US HPS lamps are not compatible at all with EU ballasts for any wattage, even if they are nominally the same arc voltage.  USA generally uses constant wattage ballasts, which are relatively immune to lamp voltage changes. Because of this the manufacturing tolerances for lamp voltage of USA HPS lamps are quite huge compared to the EU types.  If used on the EU choke ballasts, which are highly sensitive to lamp voltage variations, those lamps will be proportionally over or under run.  They will run at different power with consequences for lumens, colour consistency and life.  EU HPS lamps can however be used on USA ballasts, because their voltage will always be much closer to the nominal value.

Even though that there are some series choke ballasts specifically designed for medium wattage North American high pressure sodium lamps, what is the precise explanation of North American 400W S51 high pressure sodium lamps being 100% incompatible with European 400W high pressure sodium ballasts and North American 250W S50 high pressure sodium lamps being incompatible with European 250W high pressure sodium ballasts? From what I see, I can hardly see any problems with interchanging North American 400W S51 high pressure sodium lamps with European 400W high pressure sodium lamps and North American 250W S50 high pressure sodium lamps with European 250W high pressure sodium lamps due to those lamps in particular having a similar arc voltage drop and operating current across lamp wattages. I was wondering if the 50Hz frequency of the European power grids has to do with North American high pressure sodium lamps being incompatible with European magnetic ballasts. In that case, should there be a significant reduction in service life of North American high pressure sodium lamps on 50Hz magnetic ballasts or any harm to European ballasts?

The easiest and cheapest way is to limit the current going to the LED with a potentiometer. There are programmable potentiometers available with current ratings suitable for some LEDs, if you want to use say a Raspberry Pi or Arduino.

The most efficient way is PWM dimming, where the led is switched on and off really fast, but the length of time it's on is changed. A little more complex, but still manageable for a DIY project.

I believe these were also wired in series and they tended to piece together whatever for those circuits.

I have seen some FCO high performance optic 325s on Caltrans roads.

Probably a case of whatever they could get their hands on. The old lights were probably damaged all at once or had issues at some point and they had to get new lights ASAP. If you look closer, one of the the lights is a former HPS 325 with high performance FCO optics.

Hello fellows,
 In classic incandescent lamp, you could just "slice" the sinusoidal wave with the help of a potentiometer, diac and capacitor - or a MCU with zero-crossing circuit. EASY.

However, today we have dimmable LEDs bulbs. I'm considering make an alarm clock with AC output and a LED dimmable bulb, so it could wake-up with the light, simulating sun-rise. I'll 3D print everything... For sure, such devices already exists, however I wanna implement more features like temperature/humidity (because I can), bluetooth connection, proximity sensors, etc... So I'll make a custom PCB and firmware, and this will be my 30th birthday project.

The question is: Do you know which technique are applied to control/dim this kind of bulbs?

I'm electrician, so I already know the dangers of dealing with LIVE AC, how to mitigate and how to design Live AC safe pcbs with optocouplers --- don't worry about that.

thank you in advanced 

I have never seen something like this before!
https://www.google.com/maps/@34.1702292,-118.6033703,3a,15y,115.45h,104.33t/data=!3m8!1e1!3m6!1sB2Oji_SXNe1m8jWTlA9Ufg!2e0!5s20150701T000000!6shttps:%2F%2Fstreetviewpixels-pa.googleapis.com%2Fv1%2Fthumbnail%3Fcb_client%3Dmaps_sv.tactile%26w%3D900%26h%3D600%26pitch%3D-14.334806946776752%26panoid%3DB2Oji_SXNe1m8jWTlA9Ufg%26yaw%3D115.45247563119204!7i13312!8i6656?entry=ttu&g_ep=EgoyMDI0MTIwNC4wIKXMDSoASAFQAw%3D%3D

In this topic i will post pictures of old Polish latarens and describe it.

First will be one of my favorite lamp MESKO OURW-2250. This lamp was made by MESKO in 70's and 80's. This lamp was made for two 250watt mercury vapour lamps. The body of the luminaire is made of an aluminum alloy as a pressure casting. The luminaire is equipped with two side directional reflectors made of electropolished aluminum sheet. Mercury lamps are placed vertically. The lamp chamber is closed with a prismatic glass cover, which ensures a rational light distribution and constant light efficiency throughout the period of use. The chokes are placed in metal housings and filled with resin. The luminaire is installed on a tubular boom with a maximum external diameter of 60 mm. Here are some photos of it

@fluorescent lover 40- Thanks for the info about what Francis has.

@Baked bagel 11 - Cool. A 150w mag-reg ballasted pair.

Francis does have a few 100w M-250R2s with multi-tap CWA ballasts if you consider one of those.