Re: Spectrum of Sodium Street Lamps

Dirk Bruere at Neopax wrote:
Farooq W wrote:

Marvin wrote:

Farooq W wrote:

The yellow color of sodium lamps is well known. I believed that it was
purely monochromatic based on soidum D line until recently, when I saw
the light through a transmission grating. The result was slightly
*unrealistic* to me, there were many colors (looking much like
contiunous emission perhaps, some lines due to characteristic bright
green Hg line) in it and more importantly a "BLACK" line in the yellow
orange region where I expected highly intense D lines. This can be
attributed to the self absorption in the high pressure lamp - I assume.

The confusing part is: Our eyes are seeing the characteristic yellow
color of the Na-lamps but the image through a grating shows a black
line where the sodium D lines were expected. What is wrong with this

I don't understand what you mean by a black line. Is it the space between the yellow
doublet? If not, where is it seen in the spectrum?

In other words, there is a dark space in that continuous spectrum in
the yellow orange region where I expected to see the sodium D lines.
The rest of the region seems to continuous with Hg overlapping lines
(the violet one I can see and the lime green). Agreed that the emission
is continuous but why the lamp's emission just looks like the Na flame
in the flame photometer. The flame is said to be emitting
characteristic color attributed to the sodium D line. Am I mistaken?

The lamp contains a little Hg to start the discharge. Once the lamp warms up enough to
vaporize some sodium, the mercury lines are still there, but their intensities do
decrease, especially for lines from mercury ions. There will be a continuum emission,
just as there is in a flame that contains sodium. It comes from the recombination of
sodium ions and electrons.

What is that phenomenon called?

The Wikopedia entry at is informative.
There are some gaps between the Hg lines in the spectrum there, but nothing that should be
called a black line.

High or low pressure sodium lights?
The spectra are different

The original post mentioned high pressure, and the Wikipedia spectrum is of a high-pressure lamp.