Picture of a Red Neon Musical Note Artistic Lamp, turned on, showing the light emitted from red-based neon / hydrogen gas mixtures from within clear glass tubes.

What Determines Neon Light Color?




Picture of a green glowing neon money sign lamp, featuring mercury vapor gas, blue based fluorescence.
Green neon money sign lamp, operating, featuring mercury vapor gas, blue based neon.

Question: What determines the color of neon lights?  Answer: As in just about all gaseous discharge lamps, the color that neon lights glow is set by several factors, including:

  1. The type of gas inside the neon light’s glass tubing.
  2. The amount gas (pressure) inside the tube.
  3. The type of fluorescent coating on the inside surface of the tubing
  4. The color of the glass tubing itself.
  5. Combinations of the above factors can produce colors in neon lights that span the entire visible spectrum and beyond.
  6. To a much lesser degree, the amount of voltage applied can affect the emitted light color.

 

For an explanation of each of these color-determining factors, see below.

1. The Type of Gas Helps Set the Color of a Neon Light. There are historically and primarily, two colorless, odorless gasses used in this form of gas discharge light: neon (of course) and mercury vapor. Several other gasses can be used, either separately or in combination, to control what color you ultimately get from a neon light.  These include hydrogen for a deep red light, helium for yellow, mercury in the blue-based neon lamps and signs, and, though seen infrequently, carbon dioxide for white light.  Even though the mercury vapor, hydrogen and other gas discharge lights contain no neon, they are still collectively referred to as ‘neon lights’ as they bear most of the same functional characteristics of neon lights, except that they contain other gasses and gas combinations besides neon.

Mercury vapor, when placed inside a clear, non coated glass tube, and subjected to a couple thousand volts of electricity, glows a sort of gentle blue color. This shade can be observed in street lights when they first come on and they are still very dim.

On the other hand, use neon gas, and you get a fiery, coal red glow such as seen coming from neon light beer signs. Neon requires higher voltages to create and sustain ionization in the colorful neon light.  Further, the amount of voltage required to get a healthy red glow from neon depends on the pressure of the neon gas inside as well as the length of the tube. A five-foot long tube filled with neon requires about 7000 volts to glow. Neon lights containing neon gas are said to be “red-based” while the neon lights utilizing mercury vapor are known as “blue-based” neon lights.

Neon Dollar Sign Lamp, Turned Off. Neither green filter glass nor green colored phosphors, except for when it's operating.
Neon Dollar Sign Lamp, Turned Off. Neither green filter glass nor green colored phosphors, except for when it’s operating.

2. The Amount of Pressure Affects Color of Neon Lights.   The pressure of the gas in the glass neon light tube is crucial. Too little pressure, too much air, or too much vacuum, and you’ll need prohibitively large amounts of voltage to get the gas to ionize, conduct current,  and glow. On the other hand, put too much gas in, and neon loses its vivid red color when ionized, and becomes more of a pink or white. To get good color and solid flicker-free lighting, the pressure must be controlled very precisely.

3. The type of Fluorescent Coating Affects Color.  Neon lamps did not earn their colorful, vibrant reputation by using clear tubes of glass filled with neon gas alone; at least, not at the beginning. Without other mechanisms, the only colors you’d have would be red (neon) and blue (mercury vapor).

But neon lights come in just about every color of the rainbow, via the employment of the principle of fluorescence. Have you ever seen those brightly colored paints at Spencer’s gift shops that, when placed under a black light, glow unusually brightly? These are fluorescent paints, because they have the ability to absorb light energy of one color or wavelength, and convert it to another color. These paints convert the near ultraviolet rays of a black light into visible light. This makes them appear to glow without external stimuli. And that makes them intriguing to us younger folk. 🙂

The chemical makeup of these paints determines the optimum color of input light as well as the color they radiate after the input light is converted. Similar chemicals are applied as thin coatings on the inside of the glass tubes in neon lights, to produce many different colors. Mercury vapor radiates much ultra violet light when ionized (conducting electricity). The amount of visible light produced is rather small. But by applying a fluorescent coating, most of that ultraviolet light is converted to a more useful, visible energy. Blue, purple, pink, green, cyan, yellow, and white colors are produced this way.

On the other hand, similar effects are produced with neon in the tube. Neon produces a fair amount of infrared light that can be converted into visible energy as well. Pink is probably the most popular color produced using the neon gas and fluorescence combo inside a neon light.

 

Picture of a Red Neon Musical Note Artistic Lamp, turned on, showing the light emitted from red-based neon / hydrogen gas mixtures from within clear glass tubes.
Red Neon Musical Note Artistic Lamp, turned on, showing the light emitted from red-based neon / hydrogen gas mixtures from within clear glass tubes.

4. Coloring the Glass Tubes Affects Color.  When colored, the glass tubing Itself also Influences the emitted light color. This form of light filtration has been used for decades to get incandescent Christmas light bulbs to radiate the popular reds, yellows, greens, and blues of the holiday season.   As with the Christmas lamps, a transparent or translucent coating is applied to the outside of the glass that prevents all but the desired color of light to pass through. In neon, you might start with a white light, and apply a nova blue filter to the glass, for a really bright, deep blue light. The extra bright neon that has appeared over the past 20 to 25 years employs this method. You can recognize this when the light is off. If the tube is colored something other than white, then this filtration system is its mode of operation of the neon light. A white tube means that method 3 is how the color is produced when it’s turned on.

5. Coating Inside of Tube with Fluorescent Chemicals Changes Color Output.  Combinations of the Above By applying a fluorescent coating to the inside of the tube as well as a filter to the glass itself or to the outside of it, all the visible colors can be produced in a wide array of intensities.

6.  Applied Voltage Affects Color Somewhat.  On voltage determining neon light color: The voltage level controls more the intensity of the light rather than its color.  However, be careful not to apply too much voltage, as this can alter the chemical makeup of the neon and render it incapable of producing any light at all.  I experiment with those little neon light bulbs that you see in elevator button panels, old radios and so on.  I found that excessive voltage makes them glow quite brightly for a short time.  But they then burn out.

Picture of a Musical Note Shaped Red Neon Lamp, Turned Off. No sign of red in the clear glass and gas when OFF.
Musical Note Shaped Red Neon Lamp, Turned Off. No sign of red in the clear glass and gas when OFF.

It appears however, that LED-filled tubes are supplanting gaseous discharge neon and fluorescent lights, because they require less voltage, are easier to fabricate, are more rugged and thus, are harder to break, are more efficient, and the LED lights are fast becoming cheaper than true neon lights, though they faithfully duplicate the brilliance and vibrancy of neon lights.  Plus, LED lights in a neon-style configuration can be adjusted by the user to emit any color in the rainbow.  The light color in traditional neon lights is set at the factory and cannot be changed by the end user.  Still the transition from neon to LED is slow at present.  So true neon lights will be popular for some decades to come I suspect.  Long live neon lights.  I love ‘em!

 

References

 

Revision History

  • 2015-10-17: Added appropriate tags.
  • 2014-11-30: Added pictures of red-based and blue-based artistic neon lamps, in both the ON and OFF states.
  • 2014-11-24: Added a References section, adjusted ad placement, and added more content.
  • 2012-08-14: Originally published.



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