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

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

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.

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?

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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?

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).

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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

------------------
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

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

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

Steve

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)?

------------------
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