Three weeks ago, we embarked on the Great Nightlight Mystery: Why does my daughter’s nightlight turn off in sunlight, but not when I turn on the overhead light?
First, we looked into how a spectroscope works so that we could investigate the different spectra of the lights in my house. We discovered that incandescent light bulbs contained all of the colors of the rainbow, much like the sunlight that comes through my windows. This is because sunlight and incandescent bulbs are both examples of blackbody radiation. The fluorescent bulbs, on the other hand, were only made up of a few colors of the rainbow, through when combined, the light still looks white. Finally, we looked into how the LED nightlight itself works and what colors of light is puts out.
So where does all this leave us? Let’s look at a summary of the spectra, or color outputs, of the different lights in my house:
The nightlight must have a photodetector – this is usually a semiconductor material that detects light. Just like the materials that emit light of only certain colors, this photodetector can be designed to only detect light of certain colors. My theory (I don’t know for sure) is that the detector was picked so that it would not detect light from the LED nightlight. It would then only be affected by outside light.
We checked this – our external bright LED light did not seem to affect the nightlight (turn it off) unless the nightlight was very close to the LED light bulb. The same is true for the fluorescent bulbs. However, a small amount of incandescent light or sunlight will turn the nightlight off.
Looking at the spectra above, it’s difficult to see why this is. I thought originally that the dip in light from the LED – the 500 nm blue region must be the light that the detector is sensitive to. However, the fluorescent bulb has light there. Perhaps the detector is sensitive to light in the bluish-green – that narrow area where neither the LED or fluorescent bulb emit light? I would need more information on the detector or some more sophisticated equipment than I have in my house to be sure.
The detector does seem to be wavelength dependent, though. It seems to be a poor design to me that it does not turn off in the presence of fluorescent light bulbs since many homes are switching over to fluorescent bulbs as a more efficient alternative to incandescent bulbs. In fact, there are new standards in the US designed to help phase out the inefficient high wattage incandescent bulbs. You can read more here. Since many of the more efficient light bulbs are compact fluorescents or LED light bulbs, my daughter’s nightlight will not be very useful in most houses – it will always remain on. Of course, if I open the shades, the sunlight will turn it off. I suppose the most efficient lighting will always be sunlight anyway, especially in very sunny Colorado.
What does this mean for you and your house lights? You will need to make choices between brightness, color, efficiency and cost. Sunlight and incandescent bulbs are probably easiest on your eyes, but new, better compact fluorescent bulbs and LEDs have better spectra and are much more efficient. Here are a couple of things to keep in mind:
- BRIGHTNESS: We are used to thinking of brightness in Watts, but really that has nothing to do with brightness. The Watts are just a measure of how much power is required to run a traditional incandescent bulb. The unit of measurement ‘lumens’ is a more appropriate measurement of brightness. The Department of Energy provides the following graph to help you convert between the two:
- COLOR: The new lighting label on the left above also includes a section entitled “Light Appearance.” This compares the light output to the light output of a blackbody of a certain temperature. If you remember from our discussion of blackbody radiation, a higher temperature means light that is more blue and less red. From the PhET simulation website on blackbody radiation, we see that the temperature of a typical incandescent is about 3000 K, and sunlight is approximately 5700 K. This can help you find a bulb that is more red (warm according to the label) or more blue (cool on the label) depending on what you prefer.
Of course, as we can see with our little handheld spectrometer, none of the lights are quite as good as just opening the shades on a sunny day – that has the best color and uses the least energy, but it is not always an option (especially at night!).
- EFFICIENCY & COST: The new lighting label also includes an estimated yearly energy cost and lifetime. With that and the cost of the light bulb, you can figure out which light bulbs are most cost efficient. Of course, if saving energy is more important to you than saving money, you may just go with the most efficient option.
Hopefully you enjoyed our light filled April and now feel confident in your knowledge of light bulbs. I at least better understand my daughter’s nightlight and realize that if I want it to turn it off with the room lights on, I had better get an incandescent light bulb, open the shades or possibly explore different compact fluorescent and LED light bulbs until I find ones with the appropriate color spectrum.