Filters to Verify Traffic Lights

When far from the traffic light, especially when facing a bright sky, the achromat may need help at times with the traffic light.  For many achromats, a  quick look into a small strip of red filter will take the green light away but will enhance the red light. This gives the achromat a quick verification of red or green
 
Here we see the same traffic signal the way a "normal" would see it on the left, and the way a complete achromat would see the traffic signal on the right.

If the achromat bioptic driver is far enough away from the traffic light to have difficulty detecting which light is on, a quick view into a small red filter strip below the eyepiece on one lens can  help verify red or green.
 

Notice below that the "normal" on the left would see the world turn red, and the green light disappear. The achromat on the right will see the world slightly darker gray and the green light disappears confirming the light green is on!

If the traffic light is red, it would be seen by the "normal" as shown on the left and on the right, as an complete achromat would see it.

 Here we see another example of a red filter that is used by an incomplete achromat to drive for many years.

The achromat can look into the filter to help verify the red light on a very distant traffic signal.

The "normal" eye,  as shown below on the left, would see the world turn red and the red light appear brighter or more distinct. The right picture demonstrates how the achromat driver would see a verification that the light is red. The light does not disappear thus the red light is on.

Most achromats that have become drivers develop strategies to handle traffic lights. The use of filters to enhance one color or subtract another is just one  additional strategy.This concept is similar to the way a digital camera sees color. There are filters over the elements in the CCD that subtract all but the one color of light for each element to register.