Optimizing the lens aperture on DPI's new dVision Series Projectors

Every lens for Digital Projection's dVision sx+ and dVision HD projectors includes an internal motor driven aperture that can be remotely adjusted to optimize the balance between projector light output and contrast for a specific venue. In practice, the effect of this aperture is simple: When in its maximum open position, the projector's full brightness is transmitted through the lens and onto the screen. As the aperture is incrementally closed, the projector's brightness and optical scatter light are clipped, resulting in lower lumens but rewarding with significantly reduced black level and a higher contrast ratio.

When using a dVision projector in a high ambient light environment, the proper setting for the aperture is its maximum open position. The reason is that projected imagery in high ambient light venues will not benefit from a projector with high contrast (low black level), but will benefit from maximum lumens. Indeed, in high ambient light environments, the only way to increase the environmental dynamic range of the image is to either turn out the lights, or bring as many lumens to the table as you can possibly muster. Thus, for high ambient light environments, beyond assuring the aperture is fully open, you may also want to run both of the dVision's lamps at full power. Depending on the content being displayed, you may even want to add some white boost and select one of the more dynamic gamma presets.

However, in venues with high control over ambient light, the benefits of the adjustable aperture really come into play. The darker the venue, the more an image with ultra low black level and high contrast can be appreciated. In theatrically dark venues, once a minimum threshold of projector lumens is achieved (enough to produce 12-16 Ft. Lamberts of light as reflected by the screen), screen black level and contrast ratio become a dominant factor in creating superior image quality.

Follow the simple procedure below in venues with low or theatrically dark ambient lighting:

•  Set up the dVision as you normally would, starting with both lamps on "High" and white boost "Off" or set to a minimal level.

•  With the room lighting set to typical viewing conditions, display a test pattern or a static source that exhibits both 100% white and 100% black content, as well as a range of intermediate shades. Select the projector gamma and set colorimetry to your preference. Adjust projector brightness and contrast accordingly.

•  If the image is far too bright for the venue, reduce lamp power on both lamps, or turn one lamp off. Adjust lamp brightness until the image is still somewhat brighter than you need it to be. Re-check colorimetry and gamma if necessary.

•  Finally, start closing the aperture while carefully watching the black level in the 100% black content area of the image. Optimally, you want to achieve zero black level (no visible illumination in the black content) while still projecting enough lumens to produce the target Ft. Lamberts required for the venue. If in your search for “perfect black” the projector brightness drops too low and you can't achieve your target Ft. Lamberts, re-adjust lamp power and/or turn on both lamps.

You may need to go through the cycle a couple of times before you strike the perfect balance between screen brightness and black level. The goal is to produce perfect black and your target Ft. Lamberts while running one or both lamps (as necessary) at the lowest possible power. Lower lamp power equals longer lamp life, so by following this process, you not only achieve the optimum balance between contrast and brightness for the venue, but you are further rewarded with the lowest

Digital Projection's Screen Brightness Calculator:
A Useful Tool for Designing High-Impact Projection for Any Venue

The answers to the questions of how bright a projector needs to be, or how big a screen can be employed on a job, depend on viewing distance, ambient light level, content to be displayed and desired image brightness. DPI created the Screen Brightness Calculator to assist in defining the optimum mix of screen size, screen gain, projector lumens and contrast, for any venue.

To use DPI's Screen Brightness Calculator, the only factors you need to know in advance are the ambient light level that will be falling on the screen and the display objectives for the venue. Armed with this information, the calculator can guide you in selecting the perfect combination of projector lumens and screen size and type, to create optimum imagery in your applications.

The Screen Brightness Calculator can also assist you in determining screen dimensions (height, width and diagonal) for any aspect ratio screen. Simply select the required screen aspect ratio and define any one of the three screen dimensions, and the remaining two dimensions will calculate automatically.

To put the Screen Brightness Calculator to work, select your projector resolution and the required screen aspect ratio, then simply type the relevant numbers into the yellow boxes. Your goal is to match screen dimensions, screen gain and projector lumens, such that the system delivers enough Ft. Lamberts to overcome the venue ambient light level and produce suitable environmental contrast ratios. Target Ft. Lamberts for different types of venues are provided as examples within the Screen Brightness Calculator.

Experiment with the Calculator by trying different screen gains, screen sizes, aspect ratios, projector lumens and venue ambient light (falling on the screen). Notice the impact these factors have on image performance.

When using the calculator, be aware that projector brightness in lumens and the projector's contrast ratio must be as accurate as possible. Some manufacturers exaggerate these values in their specifications. Don't use "spec" data if it is suspect, use real data. Additionally, projectors that use Metal Halide lamps produce much less light when calibrated to D6500. Finally, all projection lamps lose light output as they are run through their useful life. Thus, the actual values you create using the Screen Brightness Calculator should be 25-to-50% higher than the target values referenced in the Calculator. This will help assure the projected imagery remains high impact through the entire life of each lamp.

CLICK HERE to start using the Screen Brightness Calculator.

IMPORTANT: The Caculator is a Microsoft Excel file and uses "macro" functions. In order for it to work properly, the Excel program on your computer must be set-up properly. This is done by setting Excel's Security settings.

If you don't know how to do this, follow these simple instructions (BEFORE opening the Calculator):

If you have any further problems, please contact your IT Manager or your DP Regional Market Development Manager.

What You Need to Know About Gamma

Ideally, images shot by a video camera would be reproduced on a display device exactly as it was recorded. Unfortunately, this is very rarely the case, as the technologies of cameras and display devices sometimes have their own nuances that slightly, and sometimes significantly, change the performance of imagery. In such cases, you need tools to correct for these abnormalities.

What is Gamma?

Gamma is a function to correct nonlinear performance inherent in CRT display devices. The CRT does not perform in a linear fashion against voltage; therefore, it might display an image not exactly as it was intended.

Voltage is the medium in which the CRT is fed the video signal. Another way to look at it is the relationship of video signal input to the CRT versus light out is not linear.

To compensate for this, a gamma "curve" is applied to the signal at the video camera during recording. This curve is defined based on the known inherent performance of a CRT. When the signal is applied to the CRT, it is already corrected for the nonlinear performance; hence, it correctly outputs the images recorded by the camera.

Since DLP devices have a natural linear function, the gamma correction needs to be removed. So to be correct in our terms, we apply a degamma function to the signal within DLP products.

What gamma do I need in my application?

Since all display devices are well documented concerning their performance characteristics, we know what gamma corrections are normally applied to a signal that was intended for use with such a device. Until recently, CRT has been the dominant display technology of display devices.

Signal types such as NTSC and PAL have gamma curves based on CRT displays. Film, on the other hand, has a gamma that is based on celluloid reproduction and film-to-video transfer. NTSC, the video standard adhered to in the U.S., has a gamma of 2.2.

Gammas also are determined based on the conditions of the environment that the display device may be used, such as a dark room (cinema, home theater), a well-lit room (conference room, sanctuary) or outdoor environment. To account for a host of conditions, gammas are created for low-light, or bright-light situations. A film-based video source in a controlled theater environment with very low lighting might use a gamma known as Film Low.

Other gammas have occurred based on nothing other than an artistic approach to the reproduction of an image. Sometimes, people just like to see images in a certain way.

Summary

Although gamma correction was born to fill the need of correcting nonlinear performance of video recording and display devices, it has also become a subjective tool for those who want a certain look, or just like the effect of one gamma curve over another. It is your choice to be objective and use the gamma, or degamma function that is intended for a certain signal type and display device, or you can be artistic and pick something you find pleasing. Different gamma curves may appeal to you for different environmental lighting conditions as well. Understand the foundation of gamma, but use as you see fit.

Total Environmental Dynamic Range - the Key to Assuring Maximum Visual Impact
in any Venue

A key factor to consider when selecting a projection display is understanding the minimum Total Environmental Dynamic Range required to achieve killer imagery in the target venue.

The term Total Environmental Dynamic Range (TEDR) describes the actual contrast ratio achieved in a venue, including the impact of ambient light. Thus, the TEDR value defines the dynamic range, or contrast, the viewers actually perceive. You will see, even in venues with a minimal amount of ambient light, the TEDR value will be much lower than the contrast ratio defined in projector specification sheets.

The method of calculating TEDR is relatively simple:

TEDR = Projector Screen Brightness / (Projector Screen Black Level + Ambient Light reflected by the Screen)

The most complicated part of the process is converting the three values in the formula to one standard of measurement. For purposes of this article, we will use Foot Lamberts (FtL). FtL is a measurement that defines the light being reflected by the screen.

Step 1: Projector Screen Brightness

Convert the lumens produced by the projector to Projector Screen Foot Lamberts (PFL) using the following formula:

PFL = (Projector Lumens / Screen Area in Sq.Ft.) X Screen Gain

Step 2: Projector Black Level

Projector Black Level (PBL) must also be defined in terms of Foot Lamberts. The approximate PBL value can be calculated in terms of lumens, by simply dividing the projector lumen spec by the projector's specified contrast ratio. As an example, a 1,000 lumen projector with a 1000:1 contrast ratio, in theory, should produce a PBL of 1 lumen. We then use the same formula we used in step one to convert the lumen-based PBL to a FtL based value (PBFL).

PBFL = (PBL in Lumens / Screen Area in Sq. Ft.) X Screen Gain

Step 3: Ambient Light

For new construction, defining the ambient light that will fall on the screen is best done with the help of the lighting designer. For existing installations, we recommend taking a real-world measurement using an accurate incident light meter positioned where the screen will exist. Hold the meter parallel to the screen surface aimed toward viewers' position. Many luminance meters measure incident light in terms of Lux. If this is the case with your meter, the Lux value will need to be converted to Foot Lamberts.

To convert Lux to Foot lamberts, use the following two formulas:

1. Convert ambient incident Lux to Lumens:

Ambient Incident Lumens = Ambient Incident Lux X Screen Area in Sq. Meters

or

Ambient Incident Lumens = Ambient Incident Lux X (Screen Area in Sq Ft / 10.56)

2. Convert Ambient Incident Lumens to Ambient Incident FtL (AIFL)

AIFL = (Ambient Incident Lumens / Screen Area in Sq.Ft.) X Screen Gain

Step 4: Bringing it All Together

Now that all of our variables are expressed in terms of FtL, we can use the formula to calculate the TEDR that will be achieved:

Total Environmental Dynamic Range = Projector Foot Lamberts / (Projector Black Level in FtL + Ambient Incident Light in FtL)

Or, stated in short form, using our acronyms:

TEDR = PFL/(PBFL + AIFL)

Putting Total Environmental Dynamic Range to Work for Your Customers

As an approximate guide, DP recommends Total Environmental Dynamic Range targets for the application categories
listed below:

- Conference Room (PowerPoint, Spreadsheets, some Video or HD): 10-20:1 rear or front-screen
- Home Media and Entertainment (Video, HD, Videogames, Web): at least 15-30:1 rear-screen, 30-50:1 front-screen
- Worship (Hymn Text, IMAG, some Video): 10-20:1 rear-screen, 20-40:1 front-screen
- Staged events and broadcast applications (CG content, Video and HD): 30-50:1
- Theater or screening room (Video, HD, CG content): at least 500-1000:1

Of course, customer preferences and content present additional variables, meaning no simple set of rules will
work for every application. However, as you start to consider TEDR in the systems you design, you will define the TEDR values that work for your customers as well as the projector, screen and lighting configurations that deliver those TEDR values.

It is all about dynamic imagery. The final simple rule: Reduce ambient light as much as possible. If TEDR values are
still too low, bring more lumens to the task and consider the use of high gain and/or rear projection screens.

In essence , the bigger DMD means a bigger light patch exiting the lens, so the classic lens
gets shorter when used on the 12000Dsx+ platform. Recalculation of the lens throw ratio is
pretty straightforward:

• Divide 1280 pixels by 1400 pixels. That equals .914
• Multiply the original throw ratio of the classic HIGHlite lens by this factor (.914)

The result is the throw ratio the classic HIGHlite lens will provide on the new HIGHlite
12000Dsx+. As an example, a classic HIGHlite 2.5 - 4:1 lens will actually be 2.285 - 3.65:1
when used on a HIGHlite 12000Dsx+ projector.

There are a couple of important things to remember. First, classic HIGHlite 1.5-2.5:1 lenses need
to be specifically denoted as "High-Brightness" to be used on a HIGHlite ProSeries projector. Specifically, the DPI part number LA00263 (the lens may be marked TL-1ZH), confirms the High- Brightness version of the lens. Use of the standard brightness 1.5 - 2.5:1 classic HIGHlite lens on
a HIGHlite Pro series projector will cause the lens to eventually discolor and fail.

Additionally, an adapter ring is required to enable compatibility between the classic HIGHlite lens and the new lens mount used on the HIGHlite Pro series projectors. The "Classic HL Lens
Adapter" is listed under "Options" on the HIGHlite Pro series page in our Commercial AV and
Home Cinema Dealer pricing. It carries a list price of $495, and provides a very efficient path to repurposing classic HIGHlite lenses for use on the new HIGHlite Pro series.

Perforated Screen Tips

In many home entertainment applications, a perforated screen is often utilized. As in commercial cinemas, this allows transparent sound to radiate from the center channel behind the screen.

Although rare, when using any fixed resolution projection device, including DLP™ technology on a perforated screen, a moiré pattern may sometimes be a visible artifact. The moiré is caused by the interaction of the DLP™ mirror array, or the source frequency, with the screen perforation pattern.

If you experience this phenomenon, you can minimize moiré by applying some or all of the
following steps:

• Slightly increase the size of the projected image. Use projector blanking to mask the overshoot of the image.
  
• Adjust the projector pixel clock for the source.
  
• If the above options do not work, slightly defocus the image.
  
• If all else fails, consider utilizing a perforated screen with a different perforation pitch.
  

Aspect Ratios and Screen Dimensions

Some of the most common questions we receive on our applications support line have to do with calculating screen dimensions as they pertain to various aspect ratio's.

With this handy table of formulas, you will be able to ascertain the screen size, starting with only one dimension and the aspect ratio required for the application.

Customized MERCURY Performance

DPI offers a full range of lenses for our new MERCURY series. A glance at our pricing reveals that each lens results in a different throw ratio when used on a MERCURY 5000gv versus a MERCURY HD or 5000HD. It also reveals that there are two versions of each lens, one optimized for high brightness - normally recommended for use on a MERCURY 5000gv or 5000HD, the other optimized to produce maximum contrast, normally recommended for use on the MERCURY HD.

However, you may have applications where mixing lenses and projectors allows you to create a projection solution more optimized for your customer. As an example, let's assume we have a boardroom application where the quiet operational benefits of a MERCURY HD are important, but the ambient light level in the room dictates the projector produce more than the product's standard 1,600 ANSI lumens. For purposes of this example, we will assume the application requires a 1.44-1.8 lens (high-contrast part number: 102-735),

By simply specifying the MERCURY HD with the high-brightness version of the 1.44-1.8 lens (part number: 001-737), the projector's brightness will increase by more than 25%. The result will be a 'custom' MERCURY HD, producing roughly 2,000 ANSI lumens with a similar, but opposite impact on contrast. In a boardroom environment with ambient light, the slight loss of projector black level will never be noticed, but the added lumens will deliver a 25% improvement in the environmental on-screen dynamic range produced in the environment.

As another example, let's assume we have an educational theater application where the 20-foot, 16 x 9 aspect ratio screen requires more than the 1,600 ANSI lumens produced by a MERCURY HD, but the theatrically dark environment requires higher contrast than the 1000:1 produced by a MERCURY 5000HD. In this case, we can specify a MERCURY 5000HD with a high-contrast version of whatever lens is required. The result will again be a custom MERCURY chassis, producing roughly 3,200 ANSI lumens with contrast approaching 2000:1. This custom MERCURY is the perfect solution to put 12-14 Ft candles onto the 20-foot screen while at the same time maximizing contrast.

Bottom line, you should feel free to mix and match MERCURY lenses and projectors to create products with custom lumen and contrast performance. By applying this technique, which has no incremental cost, you assure the most precise solution for each and every customer.