You Light Up My Screen….

Back to the old screen brightness thing again. This time, the question is, “How do I pick a projector that will light up my screen to 16fl?” I wish this were easy. Actually it is. You call Platinum Home Theaters, and we design your theater for you. We take care of site lines and sound, seating and acoustics, screen size, shape, material, projector, projector position and lens, control systems, décor, the whole works, or any part of it. You get what you really want, you don’t take a chance and spend your hard earned money for something that doesn’t work, and we both walk away smiling. You get our knowledge and expertise; we get you as a client and friend. But so much for the easy way, I know some still want to duke it out with physics, so here we go again. Got your slide rule ready?

In an earlier post, we noted that projector light output is measured in luminous flux, with the Lumen as the unit, actually a unit measurement of energy (actually is 1/683 joules per second, for those who care). ANSI Lumens are figures obtained with a defined test procedure that makes projector comparison more valid, and Home Theater ANSI Lumens defines the test procedure more tightly, and more realistically, as projectors are only tested after they are ISF calibrated to the D6500 color standard. Lower numbers result due in part to the fact that a metal-halide lamp emits a native white light that is a warmer color than D6500, and calibrating a projector can only be done by inserting precise amounts of light loss in controlled areas of the spectrum. Note I said, “light loss” – you can only reduce the intensity of a particular color, you can’t add it in magically if it isn’t strong enough. To increase the strength of one color, you actually have to lower the other two. The result is proper color rendition, but lower overall screen brightness. Sadly, not many projector manufacturers want lower Lumen numbers, so very few use Home Theater ANSI lumens.

As you look at projector data, do you find your eyes glaze over? They should. A look at any projector data base reveals tons of specs that are all hard to digest. But lets try. The biggie is Brightness, usually in ANSI Lumens. Remember, this is a measure of luminous flux, or all the light coming out of the projector. All that luminous flux has to hit a screen before it makes a picture, so we’re only part-way there with ANSI lumen figures. But it’s a start. Ignoring, for the moment, that you are dealing with an uncalibrated projector operating in “torch” mode (full uncalibrated white output) for that big beautiful ANSI lumen figure, here’s how you calculate the resulting screen brightness in Foot Lamberts:

Brightness (fl) = Projector Light Output (ANSI Lumens) / Screen Area (W x H)

So if we had a 1300 ANSI lumen projector and a 10 foot wide 16×9 screen…
Screen area is 10 x 5.625= 56.25 sq ft
1300 / 56.25 = 23.11 fl

I have no idea why water analogies work so well, perhaps it’s the fact that everyone has used a hose. So here’s what’s happening from a wet point of view. Lets say your projector is really a hose with a nozzle pointed downward, and a bucket below is the screen. Your hose sprays out exactly one gallon of water into that bucket. But now you raise your hose, or widen out the spray (take your pick) to cover, evenly, 16 buckets. You still spray out only one gallon, but it now disperses over all those buckets. How much lands in one of them? 1/16th of a gallon, right? The water in the buckets is the light on your screen. The more screen you have to fill, the less light hits any given area. It’s called the “Inverse Square Law” and is pervasive in light, sound, and all other forms of energy. And that’s what we are working with when figuring out how bright your screen will be with a given water output of your projector…or should I say light? If you get water out of your projector, write and tell us at once!

How much lower will the calibrated maximum be? Depends on the projector, but it’s always lower. Remember, you loose light in calibration to D6500. As you can see, though, a 1300 ANSI lumen projector might be a little dim once calibrated.

But then there’s contrast. Your eye likes contrast, and is indeed capable of a huge contrast ratio. Projectors, while getting better, are not as capable. But it may not matter anyway. Take a look at your projection screen with the projector off and the lights on. What color is it? White, right? Or light grey? What you’ve just seen is actually black, or at least as black as black would be on that screen with the room lights up. How do you get that huge contrast ratio of 2000:1? Simple. You gotta make the room dark…very dark. The reason is that your projector has a bright light limit that we just talked about. Even when it’s turned off, or lens capped, the blackest black will be defined primarily by the ambient light falling on the white screen. If you get that to nearly zero, you will start to realize that contrast ratio.

Here’s an example. Say your projector has a specified 2000:1 contrast ratio, you have a white screen with a gain of 1, and a 1300 ANSI lumen projector. With a little soft incandescent light in the room of only 1 foot candle (geezopete, another measurment unit!) your 2000:1 gets bumped down to 34:1. Lets cut that light on the screen down to a measly .1 footcandle. Bingo, you now have 286:1. Still not great. In fact, to get even close to that 2000:1 spec, you’ll have to get screen ambient light down to .001 footcandle.

Now lets work it from the other side. We can’t cut screen ambient light, for some reason, like windows (!). Lets say screen ambient light sticks at 3 footcandles, and that’s all the better we can get it. If we could get a bright enough projector, we could still get your 2000:1 contrast ratio back, right? Um….sort of. We could, if we only had a projector that could spit out 100,000 ANSI lumens! And then we’d land at around a 600:1 contrast ratio, which might almost be acceptable. Too bad we can’t get a projector like than in through the front door. Or plug it into a standard wall outlet. Or pay the bill for it.

The secret: you have to keep light off the screen to save projector lumens and dollars.

We’ll deal with plasma and LCD screens another time, but keep this in mind…look at a plasma screen turned off in a lit room. What color is it? Black. And that’s the black you’ll get in that kind of room light. Hmmmm!!

….and the War Continues…

Of course we’d have to declare a winner to perpetuate a war, right? That’s about what’s happened in the HD DVD vs Bl-ray Disc wars. About two months ago, we blogged that Blu-ray Disc seemed to be the defacto winner, due mostly to the news that Blockbuster had announce that they would only be renting Blu-ray Discs, based on their in-house test marketing. Added to the victory chant was the news that sales for Blu-ray software seemed to be winning. Then HD DVD claimed an equally early victory based on hardware sales, and the fact that HD DVD players are cheaper than Blu-ray Disc players (still true), and so would capture the hardware market.

I think we declared a qualified draw at that point. And if not, we should have. Here’s the latest news from the front:

Two studios have announced they will now exclusively support HD DVD. They are Paramount Pictures and DreamWorks Animation SKG. Paramount Home Entertainment claims they will publish release-day and date as well as catalog titles on HD DVD only. This decision overturns Paramount’s earlier decision to support both formats.

There are now 3 major studios exclusively supporting HD DVD: Paramount, DreamWorks, and Universal. Weinstein Company is also an HD DVD supporter, but has not singled that format out exclusively yet. Paramount also claims that their decision will include all films they distribute by Paramount Pictures, DreamWorks Pictures, DreamWorks Animation, Paramount Vantage, Nickelodeon Movies, and MTV Films, all under the Paramount Home Entertainment umbrella.

The exception to the rule? Who else but Stephen Speilberg, who hasn’t committed to either format, but Close Encounters was recently slated for Blu-ray Disc release.

Blu-ray Disc is supported, again exclusively, by Sony Pictures, Disney, Fox, MGM and Lions Gate.

There is only one studio actively supporting both formats, and that’s Warner Brothers, with their unique dual-format disc (HD DVD on one side, Blu-ray Disc on the other).

What on earth could cause studios to suddenly cuddle up to HD DVD? Could it be money? Naw…ok, maybe….ok, yes it probably was. The site www.deadlinehollywood.com seems to think that some mysterious backers payed off Paramount and DreamWorks to the tune of $150 million for their exclusive HD DVD jump. The studio’s official comments are not even worth quoting.

Ding…..Round 2…or is it 3…or 4?

Screen Brightness – Lumens, LUX, and Lamberts

I recently had a conversation with my friend Mike Lake about the brightness of theater screens, and how that brightness relates to home theater screens. During the conversation Mike challenged me to not only define target home theater brightness levels, but to come up with a way for the average home theater enthusiast (who’s that?) to measure screen brightness using something commonly owned, like a SLR or DSLR camera, or a light meter like a 1 degree spot meter.

Thanks for the challenge, Mike! Here’s what I came up with.

Screen Brightness Defined

Lumens, LUX, and Lamberts

A lumen is standard unit of luminous flux…if you wish, it’s how much light is emitted. When a projector’s light output is measured in lumens, it is an attempt to place a number on the maximum amount of light coming from the projector. It is not now bright your screen will be!

LUX is a measurement of illuminance. It takes into account the area over which the luminous flux (lumens) is spread. For example, 1000 lumens spread over one square meter results in 1000 lux. Back the light off until the same luminous flux now fills 10 square meters, and you get 100 lux over that area.

Screen Brightness, or how bright a screen will look, involves measuring the light reflected from its surface to our eyes. It takes into account the luminous flux (lumens) falling over its entire area (lux) and how reflective the surface of the screen is. It literally is a measure of the light bouncing off the screen. Luminance is measured in foot-lamberts.

How bright a screen is has impact on the image presented, and not in a small way. To help quantify screen brightness, some test method had to be standardized. In film projection, it’s the projector without any film projecting onto the screen. With digital projectors, it’s a 100% white image. It’s interesting to note that the two are not identical. Film base attenuates the light through the projector, so a white film frame would measure lower, but digital projectors use no such film, so 100% is 100%. The target luminance is between 12 and 22 foot-lamberts (fl). The target is 16fl, but a group of surveyed viewers much preferring the 22fl screen brightness. Many movie houses are dimmer, around 7-10fl. Yes, it’s a cost thing. Xenon bulbs are expensive, and last longer if you don’t burn them as bright.

Here’s your first answer: The target luminance for a THX Home Theater Screen is 16fl, same as a commercial theater, and brighter would be better…and more expensive!

Note at this point that we are talking Foot-Lamberts, not lumens. They are not the same, and don’t even really relate directly to each other. To reiterate, a lumen is standard unit of luminous flux. A projector that provides 1000 lumens of light will provide that flux regardless of how big the screen is, or how far away it is. To change the luminous flux of a projector you have to do something in the light chain, like boost the lamp current, or get a bigger (faster) lens. Think of it as the total amount of light emitted. The current standard for projectors is know as ANSI Lumens. ANSI, the American National Standards Institue, has standardized the method used to test projectors. The method involves, among other things, testing multiple areas of the light source. Home Theater ANSI Lumens is a measurement standard created by Runco International, and most significantly differs from ANSI lumen measurement in that the projector under test is first calibrated to ISF (Imaging Science Foundation) standards at 6500K, the color temperature required for an accurate video image. The projector is then targeted to a standard screen, and the resulting light falling on the screen is measured at 9 points with a LUX meter, then averaged and multiplied by the surface area of the screen. The resulting measurement is much lower than the standard ANSI lumen equivalent, but is a better indicator of projector performance than the measurement of a projector running wide-open and uncalibrated.

Back to the screen. If everything in your home theater design was correct, you should hit the same 16fl luminance target figure that theaters try for. In fact, the THX Home Theater standard is 16fl, but they talk about trying to duplicate the image seen in mastering houses, which calibrate 100% white to 35fl. That’s quite a range!

Now, how to you know you’ve got it right? The best way is to measure luminance in foot-lamberts directly. If you have the right kind of light meter (Konica-Minolta makes an industrial unit for this purpose), you just aim it at the screen, pull the trigger, and read your meter. Of course, you don’t have that meter. But, like Mike, you may have a DSLR or film SLR camera with built-in light meter. Well, you’re almost there. It takes a bit of math, though.

Second Answer: Measuring Luminance in Foot-Lamberts with a camera

With a camera, it’s best to use a telephoto lens, or get close to the screen. Your object is not to try to measure the entire screen, but try for a small section, ideally, 1/9th. Set your camera for ISO 100, and your shutter speed to 1 second. This places your camera in the range where useful EV figures can be converted to foot-lamberts with our little chart. We picked 1 second because many new zoom lenses only open to f4 or so, and we need the extra sensitivity. With a test DVD (any THX certified DVD has the THX Optimizer on it, which will work fine), put up a 100% white frame, and take a light reading by pointing your camera at the white area and noting the f-stop and shutter speed. Plug them into the formula:

EV = 3.3 Log10 (f²/T)
Where:
f=f stop
T is exposure time

We include a chart for this, if the math is to hard. The chart is limited, but you can get some useful luminance data with it anyway. (For the technologists, we’ve stuck with the Minolta recommended K of 1.3)

Your target is 16fl, which is between EV8 and EV9, or between f16 and f22 (ISO100, 1 sec)

Spot-meter or camera set to ISO100

100% white screen, Table is below…

EV 1 sec .5 sec ft-L
1 f-1.4 f-1.0
2 2 1.4
3 2.8 2 .33
4 4 2.8 .65
5 5.6 4 1.3
6 8 5.6 2.6
7 11 8 5.2
8 16 11 10
9 22 16 21
10 32 22 42
11 45 32 84

Ok, the challenge is met! The next is finding a projector that will hit 16fl reflected from your screen given its size. Oh, and is 16fl really enough, given a high level of ambient light in the room? At some point, I’ll need to get paid for this stuff….

Screen Brightness – Lumens, LUX, and Lamberts

I recently had a conversation with my friend Mike Lake about the brightness of theater screens, and how that brightness relates to home theater screens. During the conversation Mike challenged me to not only define target home theater brightness levels, but to come up with a way for the average home theater enthusiast (who’s that?) to measure screen brightness using something commonly owned, like a SLR or DSLR camera, or a light meter like a 1 degree spot meter.

Thanks for the challenge, Mike! Here’s what I came up with.

Screen Brightness Defined

Lumens, LUX, and Lamberts

A lumen is standard unit of luminous flux…if you wish, it’s how much light is emitted. When a projector’s light output is measured in lumens, it is an attempt to place a number on the maximum amount of light coming from the projector. It is not now bright your screen will be!

LUX is a measurement of illuminance. It takes into account the area over which the luminous flux (lumens) is spread. For example, 1000 lumens spread over one square meter results in 1000 lux. Back the light off until the same luminous flux now fills 10 square meters, and you get 100 lux over that area.

Screen Brightness, or how bright a screen will look, involves measuring the light reflected from its surface to our eyes. It takes into account the luminous flux (lumens) falling over its entire area (lux) and how reflective the surface of the screen is. It literally is a measure of the light bouncing off the screen. Luminance is measured in foot-lamberts.

How bright a screen is has impact on the image presented, and not in a small way. To help quantify screen brightness, some test method had to be standardized. In film projection, it’s the projector without any film projecting onto the screen. With digital projectors, it’s a 100% white image. It’s interesting to note that the two are not identical. Film base attenuates the light through the projector, so a white film frame would measure lower, but digital projectors use no such film, so 100% is 100%. The target luminance is between 12 and 22 foot-lamberts (fl). The target is 16fl, but a group of surveyed viewers much preferring the 22fl screen brightness. Many movie houses are dimmer, around 7-10fl. Yes, it’s a cost thing. Xenon bulbs are expensive, and last longer if you don’t burn them as bright.

Here’s your first answer: The target luminance for a THX Home Theater Screen is 16fl.

Note at this point that we are talking Foot-Lamberts, not lumens. They are not the same, and don’t even really relate directly to each other. To reiterate, a lumen is standard unit of luminous flux. A projector that provides 1000 lumens of light will provide that flux regardless of how big the screen is, or how far away it is. To change the luminous flux of a projector you have to do something in the light chain, like boost the lamp current, or get a bigger (faster) lens. Think of it as the total amount of light emitted. The current standard for projectors is know as ANSI Lumens. ANSI, the American National Standards Institue, has standardized the method used to test projectors. The method involves, among other things, testing multiple areas of the light source. Home Theater ANSI Lumens is a measurement standard created by Runco International, and most significantly differs from ANSI lumen measurement in that the projector under test is first calibrated to ISF (Imaging Science Foundation) standards at 6500K, the color temperature required for an accurate video image. The projector is then targeted to a standard screen, and the resulting light falling on the screen is measured at 9 points with a LUX meter, then averaged and multiplied by the surface area of the screen. The resulting measurement is much lower than the standard ANSI lumen equivalent, but is a better indicator of projector performance than the measurement of a projector running wide-open and uncalibrated.

Back to the screen. If everything in your home theater design was correct, you should hit the same 16fl luminance target figure that theaters try for. In fact, the THX Home Theater standard is 16fl, but they talk about trying to duplicate the image seen in mastering houses, which calibrate 100% white to 35fl. That’s quite a range!

Now, how to you know you’ve got it right? The best way is to measure luminance in foot-lamberts directly. If you have the right kind of light meter (Konica-Minolta makes an industrial unit for this purpose), you just aim it at the screen, pull the trigger, and read your meter. Of course, you don’t have that meter. But, like Mike, you may have a DSLR or film SLR camera with built-in light meter. Well, you’re almost there. It takes a bit of math, though.

Second Answer: Measuring Luminance in Foot-Lamberts with a camera

With a camera, it’s best to use a telephoto lens, or get close to the screen. Your object is not to try to measure the entire screen, but try for a small section, ideally, 1/9th. Set your camera for ISO 100, and your shutter speed to 1 second. This places your camera in the range where useful EV figures can be converted to foot-lamberts with our little chart. We picked 1 second because many new zoom lenses only open to f4 or so, and we need the extra sensitivity. With a test DVD (any THX certified DVD has the THX Optimizer on it, which will work fine), put up a 100% white frame, and take a light reading by pointing your camera at the white area and noting the f-stop and shutter speed. Plug them into the formula:

EV = 3.3 Log10 (f²/T)
Where:
f=f stop
T is exposure time

We include a chart for this, if the math is to hard. The chart is limited, but you can get some useful luminance data with it anyway. (For the technologists, we’ve stuck with the Minolta recommended K of 1.3)

Your target is 16fl, which is between EV8 and EV9, or between f16 and f22 (ISO100, 1 sec)

Ok, the challenge is met! The next second is, finding a projector that will hit 16fl reflected from your screen given its size. Oh, and is 16fl really enough, given a high level of ambient light in the room? At some point, I’ll need to get paid for this….

42” TVs – “the new 34”

Yes, we’re on about screen size again. Why? Simple. It’s the single biggest factor in choosing a new digital TV or projector. This post will place a “perspective” on two issues that are pervasive in the TV selection process.

First, is 42 bigger than 34? Now, you math whizzes, don’t freak out. But the answer is a qualified “NO”! And anyone that’s “upgraded” from a 34” standard TV to a new 42” plasma or LCD will agree with this.

As we’ve noted, the new shape of screens is the wide 16×9 aspect ratio. But much of what’s available on broadcast, satellite, and digital cable TV is still the old 4×3 shape. Just watching the old shape programs on your new TV causes consternation. If you do nothing, you end up with what looks like a small, square-ish picture inside your big wide screen. And it is indeed no bigger than your old 34” set! The height of the screen tells the tale. For a 34” diagonally measured TV, the screen height is just over 20”…the same as your new 42” wide screen. Hmmm! So, even though the screen is bigger, the standard 4×3 video image isn’t. Unless….

The second issue is more than a bit annoying. In fact, I’m wincing as I’m writing this paragraph. Because some people don’t like to see the little square image inside their big TV, or are bothered by the black side bars beside it, TV manufacturers build in a ‘stretch’ function that distorts the 4×3 image by stretching it to fill the new 16×9 screen. OK, fine, if you gotta have that feature. But rather than seeing the 4×3 picture as it is supposed to be, it now fills your screen with people that look bloated and overweight, car wheels that look like eggs, fat-faced newscasters, extra-wide cereal boxes, and can it really be that those pencil-thin models now look…um…normal? I’ll bet seeing themselves that way is enough to drive some of them to anorexia nervosa. So is a TV with the cubbies really a good trade-off? The purist fairly shouts “No!”, but yet you can’t go into a bar, restaurant, or retail store without seeing bloated pictures. We all know the camera adds 10lbs, but just how many cameras are on these people? 4? We should all complain, and lobby for ‘un-stretched TV”! Snatch that remote and hit the “aspect” button until it looks right. I’m stumbling off my soap box now.

The point really is, for much of what we watch, a new 42” set is no bigger than an old 34” set. Just be aware of that fact when you nervously tap your credit card on the check-out counter. You’re not really buying bigger. In fact, we think 42″ should not even be called a “big screen” at all. We know they are all now on sale, and some are even much less than $1000. Just know exactly what you are buying, and it isn’t all that big.

Here’s a link to Screen Math, a site dedicated to analyzing the size and shape of TVs. http://www.screenmath.com See our earlier posts about screen size to learn how big a screen you should really consider and why.