I have some green, blue, and purple markers which clearly fluoresce under monochromatic light with a lower wavelength. Here they are illuminated by 589nm light emitted by a low pressure-sodium lamp (SOX light):

markers under sox lamp

And here lit by the diffuse light from a a 532nm +/- 10 green laser: enter image description here

This is surprising because fluorescence is defined as:

the visible or invisible radiation produced from certain substances as a result of incident radiation of a shorter wavelength such as X-rays or ultraviolet light.

By my understanding, that means it shouldn't be possible for either the blue or purple marker to be fluorescing under either light source because blue and purple light has a shorter wavelength.

What is happening?

More Photos

Markers lit by 650nm +/- 10 red laser light. The blue marker looks like it might be slightly blue to my eye, but it's hard to say. enter image description here

Markers lit by sunlight: Markers under sunlight

Markers lit by incandescent light:

Low Pressure Sodium Lamps

The yellow light is a low-pressure sodium lamp (also known as a SOX lamp). As wikipedia describes:

These lamps produce a virtually monochromatic light averaging a 589.3 nm wavelength (actually two dominant spectral lines very close together at 589.0 and 589.6 nm). The colors of objects illuminated by only this narrow bandwidth are difficult to distinguish.

Viewing the SOX lamp through diffraction grating with 100 lines/mm:

In contrast to an incandescent light viewed through the same grating:

In contrast, "high-pressure sodium lamps emit a broader spectrum of light than the low-pressure lamps, but they still have poorer color rendering than other types of lamps" per wikipedia. High pressure sodium lamps are more common in streetlights.

The green laser shined at the wall through the diffraction grating:

Unlikely Explanations

Both prior research and answers/comments here make these seem like unlikely explanations. I have retained them for context.

I found a chemistry question here which explains that:

[Emitting light at a shorter wavelength] is certainly possible to make in a fluorescent / phosphorescent light scenario, but the engineering challenges (or, more specifically - the cost) involved in such a solution would be steep.

Another answer notes:

This method is called two-photon microscopy. Two photons of longer wavelength are absorbed by a dye molecule which then emits one photon of shorter wavelength. As mentioned before, this process is highly unlikely, but this property can be used to reduce the size of the fluorescent spot in confocal mocroscopy.

These answers provide a mechanism by which emission of green, blue, and purple light is possible, but make it sound like that only happens in exotic, expensive materials. Noting the process is "highly unlikely" also makes me expect that the blue and purple fluorescence would be much dimmer than they are.

These markers were bought in Thailand for just a couple dollars. While they glow the brightest of any green-purple objects I have, they are far from the only thing I own that exhibits this phenomenon.

  • 3
    $\begingroup$ Not two photon absorption, you would need an very bright laser source to do this. If these are dyes then just normal absorbance/fluorescence will explain observation, but if semiconductors or nano particles then upconversion is just possible. $\endgroup$
    – porphyrin
    Commented Jan 22 at 8:52

1 Answer 1


This is an interesting observation and one can appreciate that the OP thought about two photon absorption as a. A couple of observations first that challenge the hypothesis. Ordinary sodium lamps or even street sodium lamps are not monochromatic at all. If you have diffraction grating, view this lamp via narrow cardboard slit and you will see an entire line spectrum which is very beautiful. The Na-D 589 nm line is intense but other shorter wavelengths are present too. This is why you can see the markers, but they are not fluorescing at all in my opinion. If it were pure 589 nm, the markers would be appearing black just like the two on the right. You can test the fluorescence rather in solution with a yellow highlight. Dip the tip in water and view it in sunlight and in this low pressure lamp. You will see the difference.

Two photon absorption has a very very low probability. For that one needs an intense flux of photons and this is achieved with lasers as already commented above.

  • $\begingroup$ I added a picture of the light from the low-pressure sodium lamp to the original post. I also added photos of the markers lit by monochromatic laser light. $\endgroup$ Commented Jan 25 at 17:07
  • $\begingroup$ @HenryMarshall, Nice work and hopefully now you see that was not fluorescence or two photon fluorescence. $\endgroup$
    – AChem
    Commented Jan 27 at 15:43
  • $\begingroup$ Thanks. I am even more confused now. The crisp lines from the diffraction grating has me convinced that both the SOX lamp and green laser are in fact monochromatic. Does this seem true to you? If the light is monochromatic, how is it possible that the markers are showing blue and purple? $\endgroup$ Commented Jan 28 at 10:44
  • $\begingroup$ @HenryMarshall, The SOX is not monochromatic. It has green to green. Otherwise the markers would be invisible (black). $\endgroup$
    – AChem
    Commented Jan 28 at 16:02

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