Question: Ambient/Light Bulb Testing

idev247 is asking a question about spectrometry
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by idev247 | December 06, 2016 18:09 | #13752


I’m looking to compare different types of light bulbs for brightness, and color range. An example light bulb would be the Phiips Hue

Main Question

Do you think the Desktop Spectrometry Kit 3.0 would be able to measure the color spectrum of a lightbulb, and its brightness?

Secondary Questions

  • If the Desktop Spectrometry Kit does not work, do you know what relatively inexpensive tool type could be used?
  • If it can measure brightness, will I be able to compare two light bulbs? (will webcam exposure change?)
  • I was thinking of building a white box inside a black box to isolate ambient light. Good idea?

Background story

I’m new to the spectrometer world. I did quite a bit of research but I'm still not sure what tool needs to be used. I think it might be a spectrometer based on what the Sekonic C-700-U SpectroMaster Color Meter does. For a personal project I definitely don't want to spend $1,000+ on this.


Hi, I think the spectrum comparisons would be relatively straightforward -- but keep in mind that the kit requires a calibrated reference light to infer absolute measurements of brightness or intensity-calibrated spectra (#intensity-calibration), so lacking that, your comparison would be of lights relative to one another.

To compare absolute brightness overall is a harder question, but I'd suggest that you divide the "view" of the device in two so you could measure two bulbs at once, on either side of a divider.

If your bulbs are powered from the same source, and are identically aligned with the spectrometer (some kind of diffuser may be necessary, or perhaps scanning the light reflecting off a white surface, to ensure this), then scanning both at once in the same shot would ensure that the exposure compensation of the webcams is equal. Does that make sense?

Open to other ideas, and @stoft has done a lot of potentially relevant work here.

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The divider is definitely an interesting idea. I've been looking around and maybe I could use two tools: A lux meter for brightness, and the spectrometer for color. Would that make sense?

That way I'm always sure the lux meter is an absolute value. I'm just not sure if the spectrometer would be an absolute value. I might need to move it closer/further from the source depending if the light is too bright for the spectrometer's webcam.

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Oh, interesting -- curious about your lux meter --- cost or link to buy one? You could use it perhaps to confirm that you're reading equal amounts of light from each bulb, to isolate your variables in the test. Neat!

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I haven't bought a lux meter yet but what you mentioned is interesting. I could use the lux amount as a variable when doing the spectrometer measurements.

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I'll still researching. Based on what I found I'm probably better off getting a lux meter for brightness, and maybe the spectrometer for color spectrum. I'm just not sure if the light source (bulb) will be too bright for the spectrometer. Thinking I can probably move the source further away from the spectrometer to reduce brightness (in an box isolated from ambient light)

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Wow, the Phillips Hue is a 15 dollar LED light bulb with a wifi module. It requires a proprietary wifi router (~$40). The free app can control up to 50 of these lamps. I can't wait to hear about the first time someone replaces all of the light bulbs in a friend's house and starts punking him from outside the window.

I agree with Jeff that the Desktop spectrometer can produce very good spectra of a lamp. This will show you which parts of the visible spectrum are brightest. You can easily compare this type of result among lamps. For example one lamp might have 1.5 times more blue light than red light, and another lamp might have equal amounts of red and blue.

You mentioned color range, which could refer to the total spectral range of the light. It might not be possible to capture the entire range of the lamp because the range of sensitivity of the camera might not include the entire range of the lamp.

You are correct that the webcam will try to adjust its exposure to compensate for the brightness of the scene, so comparing two spectra with each other will not tell you much about how bright two light sources were. If your goal is to know which lamp is brighter, you can use any digital camera. Take a single photo of two or more lamps so that the brightest one is not overexposed. Then compare the DNs (0-255) in each color channel (e.g., Photoshop or Fiji). The photo has to be taken with extreme care so all lamps are the exact same distance from the camera. Lamps might have hot spots, so it would be best to use a diffuser or reflector to average the output of each lamp. This is not very quantitative (without an extremely careful procedure), but can tell you the relative brightness of a few lamps.

It is also possible to find software which allows manual control of exposure of the webcam. Then separate spectra can be taken with the same settings (with the lamps exactly placed, etc). Or any digital camera with manual control can be used to take individual photos of different lamps using the exact same settings and setup. These photos can be of the lamps, or of diffused or reflected light from the lamps, or of the diffraction pattern formed by the lamp light passing through a grating.


Ah, Chris reminds me of an additional way to control camera gain control -- use a camera, like the Raspberry Pi camera, where you can more tightly control or disable exposure compensation -- as the #webvalley folks did.

I asked a related question here:

Interesting. Thanks @cfastie and @warren. The answers seem to agree that the spectrometer might indeed be the right type of tool to measure color of a light bulb.

I'll do more research about comparing the different measurements from one bulb to another. Particularly, how the following might affect the reading:

  • distance from the light source to the spectrometer
  • brightness of the source vs. color reading (although not true in most cases, pretend that a light has a constant yellow value of 184; would the reading be the same if the light changes brightness)
  • manual camera exposure options
  • possible color range by different cameras
  • LED light source for cameras (some cameras flicker with LED lights)

I'll report back with my findings or questions.

There is one more issue to be aware of. In addition to automatic exposure, most simple cameras have automatic white balance. Automatic white balance can adjust the relative brightness of the three color channels (to make photos look normal regardless of ambient light color). So if you are comparing two photos or spectra of different lamps to learn about lamp color, you must make sure the camera's white balance algorithm did not alter the color balance differently for each photo. This might not be possible with most webcams.


Very good point @cfastie. Anyone know if the webcam that comes with the "Desktop Spectrometry Kit 3.0" has a fixed exposure and white balance?

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There's some guidance here, but it may be out of date: -- this'd be a good activity to document, if you have success with it -- or a question to ask separately from this one?

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