Light Bulb Spectroscopy

In college I had the opportunity to play around with the spectrometer after I had finished my work in the lab. We had used it earlier in the year to observe the Hydrogen-Deuterium isotope shift and the emission spectra of some different gases, but some of us wondered what would happen if we used it to measure the emissions of a simple light bulb.

So one day I brought in two small incandescent bulbs from Radio Shack, one rated at 1.5 volts and another at 12 volts, powered them up, and began to measure the spectra.

12v Lamp                                           1.5v Lamp

Measuring the emissions of light bulbs


The spectrometer measures the intensity of each wavelength by turning incident photons into current. The more intense the light, the more current. This current was then converted into a voltage which was measured with a voltmeter, thus the intensity is scaled in volts. I set the spectrometer to sweep through the wavelengths slowly, so I could get a high resolution curve. Eventually, I got the spectra for the two lamps:

9V lamp spectrum

1.5V lamp spectrum

I was surprised by the data. Assuming the bulbs had tungsten filaments, it seemed logical that the spectra would resemble a black body spectrum. Seeing that the light bulbs emissions peaked somewhere in the orange-yellow area (~620 nm), I estimated that the bulb's filament was reaching a temperature of about 4800 K. Considering that tungsten melts at 3600 K, this couldn't be correct. Also, incandescent light bulbs are supposed to peak in the IR region...that's why they get so hot! I plotted the theoretical spectrum of black body at that temperature using Planck's law so that I could have something to compare my spectra to:

Emissions of a perfect black body at 3600 K

Overall the observed spectra differ substantially from the theory. Comparing the spectra of the lamps to the black body spectrum indicates that something is either absorbing or reflecting a substantial amount of light in the 350-580 nm range (violet, blue and green emissions); the bulbs are dominantly red-yellow. In addition, the bulbs peak at a very low frequency compared to the black body spectrum.

Considering that tungsten is to good approximation a black body radiator and is often used to calibrate spectrometers, I can only assume that the differences between the expected and observed results lie in the spectral sensitivity of the spectrometer's detector, or the source is not a tungsten filament.

Measuring the Absorption of Colored Dyes

The colored slide is placed in
front of the spectrometer

I also brought in some food coloring to try to observe some absorption. I placed a glass slide filled with a small amount of dye in between the light source (I used the 1.5V light bulb) and the spectrometer slit. I recorded the spectra for a both red and green dyes and compared them to the original spectrum of the 1.5V lamp.

Blue: Original (No Slide)
Red: Red Slide
Green: Green Slide

Subtracting the new measurements from the original, I came up with the absorption spectra for each of the two dyes.

Red slide absorption spectrum

Green slide absorption spectrum

The red dye seemed to be more effective at blocking a wide range of non-red light, whereas the green dye block wavelengths specifically in the 600-650nm range. Despite the odd spectra of the lamps, the absorption spectra produced predictable results.

Conclusions

Since both of the bulbs I used produced the same unusual spectrum, I was sure the bulbs weren't defective. I did some research online to try to explain the shape of my spectra in the first part of the lab. Most of the spectra I found for light bulbs looked like black body curves, like the theory predicts. I did manage to find one website which had a spectrum similar to mine. The bulbs were designed for small hobbyist electronics projects, so it is possible that they weren't typical incandescent bulbs.

On the positive side, the absorption part of the experiment was successful, even if the results were a little noisy.