Light bulb temperature question

I have an infrared non-contact thermometer and I got bored earlier and started playing with it. I have an articulated lamp near my workstation and I decided to measure the bulb temperature. I found with the lamp pointed downward the temperature reads about 150[sup]o[/sup] F but with the lamp pointed upwards the reading is closer to 400[sup]o[/sup] F. I understand the heat rising thing, but glass isn’t the greatest conductor of heat and I’d expect the temperature of the nitrogen inside to be pretty much uniform after being on for a few minutes. Why the extreme difference in the temperatures of the top and bottom of the bulb?

What part of the lightbulb are you measuring?

The part of the bulb with the writing on that’s opposite from the threaded part. When the lamp is pointed down it’s the part that’s pointed down, and vice versa.

Perhaps the difference in temperature is caused by the convection currents outside de bulb; I´m not sure if I could explain it without a drawing, but… with the bulb pointing down the air around it heats and starts to flow around it, form the bottom along the sides towards the socket; so fresh air is sucked from below and flows around the spot where you measured the temperature. That should lower the temperature.
When the bulb is pointing up the air flows around the bulb before passing through the measuring point, so heat starts to concentrate there.
That´s what I think is happening

What’s the emission spectra of the bulb?

Maybe you shot at a slightly different angle, and caught more or less of a direct shot at the filiment, therefore producing a higher temp?

What an infrared thermometer sees has to be experienced to be believed - it’s a wonder anybody uses them. You could be reading a great variety of different things.
If you ever get to use a thermal imager, point it at the bulb, and decide whether the shade of the image is an indicator of the temperature of the exterior of the glass, or a reflection (things are generally more reflective in the thermal IR), or if you’re seeing through the glass, or what.
And heat rising won’t be the issue - it’s the heated air that rises, and air has such a low emissivity that the IR thermometer won’t see it.

No, these things don’t see through glass. The filament temp is around 3k, and the difference in readings is nowhere near that, plus the change in temperature takes a minute or so. If it were due to reading the filament it would happen nearly instantaneously. I like Ale’s theory the best so far. I hadn’t thought of the effects of an external convection, but that makes perfect sense.

3K is pretty cold. :wink:

So, QED you have a job now? Hope things work out for you. Should we expect your post pace to dwindle? :wink:

I can`t explain the readings you get. I would have thought you would get the oposite readings. Especially if the bulb is enclosed with some type of shade or reflector.

How many engineers does it take to change a lightbulb?

Zero, apparently.

From years of empirical evidence, the upward pointing part of a lightbulb is hotter than the other side. Using the wrong kind of light bulb in my bathroom fixture will cause the glass part of the bulb to separate from the base (bulbs pointing down), undoubtably due to excessive heat. In enclosed fixtures, such as a jelly jar light fixture, the bulb does become uniformly hot so one has to wait for it to cool(experience, again), so there’s probably something to be said about convection.

Put your hand above the light and see if you can feel much heated air flowing throught the vent holes that are almost always there.

No, no job yet. My workstation is where my computer is here at home. So, I’d expect to keep posting at the same rate for a while. :smiley:

How do you know? If you point an infrared sensor at a piece of glass, it’ll see a combination of emitted, reflected, transmitted and scattered IR. The temperature reading is accurate only if the received IR is dominated by the emission from the glass.

I know because glass is opaque to longwave IR, which is what these units detect. I proved it to myself on a hot day by measuring the surface temperature of a window and the wall next to it, and the readings were the same and both were significantly cooler than the sidewalk outside.

Pardon my hijack.

Lastr week I cleaned my (filthy, nasty) oven. I cleaned the light bulb in the oven too. It looked like a regualr ol’ bulb. Was it? Or are there special oven lightbulbs sold only at oven lightbulb stores?

No, a regular bulb will work fine, though depending on the location of the socket it may not fit. They make appliance bulbs that are somewhat smaller than standard bulbs, but AFAIK, they are not constructed any differently than standard bulbs, except for lacking the white diffusive coating on the inside of the glass envelope…

[url=]General Electric[/ur] says no.

That’s General Electric

I think he meant 3kºk (not to be confused with 3k[SUP]2[/SUP], MST3k or those cracker-ass crackers in the pointy hats) but everyone would think it’s a mutant smiley.

FWIW I usually set white balance on my digital camera for tungtsten at 3200ºk.

Yeah, he knows what I meant. He’s just being a smart-arse. :d