SI1145 results

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primexandy
 
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SI1145 results

Post by primexandy »

I've plotted Vis lux and IR lux and Insolation against the UV index produced by the SI1145.

Interesting results.

All 3 graphs are scaled to put them within the same range.

What's interesting is that IR and VIS follow each other closely until the UV gets to higher values and they appear to come closer then move apart although this could be lack of readings at the higher UV index.

Two things I am interested in, 1 is the sudden drop in VIS for no apparent reason, on each occasion there was a reading but it was between 2 and 3. Can't explain this other than a bug in the SI1145 or for some reason the value was read when the device hadn't populated both the high and low nibbles but if this was the case I should have also seen the same in the IR readings.

The second interesting thing is the relation to the insolation produced by my solar panel. The spikes in the solar panel readings match precisely the IR (very small) spikes. Am I seeing the spikes caused by FAR Infrared? I'm presuming that the insolation climbs higher quicker because of the far infrared and the little spikes in the IR from the SI1145 also reflect spikes in FAR infrared which is having some effect on the SI1145

...any one care to comment?

The Insolation readings are taking at the same time as the UV,IR and VIS. The insolation figures aren't temp compensated and if they were the insolation graph would climb quicker.

Would welcome any ones thoughts on what I am seeing here, especially an Adafruit comment about the VIS dropping. The graphs represent 200 readings throughout the day from just past midday to just past 3:30pm today. Ranging from plenty of cloud and overcast to full sun but a fair few sunny and cloudy spells mixed in.


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Last edited by primexandy on Tue Sep 09, 2014 1:35 pm, edited 1 time in total.

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adafruit_support_mike
 
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Re: SI1145 results

Post by adafruit_support_mike »

The dips in the visible light could be clouds or other distant shadows. There are lots of materials that are opaque to visible light but transparent to IR.. that's why they're measured separately.

The photoelectric effect involves photons of incoming light knocking electrons in a material up to a higher energy state. It's especially easy to see in semiconductors, which have a relatively narrow gap in energy levels between the 'covalent band' (electrons localized to a bond between two atoms in the crystal) and the 'conduction band' (electrons not localized to any specific atom).

Ideally, a photon provides exactly enough energy to push an electron from the covalent band to the conduction band. Any excess energy gets converted to extra kinetic energy in the electron, which gets distributed through the crystal as heat, and is thus wasted.

The ideal bandgap voltage for light from our sun is about 1.5eV, which is in the high infrared spectrum (825nm). Most solar cells are made from silicon, whose bandgap is about 1.25eV at room temperature, and that's down in the low infrared spectrum (990nm).

Those are just center frequencies of a statistical distribution though.. electron energy levels are called 'bands' because there's a theoretically infinite number of energy levels associated with any given orbital, each one Planck's constant away from the other. The minimum energy for an orbital is a hard limit (the Fermi Level), but it's normal to see electron energies distributed across a range of excitation levels.

The more energy the electron has in the covalent band, the less energy it takes to kick it up to the conduction band. That means photocells tend to be more responsive to frequencies near-but-below the bandgap voltage. You have to provide enough energy to kick the electron up to the Fermi level of the conduction band though, so the photoelectric response drops off rapidly as the wavelength gets longer (and photon energy gets lower).

The shorter the wavelength gets, the higher the chance that the photon will reflect off the elctron instead of being absorbed. That means the photoelectric response tapers off gradually as the photon energy levels get higher.

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adafruit2
 
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Re: SI1145 results

Post by adafruit2 »

Could be a timing error with VIS - you can take 10 samples and just use the median. The SiLabs chips are nice but...have not very 'fulfilling' datasheets - we do our best to get it all working. If you find a bug in our code let us know and we'll fix it up :)

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primexandy
 
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Re: SI1145 results

Post by primexandy »

thanks for the reply(s).

Instead of sampling and taking a mean I will write into the drive the ability to check interrupts and read the response register and make sure a sample is ready to be sure of timing.

With the first reply above, after more thought, I don't believe it will be to do with something being opaque to VIS but not IR because if that was the case a) it wouldn't have generated a UV value b) it would have also have shown up within the solar panel reading, as a spike. My software reads the solar panel (ADC), reads the VIS, then the IR, then calculated the position of the sun and the Insolation in well under 1 second.

A timing issue is a possibility but why doesn't it happen on the IR or UV readings.

I'll eliminate timing, swap the order of reading vis and ir and check interrupts and produce more data....

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primexandy
 
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Re: SI1145 results

Post by primexandy »

The drops in VIS were a timing issue.

I swapped the order that VIS and IR are read and the drops happened within IR. I think that can be sorted by reading the interrupt status before reading to make sure a valid sample is ready.

Thanks for your help.

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