I need a rainbow gobo for a show. It will be in use for about 30 sec. now here's the problem... Our gobo lights are about 100 ft directly above a staircase that goes to the stage. We have a lift but it would be a pain to use on the stairs. Would it be possible to make the gobo using an aluminum pie plate and hold it infront of the spotlight in the booth? I know its crude but its a quick 30 sec. thing and im not gonna spend an entire day replacing a gobo....
Design gobo on a spotlight?
- Thread starterelliot47
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bishopthomas
Well-Known Member
Commissioner Gordon did it...
So it'll work?
Only on TV. In the real world, not so much.
Les
Well-Known Member
Holding a gobo in front of a spotlight will not work. It needs to be at the focal point, which is directly in front of the shutters in an ellipsoidal ("gobo light") or iris (followspot).
bishopthomas
Well-Known Member
Holding a gobo in front of a spotlight will not work. It needs to be at the focal point, which is directly in front of the shutters in an ellipsoidal ("gobo light"or iris (followspot).
there is a second focal point that is external to the fixture. depending on the zoom of the fixture this could be right near the front of the fixture. theoretically if one could visualize this focal point (using smoke or haze) and it was at the fixture putting a gobo there should allow imaging. put your fixture in the widest optical range and see if the light comes together in a point at the front of the fixture. it is worth a shot. or a leko on a stand at the followspot position would work as well.
an ellipsoidal spotlight has only two real focal points:
1) where the lamp filament sits
2) the second is where the shutters/GOBO holder are located
1) where the lamp filament sits
2) the second is where the shutters/GOBO holder are located
Les
Well-Known Member
there is a second focal point that is external to the fixture. depending on the zoom of the fixture this could be right near the front of the fixture. theoretically if one could visualize this focal point (using smoke or haze) and it was at the fixture putting a gobo there should allow imaging. put your fixture in the widest optical range and see if the light comes together in a point at the front of the fixture. it is worth a shot. or a leko on a stand at the followspot position would work as well.
That is true, but I'll bet there is an extremely small margin of error
.Les: I agree, and it would be variable depending on the zoom/bone setting of the followspot, but the gate in a fixture is of a small range as well. Using a scroller frame, gel cooler or other readily available accessory that puts a frame our from a fixture by a specific distance and matching the optical setting to that may get them what they need. Church: I haven't tried this but take a 36degree S4 and see if you can image from the gel slot (or just near it). That is where an optical cross over point is located (i think) on the S4, hence a focal point. It may not work, but if someone was using a S4 on a tripod with a gobo, they would probably be using the gobo slot anyway. Though i think it would be a kewl experiment. i don't have easy access to a S4 so let me know what you find.
JChenault
Well-Known Member
First - my understanding of optics of spotlights with lenses makes me think that putting a gobo in front of the fixture and expecting to project a clear image just will not work.
However there another possibility. If the 'spotlight in the booth' is an elipsoidal - you could put a gobo in that. If the 'spotlight in the booth' is a followspot, then you could put a real gobo in the gate of the followspot.
If you look at an elipsoidal spot, you can see that the light from the lamp passes through the gate. This is a round opening just in front of the reflector where the shutters can be used to change the shape of the opening. The lens in the fixture is projecting an image of this gate on the stage.
If you look in a followspot you should see an iris assembly. When you want to change the size of the followspot beam, you will typically pull a lever which will change the size of an internal iris. If you put a gobo where the iris is, and focus your followspot - you should get a fairly clear projection of the gobo. You will not be able to use the followspot for anything else - but it should project your gobo.
However there another possibility. If the 'spotlight in the booth' is an elipsoidal - you could put a gobo in that. If the 'spotlight in the booth' is a followspot, then you could put a real gobo in the gate of the followspot.
If you look at an elipsoidal spot, you can see that the light from the lamp passes through the gate. This is a round opening just in front of the reflector where the shutters can be used to change the shape of the opening. The lens in the fixture is projecting an image of this gate on the stage.
If you look in a followspot you should see an iris assembly. When you want to change the size of the followspot beam, you will typically pull a lever which will change the size of an internal iris. If you put a gobo where the iris is, and focus your followspot - you should get a fairly clear projection of the gobo. You will not be able to use the followspot for anything else - but it should project your gobo.
As do ALL Robert Juliat followspots, but I would presume that if the OP has posed this question then they don't have a followspot with a gobo slot.
Agreed there is an optical crossover point beyond the lens train on a number of fixtures which if the lamp was a perfect point source and the optics were closer to microscope quality would be a focal point. In practice it is hard to achieve a useful gobo projection using this location. We have all experienced the challenge to get a decent gobo projection from the gobo location without using donuts to cut down the stray light and even then it does not always work.
It is always worth a try with some of these things because you can get lucky. I once managed to successfully project an image of the steel mesh in some PAR 64 onto the stage floor using some old stepped lenses in the colour frame runners. We tried it as a joke when we could not get the affect we wanted of light coming through a grill for Joseph in the jail scene - it worked and we did it in the show.
Any image taken from a secondary focal point would be of lesser quality for sure. When someone has to make due with what they have, offering ideas that may not be perfect, but good enough, may get someone past a technical stop so the show can go forward.
Compare and contrast the two diagrams below.
http://i6.photobucket.com/albums/y237/charcoaldabs/ellipsoidal_optical_path.jpg
New Klages Home Page
Which one more accurately represents the optics of the ERS?
http://i6.photobucket.com/albums/y237/charcoaldabs/ellipsoidal_optical_path.jpg
New Klages Home Page
Which one more accurately represents the optics of the ERS?
JChenault
Well-Known Member
Derek's top image is more correct.
There has been some confusion about what a focal point is and how optics work on this thread.
In an ellipsoidal spot light the reflector has two focal points based on the shape of the reflector. As shown in Derek's fixture, if you put a filament at one focus, the light bounces off the reflector and all of the reflected light passes through the second focal point.
The lens has a focal length. ( a 6x9 Altman has lenses 6 inch in diameter and a focal length of 9 inches). If you place a lighted image 9 inches from this lens, you will project that image on the wall.
Projecting a gobo does not require an ellipsoidal reflector. It requires the gobo toe at the focal length of the lens, and to have a lot of light passing through the gobo and hitting the lens. There have been fixtures that project gobs that do not use ellipsoidal reflectors ( think pareleipsphere ). Indeed if you take the reflector out of an elipsoidal spotlight and put in a gobo, you will still get a projection - it will just be very dim.
The purpose of the reflector is to gather all of the light that would otherwise not go through the lens, and redirect it through the gate to the lens. We use ellipsoidal reflectors because they do a very good job of collecting most of the light and getting it through the gate and through the lens.
There has been some confusion about what a focal point is and how optics work on this thread.
In an ellipsoidal spot light the reflector has two focal points based on the shape of the reflector. As shown in Derek's fixture, if you put a filament at one focus, the light bounces off the reflector and all of the reflected light passes through the second focal point.
The lens has a focal length. ( a 6x9 Altman has lenses 6 inch in diameter and a focal length of 9 inches). If you place a lighted image 9 inches from this lens, you will project that image on the wall.
Projecting a gobo does not require an ellipsoidal reflector. It requires the gobo toe at the focal length of the lens, and to have a lot of light passing through the gobo and hitting the lens. There have been fixtures that project gobs that do not use ellipsoidal reflectors ( think pareleipsphere ). Indeed if you take the reflector out of an elipsoidal spotlight and put in a gobo, you will still get a projection - it will just be very dim.
The purpose of the reflector is to gather all of the light that would otherwise not go through the lens, and redirect it through the gate to the lens. We use ellipsoidal reflectors because they do a very good job of collecting most of the light and getting it through the gate and through the lens.
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