Light Reflection and the Angle Of Incidence
The title sounds like an action movie but we are looking at some lighting theory of reflection, the angle of incidence and interaction of light with different surfaces. Plus the comforting constancy of physics and being able to predict what happens to a stage lighting beam once you’ve let it fly.
Last week, a Project Manager sent me an email asking for my thoughts on forthcoming plans to use white PVC covered flats to back an award show, rather than using their usual finish material of felt. The question was specifically asking about successfully uplighting the shiny PVC from the floor using LED battens to create a changing coloured background. A pretty standard technique for this kind of show, making a pleasing backing for things to happen in front of.
The set designers of today, whether creating environments for the TV studio, award shows or exhibition stands, have access to a wide range of materials. As well as traditional wood, paint and cloth finishes, they love to use polymers and glass. Perspex plinths, PVC wall coverings and glass table tops all have one thing in common. They give the lighting designer a headache.
Let’s take a look at the basic physics behind the problem.
What happens when you light a diffusing material like felt?
A felt covered flat or a Cyclorama has a diffusing surface that scatters light in many directions because at a micro level it’s, well, bumpy. A light source skimming across the surface is reflected back in many directions, including “out front” to the viewer. In fact, the useful viewing angle of reflection for a diffuse material is pretty wide meaning the effect works for the audience, wherever they are. The light reflects of the side of the bumps and out to the eye. Not having a really obvious nap or grain (like velour or suede) such a diffuse surface is pretty forgiving in terms of both lighting and viewing angles. It has a similar look from everywhere.
The light that reflects from a surface in this way is called, surprise surprise, Diffuse Reflection and is instrumental in the perception of reflected colour.
Because light spends some of it’s time pinging around on and shooting off and different angles , the lit surface also appears relatively even, with quite a lot of the original light doing it’s thing for everyone. Hooray for felt and other fluffy surfaces.
How Shiny Surfaces Take Light
By contrast, high gloss surfaces such as shiny PVC based fabrics, are very flat at a micro level. Less bumps equals less diffusion . Shiny surfaces seem like they should reflect light well, they do indeed reflect it well. So well in fact, that any poor photon unfortunate to hit this material is instantly twanged off again in a reasonably straight line based on it’s originating source. This kind of reflection, the window shape reflected in the cauldron of Meg and Mog or a cartoon bomb, is specular reflection.
Surfaces have an element of both diffuse colour and specularity about them, including our white PVC.
So how do we predict the actual reflective behaviour of shiny things, including what the specularity will look like and where the dreaded, ghost like reflections will appear.
Bring on the angle of incidence.
The Angle Of Incidence and Light Reflection
In lighting, the angle of incidence (x in the image) is measured between the angle a light source strikes a surface and 90 degree vector to that surface (called the surface Normal, if you are into 3D CGI graphics you’ll know that). The geometry tells us that using a theoretical perfect surface, the angle of reflection opposes the angle of incidence. A lighting angle of 30 degrees will bounce back of at 30 degrees, the other side of the surface normal. This knowledge can also be used to calculate the set up of mirror effects, including Pepper’s Ghost.
This geometry is actually what makes a shiny surface less diffusing than a bumpy one. The surface normals of material like felt are actually all over the place, even though the flat is.. well, flat.
Back to Uplighting PVC
OK, so we know why skimming light across a diffuse surface works for us and what happens to the reflected light because of the relationship between the angle of incidence and the surface normals.
Lighting a shiny surface from a “straight on” angle, the small source is reflected in high def back in a similar direction toward the viewer, meaning they can see a reflected image comprising of every light source in the entire rig. Frankly, a bit of a visual mess. While the area of the source reflected is extremely bright, the rest of the flat doesn’t diffuse enough to create that pleasing coloured background we are after. Using RGB colour mixing LED fixtures, the individual LED light sources are not given a change to mix on the surface as they rely on diffuse reflection for this to happen successfully.
Uplighting and skimming the light at a shallow angle across the surface, the viewer “out front” doesn’t get to see the source reflected because of the angle they are viewing from – not the opposing to the angle to incidence. Well, not unless they are clinging to the ceiling , the place which ends up being the brightest part of any low room due to all the lighting flying up there and diffusing it’s little butt off.
Lighting a shiny surface in this way, a large proportion of the light is wasted on things other than the viewer. So all in all, both physics and experience tells us that uplighting PVC flats with RGB colour mixing LED fixtures doesn’t work as we might hope.
Another dimension is added when dealing with a surfaces that diffuse qualities that vary in different directions, such as brushed aluminium. The stripey bumpiness diffuses light and colour well in one direction while light from another angle of incidence is reflected in a specular way.
This is called Anisotropic Specular Reflection and is what gives these brushed surfaces their characteristic look.
Other shiny materials and the reflection of light
Other materials popular in the conference and exhibition industry may not all be as shiny as our PVC flats, but can exhibit reflective behaviour that leads to questions such as “Why is that graphic brighter than that one? “ or “What can you do with the lighting so that we don’t get that bright spot?”
Foamex boards, vinyl graphics and even polystyrene lettering all have their own level of specular reflection, though they appear reasonably matt. The result can appear to be a discrepancy in intensity or a particular highlight, which of course alters depending on where you look from.
Perspex lecterns and glass coffee table are capable of pinging your well aimed front light all over the backing, screens and even back into the camera lens. Ranks of chrome and glass awards trophies cause havoc by muck spreading light in all kinds of unwanted places.
What can we do?
Understanding the different behaviours of specular and diffuse reflection, the angle of incidence and how colour mixing fixtures create their light on the surface, we are armed to make design choices about lighting positioning and angles. We might even be able to make helpful suggestions about set materials while in discussion with the production designer and the rest of the creative team.
But the chance to take preemptive action, like advising against the merit of white PVC flats for LED uplighting, might not always present itself. The important thing to remember is how light behaves once we release it and that we work with a medium that, unlike smoke, is basically predictable.
As predictable as the likelihood that, when those truck doors open on another day, white PVC flats will be glinting back at you.
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Stage Lighting Terms – A Guide to Lampie Slang
On Stage Lighting presents an essential guide to stage lighting jargon and UK lampie slang for anyone who needs to understand what these weirdos are talking about or needs to blag it as a member the LX department.
Every so often, lighting technicians have to talk to people in the real world (or lesser mortals like sound engineers). It soon becomes apparent that, as in every corner of life, there is a certain amount jargon. Lighting terms, slang and other gobbledygook that techies use when communicating with each other, to the exclusion of the outside world.
While a full glossary of stage lighting terms is outside of our scope today, here is the Blagger’s Guide to Lighting Jargon.
Some of the terms are lighting specific, others related to stage work in general and are based on the garbled utterings of UK based lighting techs.
Directional Terms
To an outsider, one of the most peculiar things about stage workers is their sense of direction. On stage, normal directional communication gets mangled. Any one who remembers acting in a school play will know that their drama teacher kept talking about “Stage Left” and “Stage Right. Drama teachers themselves often get this muddled up, so let’s be clear: Stage Left is your left hand side if you are standing on stage, looking out at the audience. This is the opposite of House Left (from the audiences point of view) or Camera Left (er, from the camera).
It doesn’t get any easier, though. Stagey people say Up when they mean backwards and In when they mean down. What’s that about?
Up and Down Stage are further and nearer to the audience respectively, relating to historically sloping stage floors. If something “runs up and down” it is not a fitness crazed techie (does anyone know any, BTW), it is something that has its long axis sitting at ninety degrees to the stage front. Up and down stage.
Conversely, it might “run on and off” (Onstage being nearer the centre, Offstage nearer the sides) with its long axis parallel to the stage front.
So having used up the terms Up and Down to mean something else, what are we going to do when we really mean up in the air or down toward the deck? Why, use Out (up / away) and In (down toward the stage floor) of course.
Let me be the first to admit publicly that my tiny brain uses these directional concepts by default, even when absolutely nowhere near a stage. Be warned of this before you try to move a sofa at home with your nearest and dearest techie.
Endless Numbers
Many of the different technical disciplines use numbers but the lighting department seem to barely use anything else. If you think that the world of business uses too much jargon, you should listen to two lampies outlining their plans for the next five minutes.
The trouble with deciphering the numbers is that they are used in a wide range of contexts and that context is only clear if you know what the numbers mean. Otherwise, they are just numbers. Let’s start by looking at what the numbers represent
Quantity and Capacity
Somewhere in the number jumble, there may terms that reference the quantity of items or the capacity of an item. Examples include five [insert another bit of jargon here] or a 6-way [bit of jargon] or a [bit of jargon] to 12 [other bits of jargon]. Also included in the capacity equation are references to power draw (1000 watt or 1k), current capacity or electrical rating (16 or 16 amp rating with appropriate connector type).
A final confusion factor is added to distinguish between equipment that is designed for use with 3 phase or 1 phase power. The term “3” refers to three phase power and thankfully the word “single” is inserted by thoughtful jargon spewers to reduce the shear quantity of integers in any one sentence.
Length
Lighting cables are not only denoted by their electrical rating and connector but also their usable length. The length of rigging kit is also something we might need to talk about. Rummage around in a lampie sentence and you will probably find a reference to the length of something, like 10 metres. 2 10s references two of something, ten meters in length.
So far, we have got the potential for a sentence to contain number that denote quantity, capacity, connector type, phase number. That just describes a piece of kit.
3 63 3s 2 3 63 singles refers to a piece of common electrical distribution kit: Three (of) [sixty three amp, three phase] plug to three (of) [sixty three amp, single phase] socket.
6 16 2 15, 3 10 metre 15 and 2 2 way 16s actually translates to : Six (of) [sixteen amp] plug to [15 amp] socket, three (of) ten metre fifteen amp cable and two (of) sixteen amp two way splitters.
Notice the use of the word “metre” to split up the length and the connector rating. Hey, anything else would just be confusing, right?
Manufacturer Numbers
Rather unhelpfully since the dawn of production lighting manufacture, the makers of stage lighting equipment have followed naming conventions for their various products that rely heavily on numbers. Lanterns produced by Strand Electric went from pattern numbers like 23, 264 and 243 on to ranges with real names that the subsets could only be described by their beam angles such as 16/30 or 11/26 ( 16 to 30 degrees and 11 to 26 degrees).
Modern lighting equipment manufacturers have continued on this unimaginative folly by mainly referencing fixture power such as 700, 1200 and 2000s. Fixtures made by different manufacturers get called the same thing, for instance, the Martin MAC700 profile and the Robe ColourSpot 700 both get called “seven hundreds”. And so it goes on.
4 700s on 3 at 200 translates to: Four (of) [some kind of 700] on bar three, starting at DMX address two hundred.
Reference Numbers
With lots of different parts to a lighting system, we need to be able to distinguish elements within it. In order to stop others from knowing what we are up to (other than skulking in Dimmer City) lampies have devised a cunning and impenetrable method of marking different control channels, dimmers, fixture addresses, intensity levels, gel references and circuit numbers. Yes, you’ve guessed it – more numbers.
The context that these numbers are used in is not so definable. They may be thrown into the middle of a sentence, their presence only discernible by the fact that the integer in question does not relate to either quantity, electrical capacity, length or manufacturer imposed number.
Putting it all together
By now you either knew all this already or are utterly confused. How do you work out which numbers relate to what? Here is some general guidance for cracking the number code:
Jumpers (not the woolly kind) that jump from one electrical connector into another are always referred to plug first. A 16 2 15 (often written 16 > 15 or 16 – 15) is a sixteen amp plug to a fifteen amp socket. Confusingly, in DMX cabling the number of pins takes over from the rating: a 5 to 3 is a 5 pin XLR plug to a 3 pin XLR socket.
Working out what a number it relates to is sometimes a question of knowing what it cannot relate to:
The number 101 is not one that relates to a fixture type, beam angle or common power capacity so it could either be a dimmer / control channel but chances are it relates to Lee 101 – Yellow, a vile yellow gel.
The numbers 16, 32 or 63 all relate to common UK power ratings and connectors but could also relate to anything else. You see how it’s all a question of context.
So, now you have started to get to grips with lampie grammar, let’s take a look at some other lighting terms and slang.
Lampie Rhyming Slang
The UK lighting business follows in the Cockney Rhyming Slang tradition with it’s own version of the language. In true Cockney fashion, the best proponents of this rhyming slang usually only refer to the first word of the term:
- Billys = Billy Bunters = Punters = The General Public
- Dianas = Diana Doors = When the Billys are let into the venue
- Desperate = Desperate Dan = Lighting Plan
A particularly inventive use of rhyming slang is the phrase “Onions” which is a contraction of “Onion Bhajis” – it rhymes with Zarges, the popular brand of combination ladder.
Trade names
Every industry uses trade names in their own unique language, stage lighting is no exception. Here are a few of the most common:
Spanset – Technically known as a roundsling, the Spanset, Spanny or Spanz is an extremely strong webbing covered loop using for rigging and slinging.
Grelco – Common in the UK, the word Grelco refers to the specific type of plastic 15 amp 2-way splitter which this brand used to dominate. Younger lampies have been known to call them Snappers, after a more modern trade name. The 3-way version of the Grelco is inventively referred to as a Trelco or Trellies, in case you were wondering why Dave said he needed some trellis but does not seem interested in gardening.
Socapex – The popular brand of multipin connector used for dimmer and power circuits, “Soca” has become the lampie slang term for the entire cable as well. Other terms include Multi or Snake and different connectors are referred to by their dominant brand – Lectriflex, for example.
People
In the stage world, it seem like everyone has some form of derogatory term for those who dare to work in other departments. Some used by us “lampies” include “Noise Boys” and “Hum Scum” for the our arch enemies in the sound department and “Vidiot” for anyone involved with screens, tellies and cameras.
Electricians are unsurprisingly called Sparks or Sparkies while Chippies (carpenters) are also known by other phrases, depending on the level of abuse intended. “Wood Butchers” right through to “Hairy Ar*ed Nail Benders”. In the world of trades shows, Rug Tuggers are in charge of all flooring requirements throughout the hall.
For some reason, Riggers are often known by the most derogatory monikers but only from a distance. They are generally too scary to insult in person but the term “Nappy Wearing Sky Gods” cannot be heard above the noise of a cherry picker at 15 metres. “Nappy” refers a fall arrest harness and not to the results of a particularly vibrant curry from the night before. Watch out though, riggers have a special spy called the Groundman who lurks around on deck and reports insulting language to the roof via Motorola radios. They can also drop things on you.
Animals
In the stage lighting world, you don’t get too close real animals unless you happen to be on the electrics crew for a “Horse Of The Year” show (yes, I have). To feel closer to nature, lampies occasionally use animal based jargon including:
Spiders – a many legged, cabley thing that breaks circuits out of a mulitcore. If you are feeling more rock n roll, you can call them Fan In and Fan Outs instead.
Badgers Clamps – A scaffold half coupler with an eyebolt for suspending barrel.
Pigtail – Any kind of short cable coming of something such as the power lead of a moving light or a flying lead from a breakout box.
Descriptive Lighting Terms
Sometimes, the best slang is based on an obvious likeness of something to something totally unrelated or a consequence of their presence. This is the case with:
Pickle – a pickle shaped hand held switch used to locally control electric chain hoists AKA the motors.
Ray Guns – Plastic 16a Ceeform 2-way splitter that looks like – well, a gun. “Sod it, Nev. Just gun ‘em together, I can’t be bothered to run another 50 metre 16 to this one as well”.
Trees – Plastic 16a Ceeform 3-way splitter of tree like appearance.
Binoculars – More 16a Ceeform 3-ways that look…. Well, you’re getting the hang of it now.
Tripe – A bundle of cables, taped together for easier management that ends up looking like tripe. Another term is Loom.
String – Electrical, referring to cable including the largest of heavy mains supplies.
Shin Busters – Low mounted fixtures that trip everyone up, specifically the monitor engineer (if you get it right)
Acronyms
While the financial and business sector have the monopoly on outrageous acronyms, stage lighting jargon has it’s fair share. We won’t go into all of them, but here a selection:
- AJ – Adjustable spanner, a UK term.
- DBO – Dead Black Out. Everything dark.
- TRS – Tough Rubber Sheath. A reference to the insulation on mains and dimmer cables . Used to denote a single channel of cable.
- RCD – Residual Current Device. Safety switchgear that protects the user.
- MCB – Miniature Circuit Breaker. Safety switchgear that protects the equipment.
- FOH – Front Of House. Anywhere forward of the stage line or where the general public hang out. Also used when leaving – F*** Off Home.
- FUCT- Acronym for various words including Failed Under Continuous Testing. Used to mark up faulty equipment
Unimpressed by Technology
In order for technical types to real feel that they are the masters of all the bells and whistles they survey, they resort to derogatory names for posh kit. This can be found in other departments such as the video where they habitually refer to the snazziest of 90” Plasma screens as Tellies.
In the lighting world, this practice is evident by terms such as Nodding Buckets or Wobblies for moving lights and Waggly Mirrors for scanners.
