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.

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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.