Whenever I have been looking at different 18w PL-L lamps designed for the North American market, I have noticed that some of the 18w PL-L lamps are designed for rapid start ballasts while others are designed for preheat ballasts. I am wondering if both the rapid start 18w PL-L lamps and the preheat 18w PL-L lamps run at the same voltage and current or does the 18w rapid start PL-L lamp run at a different voltage and current than the 18w preheat PL-L lamp. If the 18w rapid start PL-L lamps run at the same voltage and current as 18w preheat PL-L lamps, should I be be able to safely operate the 18w rapid start PL-L lamps on preheat ballasts?

Here is some information that I gathered regarding information about 18w PL-L lamps for the North American market:

 The FT18DL and FT18DL/RS lamps are not electrically interchangeable

In the old times of F40T12 when RS spread out and was replacing preheats, the lamps were modified as well: The RS got lower voltage, higher current filaments. The reason was, the permanent heating together with the material (the emission coating) loss over lifetime made the thermal balance very unstable on the thin preheat filaments, yielding their faster degradation. On the other hand the thicker filaments did not warm up enough during preheat, so RS lamps suffered from accelerated starting wear on preheat ballasts. But that was not considered as too big problem, when the starting itself was still reliable. After some time F40 preheat ballasts disappeared, so did the "preheat" spec lamps.

My guess the technical difference with the PLL18 would be the same, just the drawbacks became more significant to the extend the operation on the other ballast affects the lamp way too much (the thin lamp has higher ignition voltage, so it really needs to preheat it properly for a reliable start). Plus RS lamps need some means of ignition aid, so they retain the selectivity so only hot electrodes lead to ignition (so lower OCV ignites the hot electrode lamp in a reliable way, but won't ignite the lamp with cold electrodes)
But this would be just my guess.

The nominal arc current you may compare rather easily: Connect them on the same mains frequency ballast (for a short time they will survive that) and measure the arc voltage during operation. If it is the same, the rated current will be the same too, as the rated power is the same 18W.

In the old times of F40T12 when RS spread out and was replacing preheats, the lamps were modified as well: The RS got lower voltage, higher current filaments. The reason was, the permanent heating together with the material (the emission coating) loss over lifetime made the thermal balance very unstable on the thin preheat filaments, yielding their faster degradation. On the other hand the thicker filaments did not warm up enough during preheat, so RS lamps suffered from accelerated starting wear on preheat ballasts. But that was not considered as too big problem, when the starting itself was still reliable. After some time F40 preheat ballasts disappeared, so did the "preheat" spec lamps.

My guess the technical difference with the PLL18 would be the same, just the drawbacks became more significant to the extend the operation on the other ballast affects the lamp way too much (the thin lamp has higher ignition voltage, so it really needs to preheat it properly for a reliable start). Plus RS lamps need some means of ignition aid, so they retain the selectivity so only hot electrodes lead to ignition (so lower OCV ignites the hot electrode lamp in a reliable way, but won't ignite the lamp with cold electrodes)
But this would be just my guess.

The nominal arc current you may compare rather easily: Connect them on the same mains frequency ballast (for a short time they will survive that) and measure the arc voltage during operation. If it is the same, the rated current will be the same too, as the rated power is the same 18W.

When I read the data on a North American Osram Sylvania brochure that says that the 18w rapid start PL-L lamps have an arc drop of about 75v while the 18w preheat PL-L lamps have an arc drop of about 61v. If I do get my hands on the correct ballasts for the 18w rapid start PL-L lamps, I will measure the current draw from those lamps. As far as I know, the 18w preheat PL-L lamps are intended for operation on F20T12 preheat ballasts.