How to size your home theater screen.
"Implementation Guide" for the CEDIA / CTA Home Theater Video Design Bulletin CTA/CEDIA-CEB23-B regarding how to size your home theater screen.
That quotation is from the CEDIA/CTA-published recommendation called Home Theater Video Design Bulletin CTA/CEDIA-CEB23-B that is available for free on the CTA web site: Home Theater Video Design (CTA/CEDIA-CEB23-B). In their words, they try to cover recommendations for:
Optimized Room layout and environment/system design
Image performance objectives/specifications
Recommended practices for image evaluation and calibration
Compliance with international video standards and digital rights management
This post will try to collect community knowledge about HOW to act on those recommendations. The main caveat here is that this is MOSTLY applicable to using a video projector, though as 100” flat panel TVs approach the price of a projection setup, one could argue in a small room, the same impact can be generated with a flat panel TV for a similar budget.
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The table of contents is quite extensive.
So, how do these goals translate to action in building a home theater?
1-6. These sections are about defining terms, the process used to create the document, other standards being leveraged, etc. This is important stuff but not particularly relevant from an implementation standpoint, so it's not very relevant for this thread.
7. Optimizing Room Layout
Much of room layout is about acoustics, and that is defined in a separate document, RP22. I would size my room and place my seats largely based on room acoustics, as discussed in RP22, then decide on a screen size, and then return to RP22 for adjustment of speaker positions based on the selected screen size.
The main visual implications to consider are image fidelity (brightness, color, contrast and so on) and the viewing angle for the audience (or, how much a viewer's the field of vision is occupied by the image on the screen).
Viewing angle is a function of seating distance and screen size. Viewing angles and seat arrangements are extensively discussed but not rigidly proscribed in CEB23. They note that the ITU recommends a 58 degree field of view of UHD content, but most people find that too large, except perhaps for 2.35:1 style content, or IMAX content. Really, experimentation, starting at about a 50 degree field of view, and then adjusting seating or projected image back and forth, to see what works, is usually best.*
If you have to guess, and you are using a 2.35:1** screen, this graphic ends up being pretty useful to help visualize the situation. You can also pay attention to where you prefer to sit in a commercial theater, pace off (walk) the distance from seat to screen, counting the steps, and then walk the width of the screen, again counting the steps....and get a close approximation in that way.
8-10. Image Performance Objectives and Specifications, Contrast Ratios, & Practices for Measuring / Evaluating Images
You need to reach a certain amount of brightness with your particular screen with your particular projector to see a “reference level” image in your theater.
The most comprehensive study of (acoustically transparent) screen materials can be found here:
31 Acoustically Transparent Screens Compared
And while a bit out of date, the most comprehensive study of non acoustically transparent screens with gain ratings is here: https://web.archive.org/web/20220327084419/https://www.accucalav.com/wp-content/uploads/accucal_front_projection_screen_report.pdf
Crucially, these studies of screen materials have GAIN numbers for each screen.
That gain number is critical.
Combine the measured gain number with your intended screen size, the throw distance, and the projector model you will be using. Then, you can confirm whether you can reach the industry standard level of brightness in your room.
Use this calculator to see how bright your image can get with your gear:
Projection Calculator Pro - Projector to Screen Distance
The details for SDR DCI content are similar to what the CTA defines as their goals, and the best source of confirmation regarding these specification for a particular projector via objective, instrumentation-based reviews thought note that half of these performance goals are a combination of the projector, screen, room, and throw distance (and calibration).
The key hurdle is brightness. The Digital Cinema Initiatives (DCI) - DIGITAL CINEMA SYSTEM SPECIFICATION defines the output requirements in the following way, with additional standards listed for context. Note that no theaters are doing the DCI version of HDR yet and probably won’t until flat panel style emissive displays become common. A post on AVS Forum nicely summarized the specifications:
DCI D-Cinema SDR
Peak Brightness: 48 cd/m2 (48 nits) 14 fL*
Black Level: 0.024 cd/m2 (0.024 nits) 0.007 fL**
Contrast Ratio: 2000:1 (48/0.024)
Dolby Cinema
Peak Brightness: 108 cd/m2 (108 nits) 32 fL*
Black Level: 0.0001 cd/m2 (0.0001 nits) 0.00003 fL**
Contrast Ratio: 1080000:1 (108/0.0001)
IMAX Laser
Peak Brightness: 75 cd/m2 (75 nits) 22 fL*
Black Level: 0.009 cd/m2 (0.009 nits) (75/8000) 0.003 fL** ***
Contrast Ratio: 8000:1****
DCI D-Cinema HDR (this is just a proposed standard, unlikely to be used for projection-based theaters)
Peak Brightness: 300 cd/m2 (300 nits) 88 fL*
Black Level: 0.005 cd/m2 (0.005 nits) 0.0015 fL**
Contrast Ratio: 60000:1 (300/0.005)
*calculated by unitconverters.net/nit-to-foot-lambert.htm and averaged up
**calculated by unitconverters.net/foot-lambert-to-nit.htm and averaged up
***mathematically generated by dividing peak brightness with known contrast
****quote from IMAX CTO Brian Bonnick firstshowing.net/imax-laser-projection
Those output numbers are the first, and often most critical, hurdle. SDR output is non negotiable and should be achievable even with 30% light loss on your projector/screen combo. Although with modern tone mapping, some folks feel that HDR can work with less light output than the Dolby spec, let’s not compromise if we don’t have to. I strongly recommend designing for a system that can accommodate the Dolby spec even after some dimming of the lamp or laser, as well
(The proposed DCI HDR standard is derived from good real world testing but who knows when technology will enable it. I would not try to achieve the DCI HDR spec yet unless you have an emissive display like a giant flat panel TV. When DCI came up with this specification, they used a very unique, highly specialized projector and setup, in a relatively small space.)
Then comes the rest of the factors like these details for 2D SDR projection, as defined by Digital Cinema Initiatives, LLC, in their Digital Cinema System Specification, Version 1.2 (March 07, 2008.) Other than luminance, many of the other factors are also the minimums for HDR content as well. (Luckily many projectors can do far, far better than these minimum requirements, so once you reach the luminance goal, you are probably in good stead.). Note that many of these factors are not only controlled by the projector, but also by the screen, the room, calibration and setup. Almost any projector you choose from a mainstream brand that meets the output specs will also meet these specs nowadays.
ambient light level projector off 0.01cd/m2 (0.0029ftL)
white luminance 48cd/m2 (14ftL)
luminance uniformity 85% of center
minimum sequential contrast 2000:1
minimum intra-frame contrast 150:1
transfer function down to 5% peak white. 2.6
color accuracy match +/-4 delta E
Footnotes
*A little experimentation is in order. So get some chairs that are easy to move around, if you have the time in your build schedule, and try out various distances / levels of immersion, before committing, can be useful. Keep in mind that changing your seating location will change the audio performance of the room, so this is best to do BEFORE committing to speaker and (especially) subwoofer placement, if possible.
**I will admit it. I am a fan of 2.35:1 screens when a room allows for one that is sufficiently tall that 16:9 content looks large enough in the middle of it. Why?
Comparison of images on a constant height basis (2.39 screen on the left, 16x9 screen on the right):
Detailed views. Here is how different movies look on a 16:9 screen:
And then the same movies on a 2.34:1 screen that is the same height as the screen above: