Soooo, I know that sunlight is harmful to beer. I am bottling into brown glass, so that part is covered. But for primary fermenting, I'm using a clear glass carboy, and I'm keeping that in a closet. (Secondary fermentation, if I do it, will be in a dark green glass carboy.) Ninety percent of the time, the light is off in the closet, but there are occasions where the fiance or I forget to turn off the lights, or I'm going in and out frequently for a couple of hours and leave the lights on.

I found an answer regarding flourescent lights, and another answer for incandescent, but my lights are halogen track lighting. I don't know enough about physics to say for sure, but I'm guessing that the answer is similar: halogens won't have a tremendous effect.

Is this correct, or do I need to be super-vigilant?

3 Answers 3


For skunked beer, we're worried about Violet and UV light. There's a lot of information available on this, so I won't go into that science. Basically, we want to limit as much light less than 500nm as possible and all light under 400nm (UV) if possible.

Halogen bulbs follow a similar pattern to incandescent bulbs (ref). This chart compares different bulbs, but halogens are not included. Comparison of Light Bulb Spectra

You can see that there's a big spike around 400nm in CFLs, as well as a small one below 400nm. There's also a lot more energy between 400 and 500nm compared with incandescents. LED light would likely have the same skunking problems.

Compare this to the spectrum of halogen bulbs. The maximum intensity in this chart is approximately the same as the maximum intensity of incandescent light in the previous chart. Halogen Bulb Spectrum

Yes, you shouldn't leave the light on, but also yes, it won't be as bad as CFLs. I suggest you put a dark T-shirt over the carboy for those times when the lights are on all night long. Even regular light bulbs will skunk beer if the exposure is over too long a time.

  • 1
    Thomas, thank you! This information is awesome (and really speaks to my geeky side).
    – object88
    Jun 2, 2013 at 16:05

Cover the carboys in a blanket for no light exposure.

  • 2
    sometimes simplest is best
    – mdma
    Jun 2, 2013 at 16:24
  • Works as long as there isn't a blow-off cap, or other precarious gadget. ;) I could always throw a box over it, though.
    – object88
    Jun 2, 2013 at 18:06
  • 2
    I used to use the cardboard box my carboy came in. I just taped the flaps(?) at the top open, so the height was extended a few inches, and now it slides right over the carboy + airlock, sealing out the light completely.
    – GHP
    Jun 5, 2013 at 12:18

You can use opaque trash bags. Check for opacity then find the size that loosely slides over your carboys. I've daylight streaming into my brewing area and unusually long primary fermentations, yet no skunk aroma.

  • Charlie, I've edited the answer to bring it up to standard. It's appreciated if you write complete sentences in future.
    – mdma
    Jun 5, 2013 at 14:45

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