Cari and I have decided to build our own video projector. Beyond just being a hobby project to keep us busy for a few days, weeks, or months, the finished product will also be a great entertainment medium and at a price far lower than what a consumer projector costs. The projector will consist of four main components, each of which is documented in a separate section under Our Projector.

Commercial Projector Information

Commercial projectors are popular as they can provide a theater-like experience at home. They project video upon a wall or screen just as in a theater, producing enormous pictures, but requiring a large empty wall and potentially suffering from projector side effects like picture wash-out due to excessive ambient lighting. Most can be mounted on the ceiling or floor, or placed on a table.

Depending on the model, they can handle different input sources, like RCA, S-Video, component video, RS-232, and/or computer video (15-pin D-Sub and/or DVI).

Resolutions vary from a low of 854x480 up to a usual high of 1280x720. Resolutions tend to be one of two different standards: PC standard, or HDTV standard. The PC standard includes resolutions such as 800x600 and 1024x768, while the HDTV standard includes resolutions such as 854x480, 1024x576, and 1280x720. The PC standard is currently cheaper and more common. HDTV resolutions are often referred to by their y-axis dimension along with an 'i' or 'p' to indicate whether or not the picture is interlaced. Examples include "480i", "480p", "576p", "720i", "720p", and "1080i".

Price currently ranges from a low of around $750 for a simple 800x600 projector up to a high of well over $3,000 for a well featured 1280x720 projector. Lamp price is also of concern, as lamp life is usually only 3000 hours, and replacement lamps can reportedly cost hundreds of dollars.

DIY Projectors

Do-It-Yourself projectors are built using an LCD screen (usually salvaged from an LCD flat-panel monitor), a high quality bulb and reflector, and series of lenses, and a homemade enclosure.

Our Projector

Initial Projector Design Ideas -- meh, these aren't so good. They are only preliminary brain farts.

Due to our desire to better understand optics for this project, and due to the little amount of trustworthy optics knowledge online, we have ordered the following book to learn from and to assist us in our efforts: "Modern Optical Engineering" by Warren J. Smith.

LCD Panel

This section details the core of the projector, including the LCD screen, video inputs, video controls, and related electrical components.

After researching some options, Cari discovered a little gem on eBay. An LCD transparency panel paired with a video converter that may serve nicely in our project.

ASK Impact WSX (LCD transparency panel) Size 310 x 310 x 42 mm 12.2 x 12.2 x 1.65 inch Screen Diagonal 10.4 inch LCD Cell Active-matrix TFT Resolution 1024 x 768 Colors 16.7 million Response Time 30 ms Contrast Ratio 100:1 Video Converter Inputs RCA and S-Video Outputs PC Video (15-pin D-Sub)

Compared to using a screen from an LCD flat-panel monitor (the standard for DIY projector projects), this equipment has the following advantages and disadvantages:

Advantages

• Small screen size. With only a 10.4" diagonal screen size, the enclosure and some of the components inside can be much smaller than with an LCD flat-panel monitor, saving space and reducing costs in some areas.
• Remote control. This is a big advantage, as control of picture quality settings like brightness and contrast can be controlled via remote control, much like a commercial projector. Most DIY projectors are lacking in this area, and have manual controls on the projector at best.
• Video converter. A wonderful accessory to have, as it should provide the ability to have S-Video and RCA video inputs. This makes it possible to use a regular DVD player with the projector, and not just a computer. Video converters of this kind are difficult to find, and often quite expensive. Whether built as part of the projector or just used along side of it, having this piece is a definite bonus.
• Although not an advantage over LCD flat-panel monitors, it is also worth noting that the resolution of 1024 x 768 is higher than that of most LCD transparency panels. A definite plus, despite not being an advantage.

Disadvantages

• Contrast ratio is fairly low. This may result in a poorer picture.
• Response time is borderline. This may also result in a poorer picture.
• Age. Manufacture date for this item is 4/1997, which is a little old, adding further concern for overall quality of the screen.

Cari tested a different brand of LCD transparency screen and found the picture quality to be okay, so we are hopeful the same will be true for the ASK Impact WSX. Until it arrives, we won't know for sure.

Should the ASK Impact WSX prove to be inadequate, we may switch to using a 15-inch LCD flat-panel. However, the Video Converter should still be of use.

Light Source

This section details the projection lamp, reflector, and related electrical components.

We were originally considering salvaging a lamp, and it's associated electronics (such as a ballast), from a used overhead projector. Although many overhead projectors come with inappropriate lamps, some of the higher quality overhead projectors supposedly use metal halide lamps, which are more suited to the task.

However, after some research, it appears as though finding an appropriate overhead projector will be next to impossible. Instead, we opted to purchase a 400W pulse start metal halide bulb and electronic ballast from Lumenlab. The reason for selecting this option was that Lumenlab had a reasonably priced electronic ballast, which is especially hard to find. We also wanted a pulse start bulb, and the electronic ballast from Lumenlab works with pulse start bulbs. We were originally considering going with a 250W bulb and ballast, but the electronic ballast from Lumenlab comes only in 400W (which is okay).

Note: On the electronic ballast from Lumenlab, Zarva technology model LKEB-400MHD, the wiring was initially somewhat unclear. There is a small label on the side of the ballast with the wires that shows the proper orientation. Initially, I did not trust this label, but turns out it is correct. The wire bundle on the left, consisting of blue wire and a brown wire, goes to 120V AC. The wire bundle on the right, cosisting of a blue wire and a red wire, goes to the bulb. The polarity of the two wires on each side supposedly does not matter, so as long as you get the IN and OUT wire pairs correct, you should be okay. Another interesting implementation note is that the bulb would not turn on when hooked up to a particular outlet in my basement. My current assumption is that the electronic ballast and bulb do not work without a proper ground. This, in turn, is based on the assumption that the outlet in my basement, despite being a three prong outlet, is not properly grounded.

	Metal Halide "T15" bulb and electronic ballast from Lumenlab. Click thumbnail for a larger image.
	Closeup of label on the electronic ballast. Click thumbnail for a larger image.
	Light testing stand, used for testing out the initial design parameters.
	Light testing stand. The little round metal dial with the pig sticker is a candy thermometer for taking temperature measurements.

Most all DIY projectors use metal halide bulbs. They offer mostly neutral white light and low cost. When choosing a metal halide bulb, there are many factors to consider, including:

• Wattage -- DIY projectors generally use bulbs in the 200W - 400W range, depending on LCD size and how much light is actually utilized.
• Regular vs Pulse Start -- A small number of high wattage bulbs are "pulse start" bulbs. These bulbs have the advantage of reaching full brightness quicker, and having quicker hot restrike time as well. Also, they usually have reduced color temperature shift over the life of the bulb. I don't know of any disadvantages yet; my only suspicion is that they may have larger light arcs. See the NLPIP Metal Halide Q&A (this doc has lots of other useful info too) or the Pulse Strike vs Standard Metal Halide Lamps page at Atlas Lighting Supply.
• Color Temperature -- Metal halide bulbs generally offer neutral white light as desired, but there is some variance. They usually range from about 3500K to 4500K in color temperature, with most being 4000K. The Architectural Lighting Design Software site has some examples of different color temperatures in it's knowledge base.
• CRI -- This is described by ADLT Technology as "the ability to accurately depict colors". Higher values are better. Most metal halide bulbs have a value of 65. Some have a higher value such as 85.
• Burn Orientation -- Some bulbs must be used in a particular orientation, such as base up, horizontal, or vertical. However, most are universal, and can be used in any orientation.
• Base Types -- Many bulbs use what is called a "mogul" base, but there are others, and the mogul base itself has several different sizes. Some common base types.
• Ballast -- The ballast needs to be matched to the bulb. The wattage should be the same, and if a pulse start bulb is used, a pulse start ballast should be used. Ballasts can be magnetic or electronic. The electronic ballasts are supposed to be superior for several reasons, including smaller size, less risk of hum, an less risk of light flicker. However, as far as I have researched, the electronic ballasts, usually only found at fish tank lighting supply sites, are far more expensive (3 to 10 times more expensive). However, Lumenlab does sell a 400W electronic ballast that is only $100. Metal Halide Ballast Price Comparison.
• Physical Size -- This is important when designing the bulb mount and reflector. Some common Philips bulb sizes include 2-1/4"x9-3/4", 3-1/2"x8-5/16", and 4-5/8"x11-1/2" (diameter x length).
• Light Arc -- This is hard to tell without examining the bulbs, but it is important to try and pick a bulb with a very small arc width. The arc width is the little area at the center of the bulb that is actually producing the light. The smaller the arc width is, the easier it will be to properly focus the light. Having a smaller arc width may also help make for a clearer picture.
• Coatings -- Almost goes without saying, but make sure the bulb has a clear glass shell rather than a phosphorous or colored coating.
• Safety Concerns -- The outer glass shell of a metal halide bulb protects against the dangerous UV rays that would otherwise emanate from the bulb. However, most bulbs are still considered as requiring an enclosed fixture for added safety (potential dangers include harmful UV rays if the outer shell is damaged, along with flying glass and superheated gases should the bulb explode). Some metal halide bulbs are built with an additional sleeve around the light arc for added safety, and are usually considered safe for open fixtures. In the case of a projector, the projector itself is the enclosed fixture, so any bulb should do. However, special care should be taken during open box testing.
• Price -- For bulbs at bulbs.com, price ranges from $17.49 to $54.99 for metal halide bulbs in the 250W to 400W range with a clear glass shell.

See Sam's and Don's D-Lamp FAQ for more information on various gas discharge lighting such as metal halide. Also see an HID Lighting Safety Notice for additional information on UV safety concerns.

The Goal:

The goal when building the lighting source is to precisely and evenly light the LCD panel to the level needed, while wasting as little light as possible. There are any number of reflector and lens combinations that could be used. Whatever you might use however, in a standard 2 fresnel projector design, the light must eventually project such that it acts like a point source at the focal length of the first fresnel lens (the fresnel lens between the light source and LCD). This will then project a nearly perfect beam of parallel light through the LCD and into the second fresnel lens (the fresnel lens between the LCD and projection lens).

The Reality::

The reality is that adding special focusing lenses or custom made reflectors is expensive (not to mention complex), and it is our goal to make the projector cost effective in any area that does not negatively impact projector quality. We therefore intend to mount the light source at the focal length of the fresnel lens and use a spherical reflector behind the lens, a fairly cheap and effective solution. This wastes a large amount of light, but our 400W bulb is more than we should require to illuminate a 10.4" LCD, so it will probably balance out nicely.

A spherical reflector will reflect light back towards the source. This works sufficiently well with metal halide bulbs as there is not much obstruction to block the reflected light from passing back through the bulb. This will, however, cause the bulb to run hotter, possibly decreasing bulb life, so it is advisable to cut the reflector such that it is no bigger than needed.

We plan to use a 220mm focal length fresnel lens to capture the light. When using a bulb with spherical reflector, as in our case, it is ideal to have the bulb as close to the fresnel as possible in order to capture more of the bulbs light, which means using a fresnel lens with a relatively short focal length. However, the closer to the fresnel the bulb is, the more difficult managing heat will become. As you will see in the Lenses section of this document, we ordered a kit that comes with a 220mm fresnel and a 317mm fresnel. We will collect more light with the 220mm fresnel, so it will be a better choice so long as we can manage the heat. As you will also see in the Lenses section, the 317mm fresnel is better used elsewhere anyway.

Finding a Spherical Reflector:

This is more difficult than it might seem. The common cost effective way of getting a spherical reflector is to find and cut a good sized soup ladle.

Cutting a Spherical Reflector:

Once you find a spherical reflector, it will probably need to be cut to size. You can calculate how it should be cut by considering your fresnel lens focal length. Trimming the reflector to size will ensure that the amount of heat and light being reflected back through the bulb is kept to a minimum.

Light Usage:

Given the setup of a bulb at the fresnel focal length with a spherical reflector, we will only be capturing about 10% of the bulb's light. This may be sufficient, given that we will be using a rather hefty 400W bulb. If not, we may look at adding an additional lens into the system to collect more light.

Lenses

This section details the various lenses and mounting techniques used in the projector.

A projector typically requires 2 fresnel lenses and 1 lens triplet. We have purchased a 320mm focal length lens triplet, a 220mm fresnel lens, and a 317mm fresnel lens. These lenses came as a set from Lumenlab.

Variables of interest:

• F_p = projection lens focal length
• F_f = fresnel lens focal length
• D_s = distance from projection lens to screen (throw)
• D_l = distance from LCD to projection lens
• S_x = diagonal size of LCD panel
• S_y = diagonal size of projection

Equations from the DIY Projector Company:

S_y = ( (D_s - F_p) * S_x ) / F_p Note: I am not using this equation; see details below.

D_s = ( (S_y * F_p) / S_x ) + F_p Note: I am not using this equation; see details below.

Equations from a threads on DIY Audio:

1/F_p = 1/D_l + 1/D_s

My equations:

For now, I will be using slightly different equations than shown on the DIY Projector Company web site for determining the diagonal size of the projection and for determining the distance from projection lens to screen (throw). I will also be using the equation as is from DIY Audio for determining the relationship between the distance to LCD and the distance to screen.

Eq. #1)   S_y = (D_s * S_x) / F_f

Eq. #2)   D_s = (F_f * S_y) / S_x

Eq. #3)   1/F_p = 1/D_l + 1/D_s

Rationale:

For the DIY Projector Company equations, I am not sure how it is that the size of the projection would not at least partially rely on the focal length of the fresnel lens. As I see it, the size of the projection should depend on both the focal length of the fresnel and the focal length of the projection lens, in addition to the size of the LCD panel and the distance to screen.

It is fairly easy to imagine how the size of the projected image will vary if there were only a fresnel lens. However, how the projection lens will affect the image after passing through the fresnel is unclear. More study needs to be done in this area. For now, I will use some experimental data I gathered using a simple laser pointing device and the projection lens.

There is a range in which the complete image can pass through the projection lens. This range will be somewhere near the focal point of the fresnel lens, where the image is small enough to pass through the projection lens in it's entirety. However, there are several possible scenarios to consider, and more research is needed to determine with certainty what exact range will produce a quality projected image. Consider the following possible light paths through the projection lens:

As previously stated, it is unclear what range, from the extremes shown in diagrams (A) and (C), will produce quality projections on the screen. What is clear, however, is that the image size is increased slightly in scenario (A), decreased slightly in scenario (C), and roughly unchanged in scenario (B). This was confirmed through experimentation. It was also determined through experimentation that the change in image size even at the extremes will not be highly significant (in my opinion).

The reason the change in image size due to the projection lens is so small is thus a factor of two presumptions. First, the maximum change in light path through a comparatively small projection lens is far less than the maximum change in light path of the projected image passing through the fresnel. This difference changes depending on selected LCD size, but even with a smaller than typical 10.4" screen diagonal size, the difference is still quite significant. Second, we will assume that scenario (B) is ideal, and scenario (B) is where the change in light path is minimal.

Given the minimal impact of the projection lens on image size, for simplicity I will be ignoring the projection lens completely for image size calculations. We can then reduce image size calculations to a simple relationship as follows:

Solving for S_y and D_s gives us my equations #1 and #2.

My equation #3 is the same as the equation from the DIY Audio forum, which I assume can be applied in determining the relationship between distance from projection lens to LCD and distance from projection lens to screen. This equation holds true for focusing a simple lens, and I am assuming it will also hold true for our projection lens. You can find this same equation in the Modern Optical Engineering book, 3rd edition, on page 26, equation 2.4. The equation in the book is 1/s' = 1/f + 1/s, where s = distance from lens to image, s' = distance from lens to projected image, and f = focal length of lens. In their case, s is a negative number (whereas our D_l is positive); taking this into account yields the same equation as used in used in both my equation #3 and the DIY Audio equation.

We can now look at ideals and extremes for our projector given our available equipment. We have the following:

• Projection Lens - 320mm focal length, 58mm lens diameter
• Fresnel Lens - 317mm focal length
• Fresnel Lens - 220mm focal length
• 10.4" diagonal LCD (264.16mm)

Assumptions:

• Ideal focus range will be achieved with fresnel focal point near center of projection lens as in diagram (B) above.
• The furthest backward we can move the projection lens (toward the fresnel) will be where the image just barely fits into the lens as in diagram (A) above.
• The furthest forward we can move the projection lens (away from the fresnel) will be where the focal point of the fresnel lines up with the leading edge of the projection lens, somewhere between the positions shown in (B) and (C). (through some admittedly poor experimentation, it seemed distortion became a problem beyond this point.
• We want an ideal projection diagonal size of 8 feet (2438.4mm). This would give us a screen size of approximately 7 feet by 5-1/4 feet. However, this is a 4:3 aspect ratio. Assuming we watch mostly widescreen shows at a 1.85:1 aspect ratio, the resulting image would be slightly smaller; approximately 7 feet by 4 feet.

These assumptions are just that: assumptions. They may be adjusted later after more information is collected.

Distance to Screen (Throw) Using the 220mm Fresnel:

For ideal 8' screen diagonal,

D_s = (220*2438.4) / 264.16 = 2030.8mm = 6.7 feet

Distance to Screen (Throw) Using the 317mm Fresnel:

For ideal 8' screen diagonal,

D_s = (317*2438.4) / 264.16 = 2926.2mm = 9.6 feet

Fresnel Lens Selection:

An ideal throw is probably about 10 feet. The 317mm Fresnel is clearly a better selection.

Distance From Projection Lens to LCD:

We can now calculate distance to LCD for our ideal setup using my equation #3.

1/320 = 1/D_l + 1/2926.2; D_l = 359.3mm

This puts the fresnel 42.3mm away from the LCD panel (359.3mm - 317mm = 42.3mm).

Rough Estimate of Throw Range and Subsequent Image Sizes:

For this, we need to consider the thickness of the projection lens. Our projection lens if roughly 40mm thick. Our ideal setup is with the fresnel focal point in the center, 20mm in.

We made the assumption earlier that the furthest backward we can move the projection lens (toward the fresnel) was to the point where the image just barely still fits into the opening of the projection lens. This will be a distance x away from the fresnel lens focal length.

x / 58 = 317 / 264.16; x = 69.6mm (247.4mm from the fresnel)

We also made the assumption earlier that the furthest forward we can move the projection lens (away from the fresnel) was the point where the focal point of the fresnel touches the edge of the lens (317mm from the fresnel).

We now have a possible range of 247.4mm to 317mm such that the image can potentially be focused. These measurements, however, are both to the edge of the lens, and we need measurements to the center. Adjusting by half the width of the projection lens (20mm), we get a possible range of 267.4mm to 337mm, with the ideal point at 317mm (distance from fresnel). Going back to our ideal LCD location 42.3mm away from the fresnel, we get a potential projection lens to LCD range (D_l) of 309.7mm to 379.3mm. We can use this potential range to calculate minimum and maximum throw distances.

1st Attempt: Minimum Distance To Screen:

1/320 = 1/379.3 + 1/D_s; D_s = 2046.8mm (6.7 feet)

This minimum is slightly higher than what we would like. We can fix this by moving the LCD a little. We need to recalculate to allow for an appropriate minimum, disregarding the original "ideal" calculation.

1st Attempt: Maximum Distance To Screen:

1/320 = 1/309.7 + 1/D_s; D_s = -9622mm

The negative value is an indication that there is no maximum distance to screen. This will still happen when the new minimum distance to screen is calculated below (though I won't be showing this). Since we don't need a projector that can project out to infinity and beyond, it might also be a good time to consider what maximum throw we would like to have.

New Assumptions:

We need a minimum distance to screen of 6 feet (1828.8mm).

We need a maximum distance to screen of 20 feet (6096mm).

2nd Attempt: Minimum Distance To Screen:

D_s = 6 feet (fixed)

We are now fixing the minimum at 6 feet, but we need to recalculate the distance to LCD in order to recalculate the other distances.

1/320 = 1/D_l + 1/1828.8; D_l = 387.9mm

We need to increase the distance to LCD by 8.6mm putting the LCD a total of 50.9mm from the fresnel lens (42.3mm + 8.6mm = 50.9mm).

2nd Attempt: Maximum Distance To Screen:

D_s = 20 feet (fixed)

1/320 = 1/D_l + 1/6096; D_l = 337.7mm

New Projection Lens Adjustment Range:

With the minimum and maximum distances to screen now constrained to 6' and 20' respectively, the required projection lens adjustment range drops from 69.6mm to 50.2mm (387.9mm - 337.7mm = 50.2mm)

2nd Attempt: Projection Sizes:

At minimum throw of 6 feet: S_y = (1828.8*264.16)/317 = 60" (5 feet)

At preferred throw of 10 feet: S_y = (3048*264.16)/317 = 100" (8.3 feet)

At maximum throw of 20 feet: S_y = (6096*264.16)/317 = 200" (16.7 feet)

Location of projection lens (center) at minimum throw: 1/320 = 1/D_l + 1/1828.8; D_l = 387.9mm

Location of projection lens (center) at preferred throw: 1/320 = 1/D_l + 1/3048; D_l = 357.5mm

Location of projection lens (center) at maximum throw: 1/320 = 1/D_l + 1/6096; D_l = 337.7mm

Final Alpha Design Measurements:

Note: The projection lens adjustment range of 50.2mm, along with it's starting and ending points, are still largely conjecture. Also, the final range may be further constrained by the enclosure design and the physical projection adjustment track. Distance From LCD to Fresnel: 51mm (rounded) Minimum Distance From Projection Lens to Screen: 6' Preferred Distance From Projection Lens to Screen: 10' Preferred Distance - Projection Diagonal Size: 8.3' Preferred Distance - Distance From Fresnel to Projection Lens (center): 357.5mm - 50.9mm = 306.6mm

Enclosure

This section details the physical enclosure design and construction.

Until we have more components selected, we cannot do much work on designing the enclosure.

Other Elements

• Projection Screen -- At this point, the projection screen will most likely be a wall. Doesn't get much simpler than that.
• Projection Screen Border -- It can improve appearance to include some kind of black border around the projection screen. This will be one of the last things we look into.

Component List

Major Components

Component	Source
ASK Impact WSX transparency LCD, 10.4", 1024x768, w/ remote	salvage, eBay
ASK Impact WSX companion video converter	salvage, eBay
400W Pulse-Strike Metal-Halide Bulb (Mogul)	Lumenlab, bulb retail/wholesale
400W 120V Electronic Ballast	Lumenlab
Projection Lens Triplet, 320mm focal length	Lumenlab
Fresnel Lens, 317mm focal length	Lumenlab
Fresnel Lens, 220mm focal length	Lumenlab

Minor Components

Component	Source
Mogul Bulb Base	Lumenlab, bulb retail/wholesale
Overhead Projector Focusing Track	salvage

Price Analysis

Our Costs

Component	Cost	Shipping	Total
LCD screen and video converter	$ 175	$ 21	$ 196
Lamp and related electronics	$ 170	$ 12	$ 182
Fresnel and triplet lenses	$ 65	$ 7	$ 72
Enclosure supplies (estimated)	est $ 75	est $ 0	est $ 75
Totals	est $ 505	est $ 40	est $ 545

Price Comparison

Last Updated 1/20/2006 by Scott Arnold