# Primary Colors in Cecil's Spotlight?

**URL:** <https://boards.straightdope.com/t/primary-colors-in-cecils-spotlight/399618>\
**Category:** Cecil's Columns/Staff Reports\
**Created:** [April 11, 2007, 8:31am UTC](https://boards.straightdope.com/t/primary-colors-in-cecils-spotlight/399618 "2007-04-11T08:31:58Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\
**Post date:** [April 11, 2007, 8:31am UTC](https://boards.straightdope.com/t/primary-colors-in-cecils-spotlight/399618/1 "2007-04-11T08:31:58Z")

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In this thread, [http://www.straightdope.com/classics/a3\_344.html](http://www.straightdope.com/classics/a3_344.html), Cecil claims that spotlights use the additive primary colors to make white light. And true, I’ve seen this demonstrated. However, Cecil also claims that items which use pigment, _including filters_, follow the subtractive primaries - also true.

My question is: Professional spotlights use filters, but why do these spotlights follow the rules of additive primary colors? Spotlights I’ve made with colored cellophane filters follow the subtractive primaries. What is happening here?

- Jinx

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**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\
**Post date:** [April 11, 2007, 1:28pm UTC](https://boards.straightdope.com/t/primary-colors-in-cecils-spotlight/399618/2 "2007-04-11T13:28:06Z")

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[QUOTE=Jinx]  
In this thread, [http://www.straightdope.com/classics/a3\_344.html](http://www.straightdope.com/classics/a3_344.html), Cecil claims that spotlights use the additive primary colors to make white light. And true, I’ve seen this demonstrated. However, Cecil also claims that items which use pigment, _including filters_, follow the subtractive primaries - also true.

My question is: Professional spotlights use filters, but why do these spotlights follow the rules of additive primary colors? Spotlights I’ve made with colored cellophane filters follow the subtractive primaries. What is happening here?

- Jinx  
[/QUOTE]

If you add two filters to one spotlight, the resulting colour follows the rules of subtractive colours. If you point two spotlights at the same spot, the resulting illumination follows the rules of additive colours.