Each module of a color light signal has a two lens design consisting of an inner doublet lens and an outer doublet lens.  The inner doublet lens contains the aspect color and has concentric prisms.  It is 5 1/2 inches in diameter.  The outer doublet lens is clear and also has concentric prisms.  Most outer doublet lenses also have an integral center deflecting bulls-eye or close-up prism to allow viewing the indication when closer to the signal.  The outer doublet lenses are 8 3/8 inches in diameter.  The concentric prisms on the outer doublet are located on the side facing the bulb.  On the inner doublet, the concentric prisms are located on the side away from the bulb.

Above.  Outer doublet with center 40 degree deflecting bulls-eye.  Note the deflection direction marked by the arrow.  The 40 degree angle is almost a standard.

Below.  Outer doublet with clear bulls-eye for use where a close-up prism is not needed, such as a ground signal, or a signal on which a Phankill unit has been installed since these usually have an integral close-up prism.

Above.  Outer doublet lens with round prisms located on the outside surface to spread the light beam.  This came from a ground signal.

Above.  Green, clear and lunar (for the color of the moon) 5 1/2 inch inner doublet lenses used in color light signals.  Earlier lenses were made of glass.  Newer lenses are generally made from Lexan plastic.  Other colors which were widely used included red, yellow and blue.  Kerosene pink, kelley green and purple were also available but rarely used.  While the lunar lens pictured above looks blue, in operation, it just cools the warm tungsten light source to a color which approximates that of the moon at night.  In operation, it looks very similar to the clear lens.  The inner doublets are placed in the signal with their smooth concave side facing the bulb.