Engineering
The lens is the fixture. Everything else — housing, yoke, electronics — exists to hold it in the right place and keep it cool.
A plano-convex lens of the diameter a stage fixture needs would be enormously thick, enormously heavy and would crack under thermal shock. The fresnel solution, from 1822 and originally for lighthouses, is to keep only the curved surface and collapse the rest into concentric rings.
Each ring is a section of the original curve. Between them, the risers do nothing useful optically — they are dead geometry, and how they are angled decides whether the beam has visible rings in it.
If a riser is cut perpendicular to the lens face, light striking it scatters and you get a faint bright ring in the field. Angle each riser to point at the source instead and that scatter mostly disappears. The difficulty is that the correct angle differs for every ring, and it changes as the lamp carriage moves — so the design is optimised for a beam position around two-thirds towards flood, where fresnels spend most of their working life.
A stippled or lightly frosted outer face softens the beam edge further and hides small imperfections. Too much stipple and efficiency falls; too little and the edge hardens until the fixture stops blending with its neighbour, which is the entire point of a fresnel.
We settle this by eye, on a bench, with two fixtures side by side and a white cloth at nine metres. There is a numerical version of the test. The eye is faster and, on this specific question, more reliable.
Behind the source sits a spherical reflector that returns backward-going light through the filament or LED array and out to the lens. Its radius has to place the reflected image exactly on the source. Ten millimetres out and you get a doughnut in the middle of the beam — the fault most often reported to us as "the lamp is failing" when in fact somebody has refitted a reflector the wrong way round.
Cyc units use an asymmetric reflector instead: a compound curve that sends roughly seventy percent of output downward. Nothing about the shape is intuitive and it took eleven prototypes in 1996; it has changed twice since.
| Fixed barrel | Zoom barrel | |
|---|---|---|
| Edge quality | Sharper at its design angle | Very good, slightly soft at extremes |
| Efficiency | Reference | About 8% lower |
| Weight | Lighter | 1.5–1.8 kg heavier |
| Flexibility | None — the angle is the angle | Re-riggable without a new barrel |
| Best for | Repertory houses with fixed positions | Touring and hire stock |
Five emitters rather than three exist for one reason: the gaps. An RGB fixture set to a warm white leaves a hole in the spectrum where amber should be, and skin tones fall into it. Adding amber and lime fills the two worst gaps, which is why our TM-30 fidelity figures hold up under a camera rather than only under an eye.
We publish measured spectra for every fixture rather than a marketing CRI number. A single CRI figure hides exactly the failures that matter on camera.
An LED array that runs ten degrees hotter loses output over its life much faster than the data sheet suggests. Our housings are convection-cooled with no fan in any fixture below 400 W — fans fail, fans get blocked with haze fluid residue, and a fan in a quiet auditorium is the noise the audience hears during the pause.