Choosing the size of your screen, either it is a HDTV set or a front projected setup, require to take many parameters into account:

• The seating distance
• The available projector to screen distance (in the case of front projection)
• The room light levels

The seating distance

How far should you stand from your screen? The common rule of thumb is 3 times the width of the screen. This equates to approximately 37 degrees horizontally, from the left to the right of the screen from the user seating location. Less than 30 degrees of horizontal field of view is not recommended as it will not provide a good enough immersion feeling.

In large auditorium and cinemas, the recommended optimal viewing angle is between 45 and 50 degrees horizontally at the "reference seat" location. That location closely corresponds to the seat that is right in the center of the room, in both front to back rows and left to right axes. This value is for a 1:2.35 Panavision / Cinemascope screen ratio. Thus, if we follow that rule, you could in theory put your screen at only 1 time the width of the screen. So if you have a 120 inch diagonal 1:2.35 screen, you could be seated as close as 9.2 feet away from it for optimal cinematic immersion. This is pretty close indeed... maybe a bit too much? Let's continue our discussion.

Here are some concrete use cases:

As we can see, the last case is about the biggest screen we can use for this seating distance. The other thing to note is that as you use smaller screen sizes such as the RPTV cases, any seating distance ratio below 2.5 would not make much sense. Thus, under a certain screen size threshold, the range of seating distances will be reduced. We took 10 feet in the table above as it represents in our opinion the closest screen to seat distance that is advisable to use in any situation.

Screen gain vs screen seating distance

One additional point to consider is that in large cinema rooms, the screen is curved such that all the audience can perceive the same amount of light in such large auditoriums. This also has to do with the fact that they often use screen gains higher than 1.0 to save on projector lamp power and costs. Since you will probably not recreate this in your home theatre, you should remain more conservative regarding screen distance. Using a high gain screen will lead to left and right edges being much darker than the center as you cannot curve the surface of your screen.

The 3 times the width rule will put you in a situation equivalent to be seated on the last row of a large cinema. It has the advantage to be less susceptible to get you motion sick as the image moves rapidly during some action scenes. It will also take the above facts into account and ensure a more uniform screen image.

If your screen has a close to unity gain (white matt screens for example), then you could push the envelope further and locate the seats closer to the screen with less image brightness uniformity problems. As the screen gain increases, the directionality of the reflected image will force you to move further away from the screen in order to keep a satisfactory image, all over the surface of your screen.

 
Source: http://www.harknesshall.com/

The above graph illustrates the problem. At +- 25 degrees of viewing angles (a 50 degrees field of view situation), a high 2.0 gain screen will have drop to a gain of about 1.3 on its edges. This represents a significant drop in light intensity perceived. Thus, image intensity uniformity cannot be ensured in these conditions. On the other hand, if you use a 1.0 gain white matt screen, you can see that the gain varies from 1.0 at center to about 0.8 on its edges (a gain variation of 0.2 only), which remains acceptable for viewing uniformity.

The main criterion for image uniformity is that the gain difference or variation is kept at less than 0.3 from the center to the edge of the screen regarding seating location. That is, that the gain variation is under 0.3 over the full horizontal field of view of the spectators. This has to do with the fact that the human eye works in a differential way rather than on absolute light power. Thus, even small light power variations can be seen. Such is the case when watching large computer monitors running at low refresh rates (60 Hz).

What this implies is that if your room as more ambient lighting, you will have to consider putting your seat farther away from your screen as the screen gain will have to be raised.

Bottom line is: Put your screen between 1 and 3 time its width. The closer from the screen, the better the immersion, but a compromise must be considered between immersion and image uniformity. Let's keep on reading.

The available projector to screen distance (in the case of front projection)

Depending on the available distance to put the projector from the screen, your choice for a given projector will be dictated by its "throw-distance range". This range is directly determined by the minimal and maximal zoom factor of the projector lens. Inversely, if you already have a given projector, its throw-distance range will dictate the minimal and maximal screen size you can project on, based on the distance you put the projector from the screen.

It is thus critical to choose the right screen size and projector when opting for front projection so that you can get the optimal image size for your room.

The room light levels

As we said above, room ambient lighting will directly be linked to the screen gain you end-up using, thus affecting the screen position as well.

First, let's define some useful tools we will be using.

Foot Lambert

The foot Lambert value will allow us to quantify the amount of reflected light from a given surface area which is exactly what we need for a screen luminance evaluation. It is given by:

The minimal reflected power should be between 8 and 12 ftL in a well controlled light room condition. Public cinema theaters typically use 16 ftL.

Screen area formulas

The below formulas can be handy to compute the screen area from the screen diagonal measure and ratio:

And of course

Now, what we want to establish is the projector light power output required based on the room condition of a given home theater. The table below gives an approximate guide to let you decide what target foot Lambert ratings you should consider.

Here are the results for some typical screen sizes

Table of screen gain vs projector lumens vs room lighting

This table can be summarized as:

Most projectors to day produce over 1000 ANSI Lumens of optical power output. As the tables and graphs have shown above, the problem today is not to maximize image brightness anymore but rather control it so that the image does not in effect become too bright. In most home theater conditions, a 1.0 screen gain is probably going to be your best choice. An alternative is to use a gray screen which may in effect have a gain slightly lower than 1.0 and allow to boost black levels and contrasts. Try to find a dealer that has a well equipped demo room. Try the few screen / projectors that you prefer and see with your own eye which look the best to you. Bring your favorite DVD with you. Don't rely on demo videos they could show you.

Choose the projector and screen gain such that you remain inside the foot Lambert ranges that were given above. If your room is very dark and you watch movies at night, all lights turned off. If your walls are all painted dark and minimize reflections, then you fall in the 20 ftL range category. If you are using your home theater mostly at night but have white or pale painted walls that reflects light. If you don't have full control over the remaining ambient light, then you probably fall in the middle category at about 35 ftL. If you are using your system in a daylight environment, consider buying your equipment such that you fall in the 50 and up ftL range.

In Conclusion

This sounds overwhelming? Don't' worry. Break the problem in smaller sub-tasks. Here is the order in which you should design your home theatre room:

• 1) Select the maximal screen width that your room can allow.
• 2) From that, you can determine the projector allowable throw-distance range
• 3) Then find a projector that has Lumens rating that suit your room environment
• 4) Once you have selected the right projector for your screen and room size, consider the screen gain required for optimal image brightness, contrast and uniformity.
• 5) Finally, determine the seating position to be with-in 1 to 3 times the width of your screen.

The key thing to see here is that the step 5 is the least critical of the five and can be determined well at the end of your home theatre room setting up process. You can even try different seating positions and see what is best for you. After all, it is only a matter of moving seats back and forth which is a non-destructive process. The first steps are more critical as you cannot generally go back once you made these decisions.