 1950s and early 1960s. In the early 60s, carousel slide projectors with a horizontally mounted tray was released by Kodak.In the late 1950s Roger Appeldorn was challenged by his boss at 3M to 

find a use for the transparencies that were the waste of their color copy process. Appeldorn developed a process for the projection of transparent sheets that led to 3M’s first marketable transparency film. The Strategic 

Air Command base in Omaha was one of the first big clients, using circa 20,000 sheets per month. 3M then decided to develop their own 

overhead projector instead of the one they had been selling until then, which was produced by an outside manufacturer. It took several 

prototypes before a cost-effective, small and foldable version could be presented on January 15, 1962. It had a new fresnel lens made with a structured-surface plastic, much better than other plastic lenses and 

much cheaper than glass. In 1957, the United States' first Federal Aid to Education program stimulated overhead sales which remained high up to the late 1990s and into the 21st Century.

         USE IN EDUCATION

                  Overhead projectors were widely used in education and business before 

the advent of computer-based projection.

The overhead projector facilitates an easy low-cost interactive environment for educators. Teaching materials can be pre-printed on 

plastic sheets, upon which the educator can directly write using a non-permanent, washable color marking pen. This saves time, since the 

transparency can be pre-printed and used repetitively, rather than having materials written manually before each class.The overhead is typically placed at a comfortable writing height for the educator and allows the educator to face the class, facilitating better 

communication between the students and teacher. The enlarging features of the projector allow the educator to write in a comfortable small script in a natural writing position rather than writing in an overly large script on a blackboard and having to constantly hold their arm out in midair to write on the blackboard.

When the transparency sheet is full of written or drawn material, it can simply be replaced with a new, fresh sheet with more pre-printed 

material, again saving class time vs a blackboard that would need to be erased and teaching materials rewritten by the educator. Following the class period, the transparencies are easily restored to their original 

unused state by washing off with soap and water.

LIQUID CRISTAL DISPLAY(LCD) PROJECTOR

               An LCD projector is a type of video projector for displaying video, images or computer data on a screen or other flat surface. It is a modern equivalent of the slide projector or overhead projector. To display 

images, LCD (liquid-crystal display) projectors typically send light from a metal-halide lamp through a prism or series of dichroic filters that separates light to three polysilicon panels – one each for the red, green and blue components of the video signal. As polarized light passes through the panels (combination of polarizer, LCD panel and analyzer), individual pixels can be opened to allow light to pass or closed to block the light. The combination of open and closed pixels can produce a wide 

range of colors and shades in the projected image.

               Metal-halide lamps are used because they output an ideal color temperature and a broad spectrum of color. These lamps also have the ability to produce an extremely large amount of light within a small area; current projectors average about 2,000 to 15,000 American National Standards Institute (ANSI) lumens. Other technologies, such as Digital Light Processing (DLP) and liquid crystal on silicon (LCOS) are also becoming more popular in modestly priced video projection.

Projection SurfacesBecause they use small lamps and the ability to project an image on any flat surface, LCD projectors tend to be smaller and more portable than some other types of projection systems. Even so, the best image quality is found using a blank white, grey, or black (which blocks reflected 

ambient light) surface, so dedicated projection screens are often used.

              Perceived color in a projected image is a factor of both projection surface and projector quality. Since white is more of a neutral color, white surfaces are best suited for natural color tones; as such, white projection 

surfaces are more common in most business and school presentation environments.

However, darkest black in a projected image is dependent on how dark the screen is. Because of this, some presenters and presentation-space planners prefer gray screens, which create higher-perceived contrast. The trade-off is that darker backgrounds can throw off color tones. Color problems can sometimes be adjusted through the projector settings, but 

may not be as accurate as they would on a white background.

       Throw Ratio

            

               A projector's throw ratio is used when installing projectors to control the size of the projected display. For example, if the throw ratio is 2:1 and the projector is fourteen feet away from the screen, then the display width 

will be seven feet.

         History

               Early experiments with liquid crystals to generate a video image were done by John A. van Raalte at the RCA-Laboratories in 1968. His concept was based on e-beam-addressing to generate an electronic charge pattern corresponding to a video image, which in turn controlled the LC layer of a reflective LC cell.

Gene Dolgoff began thinking about different types of projectors in college in 1968 as a way to produce a video projector that would be brighter than the then-available CRT projectors. The idea was to use elements referred to as "light valves" to regulate the amount of light that passes through it, such as in traditional slide projectors. This would allow the use 

of a very powerful external light source. After looking at many different materials, he thought that liquid crystals would allow to modulate the light as planned. However, direct-driven, matrix-addressed LCDs with sufficient resolution for video images were not available at the time, so that Dolgoff could not yet complete building his projector prototype.

The first experiments with a direct-driven, transmissive matrix-addressed LCD using a converted slide projector by Peter J. Wild working at Brown Boveri Research in Switzerland were conducted in 1971. A projector was shown at the SID Conference 1972 in San Francisco. As passive 

LCDs (without transistors at the intersections) were not capable of displaying images with sufficient resolution for video pictures, a 

combination of a fixed image together with an LCD matrix for the variable elements was proposed as an LC projector for certain control room applications, with a corresponding patent filed in Switzerland on Dec. 3, 1971.

               A lot of effort went into optimizing thin-film transistors (TFT) suitable for driving active matrix-addressed (AM) LCDs. The concept was invented and early trials were conducted by teams at RCA and Westinghouse Electric. T Peter Brody left Westinghouse and founded Panelvision in 1981 to manufacture AM LCDs. Breakthroughs occurred elsewhere in new materials and thin-film structures, with Hitachi of Japan as a pioneering company. Such AM LCDs became commercially available in the early 1980s.

              Therefore, it took Dolgoff until 1984 to get a digitally-addressable LCD matrix device with sufficient resolution and contrast, which is when he completed building his LCD video projector. After building it, he saw many problems that had to be corrected including major light losses and very noticeable pixels (sometimes referred to as the "screen-door 

effect"). He then invented new optical methods to create efficient and bright projectors and invented depixelization to reduce the screen-door effect.

                   At about the same time, the German company "Bonner Ingenieurbüro für 

Optoelektronik CrystalVision" started experimenting with LCD projection devices from 1985 onwards. Although traditional slide projectors alreadyused infrared filters to reduce heating of the photographic slides, LCDs are much more sensitive to overheating. When the temperature in the nematic liquid crystal layer reaches the "clearing point" (i.e. enters the isotropic phase), the LC light valve does not work anymore until the temperature drops below again. Bernt Haastert, an engineer working at CrystalVision, found out, that placing the required polarizing filters at a 

certain distance on both sides of the LC cell allowed for efficient air cooling of the arrangement. Without applying this invention, LCD projectors with a powerful light source don't work. A commercial LCD projector based on this principle was launched in Germany in 1990 underthe trade name "Imagina 90".

With patents filed all around the world (filing his first LCD video projector patent application in 1987), Dolgoff started Projectavision, Inc. in 1988, as one of the world's first dedicated LCD-projector companies, which he took public on Nasdaq in 1990. He licensed the technology to other companies including Panasonic and Samsung. Early pioneers of LCD projection in Japan were Epson and Sharp, which launched their own color video projector products in 1989.In 1989, Projectavision, Inc. was awarded the first Defense Advanced Research Projects Agency (DARPA) contract – for US$1 million – for proposing that the United States high-definition television (HDTV) standard should use digital processing and projection. As a member of the National Association of Photographic Manufacturers Standards 

Subcommittee, IT7-3, Dolgoff along with Leon Shapiro, co-developed the worldwide ANSI standard for measurement of brightness, contrast, and resolution of electronic projectors. Since 2005, the only remaining manufacturers of the LCDs for LCD projectors are Japanese imaging companies Epson and Sony. Epson owns the technology and has branded it as "3LCD". To market 3LCD 

projector technology, Epson also set up a consortium called the "3LCD Group" in 2005 with other projector manufacturer licensees of 3LCDtechnology that use it in their projector models.Early LCD systems were used with existing overhead projectors. The LCD system did not have a light source of its own: it was built on a large "plate" that sat on top of the projector in place of transparencies. This 

provided a stop-gap solution in the era when the computer was not yet a universal display medium, creating a market for LCD projectors before their current main use became popular.

This technology was employed in some sizes of rear-projection television consoles when there was a cost advantages in mid-size sets (40- to 50-inch diagonal). In 2014, 60-inch 1080p flat panel televisions were less costly than a projector with 1080p native resolution. Projection systems were typically marketed as offering a diagonal image size of 100 to 300 

inches.

            In 2004 and 2005, LCD front projection began a comeback with the introduction of the dynamic iris and other modifications that have 

improved perceived contrast to levels similar to DLP.The basic design of an LCD projector is frequently used by hobbyists who build their own DIY (do-it-yourself) projection systems. The basic technique is to combine a high color-rendering index (CRI) high-intensity discharge lamp (HID lamp) and ballast with a condenser and collector Fresnel lens, an LCD removed from a common computer display and a triplet lens.

           SLIDE PROJECTOR

         A slide projector is an opto- mechanical device for showing photographic slides. 

35 mm slide projectors, direct descendants of the larger-format magic lantern, first came into widespread use during the 1950s as a form of   occasional home entertainment; family members and friends would gather to view slide shows.  Reversal film was much in use, and supplied slides snapped during vacations and at family events. Slide projectors were also widely used in educational and other institutional settings.Photographic film slides and projectors have mostly been replaced by 

image files on digital storage media shown on a projection screen by using a video projector or simply displayed on a large-screen video 

monitor.

       History

        A continuous-slide lantern was patented in 1881. It included a dissolving views apparatus.

    Components

        A projector has four main elements:

   • electric incandescent light bulb or other light source (usually fan-cooled)

   • reflector and "condensing" lens to direct the light to the slide

  • slide holder

  • focusing lens

          A flat piece of heat-absorbing glass is often placed in the light path between the condensing lens and the slide, to avoid damaging the latter. This glass transmits visible wavelengths but absorbs infrared. Light 

passes through the transparent slide and lens, and the resulting image is enlarged and projected onto a perpendicular flat screen so the audience can view its reflection. Alternatively, the image may be projected onto a translucent "rear projection" screen, often used for continuous automatic display for close viewing. This form of projection also avoids the audience interrupting the light stream by casting their shadows on the projection or by bumping into the projector.

     Types

  • Straight-tray slide projectors

  • Round-tray slide projectors

  • Stack-loader slide projectors

  • Slide cube projectors

  • Dual slide projectors

  • Single slide projectors (manual form)

  • Dissolve projectors

  • Viewer slide projectors                                        • Stereo slide projectors project two slides simultaneously with different polarizations, making slides appear as three-dimensional 

to viewers wearing polarizing glasses

• Medium-format slide projectors

• Large-format slide projectors for use on stages, at large events, or for architectural and advertising installations where high light output is needed.

• Overhead projectors

         CONCLUSION

                 Projected aids are useful for teaching process, these are great tools to catch the attention of audience, it is important to use variety of teaching methods for students with varying preferences. And an AV Aids should 

also not distract a student from learning experiences.

        REFERENCES

• www.wikipedia.com

• ecoursesonline.iasri.res.in

• www.slideshare.net

                        

                           THANK YOU