# Incandescent Lamp Efficiency



## Les (Nov 20, 2015)

So I got caught up in a discussion about incandescent A-style lamps the other day. Someone had a problem with their 40w appliance bulb melting the lens on the inside of their refrigerator, and one reply was _"Try to remember that 2 different 40 watt 120v bulbs can create different amounts of heat based on how efficient they are"._

To me, this is incorrect. Two 40w 120v incandescent lamps will always create (roughly) the same amount of heat. His reasoning was that they can vary depending on the resistance of the filament. In my mind, this is what determines the wattage (or voltage?) of a bulb. So you can't vary the resistance of the filament and still have a 40w 120v bulb. 

Who is right, who is wrong, and why?


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## techieman33 (Nov 20, 2015)

Hopefully one of our lamp experts can come along with some science. I could see the other person being correct though. Take the 575w HPL for example, you can buy two versions of it that are both 575w and 115v. You can buy the standard version that is rated to last ~300 hours and puts out somewhere around 16000 lumens. Or you can buy the long life version that is rated for something like 2000 hours and puts out around 13000 lumens. The wattage and voltage are the same, but they are obviously different in other ways. I don't know if the heat output is very different between the two versions or not. It does provide some food for thought though.

In the case of an A style lamp I would imagine that the various coatings on different brands and styles of lamps would be the most likely culprit for large temperature differences.


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## JohnD (Nov 20, 2015)

For the sake of argument, are they both the same size, say for instance one was an A15 and one was an A19.......


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## robartsd (Nov 20, 2015)

While the overall energy output of two 40W lamps should be the same, the distribution of that energy certainly can differ both in space and in spectrum - a change in either would be cause for a temerature diference at the lens. Also while the reply says "2 different 40 watt 120v bulbs", I think many people outside of the professional lighting industry might refer to any lamp for 110-120v service as a "120v bulb"; and a difference in voltage rating between two lamps with the same wattage rating certainly should mean that they have different overall energy output when placed in the same fixture.


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## garyvp (Nov 20, 2015)

Les said:


> So I got caught up in a discussion about incandescent A-style lamps the other day. Someone had a problem with their 40w appliance bulb melting the lens on the inside of their refrigerator, and one reply was _"Try to remember that 2 different 40 watt 120v bulbs can create different amounts of heat based on how efficient they are"._
> 
> To me, this is incorrect. Two 40w 120v incandescent lamps will always create (roughly) the same amount of heat. His reasoning was that they can vary depending on the resistance of the filament. In my mind, this is what determines the wattage (or voltage?) of a bulb. So you can't vary the resistance of the filament and still have a 40w 120v bulb.
> 
> Who is right, who is wrong, and why?



I agree with your assessment, if they both consume a real 40 watts. However, I am confused by the situation. The refrigerator light is supposed to go off when the door is closed, so why is it melting the lens? Or is the door left open that long? Or is it a commercial unit? Or is this the final scene in _'Wait Until Dark"?_


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## DELO72 (Nov 23, 2015)

So, there are couple things at play here-- the plastic should be rated for whatever heat the lamp that is specified gives off. Now- if they left the door ajar and open for hours, MAYBE something could happen, but I highly doubt it. Remember- this is INSIDE A REFRIGERATOR, so the heat is going to be less due to the fact it's in a cold environment. The only thing that I can think of that might cause it is if the lamp is old and the filament sags and starts to arc out-- at this point you might get a huge inrush of current and heat beyond what the lamp socket can handle, and that could potentially cause the heat which melts the plastic. OR- they used a bulb that is too big, and the glass was touching the plastic (if it says use an A11 lamp for instance, and they use an A19 and the glass is pressed up against the plastic....)

I agree that 40W of power at 120V is going to be roughly the same amount of heat. If it was a 40W bulb and was rated for 100V, then it would be over-driven, and could produce more heat, or conversely if they accidentally put in 60W bulbs, that could easily do it.


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