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How to measure the wavelength of a light source

  1. Feb 22, 2013 #1
    Hey Folks

    Quick disclaimer:

    I have no background in physics whatsoever but I have found myself trying to solve a problem that is seemingly based in physics so I am trying to learn.

    I also have a background in search and search engine optimisation which usually means I am a dab hand at locating information online, but in this case, I seem to be going around in circles which I believe is in part due to me not really knowing how to ask the question.


    After my wife was diagnosed with Multiple Sclerosis I have spent the last four or so years optimising our diet and lifestyle and whilst we pretty much have stress, diet and exercise dialled in I have now come to improving our sleep. I read a book called Lights Out that goes into detail regarding how our artificial lighting impacts our sleep. To cut a long story short, this all comes down to a hormone called melatonin that is suppressed by daylight and by artificial light so sitting at home, with the lights on, way after dark is bad.

    It would seem that melatonin is suppressed by certain colours of light - primarily blue light and that light over 530 nm on the spectrum of light is okay (as it is similar to more natural sources like firelight).

    This is a quote from wikipedia:

    The Question:

    It would seem there are a few bulbs on the market that possibly meet my requirement yet to be sure I need to measure the colour of the light in a room when using these bulbs. I would like it bright enough to read if possible but also dark enough and with no blue light so we get the benefits of cutting out artificial light.

    I have been trying to understand how to measure the nm value of a given light source but have found myself getting increasingly confused. I have looked at Photometry, Colorimetry, Spectroradiometry and a whole other bunch of *metry's and I am still unsure what I should be looking at.

    I have also seen a technique known as diffraction grating that again, is a little out of my ballpark and I unsure if this is correct for measuring direct colour from a bulb or the colour of light in a room.

    So, I would really love some help figuring out how to measure the light of a given light source.

    - is there a tool I can use for this?
    - are there companies or universities that could perform this for me?
    - is there anything else I should read / learn / research?

    I hope this question is well structured enough that some of you good folks around here can help .

    All the best!


  2. jcsd
  3. Feb 22, 2013 #2


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    If the light source(s) in the room does not emit light below 530nm, the room itself won't produce that (in a significant amount), so it should be sufficient to measure the spectrum of the light source(s). This can be done with a diffraction grating, and some method to direct light to that grating (everything should work, including a small slit in some paper). It might be possible to see the result by eye (is there blue/green light somewhere?) - alternatively, a simple calculation should give the wavelengths.

    A CD is a low-tech version of the diffraction grating: If there is really no blue/green light anywhere, it should be fine.

    Probably, but that could be expensive.
  4. Feb 22, 2013 #3

    Simon Bridge

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    You would use a spectrometer of some kind to find the wavelength of the light.

    The cheapest and easiest to use are the diffraction-grating versions.
    Don't be intimidated by the theory - the actual calculation is a matter of multiplying and dividing.

    You can even build your own and cheap commercial spectrometers are available.

    You can buy diffraction gratings - they will be rated with the line density (no of lines per millimeter) written on them. You need this number - call it p.

    Put the light bulb in a box so light can only come out a small hole - helps if the hole is a slit.
    Put the diffraction grating over the hole. Turn out the room lights.
    Shine the light you get onto a screen.
    You need to know how far away the screen is - call that L. You want it to be at least a meter or so away to make the math easy.

    The light will be a central narrow bar the same color as the bulb, and a series of batches of color on either side of it.
    The batches will look like rainbows for regular lightbulbs, and like collections of lines for some others.

    Use a ruler to measure the distance from the central bar to the first appearance of the color you want the wavelength for... call this distance x. (It is usually easiest to use a paper screen and mark the different color positions with a pencil so you can use your ruler with the lights on.)

    Then the wavelength is $$\lambda = \frac{x}{pL}$$
    You can get more accuracy by using the other bunches of colors, but this should work for your purposes.
    If all you care about is the absence of blue-green, then this will be obvious from inspection - there will be no blue-green in the colored bunches.
  5. Feb 22, 2013 #4


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    You can buy a simple hand-held spectroscope on ebay. For instance, this one for $60 or this one for $70. It is like a small monocular to look at and you just point it at the light source and see the spectrum against a scale. Quite accurate enough for what you need. There are others, even cheaper, I think. You really don't want to get involved in construction something when this sort of thing is available already made up.
  6. Feb 22, 2013 #5


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    Not sure you need to go to that much effort. We know that the mercury spectrum has a strong blue line, so no standard florescent bulbs, incandescent lamps, as already mentioned have a continuous spectrum which contains blue, so no incandescent lamps. That leaves sodium, sodium has only orange lines, no blue. If you can find, then tolerate, the harsh light from a sodium source it will do what you want. OR just use candles

    Beyond that I would be very skeptical about what may be a lot of new age who-ha in these books you are reading. Look for real scientific works to verify any book making these claims.
    Last edited: Feb 22, 2013
  7. Feb 22, 2013 #6


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    A filter might be interesting as well, as it can block the blue light.
  8. Feb 22, 2013 #7

    Simon Bridge

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    @sophiecentaur - that's awesome!
    @mfb - I realized after posting ... anyone really worried should just use a filter.

    The melatonin thing is a well documented effect... but there are other factors affecting sleep... OP has already mentioned diet. I think the operative question is "how much" - how much does sitting up at night affect health? If healthy sleep is the target, then just going to bed early seems to be the answer.

    Conditions like Multiple Sclerosis attract an enormous amount of pseudoscience and quackery - basically just preying on people. Maintaining skepticism is a good call.
  9. Feb 22, 2013 #8
    It would be optimal if you bought a spectrometer ready and tested. Yet, to me you shouldn't place it before your light bulb and measure its spectrum. You should place it in front of you and your wife and measure what wavelengths and at what intensities come towards you. This, mainly because there will be some scattered light which reaches the place that you sleep.
    As far as the spectrum of the lamp is concerned, some of them must have specifications from the manufacturer, so you could see exactly what is the peak wavelength and the intensity distribution.
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