I got an email from Les who picked up an older GE traffic signal for his man cave. Personally I think it’s a great addition to any room of the house, but if I want to stay married, I’ll have to keep the signals in the garage. Anyway, Les has been browsing eBay, looking for a green lens because his traffic light came with a blue lens. Fortunately, Les doesn’t have to worry about looking for a replacement. What he has is the correct color.

Wait, what? Traffic lights are red, yellow, and BLUE? Well, sort of.

Although you may not perceive it, “white” light can give off different colors. This coloring of the light is known as color temperature, which is measured in degrees Kelvin. There’s a detailed article about color temperature on Wikipedia if you’d like to learn more. For example, a common household light bulb may have a color temperature of 2400K (more of a yellow) but the light hitting you on a cloudy day might have a color temperature of 6500K (more of a blue). This is more perceptible if you take an outdoor photo using the indoor setting. The photo looks REALLY blue because the camera thinks the overall lighting is not what it expected and has compensated for the difference incorrectly.

The light appears green despite the blue color of the lens.

The green lens looks blue when not illuminated by an incandescent light bulb.

If you recall your days of finger painting in nursery school (who doesn’t?!) you’ll remember that yellow (representing the bulb) and blue (the lens) makes green (observed light output). Though not exactly the same, the principle applies. Going back to the blue color of outdoor light, this will compound the appearance of your lens and it will look even MORE blue. Because incandescent bulbs give off that warm, yellowish color, a slight correction has to be made. A bit of blue tint is added to the lens, making the light you see appear green when illuminated by the yellowish bulb.

So, Les. You can sign out of eBay and close your browser for now. That piece of glass you have is precisely what should be in there. Go buy yourself a few clear, 40-watt appliance bulbs and fire it up. Of course, now you’ll want to make it work like it did when it served the street. Enjoy!