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Thread: Solarmeter vs. a true Spectrometer

  1. #1
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    Default Solarmeter vs. a true Spectrometer

    For most people, the cost of a true spectrometer is out of reach and so the instrument of choice (and even that isn't that common) is to go with a Solarmeter instrument. I've often warned people in the past about the inherent inaccuracy about the Solarmeters, due to how they actually respond to light and give you a measure (ie. an average over a very wide range of unequally measured wavelengths) and have said they really only give a 'ballpark' measure. Little did I realize how enormous a ballpark could be.....
    I've just returned from a workshop on amphibian conservation, and one of the exercises we did was compare a Solarmeter 6.2 to a laboratory spectrometer that measured ONLY the UVB wavelengths responsible for Vitamin D3 photoactivation, and the results might be surprising to people. Under relatively weak artificial lighting (I stress, relatively WEAK), the true reading of UVB was about 30% what a Solarmeter indicated (so if you read 10 uW/cm^2 on a Solarmeter, the true utilizable UVB intensity is about 3-4 uW/cm^2). Under full sunlight the accuracy was even worse, the true reading of UVB intensity is only about 10% the value that a Solarmeter gives (so if a Solarmeter says 200 uW/cm^2, then the true reading is only about 20uW/cm^2). I wish we had the time and means to make something of a respectable calibration curve, but sadly this just wasn't possible in the time we had. Nevertheless, it is a start to getting a better understanding of scenario. So if you rely on your Solarmeter to determine if a bulb really needs changing, then I would say that if you're reading 4 uW/cm^2 or less, then it is likely your animals are not receiving utilizable UVB at all.

  2. #2
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    So, at what level would you suggest changing a bulb when using a solarmeter? I am so grateful there are people that understand the sciences of these things =)

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    Unfortunately, there's still no evidence out there on exactly HOW much UVB animals should have exposure to, or for how long. To complicate things, I know I have rehabilitated animals with hypocalcemia/vitamin D3 issues using old, mostly UVB-burned out bulbs: basically reinforcing the idea they don't need high intensity. My own preference is to buy the highest UVB output bulb I can afford, and use it until there is "nothing" left. I'd say if your meter is telling you less than 5uW/cm^2 at the level your animals would receive it, then it's time to replace the bulb for sure.

  4. #4
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    Wow. Well I was just thinking of buying a Solarmeter. I guess I will pass.
    Laura R (FL)
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    Please don't get the idea that I'm saying Solarmeters are crap, I'm not. I'm just saying that you need to keep in mind their limitations - ie. accuracy. I think they're still useful for giving you an idea where you're at with your bulbs, just not too precisely. The option is an instrument that costs a rather large sum, and for just measuring lightbulbs that doesn't make fiscal sense.

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    Thank you tupinambis for the information. I always bring my bulbs to our local reptile store ( not Petco or Pets Mart ). They check the bulbs for us.
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  7. #7
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    Tupinambis, thanks for the great info. I am assuming you were using a Solarmeter 6.2. I have a 6.5 which is a UVI meter, it supposedly measures the wavelengths that are most responsible for vit D production rather than the broader range that the 6.2 measures. Do you have any thoughts on this device related to your recent findings?

    Thanks again.

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    Indeed, we were using the Solarmeter 6.2. The 6.5 is probably even more "inaccurate" than the 6.2 for what people want. And I want to point out that this isn't because of them trying to hide "crap" equipment or anything like that, it is just that this is how these things work and people fail to understand the responses of these devices.

    For the model 6.2, go to: http://www.solarmeter.com/images/SM60graph.gif
    This graph tells us the response of this device to various wavelengths. In it, it responds MOST STRONGLY (equal or greater to 0.9 response - or in common terms more respective of the wavelengths present in this range) to wavelengths in the 275-295 nM range. However, it is still responding to wavelengths greater and lesser than the aforementioned range, but just not as strongly. What this means is if the intensity of the 285 nM wavelength is 10uW/cm^2, then for this wavelength it is getting the response 10uW/cm^2 - so for this wavelength it is accurate. However, if the 260nM wavelength is actually 100 uW/cm^2, and the 320nM wavelength is also 100uW/cm^2, the device gets a response of 60uW/cm^2 for 260nM, 40uW/cm^2 for 320nM (both of which do next to nothing for Vitamin D3 production), takes this values and averages them, and spits out 36.6uW/cm^2. Keep in mind, this is just for the three values in this example, the device is basically doing this for the entire range. So to minimize the bias in the unuseable range, maximise the bias in the useful range, it is more sensitive to the range needed, less sensitive to the useless ranges - however still sensitive and therefore not a real accurate picture of the Vitamin D3 producing wavelengths.

    When we look at the response curve for the 6.5 model: http://www.solarmeter.com/model65.html
    We see that it is highly sensitive to the wavelengths less than 285 nM (useless), less sensitive YET more sensitive than the 6.2 model for wavelengths greater than 310 nM (again, useless). So this would be even more "out of whack" from the readings on the 6.2. Plus, it spits out a value of UV Index - which I have little idea how it relates to Vitamin D3 production, just know that it is a unitless "measure" that relates to how well people get sunburn.

    This is why an accurate spectrometer is so darned expensive - it has the hardware necessary to remove more of the "noise". And in technology, greater accuracy means greater $.

  9. #9
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    I didn't ever look closely at the response curve for the 6.5, yes lots of noise at the edges. Some work has been done with respect to the interpretation of the UVI reading, however. Here's a link to the article (Tupinambis, you can probably access it, I can't unfortunately but I think it's a good start from what I've read):
    http://onlinelibrary.wiley.com/doi/1...20255/abstract

    I was thinking that if you knew the UVB spectrum of the lamp you were using, then some of this noise could be eliminated. For example, I looked at the spectrum for a 100W SB Mega Ray lamp. There is very little production of any wavelengths under 285 nM. You should be able to conclude then that most of what the 6.5 is measuring is not below 285 nM. Eliminates some noise anyway, but it only works if the lamp producer provides a spectrum and if you trust it to be accurate.

  10. #10
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    Indeed, it would be possible to eliminate some of the noise if you knew the whole spectrum and it's intensities....but then if you knew this information, you'd have little need for the Solarmeter I am "comfortable" just going with "30%" of the reading for fluorescent bulbs, "10%" for high intensity bulbs. It may not be all that accurate, but it does reduce the size of that ballpark.

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