Screen Tip Friday: Projector Screen Hot Spots Explained

Picture this – you devise your perfect Home Theater.  You install your seats, block out your windows, and mount your new 3D projector with enough glasses for the entire (extended) family.  To top it off, you get the perfect projection screen and fabric – say, a Contour Electrol by Da-Lite with their new Silver Lite fabric.  You pop in your copy of Avatar (or the like) and turn out the lights to take your system for a test drive.  And there it is, staring you right in the face – a bright ball of light right in the center of your field of view.  You’ve fallen victim to a hot spot.

What you’re seeing is a reflection of light from the projector’s lens.  This is because of how your Silver Lite fabric works in making your image brighter.

Some fabrics, such as the High Power (2.4 gain), Silver Lite (2.5 gain),  and now-defunct Glass Beaded (2.5 gain), are retro-reflective, which means the fabric will reflect the light back at the projector, along the “projection axis”, and will make your image appear brighter because less light is scattered.  This will also make the hot spot more noticeable by the audience.  These higher gain fabrics become almost like a mirror, and what you’re seeing is actually the reflection of light from the projector’s lens.

Retro-reflective fabrics have a more uniform brightness over a relatively narrow viewing angle.  In other words, the fabric will not scatter the light from the projector very much, which incidentally is why these fabrics produce a brighter image.  This lends these fabrics to hot spotting because a hot spot will be more prevalent over a wider section of the audience.

Since the optimal location of a projector when used with a high-gain fabric screen is with the projection-axis close to the line-of-sight of the audience, the incident angle of the reflection will tend to be directed at the audience.

You’d think that lower gain fabrics, which are angular-reflective, would not have this issue.  Well, they do!  Hot spots will occur on these angular-reflective fabrics, although you may not detect them.  Angular-reflective fabrics will scatter light in different directions, making the possibility of a hot spot less likely.  By scattering the light, lower gain fabrics provide a wider viewing angle, although the effect is a relatively darker image.

Also, since less light is reflected within the same narrow viewing angle, any hot spot would be dim and may not be noticeable during viewing because the fabric will scatter both the projected image and the light reflection from the projector’s lens.

The affected location in the audience is a much narrower section and usually is on a incident angle from the projector, and not along the projection axis.  In most home theaters, this location could even be in front of the audience.

There are a few things you can do to reduce the possibility of having a hot spot:

• Reduce the output power of the projector.  In some cases this might work, but if you’ve chosen a high gain fabric in the first place, you’ve done so to increase the brightness of your image.
• Use a fabric that is not retro-reflective.  Angular reflective fabrics, such as Video Spectra 1.5 and Matte White could be a better choice, but again you might have chosen a high gain fabric for the higher image brightness.
• Move the projector further away from the screen.  This will increase the throw distance of the projector, and will effectively reduce the brightness of the image.
• Reposition the projector relative to the audience.  Raising the projector’s height, or even positioning it off to the side (or both) will help relocate the hot spot so that it would only be visible from a location not within the “audience”.  Sometimes this position put the audience outside of the optimal viewing angle of the screen, so the image brightness can be affected.

If you have a more complicated setup, and you’re not sure if you’ll have a hot spotting issue, contact us and we can help you with your home theater.

Da-Lite’s High Power Fabric Changes

Da-Lite had recently changed their High Power fabric specification, much to the surprise of many home theater fans.   Da-Lite quietly changed their specifications for the High Power fabric that is used on manual pull down screens and electric screens.  The High Power fabric on fixed-frame and tensioned electric screens remains unchanged.

The “new” High Power fabric has a lower reported gain value of 2.4, and a slightly wider viewing angle of 30 degrees (half-angle), compared to the original High Power fabric (2.8 and 25 degrees half-angle respectively).

High Power is typically required when there is uncontrollable light levels in the room, or if the projector is low powered (such as an old CRT projector).  However, it has also been a niche fabric for some home theater users who set their projectors at the maximum contrast.

Da-Lite attributed the change to quality improvements in the fabric’s encapsulated glass beaded surface.  They also noted that the 2.8 gain fabric is available on pull-down screens and electric screens by request at no additional charge.

Fuxion Electronics is an authorized dealer for Da-Lite – if you’d like a screen with the 2.8 gain High Power fabric, just let us know. *Da-Lite no longer offers the 2.8 gain High Power fabric, sorry!

Screen Size Formats – What’s the Difference?

Da-Lite projection screens come in standard sizes and formats. Depending on your application, you can choose the format that best fits the media you’ll be showing on your screen.

Below is a visual summary of the standard supported screen formats (click to open in new window):

Below is a summary of some of the common users of each format:

Square Format

• Most versatile, can be used with multiple image types including all listed below.
• Portions of the viewing area would be visible during use, may not be an important issue depending if you want your image framed by black.

Video Format

• Also known as NTSC video, or Full Screen (for DVD’s).
• Standard definition movies and television
• DVD’s in the “Full Screen” format
• Gaming for all types of systems (PS2, XBox, XBox 360, PS3, Wii)

Widescreen Format

• Also known as HDTV or “16:9″
• High definition movies and television
• DVD’s in the “Widescreen” format
• Gaming for newer systems that support HD output signals (PS3, XBox 360, Wii)

Ultra Widescreen Format

• Specifically designed for PC outputs – supports screen resolutions such as 1280×768 or higher

Cinemascope

• Specifically for very wide screen movies – 2x wider than Video format.

In addition to these standard sizes, any custom size format can be ordered – you just need to let us know your viewing area size, or limiting dimensions for your screen, and we can tailor a custom screen to your specifications. Just contact me (Eric) and let me know!

Projection Screen Tip Friday: Screen Fabrics Part II

Last Friday I talked about all the properties that affect the type of fabric you should choose for your projection screen. I also went into some detail about gain, viewing angle, and what “high contrast” means.

Quick quiz, true or false: a higher gain fabric produces a better image than a lower gain fabric.

The answer is: it depends! Each fabric should be chosen for each different application based on that previous criteria I spoke about in the last post.

So, based on all of those attributes, I’ve tried to simplify and organize the process of selecting your fabric with the steps below. If you’re confused about this process, please contact me and I’ll be happy to explain it.

Step 1. Ambient Light. Can you control ambient light in your room?

• If YES, click on the thumbnail below and continue:

Controllable Ambient Light

• If NO, click on the thumbnail below and continue:

Uncontrollable Ambient Light

• If you don’t know what ambient light is, you weren’t paying attention! Read this first.

Step 2. Display Brightness. Is your display brightness greater than 25 foot-lamberts?

Step 3. Audience Location. Is the audience 30-degrees or more from the center of the screen? This is in all directions – left, right, and directly in front.

Step 4. Projected Image. Is digital text being projected?

Step 5. Projector Location. Is the projector at or below the center of the screen?

Step 6. High Contrast Desired. Is a contrast-enhancing screen desired (for movies or HDTV)?

Some attributes are not important from one situation to the next – you’ll see the symbol “–>” which just means move onto the next question.

So, did it work for you? If not, please do let me know and I’ll be happy to walk you through it!

Next week, in Part III of screen fabrics, I’ll talk about those “special” fabrics, such as rear projection and something called Silver Matte…

Projection Screen Tip Friday: Screen Fabrics Part I

Screen surface fabrics have particular characteristics that can help any application achieve the image quality you demand from your home theater. You’ll see that each fabric type is designed for particular applications, and each have specific benefits.

Each screen fabric has particular characteristics.

• Gain. This is simply a measurement of reflectivity of the screen. The higher the gain number, the brighter the image will appear. The increase in gain is based on how light is reflected by the fabric. Lower gain fabrics disperse light in a wide area, while higher gain fabrics reflect more light back along the projection axis. This causes higher gain fabrics to have a narrower viewing angle. When comparing these fabrics, the gain value is relative between each fabric type. The Glass Beaded fabric, for example, has a gain of 2.5, and is therefore 2.5x brighter than the Matte White fabric with a gain of 1.0.

• Viewing angle. This is a measurement of the angle at which you begin to lose picture quality. This is actually a “half angle”, but is expressed as an angle. It’s measured from the screen surface and along the projection axis. To determine the actual viewing area for the audience, the angle is measured out from the axis to all directions – left, right, up, down, and in between. This creates a “cone” of sorts. To determine the full viewing angle, take this value an multiply by two.

• Seamless vs. seamed. Some fabrics are listed as “seemless”. This means that the fabric is one continuous piece. When fabric sizes extend past the standard width for a fabric, two pieces must be mated together. A seam may affect the image quality, but is not normally discernable. In most cases, this is only an issue for very large screen sizes.

• High contrast. Some fabrics are listed as “high contrast” fabrics. The purpose of these fabric is to increase the contrast ratio of the image. Contrast is the difference between the darkest black and the whitest white projected on the screen. The contrast ratio is increased with the use of a gray fabric base, which enhances the black and darker colors in the image.

• High gain or High Power. High gain is a term for fabrics that are generally listed over 2.0 gain. Note that high gain is not necessarily the best for most applications, and it’s not true that a higher gain means that the fabric is “better”. High Power is a type of fabric offered by Da-Lite that has the highest possible gain – 2.8.

So what should you look for when choosing your fabric? It depends on your application and environment. Here’s some things to consider about your application when preparing to choose a fabric:

• Ambient light conditions. Ideally, you would want to have a completely dark room. However, you may be traveling with your screen and are unable to know what the lighting conditions are for the venue, and an elevated fabric would be acceptable. Or, you may not be able to block out sunlight from the room. You can define your ambient light conditions as “controllable” or “uncontrollable”.

• Projector output power. This is typically expressed in lumens. Higher output power projectors would require lower gain fabrics. Typically, LCD and DLP projectors would not need elevated gain fabrics, unless the ambient light conditions are such that a brighter image was necessary. Older projectors, such as CRT-type projectors, slide or 8mm movie projectors would benefit from elevated gain fabrics with a gain of 2.0 or better.
• Display Brightness. Related to your output power, this determines how much light is reflected back towards the audience. A display brightness of under 25 foot lamberts would require a fabric with a slightly elevated gain (1.5-2.0). The calculation for this is:

[Output Power (lumens)] / [Viewing Area (square feet)] = Display Brightness (foot-lamberts)

• Projector contrast ratio (2000:1 or better). Typically, projectors with a listed contrast ratio of 2000:1 or better would not need the High Contrast fabric, and would instead benefit from an elevated gain fabric. These would include:

• Projector contrast ratio (less than 2000:1). For those projectors with lower contrast ratios, the high contrast fabrics would be a good selection, especially for home theater use. Note that lower output projectors may benefit more from elevated gain fabrics than from high contrast fabrics. High contrast fabrics would include:

• Projector mounting location. Projectors can be located in several locations – even behind the screen itself.
  • Front projection – ceiling mounted. In most cases, this is the ideal location for your projector. However, issues with higher gain fabrics and their narrow viewing angles may arise. It is recommended that a 1.5 gain or lower fabric is used for ceiling-mounted projector applications.
  • Front projection – table mounted. Since the light from the projector is reflected back at the source, the projection axis is not perpendicular to the screen surface, and people sitting at a “normal” distance may have issues with image quality with fabrics that have a narrow viewing angles. As such, it’s recommended that the projector is mounted as close to eye-level as possible when using high gain/narrow viewing angle fabrics.
  • Rear projection. There are a few fabrics that are designed to be used for rear projection. These fabrics have to be tensioned, so they’re not available with manual or standard electric screens. Da-Tex is a rear projection only fabric, while Dual Vision can be used as a rear or front projection applications.

• Cleaning required? Most screen fabrics are able to be cleaned, with the exception of the Glass Beaded fabric. This fabric has glass beads that are impregnated into the fabric. Rubbing or rough handling can dislodge these beads, creating dead spots and ultimately affect the image’s uniformity with respect to brightness. If a high gain fabric is required, and it’s possible that cleaning may be required in the future (kids, dusty location), then the High Power fabric would be a better choice (it can be cleaned). Cleaning is done with warm water and mild soap.

When you choose your fabric, be sure that they are fire retardant and mildew resistant, as all of Da-Lite fabrics are.

So now you know the basics about projection fabrics. Ok, so now what? I’m glad you asked. Using all the information you’ve gathered about your application, see Part II next week to help choose the best fabric!

New Product Spotlight: 3D Virtual Grey Fabric

Today Da-Lite announced the introduction of their new fabric, 3D Virtual Grey.  This new fabric comes hot on the heels of their innovative 3D Virtual Black rear projection fabric introduced earlier this year for passive 3D applications .

The 3D Virtual Grey fabric is a front-projection fabric that features a remarkable 99% polar retention in 3D passive stereoscopic applications.  This type of fabric is perfect for gaming, studio applications (newscasts, sports), architectual design, and many other applications.

According to Da-Lite, this fabric solves the biggest problem facing 3D applications – the loss of polarization of the image:

“In the past, the biggest obstacle in passive 3D stereoscopic projection was the ability of the projection screen to retain or “hold” the polarization of the image on the screen.  With the introduction of 3D Virtual Grey, users are able to “hold” 99 percent of the polarized image on the screen.”

This new fabric also boasts a 11-degree viewing half-angle, and is seamless up to 16 feet high.

Please contact us for more information and pricing for your applcation.  For even more information, visit Da-Lite’s website.

Projection Screen Tip Friday: Screen Size

Everyone wants to have the biggest screen available, but let’s face it, getting a screen that is TOO big will only wear out your eyes and neck (try sitting in those front-row seats in the movie theater and you’ll get what I mean).

At the same time, choosing a screen that is too small will make seeing it from a long distance very difficult.

To solve this, there is a “rule of thumb” that you can use to choose the screen size based on your audience location.

First, take the distance between the screen and the first row of the audience, and divide by two. This is the maximum height your screen should be – larger than this and your audience will have to crane their necks or search with their eyes to capture the image on screen.

Next, take the distance between the screen and the last row of the audience, and divide by six. This is the minimum height your screen should be – any smaller and people sitting in the cheap seats won’t be able to make out text and details on the screen.

So in summary:

[distance first row to screen]/2 …is the maximum screen height;

[distance last row to screen]/6 …is the minimum height.

So let’s look at an example. Say your first row is 14 feet from the screen, and the last row is 30 feet from the screen. Using this “rule of thumb”, your maximum height should be 7 feet (84″), and your minimum height should be 5 feet (60″). So, it’s safe to say if you chose a screen size between 5 and 7 feet tall, you should be pretty well off.

This rule doesn’t take into consideration the width at all – you should choose your width based on your format (square 1:1, video 4:3, widescreen 16:9, or ultra widescreen 16:10).

This also assumes that your projector has the ability to be installed so that the screen size can be achieved.

Next time, I’ll discuss the different formats – which one should you choose now, and which one should you choose if you plan to upgrade in the future?

What the heck is a Keystone Eliminator?

Don’t know what Keystone is? Why would you want to eliminate it anyway?

Well, smaller Da-Lite tripod screens have a feature called a “Keystone Eliminator”, which is not available on larger tripod screens.

Keystone is a phenomenon where the top edge of the image is a different size than the bottom edge – effectively, your image appears to be a trapezoid on-screen. This happens when you place your projector high above or well below the centerline of the screen surface.

This is where the “Eliminator” comes in. It’s actually just an arm at the top of the support mast that allows you to tilt the screen surface so that the centerline of the screen surface is now in line with the projection axis.

Most current projectors, however, allow you to adjust for Keystone within their setup menu. Others adjust for it automatically based on the angle your projector is sitting at on the table. Applications where you would want to use one would be when you’re using an older projector where you can’t adjust for it, like a slide projector, overhead projector, or even 8mm movie projectors.

So, don’t feel like you’re missing out if your tripod screen doesn’t include a Keystone Eliminator – you’ll never know what you’re missing.