While at first glance it would appear that this is an attempt at making a halogen bulb, this experiment serves no practical purpose other than to observe optical transitions in iodine vapor when shining a green laser through. Iodine has several transitions at the wavelengths of green laser pointers (532 nm) and green helium neon lasers (543 nm), and by shining one of these lasers through iodine vapor the presence of these transitions becomes apparent by the reemission of green light in random directions, observed as a green beam in the vapor. Rather than making a simple cell containing iodine, I decided to add in a filament to allow the iodine to be easily heated.

The bulb was constructed in the same manner as discussed in Homemade Light Bulbs, up until the vacuum seal is performed. After flashing the filament to burn off contaminants while the bulb is attached to the vacuum pump, the bulb is removed from the vacuum pump and iodine crystals are dropped into the bulb. The bulb is then pumped down again, and the bulb is heated slightly with the torch so some of the iodine vaporizes to help push out residual gas in the bulb. The vacuum seal is then performed. The resultant bulb is shown below.

The bulb is made of soft glass, so it is not possible to operate the bulb at high enough temperatures in order for it to function as a halogen lamp. This bulb also has much more iodine than would be used in a halogen lamp. Even at room temperature, there is enough vapor in the bulb that it is brownish in appearance.

Upon passing a green helium neon laser beam though the bulb, the beam becomes clearly visible in the vapor, as seen below.

As a simple "proof" that the beam observed is the result of an optical transition in iodine, rather than backscatter off small particles, a red laser may be passed through the bulb. In the picture below, a red diode laser (somewhere around 650 - 630 nm) is passed through the bulb perpendicular to the green laser. While the green laser is clearly visible in the iodine vapor, the red laser is not.

In order to increase the concentration of iodine in the bulb, the filament may be lit, heating the bulb and evaporating more iodine.

The as vapor concentration increases, a purple hue in the bulb becomes darker.

When I burned this particular filament brightly, it resulted in a light deposit on the bulb walls, likely due to contaminates left in the bulb (the filament was previously lit at lower voltages, which did not result in any deposit). When I performed the final pumpdown of this bulb, I used a refrigeration compressor (see Refrigeration Compressor Vacuum Pump), rather than the two stage rotary pump I have been using for light bulbs (a Welch DuoSeal 1400). I was not sure how the vacuum sealing process would go, and did not want to contaminate my good vacuum pump, so I thought I could get away with the refrigeration compressor. While the refrigeration compressor can only pump down to tens of Torr at best (the DuoSeal is likely taking me at least into the 10-3 Torr range, although I don't have a gauge to measure this), my hope was that by heating some of the iodine during pumpdown, the vapor would push out any residual gas. However, it is likely that after heading the iodine and the bulb cooled, gas was pulled back into the bulb before the vacuum seal was made. The residue left on the bulb after cooling to room temperature is shown below (notice the glass looks slightly foggy when compared to the first image on this page).