@briandegger @jeffh - thanks for the tips on LED sourcing! Thought you'd enjoy seeing this.

@dhaffnersr and @stoft, if you'd like I could send you the lower-wavelength LEDs as well for comparison testing?

Sure ... what's the LED's packaging style/type? SMD, leaded, ...?

Hey Jeff! Sure I'd be happy to test any LEDs you want to send. I do want to throw an idea your way, I think using the higher wavelength LEDs are still a good idea and here's why, it's easy to design a circuit where you can control the output dc voltage like in a step motor, the concept begin that voltage is relative to wavelength and so we can control that, a lot easier than replacing LEDs all the time. What do you think?

Unfortunately, LED wavelength does not change with applied voltage or current -- their center peak wavelength is not variable or controllable and PWM only controls intensity. Any correlation between observed Vf (forward voltage drop) and peak center wavelength is purely manufacturing variation.

That manufacturing variation results in different center peak wavelength as a result of bandgap variations in the semiconductor material; once constructed, it doesn't change. Also, the Vf forward voltage drop can be different between devices of the same product; same reason. The only two spectral changes are 1) a very slight aging at a rate of about 0.0001nm/hr of on-time and 2) with some devices, an increase in the FWHM (bandwidth) as current increases from the initial threshold voltage point to primary conduction with 20mA or more. With UV LEDs this later effect seems to be primarily a spread of the LED spectrum slope on the longer-wavelength-side, with little on the shorter wavelength side. However, once the current is set (fixed) even this effect does not change.

Hey Dave, well I was thinking along the lines of when I built a wind speed indicator with a small dc motor and LCD screen attached and calibrated it by having a friend drive at different speeds while I logged the voltage as related to that speed (rough way of doing it but it worked.)

I understand what you are saying about the PWM increasing intensity, but that's the whole point with fluorescence, that's also why I hooked up with this person, Dr. Friedrich Menges in Austria, the developer of spekwin32. I also understand the manufacturing process, I use to be an assembler level 3 at KC automation (which is know bought out by Borgtwalt in Germany.) we did 90 percent in house, I built from the ground up, the 20 port cigarette smoke analysis machine and the Side Stream smoker analyzer(which analyzes second hand smoke.) that's what we did there building Lab and industrial quality high precision equipment.

This kind of work I have never done but I have always been very facsinated by it and a friend of mine had a sample of something that they wanted to see if I could find out what it was, and I just happened to come across this spectrometer kit, since I just didn't have 30,000.00 to get a decent one. now I'm hooked and I'm learning a lot, and I'm sure I will continue to learn and refine my prep and technique.

Perhaps your correct about voltage and wavelength begin separate, I'd like to know how some of these spectrometers can have their wavelengths adjusted, I'll find out.

Hey Jeff, I saw your spectra of the nail polish machine and there it is the tell tail "hump," so I exported it and processed it and here is what it really looks like:

Thought you would be interested.

Hi, Dave - what is the tell-tale hump? Do you mean from the LED input light? If so, it's at 398nm, not 385nm. What processing did you do?

Hey Jeff, I use spekwin32 for all my spectral data processing, first here are 2 references that may help you

https://www.researchgate.net/topic/uv_spectrometer http://pubs.rsc.org/is/content/articlelanding/2015/ra/c5ra08729e#!divAbstract

below is a new graph detailing what I am talking about and below that will be another reference graph detailing it even further

iol_hump2.bmp

It's me again Jeff, Dave H, sorry I posted the wrong graph i made, this is the right one