Wow, interesting. The Kr red line is so strong, it's certainly odd to see a mixed-gas unit without red but with yellow. Very cool!
The KR red line shares a upper state with the 568 yellow line, and which of the two lines you get depends on the optics, tube fill pressure, and magnetic field. Since the Omni 643 series tube has no magnetic field, it comes down to optics, and to some extent, the amount of argon added if its a mixed fill tube.
Odds are you have a laser set up for flow cytometry, the beam is focused on a narrow jet of fluid containing various cells harvested from tissue for analysis. The cells are stained with tissue specific dyes, and these need yellow, green, and blue excitation. The dies flouresce with wavelengths longer then the excitation, ie the blue excited dyes usually glow green, the green excited dyes glow yellow to orange, and the yellow excited dyes glow in the red. Now some blue excited dyes also glow red.
So killing the red line by making the OC optic extra transmissive in the red does two things, one, the red and yellow lines are not fighting each other for gain, so the laser is more stable. Two, its easier to filter out the weak flourescence if the extra red is not there.
The 643 tubes I've seen repumped, needed to be filled to a pressure right on the border between stable red and stable yellow, so you got both when the tube was new and then things faded to yellow with dim blue and no greens as the pressure fell. If the tube was overfilled, all you got was red and a bit of 462 blue.
If you really clean and peak the optics of that laser, and if it does not have a lot of hours on it, you may see a weak 575 nm Yellow.
The OMNI HR used in the KR and Mixed Fill 643s supports every visible ion line in the universe, its a true broadband 400-700 nm optic, so you can get other lines by switching OCs. I have a few sets, they are superpolished and ion deposition, very state of the art in precision optics. There are red only, RYGB, RYB, and YGB OCs out there. The lines shift around a bit with how well the cavity is tuned, so you might wish to gently touch up the mirrors.
Also the 530 nm green fights a bit with the 568 nm yellow, so you may see a tuning dependent balance change, there as well.
Keep the current down on that beast, they eat gas like there is no tomorrow at high currents.
Steve
