# SPD curves for standard light sources



## tedfine (Apr 16, 2018)

I'm looking for suggestions for commonly available (like from Home Depot) and inexpensive lamps that can be used as a standard calibration source for an inexpensive, low resolution, slit-less spectrometer. Ideally the lamp would be small (like an LED) and easy to wire up to a 9 volt battery (preferred) or 110 VAC.

We need this lamp so students can do two types of calibration:



*Relative flux calibration *(to remove non-linear instrument sensitivity across wavelengths) : The light source needs to have a visible continuum rather than just a few broad emission features.
*Wavelength calibration*: The light source needs to have one fairly defined emission feature.

I don't need a high degree of accuracy. Students will be capturing spectra and determining their approximate color temperature.

I've thought about using the flashlight (torch) on an iPhone, since they're, um, pretty common.

I know that some lamps change in SPD over time, but I don't know to what degree this is the case with a cell phone or other possible standard lamp.

Any suggestions on lamps?

While researching this a few years ago, I found a site (GE?) that provided SPD graphs for a wide range lamps. I am unable to find this site again. The graph below (from another thread on this board) looks like it is from that site.

This is the kind of graph and standard lamp I'm looking for. I can smooth the features for #1 above. And it has some features with a known wavelength.


----------



## maukka (Apr 17, 2018)

I can't recommend any common light source found from hardware stores on that side of the pond, but here's measurements for iPhone 6 and iPhone 6s. They're pretty similar. I wouldn't assume similarities between different phone generations though.











Here's the 6s data in numerical form at 10nm intervals:


3800,0029833900,0038524000,0055294100,0298554200,1544384300,4679044400,8942124501,0000004600,4479464700,1682014800,0900344900,1086035000,2307995100,4488575200,6596585300,7963965400,8746335500,9135825600,9326125700,9190115800,8693965900,8019036000,7269716100,6481196200,5671946300,4875316400,4106596500,3395086600,2762176700,2174316800,1696066900,1324927000,1042067100,0813777200,0659257300,057547


----------



## ssanasisredna (Apr 17, 2018)

I would go with a UV pumped LED. The phosphor has a well defined output characteristic

I would not use an LED for wavelength calibration. You are better off with a laser as you have a narrow line, and ideally pick up a used HeNe as the wavelength will be more accurate.


----------



## tedfine (Apr 17, 2018)

Maukka: THANKS so much. GREAT data. Double thanks for the data points, which saves me from having to digitize it myself.

Where did you get the from? Do you know of on-line sources of this kind of information for other light sources?

ssanasisredna: Thanks for the suggestions. Seems like both of the light would be great for my needs, but I'm hoping to find standards that would be available locally. Our target audience is teachers.

Thanks again to you both for your help.


----------



## iamlucky13 (Apr 17, 2018)

The graph you show looks like a standard incandescent light bulb, which will produce an almost perfect black body curve around 2700K, plus a comparison of the GE Reveal series incandescents, which is an incandescent with a color filter incorporated in the bulb to make the light appear less yellow-hued.

I don't know if having a spectral peak outside the visible wavelengths will work for you, but you can usually find incandescents by asking around if people you know still have some spares. Or you can still buy halogen incandescent bulbs. I think the major home improvement stores still carry them.

Out of curiosity, I'd like to ask what the spectrometer you're referring to is? It sounds like a step up from the diffraction grating and digital camera project I've seen shared online (which I think might originally have been a NASA educational project)?


----------



## ssanasisredna (Apr 17, 2018)

iamlucky13 said:


> The graph you show looks like a standard incandescent light bulb, which will produce an almost perfect black body curve around 2700K, plus a comparison of the GE Reveal series incandescents, which is an incandescent with a color filter incorporated in the bulb to make the light appear less yellow-hued.
> 
> I don't know if having a spectral peak outside the visible wavelengths will work for you, but you can usually find incandescents by asking around if people you know still have some spares. Or you can still buy halogen incandescent bulbs. I think the major home improvement stores still carry them.
> 
> Out of curiosity, I'd like to ask what the spectrometer you're referring to is? It sounds like a step up from the diffraction grating and digital camera project I've seen shared online (which I think might originally have been a NASA educational project)?




That 2700K is "around" 2700K and highly dependent on line voltage (or battery) voltage both which vary greatly and the OP was looking for a spectral reference. You don't get that with an INCAN without being calibrated to a voltage.

w.r.t. "Local" ... Digikey, Mouser, Farnell, and number of places ship overnight or in a few days fairly inexpensively. A UV pumped LED is going to have a relatively consistent spectra somewhat independent of drive current hence why it works well as a somewhat "calibrated" source. Soraa bulbs are getting a bit easier to find these days too.


----------



## tedfine (May 2, 2018)

iamlucky13 said:


> Out of curiosity, I'd like to ask what the spectrometer you're referring to is? It sounds like a step up from the diffraction grating and digital camera project I've seen shared online (which I think might originally have been a NASA educational project)?



This is a project using a DSLR with an objective grating: https://www.fieldtestedsystems.com/starspectra/. It's a slit-less spectrometer originally designed for astronomy. 

However, it can be used on any compact light source, which means street lights at a distance (with dark surroundings), or closer objects in a dark room and/or behind an external slit (e.g. behind a closet door which is open just a crack...)

The software can show a corrected color curve, adjusting for instrument response in real time (video 30 and 15 here: https://www.rspec-astro.com/more-videos.)

AFAIK, most webcams can't be used for this kind of work because they have automatic gamma correction and other automatic exposure corrections that constantly change camera response. 

However, since you can turn off these automatic features in a DSLR, a true black body (SPD) curve can be created. However, in order to do that (as shown in the videos), one needs to know the QE of the _entire _system (not just the CCD, but the lens and the grating too). THAT'S why my original question looking for standard curves, which is used to create a system-specific QE curve.


----------



## ssanasisredna (May 2, 2018)

tedfine said:


> This is a project using a DSLR with an objective grating: https://www.fieldtestedsystems.com/starspectra/. It's a slit-less spectrometer originally designed for astronomy.
> 
> However, it can be used on any compact light source, which means street lights at a distance (with dark surroundings), or closer objects in a dark room and/or behind an external slit (e.g. behind a closet door which is open just a crack...)
> 
> ...



Why not just calibrate against a known star spectra and atmosphere density profile for your location and viewing hour? ... assuming a clear sky of course. I have to expect that is going to be as accurate as any uncalibrated source though I still think UV pumped phosphor may be ideal, though the issue will be with multiple phosphor coatings and the relative ratios screwing up the accuracy. I can't remember where now, but someone was selling phosphors online in small quantities. That may be a solution for you.


----------



## tedfine (May 2, 2018)

ssanasisredna said:


> Why not just calibrate against a known star spectra and atmosphere density profile for your location and viewing hour? ...



Yes, that's a reasonable approach, but requires the student to capture a star's spectrum, which is surprisingly difficult, involving finding the star in the sky, pointing the camera at it, _keeping the camera on the star _as the sky moves but keeping the star on the same pixels for 3 to 30 seconds (in other words, "tracking"), setting focus, exposure and ISO levels for a very dim object usually not visible in the view finder (only visible in the jpg/raw output files) and perhaps learning how to operate the manufacturer's camera control software. 

Phew! That's a lot!

If a student is doing astronomical spectroscopy, then he or she will have already learned how to do the above by the time they need to create a system's instrument response (QE). 

But if the student is just doing terrestrial observations, it's much easier to use a lamp of a known SPD rather than meeting all the astronomical challenges.  

Thanks for your suggestion of phosphor lamps.


----------

