I've seen some discussions on this board about using sodium sources and filtered diode sources for calibrating the wavelength scale in a prism spectroscope, but I've not seen anyone mention the cheapest, easiest source to use for calibration. If everybody already knows about this source, I'll be happy to go back and delete this comment.

A prism spectroscope has a nifty wavelength scale, but that scale needs to be adjusted. The scale contains a vertical line labelled D, and the usual adjustment is matching that line to the bright, bright yellow line appearing in a sodium lamp's emission spectrum.

Sunlight is a good alternative source. In sunlight, the sodium D-line appears as a narrow absorption band. It is the darkest of several narrow absorption bands appearing in sunlight. To use this source for the first time, make sure the switch is set to bright, sunny day. Then point your spectrometer toward any white or off-white surface exposed to full sunlight. Close the variable slits until, in the spectroscope, you see a shadow appear at the border between yellow and red colors. The more you close the slits, the narrower and darker (relative to the rest of the spectrum, which is also getting darker) the D absorption line becomes. And voila, a simple D-line source.

Once you are practised at looking for the D absorption line, you can find it just by pointing the spectrometer at the blue sky or at a white cloud. And you even can find it on significantly overcast days.

How about a candle with some salt thrown into it for a sodium light?

The candle and salt works well too for an absorption spectrum. For a nice demonstration that it is the same D line as in the sodium lamp, check out this video
http://physics.kenyon.edu/coolphys/Fran ... bspec.html

However, I still think sunlight is cheaper and easier.

I'm guessing that video wasn't made last week .

Yes, sunlight is cheaper ... but it's not always available .

Yep that video is around 40 yrs old. You couldn't see his pocket-protector, so what gave it away? And before you say "crew-cut and black-plastic frames" you'd better realize what an enduring fashion statement that is among certain scientists, almost as popular as the "Einstein" look.

For me, what gives it away is the suit and tie. I've never seen any guy in a suit step that close to experimental equipment. Although once, we had this female professor come work for us as a visiting professor. She showed up in my lab on first day in pearls and high-heels. She eventually switched to flats, but she didn't give up the jewelry.

The solar Double D line is among lots of lines. Some hand spectroscopes cant resolve the two lines because they dont have enough dispersion. Too much dispersion makes some lines turn into bands or disappear.

One of my first ebay purchases was a spectroscope just like the one in the video. $54.00 delivered. A Bausch and Lomb unit from probably the 1940s
Knobs of Bakelite instead of metal. War effort.

I still think a candle or alkeehol burner is better for initial cal. Then use the solar line as a check.

Yes, the Fraunhofer absorption lines in the solar spectrum are amazing... there's a great picture of them in Wikipedia. In my first edition of this post I wrote more about Fraunhofer lines, but I decided the post was way too long and did some major cutting.

As for the spectroscope in the video, I've never seen one of those before. With enough resolution to split the sodium lines, what is the dispersing element? Does it have a mirror or additional port to introduce the comparison spectrum, or is that a bit of early television magic? Oh, and can you take it apart?

Thanks

Being too long never stopped me before

If I remember correctly its a Pelin-Broca prism but I'll have to dig out my 1931 B&L catalog to be sure, It is an irregular shape. Ninety degree deflection all in one prism only one moving part the prism table. No mirrors inside. Prism is rather tall giving lots of aperature.Mine doesnt have the comparison mirror but they made them. Actually if you watch the video he doesnt even need one. He just has the light shining through the flame and carefully aimed at only half the slit.
I have never seen that demo done with white light. Only an old engraving in an ancient physics book showing the original eleaborate demo apparatus.
And then the absorbtion lines in HighPressure Sodium lamps on I. Galidakis website. http://ioannis.virtualcomposer2000.com/ ... index.html

I have had the cover off the unit which was clean internally so I stuck it right back on. The ocular is missing the interior components of a filar micrometer that you could measure some fine deviations with. and the slit got knocked off the telescope tube during shipping but just needs some loctite or a helicoil to make it nice again. Neither prevents its use. I think the prism hold downs may have been loose when I got it so I will need to reset the double D line or some other line.
Gene

This request for information on sodium light filters appeared on a faceting list which I am also a member of. As many of you are aware, this is a subject that Gene, is a bit twitchy about, so I collected all the current info I have on the subject, and posted it. I was pleased enough with myself, that I decided to post it here too. The original poster, is a well known facetor and nice fellow, but I deleted his email and last name to protect his identity.
My reply is thus:


And then this showed up this evening for anyone that wants to manufacture.

Sadly, I too have more than a passing interest in interference filters, since I use several different custom filters to transmit light of some wavelengths and reflect light of other wavelengths (dichroic mirrors and bandpass filters).

Gene mentions one high-end maker who has custom-fabricated some filters for me, Omega Optical. I've also ordered custom filters from two other companies: CVI (now CVI - Melles Griot) and Chroma Technologies. All these guys provide excellent documentation even for their stock items. And if you need their custom work, you can get some crazy marvelous glass from all three.

The problem with interference filters is that hopefully you never need to clean them. Interference filters work by depositing single-molecule-depth layers of material on a piece of glass. Wiping the glass with a kimwipe will dig trenches through the layers, reducing the optical effect you want. And a fingerprint on the glass is a layer of oil much thicker than the optical coating, which again reduces or even destroys the optical effect you want.

One other dirty little secret of interference filters (not intending to pun Brians cleaning comment above) is the less than widely known fact, that they only work the way they are supposed to, when the light passing through them is very well collimated (that is parallel instead of radiating in a cone shape). The filtering wavelength is different for rays that are not parallel and normal to the surface, that is 90 degrees straight on.
The best exemplar of this is of course a laser beam where an interference filter can reduce other emissions that may be present (as in gas lasers) and sidebands (as in solid state lasers) But laser light is about as collimated as it gets which is why you can dazzle an airline pilot with your souped up green laser pointer from five miles away. (and rightly go to jail for it)
But even my sodium lamps which are supposed to be monochromatic are nothing of the sort. They have Argon and Neon in the bulb and when you look at the light through a spectroscope you see lots of weaker lines. A glass or plastic filter as above will eliminate all the shorter wavelength lines but neon has lots of red line and those longerwavelength ones are still there.
So you can use Mr Omegas sodium isolation filter and eliminate them on BOTH sides. This gives you a nice double D line without any other lines but at the cost of some of the light at your wavelength of interest. (actually thats is the OTHER dirty little secret. Their transmission at your wavelength of interest is reduced compared to glass or plastic simple filters)

But getting back to the original point, If you hold an interference filter in your hand and look through it, if you tilt it slightly the color you see changes. This is why I am not interested in "sodium" light sources that use a quartz halogen source. It is almost impossible to collimate (make parallel ) such a source sufficiently to make the light monochromatic and accurate at 589. LEDs at 590 nanometers with an interference filter would probably be better. (on their own their bandwidth is 20-30 nm, better than white light but not as good as a sodium lamp)

Mr Omega has a sharp 632.8 nm rejection filter that allows you to see that the plasma discharge in a HeNe gas laser is actually blue.

Since a real sodium lamp emits mostly at the double D line An interference filter can do a good job of isolating the lines because even if not parallel and normal rays, the other lines/colors are too far away and are still blocked. That is why there is no substitute for a real sodium light for serious refractometry. It IS more trouble than most folks want to go to though.

Gene is spot on (pun intended) about the need to use collimated light with interference filters. I tend to forget about this requirement, since I am using them with an assortment of laser lights. If you do have a collimated beam, you can choose the angle of incidence that the light hits the filter to be either zero degrees from the normal to the surface or 45 degrees from the normal. The latter choice is quite useful, allowing me to merge different wavelength beams along the same beampath.

As for the sodium lamp, if you look at the spectrum right after turn-on, then yes you see lots of argon lines. But if you let it heat up for twenty minutes, those sodium D lines are going to dominate everything. Most of the background argon and neon will be driven into the lamp's glass walls. So the next biggest emission is a green doublet in sodium, way way down the intensity scale... looking at spectra my students collected this past spring, the green doublet has 100 times less intensity than the yellow doublet.

So Gene, what are you doing that requires a spectral purity better than 99% !

The answer reminds me of something I saw in the latrine at Forbes Air Force Base as a 13 year old Civil Air Patrol Cadet.
There was a machine made placard on the paper towel dispenser that asked, "Why take two , when one will do?"
Underneath which some Airman wit scratched into the dispenser in a neat workmanlike hand, "For Gracious Living."

Refractive index is nsubD. If D is available why use anything else?
When I watch my Sodium LabArc warm up the non D lines don't SEEM to become any weaker. The D doublet just gets monster bright. The neon and argon lines dont get any fainter. I cant say I have ever noticed any of those lines while reading an RI on a gemological refractometer. You can see them as ghosts on the Mighty Bausch and Lomb Precision Refractometer, which of course has a GE sodium LabArc permanently mounted. The National Burea of Standards used to have a bunch of those.
Its like a cross between a table spectrometer and a critical angle refractometer. Stopped making those about the mid 70s. Both the Precision Refractometer AND the GE Sodium LabArc.

So the answer to your question is "Nothing" or "I am doing it because I CAN." or "Some guys go out in their garage and soup up their Chevy or their Supra GT, or their Porsche. I soup up my lab equipment. So I'm a squirrel."

If they stopped making the Lab-Arc in the 70s, where do you find replacement lamps? I actually know better than to ask this question... you probably have a stockpile of 'em.

But I recall seeing some time ago on some physics listserv where someone was looking for one of those lamps and having difficulty finding one.

And lo and behold, doing a quick google search to find out what this lamp looks like, http://www.lamptech.co.uk/Spec%20Sheets/GE%20NA1.htm , I think I may have one in a black metal housing. Pretty distinctive toadstool appearance and connects at top, so that it hangs upside down? One of the things I didn't throw out in the major purge of old equipment from my lab.

I have a mercury lamp of similar vintage, probably doesn't have a UV absorption coating on the glass, so I don't use it.

They stopped making them in the seventies but they started in the thirties and sold em to every science department at every University plus lots were sold to industry. So they show up , you know where, with some degree of regularity (actually I havent seen one in a while) And though GE gave the bulb a relatively short hour rating they never fail to ignite. I have one where one of the two heater filaments is burned out and it still ignites fine it just takes longer to light and warm up. And lets leave my little stash out of this shall we?



If you run across that person you may refer them to me if they are still looking. I might be persuaded to separate myself from one of my precious NO! NO!NO! Na lamps. Or help him find one or advise him of the couple of alternatives. (Sox18 or SOX35 LPS lamps, or Sox35 based Thomas darkroom safelight or an OSRAM DUKA safelight that has a 10 watt Na spectral lamp in it but the retail it for a fraction of the price of the scientific ones and they show up you know where, for next to nothing, and are dimmable. There is a little Philips four watt that Rayner used in their Sodium illuminator. A replacement bulb cost almost as much as a refractometer. I have that Rayner Illuminator and it is my favorite. You just switch it on and sit the refractometer of your choice on it and let it warm up and off you go.



The proprietor of that award winning website (which has everything and I do mean everything you could ever want to know about any lamp) is an internet pal of mine. We have cooperated on auctions , traded items and he sent me a copy of his excel spreadsheet of his lamp collection. Excuse me it was TWO excel spreadsheet files. He has a Ph.D in some kind of lampology probably engineering physics. He is the Director of Research for Osram or Philips I forget which. Osram I am pretty sure. Nice guy.

Your black housing was manufactured undoubtedly by the George Gates company of Long Island New York. They went under probably in the late eighties. If you open the housing you will find a clear unsilvered thermos housing to keep the sodium vaporised. and a big clunky ballast that wil outlast us all (that was made by GE) inside the wrinkle painted housing.
Hook it up to your OO spectrometer and see what you get.

Its probably either a GE Uviarc, which could be anywhere from 85, 100 or 200 watts or it could have a Hanovia lowpressure Hg tube of about 30 watts or possibly an Aristo neon lamp looking long tubular style UV source.
All these venerable old companies were in the New York City area once upon a time when we actually built stuff here in the US. All were SW sources.

And, if you are really really good, children, uncle gene might tell you about ANOTHER type of lamp that they don't make anymore that I think will be handy very handy (in my gemology lab anyway).
Handy enough that my pal , the director of research at Osram has threatened to make a few to try out. (and he could care less about gemology but boy does he like arc lamps. So do I, A very ahem, bright idea)