8mm Forum: How many lux from GS1200? A value list...

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Posted by Ugo Grassi (Member # 139) on January 09, 2004, 07:22 PM:

Some months ago Gianni and I had the curiosity to measure the lux from our Elmo GS1200. The "stock" was inclusive of:
GS1200 I ver. (shutter with blades at 50°)
GS1200 II ver. (shutter with blades at 53°)
GS1200 II ver. with two blades shutter (45°) from ffr-film (Germany)
GS1200 II ver. with two blades shutter and over supply of the lamp (24,8 v. instead of the original supply at 23,7 v.)
GS1200 II ver. with two blades shutter and over supply of the lamp (25,1 v.)

Elmo lens 1.1
Elmo lens 1.0
*********************
I ver. / lens 1.1

I pos.: -----
II pos. 310 lux
-------------------
Ver. II / lens 1.1
I pos.: 200 lux
II pos.: 270 lux
-------------------
Ver. II / lens 1.0

I pos.: 220 lux
II pos.: 300 lux
-------------------
Ver. II / two blades shutter/ original supply (23,7v.)/ lens 1.0
I pos.: 310
II pos.: 410 lux
-------------------
Ver. II / two blades / lens 1.0 / over supply at 24,8 (pos. II)
II pos.: 470 lux
-------------------

Ver. II / two blades / lens 1.0 / over supply at 25,1 (pos. II)
II pos.: 528 lux

[ January 10, 2004, 06:25 AM: Message edited by: Ugo Grassi ]

Posted by Paul Adsett (Member # 25) on January 09, 2004, 08:57 PM:

Hi Ugo,
This is really great information-nothing like actual measured data rather than subjective opinions. Based on your data, I am inclined to retrofit my GS1200 with a 2-bladed shutter, but am concerned about flicker. I know there will be obvious flicker at 18 fps (which I never use so this is no concern to me) but is there any noticable flicker at all at 24 fps?

Posted by Ugo Grassi (Member # 139) on January 10, 2004, 05:20 AM:

No flickering! Don't worry! I have the SMPTE test film with ghost targets to check the flikering and there aren't problems.
Do you know how to do the work? Or do you know a technician ables to do it?

[ January 10, 2004, 09:08 AM: Message edited by: Ugo Grassi ]

Posted by Paul Adsett (Member # 25) on January 10, 2004, 09:53 AM:

Yes, I have a GS1200 service manual and have stripped the machine down to the "Engine Block" before , in order to clean and lubricate the claw mechanism. My only problem is, where do I get a 2-bladed shutter for the GS1200?

Posted by Douglas Meltzer (Member # 28) on January 10, 2004, 10:14 AM:

Hi Paul,
The Reel Image sells two bladed shutters for the GS-1200 and also for the ST-160, ST-180, ST or GS 800. The cost is $18.00 postpaid. Steve Osborne's # is 937-296-9036
Doug

Posted by Paul Adsett (Member # 25) on January 10, 2004, 11:52 AM:

Thanks Doug, I'll give Steve a call today and place an order. I was hoping there would be a supplier somewhere in the US. Sounds like an easy way to boost screen illumination.

Posted by Ugo Grassi (Member # 139) on January 11, 2004, 04:24 AM:

Well Paul. When you'll do the work, you'll need atest film to regulate the phase of the shutter in according to the pins. If you have not the SMPTE film, you may shoot a picture with white targets (squared) on black ground.
The 45° shutter is enough to close the light when the frame is moving, but it needs a little bit of time to regulate it.

Posted by Maurizio Di Cintio (Member # 144) on January 11, 2004, 04:49 AM:

Sorry Ugo [Embarrassed]

: though I agree with you 100% about the fact that the three to two blade shutter modification does not adversely affect flicker, I do not think your explanation thereof really addresses the question.
First off a foreword might be necessary and I beg everyone’s patience for the lenghtiness of this post. The SMPTE S/8 test film is meant to assess the following in projectors’ performance:
a) image steadiness (both horizontally and vertically)
b) frame edges coverage and alignment
c) lens definition and contrast handling capability
d) combined lens/mechanics definition
e) film flatness in the gate
f) lens’ flatness of focus
g) light output evenness

The RP 32 features several elements to evaluate all of these factors. But the “Travelling Ghost” you mentioned stands on its own in that its real function is to reveal even the slightest lack of syncronization between shutter blades and claw intermittent movement. If film transport is carried out when the lamp is not yet totally covered by the shutter, then some sort of vertical trail will be displayed by the travelling ghost’s squares, as if someone had smudged the white part of the image against the black background.
Now such a circumstance is impossible to happen on a S/8 machine, because of the S/8 inherent intermittent movement design, unless, of course, it is really poorly engineered (and despite all the machines I tested, I have never stumbled in a case like this, even with bottom of the line projectors). In any case, it wouldn’t be a problem of ageing or wear. It is possible to see a good example of trail from TG by running the SMPTE RP 32 S/8 test film in reverse on a Eumig Mark S 804. Because of its really thin claw, when running in reverse not only does the frame line shows well above the gate mask, but the portion of the image under the frame line features highlights that are ‘smudged’ like a ghost image. This happens because of claw thinness and the fact that in reverse the shutter covers the lamp a little bit too late after film transport has already started, rather than the other way around. Of course I am not implying the Eumig is poorly engineered (forward projection is always excellent); this is just the kind of compromise ona has to accept on a dual gauge machine like this. Nonetheless it is a good example of what a TG is. By the same token the higher-end Bauer’s and Silma’s are equipped with variable width shutter blades: in reverse mode, the blades grow a little wider and the problem experienced on the Eumig is eliminated (of course the image is a little bit darker).
In short in my opinion the TG is directly derived from 35 mm test film, a gauge with which a problem like the one I described may occur, because the sync between film transport (Maltese cross) and shutter movement are assured by mechanical organs that are not phisically in contact: indeed they are often linked to each other by means of a toothed belt (rather than gears, as was the case with older machines), and if the gears the belt has to go through, are not properly oriented, the sync between the Maltese cross sprocket drum and the shutter blades will be adversely affected. Also the TG on a S/8 test film may be of use on printers, but on S/8 projectors, as you see, it is probably not that meaningful (save for what you said in your latest post, Ugo), and surely it does not say anything as regards flicker.
Flicker in fact is the outcome of alternation between dark and bright moments on the screen in case that alternation drops below a certain threshold, i.e. it is not fast enough for the human eye to be perceived as a seamless illumination or, for that matter, as a sequence of photographs blending into a seamless movement. This threshold lies about a frequency of 40 (more or less) combined moments of light and dark on the screen in a second, which is way below the actual rate of a film screened at 24 fps with a two bladed shutter (= actual frequency: 48/second). Obviously if you project at a frame rate of 18 fps, you wil get an actual frame rate of 36/second which would result in flickering (but that would be bothersome only for the brighter areas of the image). Not to forget: for the purpose to just move frames in the gate, one blade only would suffice: the other two (for 18 fps) or one (for 24 fps) are just meant to compensate flicker.
That’s why S/8 projectors have three blade shutters as standard, so they can handle both speeds with no viewing stress. But if one just uses 24, then not only is the third blade useless, it is ‘detrimental’ to light output performance, as it steals some 30-40% therof, depending on blade width/shutter’s cam design for the purpose of claw movement fastness (the faster the film transport of each individual frame, the smaller the width of the blades required).
In conclusion, like Ugo (who succesfully modified my Bauer T610) I would say go for it to whomever wants to screen at 24 fps with blade modifications, because for said reasons, there is nothing to fear in terms of flicker (though the RP 32 won’t be able to witness that).
Happy screening to everyone.

Posted by Ugo Grassi (Member # 139) on January 11, 2004, 05:01 AM:

Yes, I agree with you. With the term “flicker” I mean a sort of ghost tail, i.e. (=id est) a problem which occurs if the claw’s intermittent movement is not in phase with the shutter’s, or if the blades are not wide enough. In fact the right meaning of “flicker” is “fast flash” but, of course, there’s no such problem at 24 fps. Indeed I was speaking about the other problem.

[ January 11, 2004, 11:54 AM: Message edited by: Ugo Grassi ]

Posted by Kevin Faulkner (Member # 6) on January 11, 2004, 07:06 PM:

Paul, As you have the Elmo GS manual then follow the instructions in that for installing the 2 blade shutter. It worked well for me. One thing to be aware of is that the original Elmo shutter is riveted to the centre pulley so you will have to file the rivets down to get the shutter off. I then superglued my new shutter in place and followed the instructions for setiing up in the manual and it has now been working for about a year without any problmes. The shutter I obtained was the one sold by Wittners is Germany. It has little holes in it which fit over the stumps of the original rivets to make centralising of the shutter on the pulley very straight forward. If the shutter is not centred correctly then the whole assay could be out of balance and cause vibrations etc. I have used the SMPTE test film after the mod and noticed no artifacts.
Anyway have a go it's really worth it and the increase in light is very noticeable. [Smile]

Kev.

Posted by John Clancy (Member # 49) on January 12, 2004, 02:53 AM:

I just got that Wilton to make me a two-bladed shutter and slap it in. He even came round to do it. He has his uses. However, he needs to do another machine for me now so he's got his orders and is considering cutting a shutter out or the lid of a biscuit tin.

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