Image projectors: Taking a closer look

If you're not one Samsung firm that can afford a 30 ft wall display panel system for that awesome presentation you've been prepping for, then I bet you're planning on doing so with an image projector. It has been a life saver right from time and to a great extent, it helped me drive my point home during my final stage in the university. Projectors have grown in popularity with many firms coming up with their own tweaks, I was motivated to write this post when I stumbled upon the world's smallest projector; the pico projector, while watching a technology documentary.

The pico projector with thickness of less than one quarter of an inch can shoot image with depth of focus up to 800x480 pixel! Though not that bright, but it is always handy. With advancements in technology, varieties are bound in the world of image projectors.

^{[The pico projector capable of very powerful image projections from our handheld devices. Credit:Flickr. CC BY 2.0 licence. Author: Cheon Fong Liew]}

The image projector has been in constant evolution since the year 1800 starting with the pinhole camera or the camera obscura. The first attempt of build a functional projector was in early 1800 with the introduction of magic lanterns. Though very primitive but the idea was a very simple one and it involves the projection of paintings produced on a transparent surface like glass or liquid, such as oil. The screen for the projection was a white wall and the projector kept at a reasonable distance depending on the required magnification.

In late 1920, a better version of the magic lantern was developed and it was called the filmstrip projectors. The working principle was almost the same with the magic lantern but with the advancement in photography, the filmstrip projector uses a sort of filmstrips from cameras or other imaging devices, and this strip was inserted in front of the projector aperture and a light source was made to focus on this filmstrip projecting a brilliant and enlarged image. The application was immediately found in education sector and they were very popular in schools.

^{[The filmstrip projector. Credit:Pixabay. CC0 creative commons licence. Author: PIRO4D]}

From the filmstrip projector we had the overhead projector which featured a fix for the major problem that faced the earlier projector; heat, this was later replaced by the data projector in the late 1980's which had the capability of video projection instead of picture projections with sources from computers and multimedia devices. Today we have more sophisticated projectors including the 3D projectors and the 4k projectors.

The physics behind image projectors

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The light bulbs in our homes are constantly toggling between on and of just as current moves between crest and the trough in its sinusoidal pattern. These lighting materials operate at either 50 Hz or 60 Hz, which means the lights toggles on and off, 50 or 60 times in a second, hence, our mind informs us that the light is constantly on. This is called the persistence of vision.

Even the videos we watch are pictures thrown up at certain speeds (usually 680 image frames per second) with a background audio. The output of a projector consists of sets of images that are arranged in the form of reels and these images are made to leave the reel in a space that is in between a light source and a display system (which includes series of mirrors, prisms and lenses).

The speed at which the image leaves the reel determines whether its a motion picture (video) or a slide show. According to the principle of persistence of vision, our eyes will fail to detect changes which occur in less than 0.0625 secs. Also colors are produced in the system using a mechanism called the spinning color wheel. Here, light projected from each frame from the reel is made to pass through series of lenses and prisms at a very high speed and at such speeds, the human brain is only able to interpret the output as a single color.

Also the projector is incapable of projecting black color onto the screen because there are no such thing as light with black color. This is one of the failures of the projector. Hence, the projector is able to make a "black color" impression by not projecting light at all for black areas of an image and the natural contrast in our eyes perceives this as a black color and as such, the image projected by the projector is as bright as the darkness of the room permits and also as the intensity of light source permits.

Operating principle

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The RGB

RGB is a popular abbreviation representing the three natural colors which are Red Green and Blue. Creating images in their right color involves a series of color operation by a specialized chip, this operation is known as the additive color mixing. Irrespective of the color of the image we see from the projector, this color emanated from the three basic natural color, the RGB.

With the right mixture of these three colors, we can come up with virtually thousands, if not millions of different colors. Depending on the proportion, when we mix the three colors in equal proportion, we arrive at the color white, when we remove blue from the equation leaving only red and green in equal proportion, we arrive at the color yellow, removing red from the equation with the remaining color at equal proportion, we would have cyan while removing green from the equation leaving red and blue in equal amount would produce a magenta color.

The video signals reaching the projector is converted to light by the light source and this light is in primary colors which undergoes rapid color spinning which mixes these RGB colors in their right proportion with the help of an imaging chip to produce beams of light that can be interpreted by human brain.

The light source

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The light source for a projector forms a major part of the projector. There are various types of light sources for the projector which include the Metal halide bulbs, LED bulbs and lasers (why not) while some spots both lasers and LEDs in a more sophisticated manner to produce even a better quality output.

The quality of the light source is measured in Lumen. The lumen is a measure of the amount of light produced by the projector lamp just as we could use the number of cars that passed an area per minute to measure the amount of traffic in such area.

The focus of this work is mainly on the LCD projectors, hence, I will only discuss the metal halide lamps (MHL) which are mostly used in the LCD projectors. The MHLs are also known as the high or ultra-high intensity discharge lamps. MHL consists of a pressurized glass which contains an electric arc. Within this enclosure we have metal vapors such as the mercury and sodium iodide.

The metal halide lamp is very light efficient and have a very lengthy lighting life time but due to the fact that these metal vapors do not cool so easily, this could lead to the bulb burning out before its speculated lighting time. Hence, specialized cooling fans, which sometimes, if not properly designed gives off some offensive noise, are employed to cool the bulb and would need a minimum of 1 min to cool before complete power off, removal of power source before complete cooling greatly reduce the life expectancy of the bulb.

The dichroic filters and the diffusers

Once a bright white light has been produced from the high intensity discharge lamp, the next step is to split this light into the three basic natural colors explained above, this is the main function of the diffusers and the dichroic filters. Most projectors are coupled with two diffusers which ensures that purple or violet component of the light produced by the halide lamp does not reach the optical chambers, hence, the diffusers can be seen as a low-pass filter which traps or reflects backwards any purple or violet light from the halide lamp and in doing so, ensures that purer light reaches the dichroic filters.

^{[The three basic dichroic filters. Credit: Wikimedia. Creative commons license.]}

The dichroic filters are made by depositing very thin layers of dielectric material in a vacuum onto a material with a very low thermal expansion. This is due to the fact that halide bulbs naturally generates a lot of heat and this could easily alter the operation of the optical arrangements. The material which is best suited as a substrate for this operation is glass.

The dichroic filters are capable of splitting white light into three components, Red, Green and Blue. In projectors, three dichroic filters are included, each capable of reflecting one color and allows rest of the component to transmit. For example, a green dichroic filter would reflect only green lights and allow the rest of the component to pass. This filtering is possible due to the fact that different colors exists at different wavelengths hence, the dichroic filters are seen as a very narrow range wave filters.

The filtering process are not just random for projectors that are coupled with only two dichroic filters, here, the projectors allows light from the diffusers to first hit the blue filters which reflects blue beams and allow yellow beams to transmit, this reflected yellow beams are then made to pass through a green filter which splits the yellow beam into red beams and green beams.

The Trichroic prism and the LCD screens and chips

The Liquid Crystal Display chip of a typical projector is truly an amazing piece. The LCD screens are composed of millions of very tiny liquid crystals each representing its own pixel. Most of our gadgets spots a display system with a resolution of 1920 by 1080 pixel, this means that for an LCD projector to accurately represent images from our gadgets, it needs crystal displays with a minimum of 2 million (1920 * 1080 = 2, 073, 600 approx 2 mega pixels) tiny crystals.

Remember the explanation I gave about the filmstrip projectors and the image reels? This liquid crystal screens acts like the filmstrips used in the filmstrip projectors but in this case, they are not inserted in the aperture of the projector but inbuilt and images are imprinted on them by the dichroic filters.

The function of the LCD processors are to create three separate channels for the three primary colors, the RGB. They do so by controlling the states of these tiny liquid crystals, i.e. either on or off at different rates depending on the optical signal it receives. Hence, the LCD produces images by forming a kind of image masks at which light is projected at to form shapes that could be interpreted by the human mind. Once the mask has been created, a recombination process is required to form a mixture of beams which then is projected to the screens in our classrooms.

This recombination process is done by the trichroic prisms. The trichroic prisms are meant to reverse the role played by the dichroic filters in splitting the white light as stated above, hence the prism is composed of two dichroic prisms. Green and blue components of the emerging light from the LCDs are combined by the first prism to form a cyan light, this light is further combined with the red light which escaped the first prism by the second prism into a complete image.

If you’re bold enough to dismantle your LCD projector and remove the trichroic prism, the output of the projector would be just three separate light beams and these would be the three natural colors explained above.

I hope that this has been informative for you and I would like to thank you for your time

REFERENCES

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1. Projector -Wikipedia
2. brief history of projector in pictures -netmanias
3. projector working principle -howwhywhat
4. pico projectors -omnisecu

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