# Why do you have to put a gobo in upside-down?



## derekleffew (Sep 17, 2009)

1.) Why must a gobo containing text be placed upside-down in an ERS to display correctly? 
2.) Why not also backwards (reversed/flipped L&R, like a 35mm slide in a slide projector) ? 

3.) Why does the rule "So the lamp can read it!" apply to moving lights?


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## shiben (Sep 17, 2009)

I can answer number one pretty easily. The way and ERS projects a template in the focal plane (sp?) results in the entire image being flipped upside down. I am not sure about the moving light one, and I am not really familiar with how slide projectors work too.


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## philhaney (Sep 17, 2009)

derekleffew said:


> 1.) Why must a gobo containing text be placed upside-down in an ERS to display correctly?
> 2.) Why not also backwards (reversed/flipped L&R, like a 35mm slide in a slide projector) ?
> 3.) Why does the rule "So the lamp can read it!" apply to moving lights?



1. shiben answered this.

2. For the same reason that a mirror flips text from right to left but not from top to bottom.

3. See Number 2.


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## bdkdesigns (Sep 17, 2009)

1) Not really sure how to explain it but the first focal point is at the lamp. From the reflector, the rays of light cross at the instrument gate inside of the instrument...which is also the point where we find our shutters and gobos. From this point, the rays then continue onto the lens where they are refracted and change directions once again where we find our third focal point, just outside of the instrument. But these refractions essentially reverse the image which is why we need to flip the gobo.

2) I'm not entirely sure what you are asking here unless this is a trick question. The easiest way to figuring out how a gobo is projected is to hold it in front of you with how you want it to read. Flip it once verticly and flip it again horizontally to get it to how it should be placed into the instrument. After projecting it, you then get the proper orientation. 

3) Never heard that expression so I don't know.


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## SHARYNF (Sep 17, 2009)

Derek and Ship have years of experience with this, and I would suggest that if they had the time to make up a drawing of the light path and focal points it would be extremely useful.

It would be interesting to show for instance the difference in placing the gobo in the accessory slot typically where the iris or rotator goes, and the gobo slot, where Dichro filters can be placed, and gel filters.

As an aside but somewhat related, I was using one of the Multiplexers that work great in a Leko, and asked the inventor how it would work in a slide or lcd projector. She was stumped and decided to try it, and was surprised that it did NOT work for a slide or Video projector. It all relates to where the various focal points are and the size of the "image" at these points

It is really a very interesting and not trivial discussion

Sharyn


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## bdkdesigns (Sep 17, 2009)

So I tested it in my shop just to make sure that I wasn't mistaken and simply turning a gobo with text upside down produced an image with backwards text. After flipping it upside down and L&R, I produced the word in the proper orientation.


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## derekleffew (Sep 17, 2009)

philhaney said:


> ...2. For the same reason that a mirror flips text from right to left but not from top to bottom. ...




bdkdesigns said:


> So I tested it in my shop just to make sure that I wasn't mistaken and simply turning a gobo with text upside down produced an image with backwards text. After flipping it upside down and L&R, I produced the word in the proper orientation.



Two opposing answers. Which one is the more correct?


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## Ric (Sep 17, 2009)

derekleffew said:


> 1.) Why must a gobo containing text be placed upside-down in an ERS to display correctly?
> 2.) Why not also backwards (reversed/flipped L&R, like a 35mm slide in a slide projector) ?



As far as I've ever seen, the image is flipped upside down & mirror imaged in any profile/leko/ellipsoidal. So in answer to 2. it *IS* backwards!
Due to the nature of light being focused through a lens, I don't see any way that only one axis of a gobo could be flipped, both MUST be.

Here's some good links that may help to explain it.

WikiAnswers - Is the inverted image created by a lens also inverted horizontally

From Lenses To Optical Instruments


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## 00AVD (Sep 18, 2009)

philhaney said:


> 2. For the same reason that a mirror flips text from right to left but not from top to bottom.



LOL! I hope you're not suggesting a mirror only works in one plane?

If you look in a mirror, things appears "backwards", but that's just because of your perspective is different. Your brain doesn't like the apparent left/right reversal. It never actually swapped sides, only your perspective did.


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## shiben (Sep 18, 2009)

bdkdesigns said:


> So I tested it in my shop just to make sure that I wasn't mistaken and simply turning a gobo with text upside down produced an image with backwards text. After flipping it upside down and L&R, I produced the word in the proper orientation.



I will perform the same experiment today at work. If I get the same result, I would say this one is more correct than the mirror answer. Also, for number 3, what does that mean? I have never used MLs as such have no experience knowing what they need, but now I am curious.


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## mstaylor (Sep 18, 2009)

As answered already you have to flip a gobo in two directions for the same reason the shutters are all reversed. If you only had to flip it on one axis then all four shutters wouldn't be backwards, only two would.


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## icewolf08 (Sep 21, 2009)

Backwards is relative. Consider that most of the time with patterns like breakups, the orientation doesn't matter, so we don't really pay attention to the orientation of the template in the holder/fixture. However, every patter has a front and a top. Usually you can find the "correct" orientation based on the manufacturers markings. Usually the side with text (pattern Number & Manufacturer name) is the front and when you can read the text correctly you have it right side up.

If you are standing behind a lighting fixture looking at the stage and you hold up the pattern in its "correct" orientation then you do have to flip it along both axis to make the image appear "correct" when projected. However, if you are standing in front of the fixture, holding the pattern so that you see it in its "correct" orientation then you need only flip the pattern "upside-down" to get the image to show up correctly on stage.

Thus, if you define FORWARDS as having the "front" of the gobo facing the stage, you need only flip the gobo along the horizontal axis to achieve the correct orientation when projected.


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## philhaney (Sep 21, 2009)

Bingo! (what icewolf08 said)

To follow up on my previous comment, "For the same reason mirrors only reverse text from right to left and not top to bottom." If you wrote a message with a white grease pencil on a fairly clear gel and held it up to a mirror with the text facing you so you could read it, you could read it in the mirror, too.


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## derekleffew (Sep 22, 2009)

So I was very confused until I watched the video. I kept saying to myself "you don't have to flip twice." It's semantics. I was _rotating_ the gobo 180°. The same final orientation is achieved by _flipping_ the gobo top to bottom _and then_ right to left, (ala the upside-down-and-backwards of the slide projector world).

By the way, I disagree with one point made in the video. The reason for the use of a donut is not because "lenses can be dirty or smudgy." Who knows a better reason for why a donut sharpens the projected image??


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## bdkdesigns (Sep 22, 2009)

Donuts reduce the flare, or ambiant light, or whatever you want to call it from around the gobo surface thus producing a sharper image. The way I understand it, even though the gobo is at the gate, the light still gets refracted again at the lens causing that slight blur. The donut essetially blocks that extra refracted light out to give us a sharper image.


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## gafftaper (Sep 22, 2009)

I would say the main need for the donut is impurities in the glass which allow a small percentage of rays to spread beyond the normal field angle. True dirty lens could do the same thing. No glass is 100% perfect.


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## icewolf08 (Sep 23, 2009)

In theory, maybe only 50% (less in reality) of the light from the lamp is reflected off the reflector into the optical train in an "organized" manner. Of the rest of the light from the lamp, only some will pass through the optical train in an ideal fashion (some won't at all). Because of this scattering of light, you have some inefficiencies in the optics. Then consider that in a source four, the lenses don't fill the entire tube, they are set in standoffs, leaving gaps for stray light. Using a donut reduces the amount of stray light that makes it from the fixture to the projection surface.

This is very similar to how when you "stop down" (reduce the aperture) the lens of a camera you increase the depth of field (more foreground and background become in focus). Why? You cut down the amount of stray light. You cut down the total amount of light also, which is why when you install a donut in a light the image gets dimmer and when you stop down a camera lens you need to use a slower shutter speed.

Anyone who wears glasses or contacts can kinda test this principle pretty easily. When you are not wearing your corrective lenses make as small an opening as possible using your thumb and index finger and hold it up to your eye. You should be able to see things more in focus yet dimmer.


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## Kelite (Sep 23, 2009)

bdkdesigns said:


> Donuts reduce the flare, or ambiant light, or whatever you want to call it from around the gobo surface thus producing a sharper image. The way I understand it, even though the gobo is at the gate, the light still gets refracted again at the lens causing that slight blur. The donut essetially blocks that extra refracted light out to give us a sharper image.





Yes. And bear in mind KC Hooper (the demonstrator in the video) spent a number of years teaching theater classes at the college level and has seen his share of less-than-stellar lenses. True, an optically clean lens will still suffer from spill, thus a donut or tophat cleans up the mess.


Great discussion-


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## derekleffew (Sep 23, 2009)

bdkdesigns said:


> ... The way I understand it, even though the gobo is at the gate, the light still gets refracted again at the lens causing that slight blur. ...


Might I suggest that the FIRST time the light rays are refracted is when they hit the first lens?


Kelite said:


> ...True, an optically clean lens will still suffer from spill, thus a donut or tophat cleans up the mess.


A tophat can affect the image shrapness? First time I've ever heard that.

No one has yet addressed what I consider the primary reason a donut helps.


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## Kelite (Sep 23, 2009)

Sorry, I kind of steered off the path on that one. The donut certainly lessens the spill and sharpens the image (often in a bigtime way!). The tophat reduces spill but does nothing to sharpen the hard-edge focus of a gobo or shutter cut.

Thanks Derek!


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## bdkdesigns (Sep 23, 2009)

derekleffew said:


> Might I suggest that the FIRST time the light rays are refracted is when they hit the first lens?



True, just a little typo on my part. I was thinking to myself about the light leaving the lens in which most cases involves two refraction points from two plaino convex lenses. That however clearly didn't come out that way in my description. 

Another need for it (although probably not what you were looking for) still involves the flare. Our current national tour, To Kill a Mockingbird, deals with a lot of rear projection onto the back walls. During the jail scene, we are projecting Tom's head inside of a jail cell onto the back wall. We are using a Source 4 19º EHD barrel. The gobo itself looked great and dropping in the donut did not enhance the gobo at all. However, there was still a bit of flare that allowed us to see Tom's entire body step into the light. The donut eliminated that completely while the gobo itself went unchanged.


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