Tungsten Fresnels

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You might not think of them this way, but tungsten Fresnel lights are complicated little buggers.

They are the workhorses of the movie business. We use them all the time but never give their mechanics or design much thought, because… they just work. They aren’t perfect for many things but are close enough for almost everything. They have some drawbacks – heat, power, weight – but they are also one of the best possible light sources. Not only do they reproduce a continuous spectrum but they have an appealing ‘soft’ quality.  If you do any post production color you know this.

There are three basic elements to a Fresnel fixture: the lens, the globe, and the reflector behind the globe. The globe and the reflector are mounted at a fixed distance from each other on a sled inside the housing. The distance between the sled and lens is variable and determines whether the light is in ‘flood’ (closer to the lens) or ‘spot’ (farther from the lens) mode. In general use, Fresnel lights are most often used in flood mode, spot being used mostly for aiming. Usually. For good shadows we want to be in flood mode.

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The Globe
As far as Fresnel fixtures are concerned, the globe itself is not a major variable since the light is doubled, reflected and then refracted and collimated through the lens. There is an argument sometimes made that different manufacturers produce globes in which the filament is slightly more compact (more like a point source) and therefore better for shadows. Given all the other factors involved, I don’t think the difference is significant. The more important factors are the lens and the reflector.

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(https://en.wikipedia.org/wiki/File:Fresnel_lens.svg)

Fresnel Lens
Fresnel lenses were originally developed by  Augustin-Jean Fresnel in 1823 for lighthouses – to minimize the bulk of lenses required to throw a collimated beam a long distance. A Fresnel lens is essentially a reduction of a plano convex or concave lens with all glass removed that does not contribute to refraction. The removal of the mass makes it lighter and thinner than a conventional lens but also reduces some of the sharpness, which contributes to the ‘soft’ look of Fresnels (there are multiple factors involved in that but we’ll confine this discussion to the practical effects). And if you’ve ever opened a Fresnel fixture and looked at the reverse side of the lens you’ve noticed that it is stippled. This also diffuses and flattens the light entering the lens. There seems to be little difference in the amount of stippling between various lenses in different fixtures – they all look about the same.

According to Wikipedia, a “Fresnel [lens] reduces the amount of material required compared to a conventional lens by dividing the lens into a set of concentric annular sections.” That is to say, a Fresnel lens is derived from a conventional lens and will have the same properties (more or less) as the lens it is derived fromLacking design specifics – which the manufacturers are unlikely to make available – we can only test for performance. We also have to consider that whatever performance characteristics we find in a light fixture are probably not solely the result of individual components but more likely a result of the of the relationships between them.

A Fresnel lens derived from a plano convex lens will act as a collimator (used grooves out, as Fresnel fixtures do). This means that ideally the light leaves the source, hits the lens, is refracted and leaves fixture in parallel beams without divergence, but that’s not the case in the real world. It diverges much less than an open face but the spreading of the beam is clearly visible in fog or smoke. The only truly collimated light is a laser.

What is a bit confusing in conversations about Fresnel lenses is the concept, occasionally encountered in technical papers, of ‘focal length’. The term is used inconsistently but sometimes refers (explicitly) to the distance at which a beam of light converges to a point. If this strikes you as a contradiction of the principle of collimation, well… me too. It may be analagous to the concept of ‘short throw’ and ‘long throw’ (see The Altman 65Q) so there may be something to it. From what I have observed, however, there is divergence in the beam from Fresnel fixtures but never convergence. For more on this, see section 1.1 of this paper.

Reflector
At first glance the reflector appears to be a simple concave mirror redirecting light forwards towards the lens but it’s actually quite a bit more complicated than that.

 

fresnel_doubling

The reflector in a Fresnel fixture is designed not to simply project parallel beams forward to the lens, but to create a secondary focal point, a virtual light, next to the actual globe in the fixture. This virtual light increases the overall output of the globe but it also creates secondary shadows (see The Venetian Blind Problem).

“It is a common misunderstanding that the reflector collimates the light of a Fresnel head. In fact, the purpose of the reflector is to double the intensity of its’ output. When the light-emitting filament of the bulb is placed near the center of curvature of a spherical, concave polished mirror reflector, the reflecting surface creates an image of the filament. That image is located in the same plane, but slightly displaced from the filament itself. This has the effect of doubling the amount of light forward projected from the locale of the lamp filament. [italics mine]

 

In other words, without the reflector, “this reflector light” (the dashed lines in the illustration above) would have been lost in the back of the lamp housing. With a reflector, these rays of light are collected and sent back to their point of origin where they emanate forward, parallel with the direct rays of light from the filament (the solid line in the illustration above), towards the back of the Fresnel lens where they are together collimated by the lens (for this reason the filaments of the bulbs used in Fresnel heads are designed with an open geometry to minimize blocking of the retro-reflected light – making them not quite an ideal point source.) Now that all the light that emanated forward and back, emanates forward from a single point within the fixture (the filament and its mirror image), the light projected forward is doubled. Quantum dot LED fixtures like the one being discussed here do not benefit by this light doubling action which is why they tend to be weak by comparison to traditional tungsten Fresnels.”
Guy Holt, Gaffer
ScreenLight & Grip
http://www.screenlightandgrip.com/html/emailnewsletter_generators.html#anchorHigh%20Output%20AC%20LEDs
It was from this information that Andy surmised the ability to block secondary shadows with the Voodoo Stick, described in The Venetian Blind Problem. Since the Voodoo Stick works exactly as theorized, I’ll take it as a proof.

 
The relationship between the reflector, the lens and their specific designs and flaws will be the determining factor in the performance of each light. Each combination will have anomalies. Even different lights made by the same manufacturer will likely not share the same anomalies. For most general purposes – bouncing on a bead board, pushing it through diffusion, general lighting on set – none of the above is important, but for a specific task like shadow control the only way to know what a light can do is to test it. For most Fresnel fixtures – used normally – the optimum distance is around 25 feet, the distance at which doubled shadows begin to blend together and become crisper (which does not constitute ‘convergence’)..

More documentation of the ‘virtual’ lamp.

 

 

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