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March 2004
Getting the Most From Your Fixtures
By Stephen Ellison
Let's start out with a quick review of the available types of fixtures. There are only 2 basic types of fixtures, the wash and the spot. All fixtures can be described as a source of illumination (lamp), housed in a metal container with a reflector behind the lamp and a yoke to hang the enclosure from. Most fixtures also have at least one lens, and some have two. Beyond that, you begin to get into the bells and whistles of fancier fixtures.
This simplification also applies to the "intelligent" fixtures. The difference boils down to whether you can project a hard edged image- like a slide projector, or if the fixture can only produce a soft edged shape.
The type of fixture is based on simple geometry, really. There are only so many shapes that can be used as a reflector. If we go back to our tenth grade geometry class, we are reminded of several three-dimensional shapes; the sphere, the ellipse, and the parabola. Wash fixtures use either a portion of the sphere or the parabola; or in the case of a cyc fixture, a mixed geometry reflector. The spot fixtures use the ellipse for their reflector. The choice of reflector influences how much light is sent out the front and not wasted. A look at a drawing of each reflector illustrates how efficient they are.
People have been using reflectors for a very long time to capture light and aim it in the direction they want it to shine. I am not sure when it was that someone really put some thought into creating the very best reflector, but with the advent of electric light, the theatre industry started to come out of the dark, so to speak.
The spherical reflector is the least efficient, and allows light to bounce around inside the fixture and never reach the stage. The most common fixture that uses this reflector is the fresnel. In this case, the lamp and reflector are mounted on a sled that can be moved in relation to the lens, which is fixed. By moving the sled, you adjust the size of the circle of light produced, but it always has a soft edge.
The parabolic reflector does a good job of capturing all of the available light and sending it out the front of the fixture. An old fixture, the Beam Projector, was the main use of this reflector. Nowadays it is more commonly found in Parcans.
The Parcan utilizes a lamp module which includes the reflector, lamp and lens all in one assembly, designated a PAR xx. The xx represents the diameter of the lens in 1/8th of an inch, so a common PAR 64 is 8" wide. In order to change the size of the field of light produced, you change the complete lamp module. They come in four standard beam spreads, very narrow, narrow, medium, and wide. In the newer fixtures that are taking over the market, all four lens types are provided and you only have to change the lens.
The problem with both the spherical and parabolic reflector is the ability to focus the light with a lens. All of the fixtures that use these reflectors create soft edge beams of light. Conversely, the infamous Batman symbol being projected into the night sky by a searchlight is impossible.
Another name for a searchlight is a beam projector, with a really big lamp. If you look at the drawing; (Figure 1- print version) the parabolic reflector directs all of the rays into a parallel pattern. It's great to create a shaft of light, but it's not very good for projections.
The Ellipsoidal reflector is different. Not only does it capture all of the light, it also focuses the light through a point. If we step back to geometry and look at how an ellipse is made, an ellipse is created from two focal points. The center of the lamp is placed at the first foci and the light is directed through the other foci. By adding a lens system in front, the light can be focused one more time and projected onto the stage. In an Ellipsoidal Reflector Spotlight, the image to be projected is placed just past the second foci. At this point, there is a plate that blocks any extraneous light rays. This plate is the round circle that is seen when there is no other image. This point in the fixture is also where the shutters are located.
It all leads up to this part of the article: How to get the most from your fixtures. In fixtures that have the first two types of reflectors I mentioned, the lamp alignment to the reflector is not adjustable. This is not the case with an ellipsoidal fixture. I was doing the lighting for a play in a church that I was visiting, using the fixtures that they had bought two years previously. I turned on the first one to start focusing it, and noticed that the output was dim, so I pulled out my screwdriver and proceeded to bench focus the fixture. After I finished, the church techies wanted to know how I got so much light out of it. The answer- the manufacturer does not adjust the lamp to reflector relationship, because you have to align the lamp to the reflector each time you install a lamp.
Figure 2- print version is a drawing of an exploded view of the Lamp Socket Assembly from a 360Q series Ellipsoidal manufactured by Altman Lighting. These fixtures dominated the lighting industry for over 20 years, and were the exact type of fixture in my story. In the exploded view, you see the lamp socket is mounted to a plate, and that plate is held in place by 3 screws (only 2 are shown). In order to keep the mounting plate steady, a fourth screw is used to press on the plate and then it is held in place by two nuts. This arrangement allows the user to adjust the plate in all three directions, which enables you to position the center of the lamp at the exact focal point.
Every time you install a lamp, this alignment must be checked. As easy as this process sounds, it is time consuming and not easily accomplished when the unit is hanging in the air.
The modern fixtures have simplified this operation, by reducing the components you have to adjust down to two: putting in a knob for moving the lamp in and out, and including a ring around the knob for x and y adjustment.
If you have never tried to do this, then I would recommend experimenting. Take a fixture down so it is easy to see all of the components. Lay it on a table and aim it at a wall, then turn it on. By moving the lamp around, you will be able to see the hot spot move around. As you move the lamp in and out, the hot spot can be increased or decreased. The objective is for the hot spot to be in the middle, and to have a flat field.
If you do not take the time to do this when you first buy the fixture, there is a very good chance that you will not be getting the maximum output.
Another problem with poorly aligned lamps is when the hot spot is focused at the gel. A medium blue gel in that fixture will have a hole burned in it in a matter of hours.
The last tip I have for now is to periodically take your fixtures down and clean them. The lenses will develop a layer of dust over time, and because it is gradual you may not notice the light output becoming less. This also applies to the reflectors. A little time spent cleaning will go a long way extending the life of the fixtures. While cleaning, check the lamp sockets for pitting, the connectors for signs of overheating, and any other component for signs of wear that may need to be replaced. Time invested on a Saturday will reward you with relaxed Sundays. It's also a good time for the tech crew to all get together and fellowship in a non-stressful environment.
If you have any questions or would like more clarification on your specific requirements, please send me an email,lighting@tfwm.com, I am always looking for questions to include in this column. Feel free to contact me.
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