Theatre Lights and Efficiency.

[Crenshaw]

I am not entirely sure that this thread belongs here...but...:thinking:

I recently had the oppotunity to take a look inside a Light that is typically used in a theatre as a spot light/ or as effects...etc.
My First discovery was the HUGE Osram bulb inside. It was literally like 3 times the size of a Wa11185 or ROP bulb.

I always wondered how they get a nice, even hotspot, with no spill, or if they wanted, a hard hotspot. The particular one i looked at had a Fresnel Lens.

Question: Doesnt a Fresnel Lens Project an image of the light source? meaning, it should project the image of the glowing coils within the Bulb right? where then , does the Even spot come from?


Somethign else i notice. inside the light, is a VERY small reflector. Its a very fine, heavy OP, but it only covered one side of the bulb, leaving most of the bulb to just emit light in whatever direction.

Our lights have around 65-80% efficiency in the light out the front. In that case, these theatre lights, have perhaps, 30-50% out the front?

My gosh that is low efficiency. Is it then possible to make a portable light, Mag sized, with WAY higher efficiency (so less bulb lumens are needed) and still have the nice even hotspot?

Crenshaw

 

[greenlight]

Are you referring to a true theater spot light or some fancy programmable intellibeams?

 

[Crenshaw]

A true "Theater" spot light... the kind used for the stage....

Crenshaw

 

[Timothybil]

"Glass Fresnel lenses also are used in lighting instruments for theater and motion pictures (see Fresnel lantern); such instruments are often called simply Fresnels. The entire instrument consists of a metal housing, reflector, lamp assembly, and Fresnel lens. A holder in front of the lens can hold a colored plastic film (gel) to tint the light or wire screens or frosted plastic to diffuse it. Many Fresnel instruments allow the lamp to be moved relative to the lens focal point, to increase or decrease the size of the light beam. The Fresnel lens is useful in the making of motion pictures not only because of its ability to focus the beam brighter than a typical lens, but also because the light is a relatively consistent intensity across the entire width of the beam of light."*

The small reflector built into the lamp is designed to basically direct as much of the produced light as possible out only one side of the lamp. With a very small light source such as we are used to, it is simpler to place the source at the focus of the reflector parabola, so that all light produced is reflected forward. Given the size of the typical theater lamp, especially the size of the filament, it is not practical to do this. Thus the reflector in the lamp itself, the usually silvered inside of the instrument barrel, and the fresnel lens at the front all work to shape the beam of light. As an added side effect, this tends to 'defocus' the lamp filament within the beam, thus reducing any 'artifacts'.

* Quote is from Widipedia article on Fresnel Lenses

 

[Gunner12]

I think for those lamps effect trumps efficiency. Those lights do give no spill, which is hard to do with a reflector. The bulb isn't at the focal point of the lens so it doesn't project the light source.

 

[65535]

Actually respectfully there are many types of lamps and most of them DO use a lamp at the focal point.

Some lamps use a few optics to produce the proper sized beam, then they have dampers on all 4 sides to produce the shaped beam. Fresnel lamps are used to produce a mid sized wash, they have a orange peel on the flat side and a reflector on the reverse it produces a nice flood but like you said isn't terribly efficient at emitting the light.

For large flood there are Par64 housings which just use pre-focused Par64 bulbs.


There are also other Par sizes.

Then there are digitally controlled "smart" lights which do everything through a computer output. Totally programmable completely automated once programmed. They often use xenon short arc lamps with projection optics.


It's amazing what you can do with optics, keep in mind that these lamps aren't slapped together like flashlights, optical engineers design them to produce the kind of light they want. They look simple but many hours of work went into the simplest parts.