# Blackbody Radiation and the Incandescent Light Globe

Hi. I am wondering how well the filament in an incandescent light globe follows the basic shape of the classic blackbody temperature/spectrum curves.

The reason I ask is that I just set a small science project for a student to measure the voltage and current of a small torch globe (4.8V 0.75A Krypton type) at various voltage levels and observe the change in resistance as the filament temperature increases.

The basic experiment went smoothly enough and gave the expected results. However I then asked the student to attempt to place an approximate temperature scale to the data by observing the color of the filament at the various operating voltage levels and to estimate the temperature from these observations.

To do this part I gave the student a “color chart” that is good for anything that approximates the shape of the blackbody curve. Charts of this type are widely available and though the color judgement is somewhat subjective they should be able to provide a reasonable estimate of temperature.

See http://www.processassociates.com/process/heat/metcolor.htm for example (or as reproduced in part below for those who dont like clicking on links)

Color Approximate Temperature °C

Faint Red : 500
Blood Red : 580
Dark Cherry : 635
Medium Cherry : 690
Cherry : 745
Bright Cherry :790
Salmon : 845
Dark Orange : 890
Orange : 940
Lemon :1000
Light Yellow :1080
White :1205

The thing that I’m now trying to figure out is why we couldn’t get any color “cooler” than orange from the light globe. Even making observations in the dark to help see the lower light levels we were still unable to perceive anything below orange.

So I wondering if this is a result of
1. The filament for some reason not radiating in the same shape curve as a black body.
2. The light levels in the red region being too low to see for such a small object as a torch globe filament.
3. Perhaps (by design) the lamp is made to produce predominately yellow/white light. I don’t know how, perhaps something to do with the surrounding gas.

Does anyone have any idea why I can’t adjust the voltage to get less “temperature” than orange?

Thanks.

Last edited:

uart said:
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Does anyone have any idea why I can’t adjust the voltage to get less “temperature” than orange?

I am no expert; just a guess here. The problem probably lies in how you measure the color temperature.

As far as I know, the incandescent lamp is very close to black body radiation. Quoting your figures, at 500C, you should expect faint red. What that means is the spectrum has a peak at red (about 700nm wavelength). But there is also considerable energy in both the infrared range and wavelengths below orange.

How you measure the color temperature affects the color temperature you preceive. If you have a probe that measures the peak spectrum, you should indeed get a red reading. If you use your naked eye, then the green cones in your retina will be stimulated by the spectrum from orange onwards. As a result your brain will not intrepret the light as red (which won't stimulate green cones) but orange instead.

If you want to measure the peak spectrum, maybe you can focus the light onto a triangular prism and then observe the emerging beams.

Wai Wong