I Spectrometer Shows More Blue light on cloudy Day....why?

[Newtons Apple]

This is sort of a sister thread to my other thread. But I've bought a spectrometer and have been playing around with it. I pointed it at the sun on a clear day and got as expected a nice full spectrum of color with a large spike of blue:

1. My first question is.. is the spike in blue range because of the scattered light from the sky? So all of that blue being scattered by the atmosphere is interfereing with the spectrometer and it's picking up more blue? OR is it because the sun emits such a large amount of ultraviolet, and this spectrometer doesn't really show deep violets?



Now, when I repeated the experiment on a cloudy day with no blue sky visible, I got this reading:

Far far lower direct sunlight so I much decreased LUX. However there is STILL a big spike in blue. So does this answer my previous question that the spike in blue was due to the atmosphere scattered the blue light? Why though am I still seeing this sharp blue spike when all of the other colors are depressed from the cloud coverage?

 

[DaveE]

There shouldn't be a blue spike if you measure the sun, as you can see here. Maybe try and also test it on an incandescent light bulb, that should also look like a blackbody radiator with a similar temperature.

 

[berkeman]

There shouldn't be a blue spike if you measure the sun, as you can see here. Maybe try and also test it on an incandescent light bulb, that should also look like a blackbody radiator with a similar temperature.

@Newtons Apple -- You were asked multiple questions like this in your previous thread about your sensor. Why have you not replied to those questions and suggestions in that thread?

 

[Tom.G]

Are you in an area with significant smog?
That could be the cause of the peak around 370nm.

Page 4, top left right, of this report:
AN-70302-Fluorescence-Method-Development-Handbook-AN70302-E.pdf
shows a peak around that wavelength that is labeled Pyrene. A Google search finds that Pyrene is a hydrocarbon, C₁₆H₁₀, consisting of 4 benzine rings.

It's source (according to wikipedia) "...it is produced in a wide range of combustion conditions."

Cheers,
Tom

 

[hutchphd]

The spectrum looks suspiciously like that of an LED (white) light bulb. I will bet money on it. Please desribe your system in detail.