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So you keep hearing people talk about gobos, you keep seeing the word ‘gobo’ in the technical specification for lighting equipment…and you haven’t a clue what one is, but didn’t like to ask.
Read on.
A gobo is a piece of thin metal that has a pattern of holes cut into it. This piece of metal is placed into the ‘gate’ of a profile lantern so that the lantern projects an image of the gobo. The principle of this is exactly the same as a slide projector.
Just like with a slide projector the image projected will be upside down and backward….so remember that, when putting the gobo into a conventional lantern (fixture).
Gobos came to be used in stage lighting equipment rather than slides largely because the very high temperatures inside halogen theatre lanterns would melt a slide (or crack a glass one). Recent advances have made it possible for some of the commercial manufacturers to produce patterned glass gobos but that’s all at the rather expensive end of the market.
Many of the disco effects you will run across incorporate gobos, as do most of the intelligent effects fixtures in use today. You’ve seen the results on virtually every variety show and game show being broadcast on TV, where (usually random or geometric) patterns are plastered all over the floor and/or walls.
In intelligent fixtures and disco lanterns the manufacturers have usually arranged several gobos in an arc on one larger piece of metal so that the unit can move each cut-out in turn into the gate of the lantern. Many more expensive fixtures have the means to rotate the gobo while it is in the gate, to provide spinning or animated effects. You can still get gobo rotators for theatre lanterns too (at a price!)
Naturally each manufacturer’s gate size is different, and each type of lantern within a manufacturer’s range is likely to have a different gate size too. The result of this is that there’s an excessive number of permutations. The industry has dealt with this by settling on half a dozen standardised gobo sizes (‘A’, ‘B’, etc.) and producing a range of holders that will take these standard sizes and fit the gate of the different lanterns. If you are ordering off the shelf patterns / designs produced by one of the major manufacturers you will need to specify what lantern you intend to use it in to make sure we supply you with the right gobo-holder and gobo.
The three most usual sizes are shown below, with the overall diameter, followed by the image diameter in brackets:
Size ‘A’ 100mm (75mm)
Size ‘B’ 86mm (64.5mm)
Size ‘M’ 66mm (49.5mm)

All that said you’ll probably want to experiment a bit, and it’s perfectly possible to cut your own patterns.
The best material is probably printers off-set litho plate. This is thin aluminium sheet, but rather more robust than baking foil! Most litho printers will be willing to let you have one or two cast off litho plates. The size of the plate will vary depending on the printing machines they use, but you will get ten or a dozen average sized gobos out of even the small ones.
If cutting your own gobos it is usually most sensible to cut a rectangular plate, rather than a circular one, and make this the right size to fit directly into the runner in the gate of the lantern you are using. This obviates the need for a gobo holder (and saves you money), although it does lose you the facility to rotate the gobo to a suitable orientation when focussing…so make sure you cut your gobo upside down.
Litho plate is easily cut with scissors, or you can punch holes with a variety of cutters.
Random patterns and ‘break-up’ gobos are very easy, and will keep you amused for hours. Once you’ve got the idea try cutting intricate shapes.
Manufactured gobos are etched into steel. The amateur may find it possible to etch sheet copper using printed circuit board techniques to create complex images. However it is rarely worth the hassle. Remember you are unlikely to be projecting this image onto a cinema quality screen, so precision detail is frequently lost on stage.
All gobo manufacturers will produce custom gobos to your design to order if your budget will stand it.

Several gobo images, from several different lanterns, can be laid on top of each other to produce complex pictures. (I’ve still got a three gobo set, hand cut in 1974 to fit Strand Patt 23s, of a paddle steamer, with all its windows lit, with steam coming from the funnels and a wake) If you use several lanterns on separate channels you can create the illusion of movement by sequencing (chasing) the circuits. This is frequently done to simulate fireworks, fountains and waterfalls, fire and the like.
Obviously you can focus the lantern with a gobo in it to produce either a sharp image, or a slightly out-of-focus diffused one. Break-up gobos are frequently used slightly out of focus to create the effect of sunlight through the leaves of trees etc.
If you put a ‘split gel’ (that is two or more pieces of colour in the same colour frame, arranged so that half the frame is on one ‘flavour’ and half in the other) in front of a lantern with a slightly out of focus gobo in it, you will get a multi-coloured image on the stage. Each piece of the pattern will have one side in the first colour, and the other side in the second colour. Additionally the centre of the individual hole will show the colour that results from combining the two original colours. (for example: if you have a split gel of primary red and primary green the centre of each bit of the image will come out yellow) Where the images of the hole overlap on the stage due to being out of focus similar colour mixing will occur.
It is usual to split the gel diagonally across the frame and stick the two halves together with some suitable clear tape.
Don’t be misled by the claims of some of the disco effect manufacturers that the effect lantern ‘includes split colour gobos’. Most budget disco fixtures produce colour by sticking little bits of di-chroic filter onto the back of the gobos inside the unit. Sometimes they use up the tiny bits by butting two bits of di-chroic together so the join comes in the middle of a particular gobo shape (usually the ‘windows’ one). Because this colouring is being done at the gate of the lantern the mixing effects I mentioned above will not happen, although you will get two colours at once on that particular gobo.

Many small scale shows give up on gobos because they find that they cannot see them once the main lighting state is faded up. Naturally a full up open-white wash will swamp a gobo in dark blue!
To get the most effect try the following:
Limit the gobo colours to pale tints, to maximise the light output.
Use the highest power profile lantern you can lay your hands on for gobos.
Cut the gobo so as to maximise the light output. Make the image as big as the gate will allow and move the lantern nearer the stage, rather than cutting the image small to make the image fit the stage. (If your gobo is half holes and half solid you will ‘lose’ half the light output of your lantern…..this means a 1KW lantern becomes equivalent to a 500W)
Make the gobo image hit something that your audience can see! Those of you working in flat floored small halls and theatres must remember that the audience can’t see the floor. TV and professional theatre doesn’t have this drawback, as the cameras/audience are raised in relation to the stage and look down on it. Amateur theatre generally suffers from a reversal of this arrangement.
If your show is anything other than straight drama, and you are trying to use gobos to give the lighting some excitement, try to ensure there is some smoke (or haze) in the air so that the beams of light show up. If doing this consider using the gobo lighting as backlight instead of front light. This will mean that the beams are coming toward the audience and are much more visible. (just like oncoming car headlights in fog).

Much of the article above was written before ‘movers’ became commonplace. At the lower end of the budget ladder for intelligent fixtures you will probably be stuck with the range of gobod selected for a particular fixture by the manufacturer. Some models will allow you to change the gobos, but remember that in most movers these are so small and fiddly thathome made (self cut) patterns are likely to prove too much of a challenge. Note too that much of the text above about heat becomes irrelevant once you buy into the LED world.

Go on experiment! make your lighting more exciting. Any questions welcome, just email me.

In conclusion… just a few thoughts about ‘projection’ in general:

Projection is a rather loose term, and almost any focussed beam can come under it as a general heading. For many years gobos were the chief way of achieving a projected image on stage. Recently gobos have appeared in almost every bit of disco equipment, and are frequently animated so that they wag, rotate or whatever too. The ability to move the image has led to this becoming an obsession with both rock concert lighting people and TV programme makers.
However ‘projection’, in the strict sense implies a more complex image than most gobos could cope with.
This sort of image projection almost always used to be done with slide projection, but is now frequently achieved by digital projection. Slide projection, using old style, but still available, 35mm slides was generally done with a ‘carousel’ type prjector.
This technique was used for both text images… for many years the captions for ‘Oh What a Lovely War’ were available to hire as 35mm slides…. and as a means of indicating place. A sort of scene setting! Now, of course, we can use digital projection for both these purposes, and for moving images and for live images. (We’ve used cameras on the audience projected via a digital projector so they can see their own reaction and on performers so they appear behind themselves larger than life…. as a budget version of the current LED wall craze.) Digital projection usually uses the SVGA output of a PC, (although you can link straight to DVD or VCR players and the like) and this means that anything you can create on the screen of your PC can be projected.
Regrettably many users fall down at this point as what goes on the screen of the PC goes on the screen that the audience sees. All too often these days an audience is treated to sporadic glimpses of the desktop.. and continual distracting views of the mouse cursor wiggling about as the operator readies the next slide, or whatever. The solution to this is to use a simple switch in the SVGA line to select either the PC’s screen or the projector. This allows you to set up a full screen image, and to cut it in on cue. (There are ways of doing this using programs like ‘Power Point’, or better the free ‘Open Office’ suite which includes a ‘Presentation’ facility with lots of fades, disolves and editing toys. They all assume that you will never want to change the sequence, and that you and the show will never get out-of-step and need to adjust ‘on the fly’….and life isn’t really that perfect is it!) so, after all that preamble, to the difficult question. “What can be said about back projection as opposed to front projection. Well back projection is a good solution if (and they are big ‘if’s) you have adequate space behind the screen location. In other words the distance behind the screen is directly related to how big you want your image, and a big image, for stage use, impies a very long throw. Bear in mind that this space is totally useless to you as a storage area for spare scenery, or as an access way for your cast…both of which throw shadows on the screen. You also need a suitable fabric for the screen. Commercially produced screens for back projection are usually covered with a sort of translucent plastic, available in a variety of shades from white to dark grey. This material isn’t too easy to come by… but the big snag is that the screen needs to be framed and stretched really tight. This implies a frame with good structural strength along its sides and no support members across the area of the screen (otherwise you’ll get shadows).
Commercially produced screens are usually aluminium framed. If you can contend with these snags then go for back projection every time.
This is because there are similar difficulties with front projection. Although the screen problems go away (any reasonably flat white surface will surfice, and I’ve even done digital projection onto black masking flats perfectly successfully!) the shadows snag is a major hassle… the cast will constantly be a problem passing in front of the screen…. and the siting of the projector itself is frequently more difficult. For a conventional centre stage image the projector really needs to be flown on the centre line at a suitable throw distance from the screen. If you can get the projector high enough up to come down at the screen at an angle you will reduce the area of the stage where passing actors get in the way. Fortunately most digital projectors will correct for the paralax error created. Many of them call this ‘keystoning’. (few digital projectors will correct for left-right paralax) This keystoning is the effect where the image is distorted to be  bigger at the bottom (or top) of the screen due to a longer throw where the beam is angled up or down. If you run into this problem with slide projection you really need to create paralax corrected slides by shooting a print of the image from a position which is the same angle off centre as you intend to project from. A few digital projectors are designed for very short throw, which is a god send in reducing the stage area affected by the beam.
The intenstity (brightness) of the image is a consideration in both forms of projection. The lamp for a slide projector is rated in watts. The output of a digital projector is shown in ANSI Lumens. This is of course purely to make it impossible for any of us to understand what it means. As a guide we use digital projection in the order of a couple of thousand ANSI Lumens, and most of our work is done on small stages. We get perfectly good results on image sizes up to about 15ft across (The inverse square law applies here as it does with all lighting, ie. double the size of image and you divide the intensity by 4) even against stage lighting. We have met a really bad situation where we were asked to project a video in a marquee in daylight. We managed this eventually with some major masking out of the surrounding area, but let me tell you that a white, unlined marquee at midday on the brightest sunny day of the year is not an ideal location for running a projected video!
In terms of size of image the mathematicians among you might want to use the formula X=D*(2*tan(f/2)) where d is the horizontal distance from the projector to the screen and f is the beam angle. Alternatively you could just try the projector on site to determine the best position!