Lamp shade and housing 'bulb' rating?
Any form of enclosure around an incandescent light bulb has a maximum lamp wattage rating.
I assumed this was based on the maximum temperature the housing could safely handle as most of the power to the lamp was dissipated as heat.
Along came the whole new range of LED replacement lamps. What determines the maximum power rating of the shade or enclosure? Has this now shifted to maximum temperature the LED lamps can safely handle?
I have not seen a temperature rating for lamps of any type, but know from experience that flexible LED strips "don't like it hot"!
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There are several issues. The first is safety (fire risk), the second is lifespan.
Lamp fixtures installed in ceilings where they may be surrounded by insulation need to be IC (In Ceiling) rated. These fixtures are designed with internal space which keeps the hot parts sufficiently far from the exterior surfaces of the fixture to prevent ignition of adjacent insulation. The bulb itself will still tend to get hotter than in a non-enclosed fixture, which can result in reduce lifespan (especially for LEDs). There are some LED bulbs are specifically rated for use in enclosed fixtures, though. I ran across this link that has more on the topic:
Lamp shades are usually on non-enclosed lamps, where air flow can dissipate some of the heat.
Most incandescent-replacement LED bulbs generate significantly less heat than the incandescent bulbs they replace. With any bulb, some of the power used gets converted to light. The remaining power has to go somewhere, primarily in the form of heat.* Some of this heat from/through the spherical portion of the bulb, other portions of the waste heat are from the base of the bulb. With LEDs, most of the heat is at the base of the bulb, which may reach 60-100C, according to https://www.razorlux.com/light-bulb-heat-temperature-chart.html. Little of the heat comes from the spherical portion, which usually remains cool enough to touch on the LED bulbs, but can get quite hot on incandescent bulbs.
* A very small amount can end up as vibration (sound), either in drive circuitry components like ballast coils, or in vibration of the filament itself in an incandescent bulb. But this energy, while sometimes enough to be an audible annoyance, is not a fire hazard.
@BradLevy Thanks you have confirmed my thoughts and the links are good thank you. The paper globe style lampshade is what started these thoughts, rated at 60W conventional bulb, the bulb gets very hot, but the ventilation is obviously enough to prevent a fire hazard. The rating for an LED bulb I recalled is much lower say 8W ( I need to check) but that runs much cooler (less than 60C), so has the lamp manufacturer been conservative? Or is the ambient temperature experienced considered to approaching an LED life/failure temperature? Unfortunately, I do not have access to thermocouples to 'live' testing. Some early LED bulbs had a nice finned heatsink on the base, but these are far less common now as the shape emulates a conventional bulb, but wonder if these with more modern LEDs and electronics would have actually provided the published lifetimes? And then there is my PIR wall light enclosure, think I'll look for the higher temperature rated version provided it has a suitable light radiant pattern. It makes me smile with some house lights with LED bulbs mounted with the base downwards providing next to no lighting on the ground...
@Boss If the lamp specifies an 8W max for LED, it is probably based on worst-case assumptions about the amount of heat from the base and the capability of the socket to withstand it, with the shade not really coming in to play. I don't know how much of the heat of an incandescent bulb ends up in the base - different, I'm sure, depending on base down or base up operation. Here is a related link. A bit old (2013), but I think still pertinent. https://www.edn.com/that-60w-equivalent-led-what-you-dont-know-and-what-no-one-will-tell-you/
@BradLevy thanks. More searching does confirm there are LED lamps specifically designed for enclosed operation. Also some of the bigger specialist brands (not those generally sold at DIY stores) available from RS do have maximum LED case temperatures specified, so another aid to what may be suitable for use in a lampshade. Looks like there could be a market for a new bulb holder with a thermal design to pair with the lamp to remove the heat more efficiently and provide longer bulb life. Wonder if this will ever appear? Should be quite easy for an ES fitting.