Film-Tech Forum ARCHIVE: What size Xenon to use????

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Posted by Matt Close (Member # 996) on 07-28-2002, 04:23 AM:

Please excuse my ignorance everyone, but is there a chart or calculation to determine what wattage Xenon is required to light a certain size screen? In this case the screen is 3.35 meters high, and the throw is 15.5 meters. How do I work this out?

BTW , haven't been around lately because .....

I was at one of our other cinemas last weekend replacing a sound system, where I hurt my leg I was on the platform that supports the right stage channel speaker ... (a sturdy construction .. or so I thought) .... when it collapsed with me on it!! Falling 2m or so and been slammed in the legs with snapped timbers isn't my idea of a fun weekend! Luckily the old speaker fell away from me!

Posted by John Pytlak (Member # 331) on 07-28-2002, 06:30 AM:

A "rule of thumb" is 5 watts for each square foot of screen area for a matte white (gain=1) screen. Assumptions include a 2-blade (50% efficient) shutter, modern lens design, good alignment, etc.

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: +1 585 477 5325 Cell: +1 585 781 4036 Fax: +1 585 722 7243
e-mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

Posted by Matt Close (Member # 996) on 07-28-2002, 06:47 AM:

Thanks John. Do I calculate based on 'C/S' or 'W/S' screen area? Also, the screen looks to have a 'slight sparkle' to it... some sort of coating, but I wouldn't know what type. Any advice on how I wouls 'guesstimate' what the gain of my screen is?

Posted by John Pytlak (Member # 331) on 07-28-2002, 07:08 AM:

I believe the scope area should be used (assuming common image height among formats). You probably have a moderate gain "pearlescent" screen, maybe around 1.4 gain. You can measure the gain by comparing the light reflected from the screen to the light reflected from a sheet of white paper of near "100" brightness rating. A gain screen will look brighter "head on", but dimmer when viewed at an angle.

Posted by Matt Close (Member # 996) on 07-28-2002, 07:26 AM:

Thanks again John ... I will work on your advice.

Posted by Steve Guttag (Member # 268) on 07-28-2002, 02:07 PM:

Depending on the projector and lenses used, I prefer 12h^2 or 13h^2. Where h = the height of the 1.85:1 image. Thus an older system with a 10 foot tall screen would need 1300 watts. If you have an efficient projector with one of the newer efficient lenses (like the ISCO Ultra Star PLUS lenses) that could be dropped to 1200 watts.

As to gain screens, if they are pearlescent, they are normally in the range of 1.5-1.8. If it is a Stewart brand screen it will be as it is claimed (they offer several gains).

Steve

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"Old projectionists never die, they just changeover!"

Posted by Frank Angel (Member # 248) on 07-28-2002, 03:59 PM:

Sure would be cool if Hurley or Harkness had a nice computer program where you could plug in the screen size, lens speed and shutter efficency, and it would spit out the bulb wattage needed for each screen type. Or what screen type you would need for an existing lamp size....like Schneider and ISCO have for their lenses. It would be a good marketing tool for them and it would make life easier for the rest of us.

Posted by Steve Guttag (Member # 268) on 07-28-2002, 10:05 PM:

There would be enough unknowns in there to beg the question that if you are that savy, just use your calculator.

Also, it would force Hurley and other common screen manufacturers to have a consistant gain.

Steve

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"Old projectionists never die, they just changeover!"

Posted by Pat Moore (Member # 380) on 07-29-2002, 07:59 AM:

The other variable, of course, is lamphouse efficiency. I think the above are assuming current horizontal lamphouse designs.

Pat

Posted by David Rigby (Member # 1301) on 07-29-2002, 08:50 AM:

Speaking of lamphouse efficiency, are vertical lamphouses always a poor second to horizontal ones? Are there any advantages at all of a vertical one?

David

Posted by John Pytlak (Member # 331) on 07-29-2002, 10:38 AM:

Steve's formula and my "rule of thumb" give about the same answer:

For a 10 x 23.9 foot screen (239 square feet)

My rule of thumb: 5 x 239 = 1195 watts

Steve's 12h^2 = 12 x 10^2 = 1200 watts

Pat is correct that lamphouse efficiency is assumed to be typical of a modern lamphouse with good optical alignment.

Posted by John Pytlak (Member # 331) on 07-29-2002, 10:44 AM:

The vertical lamphouse design of the ORC Optimax theoretically was more efficient in reducing infrared radiation, by using a 45-degree heat mirror that reflected visible light and passed the IR to a heatsink. As others have said, alignment was difficult, and the vertical lamp was prone to "candlelight flicker" due to a relatively unstable plasma.

Likewise the vertical lamp design of the old Zeiss Ikon Xenosol III required two mirrors and "honeycomb" condensers, complicating alignment. This was the first xenon lamphouse I ever had to work with.

Posted by David Rigby (Member # 1301) on 07-29-2002, 11:37 AM:

I hadn't thought about the visibility of the arc length causing a shimmering effect, but obviously it could do! How do vertical lamphouses get around this (and if it makes them a more complex engineering task why does/did anyone bother - were vertical lamps the only available type at one time?). Are any modern machines still supplied with vertical lamphouses?

Thanks for the info guys - this is interesting stuff!

David

Posted by Richard Fowler (Member # 893) on 07-29-2002, 11:54 AM:

The shimmer effect is more noticable on the vertical lamphouses with a 45 degree folding mirror due the reflector collecting the arc image 360 degrees around the bulb of the vertical arc plasma vs the older type ( if properly adjusted ) vertical type with a butterfly type reflector collecting the arc plasma from the side at 120 to 180 degree. The advantage of vertical is longer bulb life but in today's competitive market, bulbs are cheap. Basic rule is horizontal should be 30% more efficient than old style butterfly verticals and the one with the folding mirror "should" equal horizontal iif everything is perfect

Richard Fowler
TVP-Theatre & Video Products Inc. www.tvpmiami.com

Posted by Frank Angel (Member # 248) on 07-29-2002, 01:18 PM:

John said: >. Assumptions include a 2-blade (50% efficient) shutter, modern lens design, good alignment, etc.<

But what difference would the age of the lens make if the speed is the same? Wouldn't an old SnapLite with a stated speed of f2 deliver the same light pass-thru as a modern lens that is also rated at f2? In other words, isn't the f rating of a lens a constant?

Posted by David Rigby (Member # 1301) on 07-29-2002, 01:28 PM:

f numbers are just functions of physical lens dimensions e.g. focal length and (lens) aperture size (correct me if I'm wrong here guys...) - so two lenses identical except for one having blue glass would have identical f numbers...but would obviously put very different levels of light on the screen! Quality of the glass manufacture isn't reflected in the f number.

David

Posted by Phil Hill (Member # 371) on 07-29-2002, 03:12 PM:

f-stops are a ratio-function of the physical dimensions of a lens (or other optical device) as David mentioned. t-stops are a function of the actual amount of light transmitted (through the device).

Camera lenses commonly have f-stop scales so one can predict the physical performance like depth-of-field...and a t-stop scale to critically match exposures between various lenses.

Both scales usually are numerically close and use the same f-stop numbering system even though the t-stop has nothing to do with a physical ratio.

>>> Phil

Posted by Pat Moore (Member # 380) on 07-29-2002, 03:33 PM:

Hi to all; Vertical lamps were first on the front basically because that was all the industry could make work reliably in the infancy of xenon technology, especially in larger lamp designs. The first few years of horizontal lamps were an "iffy" proposition indeed and many were regarded as hand grenades. But that's a long time ago and horizontal operation is most common now, and very reliable.

Vertical lamps never overcame (in my opinion) the problem of "arc wander" or waver. This is not flickering, but a movement of the plasma ball on the cathode and/or the arc moving around the anode. The shift in light whould show up on screen, with the bright spot seeming to dance around the screen randomly. Vertical systems never seemed to be able to cure that problem.

We referred to "shiver" as a mechanical component that was also visible in some vertical designs. The reflector's outer diameter was not well supported so it vibrated according to however off-balance the cooling fan(s) was. This produced a soft-looking image on the outer edges of the picture, depending on how defocused the system was.

My interpretation of f-number is the angle of light projected or accepted by an optical system. The higher the f-number, the sharper the angle of light. Most optical systems nowadays are somewhere around f2 to f2.2.

A very significant factor in lens design is the coatings used on the elements. Great improvements in this field have allowed different optical coupling techniques to be used to improve the lens's light efficiency. That's why an old Snaplite or something similar, even with a faster optical speed, can't match a new design lens with a slower optical speed (and its far better resolution and depth of focus and contrast and so on) in picture quality or light throughput. The f-number might tell you the angle of light a lens might pass, but not how efficiently it passes that light.

Pat

Posted by Stephen Furley (Member # 1277) on 07-29-2002, 04:43 PM:

"The shimmer effect is more noticable on the vertical lamphouses with a 45 degree folding mirror due the reflector collecting the arc image 360 degrees around the bulb of the vertical arc plasma vs the older type ( if properly adjusted ) vertical type with a butterfly type reflector collecting the arc plasma from the side at 120 to 180 degree."

I am not very familar with vertical lamps, and have never seen anything with a 45 degree mirror. I can only imagine that you are talking about something similar to a horizontal setup, but turned through 90 degrees, with the bottom end of the lamp passing through the centre of a parabolic mirror, and the light being turned in a horizontal direction by an angled mirror above. Is this the case? If so, it would require the lamp to be positioned well below the optical axis of the projector. This is nothing like any projector I have ever seen, Is it done in those 'console' type machines? These are not very common over here.

"The advantage of vertical is longer bulb life but in today's competitive market, bulbs are cheap."

Why does vertical give a longer lamp life?

"Basic rule is horizontal should be 30% more efficient than old style butterfly verticals and the one with the folding mirror "should" equal horizontal iif everything is perfect."

By 'butterfly', I assume you mean the type where the vertical lamp has a small, curved mirror behimd it, and sometimes a smaller one still in front. This is the only vertical type I have seen.

Do vertical lamps operate with the positive up or down?

I assume (and hope!) that the the old three-electrode lamps like those used by BTH at one time, are now a thing of the past. I haven't seen one of these for many years, except in the Projected Picture Trust collection at Bletchley park museum, Ghastly things, unstable arc, had a habit of going strange colours when they got old, and, so I'm told by those who used them, of exploding.

Modern xenons are a vast improvement on the early ones, but I still prefer carbons.

Posted by Phil Hill (Member # 371) on 07-29-2002, 06:24 PM:

The "angle of light" is a function of the EFL, effective focal length. The f-stop is measurement of the *amount* of light. Format and EFL determine working distance.

The f-stop does not change with formats but physical size does...to obtain the same focal length. Meaning, a 1570 projector needs a much larger diameter reflector than a 16mm projector even though both lamphouses have a 2.2 f-stop.

Usually, the focal length/working distance increases with an increase in format size so as not to present a too-short light cross-over point or a too-steep light-angle to the film plane. That is to prevent the light cone from focusing before the film plane. I should say that a good reflector will never focus to a "pin-point" but rather a "minimum" cone diameter relating to the aperature it must cover. If it did focus to a pin-point...think ants and magnifying glasses.....hot, very hot and destructive...much like Dr. Evil's "Laser". Some correction in focal-length/spot-size can be obtained with lamphouse "beam-spreader" lenses to allow 70mm frame coverage with a "35mm" lamphouse.

Also, if the f-stop of the lamphouse is less (higher number) than the projection lens, the image may vignette, especially with short focal length lenses.

> Phil

God! I sure do love them hyphens, huh?

Posted by Richard Fowler (Member # 893) on 07-29-2002, 08:34 PM:

The Vertical tend to have the positive ( anode ) side up. The reflector generally has cuts at top and bottom to allow the F1 ( xenon light focal point ) to be met by the bulb being set toward the rear of the reflector....so the reflector looks like a butterfly. The smaller front mirror is used by some lamp houses to raise the efficiency of the light output but can cause other problems if the operator does not have it correctly focused, such as focusing excessive heat on the bulb and short life or failure. The Shimmer on the folding mirror set-up ( as Pat noted ) starts at the edge of the reflector vibrating ( like a salad bowl on a table ) + the chance that the folding mirror has vibration from the mounting points and
blowers cooling the folding mirror

........but when these where first developed they where state of the art compared to the early horizontal designs and condenser set ups by others.
Richard Fowler
TVP-Theatre & Video Products Inc. www.tvpmiami.com

Posted by Brian Tristam Williams (Member # 1235) on 08-06-2002, 11:57 AM:

5 Watts per square foot?

Oh oh.

We have a screen of over 3000 square feet, so we should have a lamp of over 15kW? We're only using a 7kW lamp.

Lamps are expensive though. Any idea what upgrading the screen from a matt white will cost, ballpark?

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"One man can make a difference."

Posted by John Pytlak (Member # 331) on 08-06-2002, 04:15 PM:

Brian said: "We have a screen of over 3000 square feet, so we should have a lamp of over 15kW? We're only using a 7kW lamp."

That's what gain screens are for! Just be sure it is properly curved so you still get uniform luminance (SMPTE Recommended Practices RP 95 and RP 98).

Posted by Frank Angel (Member # 248) on 08-07-2002, 06:04 AM:

Is that formula applicable to carbon arc also?

Posted by John Pytlak (Member # 331) on 08-07-2002, 12:51 PM:

Frank asked: "Is that formula applicable to carbon arc also?"

Don't know. It would apply if the "Lumens per Watt" efficiency of the lamphouse was the same.

Check out the National Carbon Bulletins in the Film-Tech "Manuals" section, which contained light output data for a variety of carbon lamps/trims in issue #3.
National Carbon Bulletins

Posted by Lindsay Morris (Member # 1430) on 09-12-2002, 06:35 AM:

Hello All,
My first go at this reply stuff.. but is really also a query for the people out there.
My current setup is carbon arc 6 & 7 mm running at around 48 amps which according to the carbon book produces around 100K lumens with a lamp wattage of 1700 watts.
The projectors are Simplex almost an E7 at least that is what we call them in OZ but going by various pictures I have seen are more than likely modified front shutter units prior to E7. The light is good the picture is good up onto a 32 foot x 16 foot outdoor SOLID screen painted flat matt white.
I intend pulling these out and installing a Century CC (twin shutter) which is equippped with a Lumex 2k Xenon lamp powered by a Strong single phase power unit.. old technology, heavy but rugged.
The 2kw lamp at 85 amps is supposed to produce 75k lumens which could be seen as a backward step... ie less light on the screen.
Using the rule of thumb talked about here of 5 watts per square foot in theory I need 2560watts to light up the screen face area
(in scope) of 512 sq feet.
BUT the Century has a twin shutter and back in my memory lurks an item that says that twin shutters have an efficiency of some 20-30% over single shutter units.
Questions are:

1. Does the 5 watts per sq foot apply to a sound screen or a solid screen?

2. Are the twin shutters more effective in passing light to the screen than the single shutters and if so by how much?

Thoughts on whether the 2kw Xenon is big enough to fill the shoes of the old carbon arc lamp setup I currently have.
I do not wish to go backwards in terms of screen illumination but the possibility of using the Century with the Xenon and the Eprad Mut and NO MORE changes every reel is quite attractive after 6 years of operation with carbons and very reliable old Simplexes.
Lindsay

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Posted by John Pytlak (Member # 331) on 09-12-2002, 07:13 AM:

Lindsay:

For a 16 foot high matte white screen, you probably should have at least 3000 watts if you hope to maintain 16 footlamberts.

The counter rotating twin shutters of the Century Model CC are slightly more efficient, as they produce a slightly more rapid "scissoring" action of blocking the light. However, unless you have a projector with a 5:1 "high speed" or "drive-in" intermittent, your efficiency is probably still close to 50% if you want to avoid travel ghost. A 5:1 intermittent gets you up to about 60% efficiency.

The difference between a solid screen and a perforated screen made of the same material is only a few percent.
------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: +1 585 477 5325 Cell: +1 585 781 4036 Fax: +1 585 722 7243
e-mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

Posted by Steve Guttag (Member # 268) on 09-12-2002, 10:08 AM:

A Century running a double shutter has an efficientcy greater than 50%

Steve

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"Old projectionists never die, they just changeover!"

Posted by John Pytlak (Member # 331) on 09-12-2002, 10:45 AM:

The technical paper that introduced the Century C and Century CC projector was published in the March 1942 Journal SMPE: "Projection Design Developments" by Ewald Boecking and L.W. Davee. It claimed "The efficiency calculation differs slightly from that of a single shutter and equals 59 percent in a light beam position C (shutter 4 inches from film plane), 56 percent in a position B (shutter 5 inches from film plane), and 53 percent in a position A (shutter 6 inches from film plane). The double shutter mechanism CC will deliver approximately 16 percent more light than the single-shutter mechanism C". The paper then notes that expected gear train backlash must be compensated for by additional degrees of covering to avoid travel ghost.

So, the double shutter mechanism can be up to 16 percent more efficient, but shutter position and gear backlash reduces the efficiency from the theoretical maximum. The paper concludes "Whatever advantage there may be in these mechanisms from this factor has been used in improving the appearance of the picture, RATHER THAN IN REDUCING THE SHUTTER ANGLES, to give the additional illumination which could be obtained."

That is why I said: "your efficiency is probably still close to 50% if you want to avoid travel ghost".

Steve: So in practice, with a standard 4:1 intermittent and shutter blades large enough to avoid any travel ghost, what efficiency have you been able to consistently achieve with the Century CC? (Screen Luminance With Shutters / Screen Luminance Without Shutters X 100)?