Focusing fixtures for an even overhead wash

The slinky method proposed by Steve Shelley in A Practical Guide to Stage Lighting states:

Most theatrical lighting instruments are optically designed in such a way that the degree spread of the beam angle is almost always at least 50% of the overall field angle. This can be expressed in an equation known as the slinky formula:

field angle/2 = beam angle

To provide proper overlap between two lights and achieve a smooth blend, the edge of the second beam should land in the middle of the first beam

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The principle he advocates for achieving a smooth blend seems to be contradicted by his ground plan in Fig. 6.9 & 6.10 (for those of you who have the book), which shows that it is the edge of the field that should land in the middle of the adjacent beam. Assuming the fixture's photometrics adhere to the slinky formula, this results in the beams overlapping marginally. So firstly, which is it? Is it the beam or field edge that needs to bisect the adjacent field/beam?

Secondly, does the slinky formula still hold true, 20 years after the book was published? Having only been in the industry for a few years, I haven't noticed a change in field to beam ratio, but my understanding is that most modern fixtures are generally able to achieve a beam angle greater than half the field angle.

How is this field to beam ratio affected by focus? Does an ERS maintain the same ratio across its focus range? I.e. is the ratio the same for a sharp focus vs unfocused (softened edges)?

My takeaway from the book is that I don't need to be too concerned with the field overlap but just ensure that the beams themselves are at least slightly overlapping. Interested to hear what methods other people to take when it comes to focusing fixtures for an even wash.

ececec said:

Secondly, does the slinky formula still hold true, 20 years after the book was published?

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The short answer to this is no, it doesn't really hold true any more. Conventional incandescent lights have been able to produce a much more even field of light more and more as years have passed, and one of the things that manufactures push about LEDs is how bright and even the field is. It is still worth knowing this formula though depending on the age of your system, and how good of a bench focus you can get from your lights.

aeh20s said:

The short answer to this is no, it doesn't really hold true any more. Conventional incandescent lights have been able to produce a much more even field of light more and more as years have passed, and one of the things that manufactures push about LEDs is how bright and even the field is. It is still worth knowing this formula though depending on the age of your system, and how good of a bench focus you can get from your lights.

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Thank you.

Doesn't "unfocusing" the fixture increase the field while decreasing the beam? Does the technique of having the beam edge bisecting the center of the adjacent beam still hold true?

It depends on the fixture really for how much it rings true. There is a chance that it will work exactly as written, there is a chance that you'll overlap the fixtures and find that the intensity where they overlap is double that of the light 2 ft next to it.

With so much variance, then how does one go about choosing fixtures, beam angle, distance between fixtures, etc, when designing the plot? I was hoping for a general rule or mathematical methodology I could follow which would yield consistent results. I thought I had struck gold with the slinky method, so slightly disappointing to learn it doesn't really apply anymore.

The reality of the situation is that you'll most likely be working with the same units and/or in the same space fairly frequently. You'll hopefully get to know the ins and outs of the fixtures and be able to do the best you can with what you can get. That is where your consistent results comes from. Your fixture choice in relation to beam angle and distance between fixtures will also stay consistent. But every person who has done lights for more than a few shows will understand that you can lay out your entire plot the way they explain it to you in a textbook, and you'll always have to account for things you weren't expecting. Set pieces, building steel that wasn't indicated in the drawings you got, your last 15' jumper was actually only 11' and you have to slide your fixture over and hope it mostly covers what you wanted it to. That's just the way it goes.

aeh20s said:

The reality of the situation is that you'll most likely be working with the same units and/or in the same space fairly frequently. You'll hopefully get to know the ins and outs of the fixtures and be able to do the best you can with what you can get. That is where your consistent results comes from.

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I see. Sounds like good ol' trial and error. Is there, however, a sensible starting point I can use for overlap before I gain enough experience to space my fixtures instinctively? For example, overlap by 50% as the slinky method suggests?

Do you know what type of fixtures you're using?

One thing that smoothed out our LED top lights was the Holographic lens material from ETC. Looks like diffusion gel, but you don't get the light loss.
All I can say when I walk across the stage looking at the back of hand meter is WOW.

aeh20s said:

Do you know what type of fixtures you're using?

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Most likely fresnels or the Source Four Par. Am I right in thinking that fixtures with a soft beam (such as pars) require more overlap than fixtures with a sharper beam (e.g. ERS)? I think it would be beneficial to understand the relationship between appropriate pool overlap, beam angle, and field angle, even if I can only rely on it approximately.

As an example:

Let's say I'm able to achieve a very even wash on my stage by spacing Fresnel fixtures such that there is a 50% overlap between their pools. Does it stand to reason that an ERS with a soft focus will need less overlap to achieve the same wash, by virtue of the fact that its beam angle to field angle ratio is likely to be less than that of a fixture like a Fresnel?

I think establishing the relationship will be very helpful.

For a top wash I would use the Source Four Pars. With soft beam lights you tend to need very minimal overlap with the fields. You can usually get away with just touching them. An ERS is what usually needs more overlap. And to help you understand how many lights you need per pipe and their spacing, the best thing you can do is hang one on a pipe and see what it does, then extrapolate from there.

aeh20s said:

With soft beam lights you tend to need very minimal overlap with the fields.

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Interesting, I would have thought it would be the opposite. Surely for a field that dims fairly consistently from the focus point to the field edge, a minimally overlapping pool would create significant intensity dips between the focus points. Whereas the "fade away" portion of the beam on a softly focused ERS will typically be narrower, therefore not requiring as great an overlap. Where am I going wrong?

aeh20s said:

And to help you understand how many lights you need per pipe and their spacing, the best thing you can do is hang one on a pipe and see what it does, then extrapolate from there.

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In an ideal world, I'd agree. However, I have no access to the venue or to the fixtures, which is why I'm trying to understand the "science" so I can put together at least an approximate plot.

ececec said:

In an ideal world, I'd agree. However, I have no access to the venue or to the fixtures, which is why I'm trying to understand the "science" so I can put together at least an approximate plot.

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So, this makes it sound like you are designing lights for a show without ever having worked in technical theater before. Do you have info on the space? Grid height, lighting positions, drawings of the space, an inventory of the light fixtures, etc.

aeh20s said:

So, this makes it sound like you are designing lights for a show without ever having worked in technical theater before. Do you have info on the space? Grid height, lighting positions, drawings of the space, an inventory of the light fixtures, etc.

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Not quite. I've been asked to light a conference at a venue I haven't worked at before. My general approach is to eyeball it on the day, depending on fixture availability, lighting positions, etc. I'm usually able to rely on intuition and experience to achieve a suitable wash. Since I actually have the venue parameters this time, I'm want to attempt a light plot. Probably unnecessary for this job, but more for my own learning. However, this question isn't about this particular case. I want to delve into the theory and develop a (reliable?) system I can apply to such jobs. It doesn't have to be perfect, but at least give me a suitable starting point from which I can tweak during focus.

My tech theater experience is mostly in rigging, and I have only a fairly elementary knowledge of lighting. At least enough to take on such jobs.

I used to have hard and fast rules when I was working in similar spaces with similar equipment.
When I started doing corporate work, every venue or venue group would purchase different LED fixtures that were either well suited or completely unsuitable. Not much in between.
So I resorted to just "doing my best."

If you're trying to come up with a plan without seeing the space or knowing the exact fixture model, just overspec.
Is the stage 10x10'? Add a few percent more than you usually would.
If the stage is 40x75, double what you'd usually design.

Then when you get into the space, you can open the roadcases and see if the "professional LED pars" are actually way too wide a wash to use as toplight from 28' trim, but you set wash are non-lensed LED cyclights and might work better, so swap the fixtures.
This example is one I came across a few times. Colorado ColorOne was "normal" as toplight and ColorForce 12s were "normal" for set wash. In reality the Colorforces threw way further and made much more sense as back/toplight.

Is there a reason that you are using approximations and assumptions about fixture beam profiles instead of building your plot based on the published photometric data from the fixture manufacturer? Field angle is defined as the angle to 10% ("full width 10% max"), while beam angle is defined as the angle to 50% ("full width half max"). So for maximum uniformity, you design around the beam angle and ignore the field angle.

Assuming the fixtures all have the same beam profile, and that profile is symmetric, spacing your downlights so that the cones defined by the beam angles just touch will give you the best possible uniformity along the line directly under the fixtures. To move this into more dimensions, you are simply packing the cones into the area you are trying to light (and here you will find that you can only make this work for a single plane). Basic geometry and trigonometry.

kluge_meister said:

Is there a reason that you are using approximations and assumptions about fixture beam profiles instead of building your plot based on the published photometric data from the fixture manufacturer? Field angle is defined as the angle to 10% ("full width 10% max"), while beam angle is defined as the angle to 50% ("full width half max"). So for maximum uniformity, you design around the beam angle and ignore the field angle.

Assuming the fixtures all have the same beam profile, and that profile is symmetric, spacing your downlights so that the cones defined by the beam angles just touch will give you the best possible uniformity along the line directly under the fixtures. To move this into more dimensions, you are simply packing the cones into the area you are trying to light (and here you will find that you can only make this work for a single plane). Basic geometry and trigonometry.

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Right, that's exactly what I'm trying to understand! For a fixture's beam and field angle, what is the required overlap for an even wash. Having the beam pools just touching, as you suggest, was what I took away from A Practical Guide to Stage Lighting.

Remember that beam angles touching at the floor means gaps at head height - Traditionally, you'd plot the beam pools touching at 5' or 6' off the ground, to avoid dark spots on faces. This does mean that you'll end up with bright spots on the floor.