High-pressure sodium light bulbs with a mogul base have what people typically call a yellow color. They're used in warehouse lighting, roadway and security lighting such as shoebox and flood light fixtures. You've probably seen them in high bay warehouse situations where energy efficiency is critical. If you need a light fixture that uses these, they are sometimes labeled as HPS lamps as well. One thing to keep in mind with these lamps is that when they need to be changed is when you see them cycle on and off. These operate on ballast, and that ballast sometimes can't supply enough voltage when the lamp degrades. The lamp will be asking for more voltage than the ballast can provide and that will cause cycling. It means you need to change it soon so that it doesn't cause the ballast any damage or short life. These were always used because they were so energy-efficient and a lot of people have changed to white light such as metal halide or LED now. But they still have a place for many types of lighting such as security lighting or tunnel lighting, roadways and such. The mogul base is an E39 base and is the large industrial size base that you'll see used in all types of overhead lighting in warehouses or production facilities. These are often used in the higher wattage version of these, but there are also some of the lore wattages that use it as well. Look for the mark on the glass as to what wattage you have to make sure you get the right one. If you're having any questions regarding the replacement of these light bulbs or the proper operation, you can always give us a call. We've seen about everything you can see in regards to how these function into what might be a problem that you already have. We know the solutions, and this can be a little tricky at times to determine exactly what you need to do. But we can help you do that so that you spend the least amount of money in trying to fix the light fixture with these light bulbs. It's still a great way to keep energy costs down while getting maximum light output. And although there are more efficient lighting systems available now, there is nothing wrong with having a well functioning high-pressure sodium light fixture.