Quote Originally Posted by ParkerSmithPhoto View Post
The Rosco #19 Fire is a very red gel, and despite that, the light that comes out of the unit is yellow all the way.
The spectrum of a sodium vapor lamp consists of two very narrow energy peaks at about 589 nanometers.

The transmissivity of the Rosco #19 Fire gel is about 50% at that wavelength, which is the only one that matters in this application.

Due to its cutoff characteristics the gel will look red to the human eye.

- Leigh