How do you do that Spotlight/Light Beam effect

Started 4 months ago | Discussions thread
tugwilson Senior Member • Posts: 2,929
Re: How do you do that Spotlight/Light Beam effect

RobzLondon wrote:

Rico Tudor wrote:

Projection attachments like the Dedolight DP1 and the Godos SA-P have kob which lets you tilt the front of the projector to correct the distortion in cases like this.

DP-1 cannot be tilted. A perfectly circular spotlight when hitting the b/g at an angle can be achieved with an elliptical gobo although the focal plane will not fall correctly for sharp edges all around. DOF can be increased by pulling the fixture way back but illumination efficiency will suffer the inverse-square law.

Inverse square law does not apply to focused or collimated light!

That's an oversimplification and not a good explanation of the physics.

The inverse square law applies to light emitted by a point source. In the real world we never have a point source so the inverse square law is an approximation to the effect we can observe. In general it's a very good approximation but in some cases the nature of the light source produces affects which seem to break the law (it doesn't break the law).

An ideal parabolic reflector will have a point source of light at the focus and the reflector will produce a set of parallel beams. The is equivalent to having light emitted by a point source infinitely distant from the observer. The inverse square law still applies but is the source is infinitely far away you get no dropoff. In practice things are lots messier than this and if you try an model the behaviour you end up with a very large number of point sources at various distances and positions behind the reflector and you do get measurable dropoff as we all know.

With most modifiers the effect on dropoff is quite small. In the last couple of years LED light manufacturers like Namlite have produced reflectors designed for specific lights which are dramatically effective. These are faceted reflectors and I don't quite understand the physics behind them. I imagine they are tuned to the size and shape of the COB.

Projectors are a special case too. Lights like the Dedolight have a focusable emitter and a set of very high quality optics that produces a beam of light with a precise spread. It does this with very little light loss. From an inverse square law point of view the light comes from a point source at some distance behind the light and that distance can be changed by using lenses of different focal length in the projector. The effect of this is that you can throw the light surprisingly long distances with very little falloff. However they don't violate the inverse square law.

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