Tea light of another color

kw: analytical projects, lamps, spectroscopy

Recently a friend gave us a goblet she made that she calls Tree of Life. Perhaps there are 12 colors on it to match the 12 fruits mentioned in the book of Revelation (I didn't count). She also gave us a candle to put in it, but the candle was in a large jar and didn't illuminate all of the goblet. So we tried a tea light candle, which worked nicely. However, we don't usually burn candles, and we wanted something safer (the cat might knock it over). So we tried an amber-colored LED tea light. That was a poor choice! We had a different kind of LED candle, with a whitish-yellow colored light, so we tried that, with mixed results. Here is the goblet with the three lights inside in order: flame, whitish LED, and amber LED.

These are the lights in the order shown above. The flame is clearly the best all around. The whitish-yellow LED candle is too tall to illuminate the whole goblet, but it show the colors well. It also uses a moving reflector to make a flame effect, but that blocks most of the light that would go out the back. The amber LED tea light, while it illuminates the whole goblet, has no range of color! (There are blue reflections in the second and third photo from a nearby computer monitor.)

It is clear that the amber LED has a narrow spectrum. How narrow? I determined to find out. Here are the results of spectroscopy of the three lamps, and also an incandescent lantern bulb.

While the flame (second spectrum) is whiter than the whitish-yellow LED, it has a broader color spectrum, though not as broad as the incandescent lamp shown first. The bright peak at the blue end of the third spectrum is normal for an LED using phosphors to add the red through green and light blue colors. LED lamps for home use use the same principle.

The amber LED (fourth spectrum) does not use phosphors. It is a low-voltage LED that has a color peak in the orange-yellow area (near 590 nm). Its bandwidth is similar to that of the blue excitation band of the other LED that does use phosphors. There is just a trace of red and a bit of green, but they are overwhelmed by the yellow-orange peak. So all the colored blobs on the goblet just look yellowish.

The lantern bulb has a full-width spectrum, from below 400 nm to beyond 700 nm; the visible portion is 300 nm wide. The candle and the whitish LED have bandwidths nearly as broad. But the amber LED's bandwidth is a mere 75 nm, and the brightest portion is no wider than half that. It mostly just makes amber-colored light and nothing else.

I plan to carry a hand spectroscope with me the next time I go to buy LED tea lights, to find a brand with a broad spectrum that'll illuminate this goblet properly!