Followspot Lighting Gel Attenuation

[Don Allen]

I loaned a pair of Strand Patt 765 followspots to a community theatre group for a production of Annie. Went to see the show last night and collect the followspots afterwards. They only used one followspot, it seemed bright enough in Act 1, as it was open white, but I could hardly notice the beam in Act 2.

 

The had put seven pieces of Lee 003 Lavendar Tint with Y = 75.7% and one piece of Lee 203 Quarter CTB with y = 69.2 %.

 

Am I correct in calculating that the light output would be reduced by 75.7% 7 times, then 69.2% of the subtotal so the output of the followspot would be 9.8% of 2000 lux at 20 metres from Strand 765 datasheet = 197 lux !

[adam2]

With a neutral density filter I believe that you are correct.

 

Each layer of gel blocks a percentage of the light, so two layers of 50% neutral density gel would pass 25% in total as you describe.

 

With a colour filter though it becomes more complex and not easy to calculate. Consider a simplified but illustrative example of a light magenta filter that totally blocks certain green wavelengths, but passes all other colours. Two layers of that filter will give the same result as a single layer. Still no green passed, but all others passed.

 

Real world filters are of course imperfect, so let us now consider a light magenta filter that blocks 90% of the green and passes 90% of the other wavelengths. Two thicknesses of such a filter will pass 1% of the green and 81% of the other colours. That does not give us the total transmission without knowing what percentage of the light is considered to be "green" for this purpose.

 

And of course different filters vary, so in general I feel that multiple layers of filter will undoubtedly reduce the light transmitted, but not always in any simple or intuitive way.

 

Experiment probably beats theory.

 

 

 

 

 

 

 

 

[sunray]

I used to be a volunteer at a little theatre. A scene needed to change from a deep blue (using a couple of floods and a couple of 123's) to purple using about 20 fittings with blue and red gels used together. the light level was adequate in blue but the stark dimming to purple was ridiculously silly. Fortunately I found enough purple gels to create a clone of the blue scene which avoided the need to replug our limited arrangements during the show.

[timsabre]

Uh... It's subtractive, you'd never make purple light by combining red and blue filters. That's how you make darkness...

 

The red takes out everything but red, then the blue takes out the red as well.

[andy_s]

Uh... It's subtractive, you'd never make purple light by combining red and blue filters. That's how you make darkness...

 

The red takes out everything but red, then the blue takes out the red as well.

 

indeed. Additive lighting is when you take two (or perhaps three) lights, one in each colour, and point them at the same thing. three lights in primary colours pointed at a white (or neutral) surface will produce the secondary colours where the beams overlap. This is how cyc compartment battens work.

 

Put all colours in the same light and you will end up with darkness eventually. This is not to say that putting two filters in one source shouldn't be done, but it doesn't work in the same way as mixing pigments on a painters palette.

[sunray]

Oh yes, I was fully aware of the problem at the time, ironically we'd done a lot of spectrum work in physics not long before.

 

The theatre had a fairly big homemade wooden switchboard, 40 or more channels, and a dimmer rack with around half of that. The plan was to replug dimmers before and after this scene.

 

As soon as I saw the stage lit in purple during a rehearsal (I hadn't been involved, with this play, in any way prior to this) I was aware it was far too dim and went to look at the lights, when I saw the double gels I removed some blue's and planned to run additive colours (would have been easier to fade 4 up rather than 20 up and 4 down simultaneously on the chain driven dimmers). Then I looked through the several boxes of gels dotted around the lighting rig and found enough purple for 4 lights, the same as the blue scene. It roughly halved the number of fittings used for the whole play.

[indyld]

Uh... It's subtractive, you'd never make purple light by combining red and blue filters. That's how you make darkness...

 

The red takes out everything but red, then the blue takes out the red as well.

 

I've done similar to create a makeshift followspot shutter when no other options were available. :-)

[timsabre]

Uh... It's subtractive, you'd never make purple light by combining red and blue filters. That's how you make darkness...

The red takes out everything but red, then the blue takes out the red as well.

 

I've done similar to create a makeshift followspot shutter when no other options were available. :-)

 

Also red+blue makes a good infrared filter for the stage relay camera.