Miata Mailing List: May 1995, Message #32
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   From: GrumpyOldMan 
Subject: Additive/subtractive light mixing (was buld color)
   Date: Mon, 1 May 1995 22:03:05 -0400

 Some statements recently about the colors of bulbs to go with the green
instrument facia are incorrect:
>   My guess is that with Red and Green, it should be like reddish orange.

Someone posted that with certain colors you may get grey...

The primary colors of light are red, blue, and green. If you have an red light
and a green light the resultant light will be amber (yellow). This is additive
mixing.

The instrument facia that is green is essentially a green filter, subtracting
all colors of light except the green wavelengths. Placing a different color
lamp behind this filter will not change the color of the light coming through
instrument facia! Only the green light will still come through. To change the
color of the instruments it is necessary to change the color of the filter (the
instrument facia). For those who might want to change the color several times
the best thing to do would be to find intrument faces with clear or white
plastic/numbering/lettering. With clear or white facia, the color of the lamps
will be the color of the light you see. You could use clear lamps for white
light (that will become more amber as you dim the lamps), orange lamps for
orange instruments, green for green, etc. You could even get creative and
paint your own lamps in any color you wish (be careful not to coat the lamp
to thickly)!!

If you want to try your own lighting experiments head down to your local theatre
supply shop and get a gel swatch book (about 2x3 inches). The swatches are of
various color filters for theatrical lighting. You can take a green filter and
put a white light behind it get green. Next put a piece of orange gel between
the light source and the green filter and see what color you get! (this
simulates the orange lamp and green facia).....
-alex-