Quote Originally Posted by brucemuir View Post
So are you saying roscoe "fire" is good to cut blue & green and rubylith is good if you only need to cut blue?
Hi Bruce,

Both will block transmission of blue and green. The difference is where in the visible spectrum they begin to allow transmission.

The Roscoe #19 Fire begins transmitting in the orange/amber region, also allowing everything lower in frequency (such as red) to pass. Since sodium orange is the color we really want from a Duplex (and nothing higher such as greens and blues) this material is a good match.

The Rubylith doesn't begin transmitting until you get down to red. It blocks everything higher than red, including orange/amber. Since the LEDs are mostly red to begin with, it doesn't matter that the material also blocks orange/amber. It's the red we really want.

The reason for choosing the Roscoe for the sodium light source is that it largely passes the desirable orange/amber, while the Rubylith does not. Using Rubylith on the sodium would unnecessarily darken the orange/amber output for no additional benefit, as both filters equally remove the undesirable greens and blues.