Before a spectrometer can do any useful work, it needs to be calibrated to identify wavelengths correctly. This is usually done by detecting several characteristic peaks or dips in a well-known light source and using these as a reference to identify other wavelengths. The most common reference for hobbyists is the pair of peaks produced by a mercury-vapor fluorescent light, but a more versatile option is a xenon-bulb light source, such as [Markus Bindhammer] made in his latest video.
A xenon gas discharge produces a wide band of wavelengths, which makes it a useful illumination source for absorbance spectroscopy. Even better, Xenon also has several characteristic spikes in the infrared region. For his light source, [Markus] used an H7 xenon bulb meant for a vehicle headlight. The bulb sits in the center of the source, with a concave mirror behind it and a pair of converging lenses in front of it. The converging lenses focus the light onto the end of an optical cable made of PMMA to better transmit UV. A few aluminum brackets hold all the parts in place. The concave mirror is made out of a cut-open section of aluminum pipe. The entire setup is mounted inside an aluminum case, with a fan on one end for cooling. To keep stray light out of the case, a light trap covers the fan’s outlet.
[Markus] hadn’t yet tested the light source with his unique spectrometer, but it looks as though it should work nicely. We’ve seen a wide variety of amateur spectrometers here, but it’s also illuminating to take a look at commercial scientific light sources.
I’m a little bit surprised by the way the two condensor lenses are put in the optical path. I always thought that condensors of this type is desigend with flat side pointing to the lamp, an the (strong) conves surface to the side, where the light rays ar more or less parallel. Compare to the design of the illumination system in a slide projector.
I’m a little confused about the optics too. I think I’d have to do a ray trace and understand the fiber coupling.
Not that it matters much but I do wonder what the lenses are made out of. One of the advantages of these xenon lamps is they emit some UV, while LED sources don’t. I think those are pmma lenses, big thick ones. I bet there’s a good deal of intensity loss in the uv because of the lens choice here. It’s probably still sufficient, but that looks like 3cm of lens path length.
All that said I do see bright light coming out of the fiber so it’s a win.
he mentions borosilicate lens in the video… not that I have any idea on what is good for UVs…
Didn’t it say to focus UV? That would be quartz. But as he says in the video, quartz is too expensive, and quartz that size would be way too expensive. So boro-silicate it is. It will calibrate out.
Really nice metal enclosure work here. Seems kind of overkill but I bet the inside of the box gets pretty hot. I wonder if they used a CNC?
I wonder how effective the aluminum pipe reflector is, I’d imagine the pipe should be much closer to the bulb and be more reflective? Then again once you couple to to the fiber optic I think you’re just admitting you’re losing 90% of the intensity so it doesn’t matter. For the end application I doubt it matters at all.
I wonder if the fiber has a ball lens or if it’s just raw.
All in all it looks like another nice project from Markus.
Vehicle headlight bulbs are not xenon, they are metal-halide vapour bulbs. “Xenon” is nothing more than marketing lie for vehicle headlight bulbs.
Real xenon bulbs, you could find around without problems, are those U-shaped (or small I-shaped) flash lamps.
Most commercial visible range spectrometers with xenon lamp use those flash lamps as a light source. At the time, when there was no powerful white LEDs (actually UV LEDs covered with white phosphor), xenon flash bulbs was relatively long-lasting , reliable and stable ligth source option, compared with halogen incandesced bulbs, that degrade fast. Spectrometer manufacturers that cared about reliability used xenon bulbs (switching to white LEDs today), those who want to make profits on expensive and regular incandesced bulb replacement and recalibration service still put halogen incandesced bulbs into their spectros.
Also, you absolutely don’t need “known spectrum” for spectrometer light source. You could use any light source you have, including white LEDs for that. To exclude light source spectrum from measurement results, spectrometers have calibration procedure with measuring white tile with known reflectance/absorbance values and a black trap as 0% reflectance/100% absorbance. Wavelength calibration of sensor could be done with cheap with red-green-blue lasers or even just tiles with different colors of known spectra.
I mostly agree with what you are saying but there is a very real reason to not only use white LEDs for spectrometers. Especially for people who are interested in anything below 400nm. A lot of analysis benefit from 250nm or thereabouts.
Also it is true you can calibrate with lasers and other samples, but there is nothing wrong with using a light source as a calibration. Tons of instrument manufacturers do that. So do scientists. There are various reasons why it’s a thing and it’s not just for wavelength calibration and resolution/QA concerns. Intensity calibration is also a thing. There are even standard light sources you can buy for these purposes.
There’s nothing wrong with this project in any of those regards.
“Xenon is nothing more than a marketing lie for vehicle headlight bulbs.” Can you back up this claim in any way? I can also see typical spectral lines from xenon in the spectrum. Yes, the lamps also contain metal halides; no one disputes that.
https://g2voptics.com/wp-content/uploads/2021/12/comparing-spectrum-01.jpg
You are off by a dozen nanometers. Metal-halide have spikes similar to xenon, but they are at different wavelenghts.
There are only two types of continuous xenon lamps – short-arc and long-arc. Short arc xenon lamps are in kW range with lifetime around 1000h at best. With ~20-30bar pressure inside – they realy explode with glass shrapnel if dropped. Used as a light source in cinema projectors, mostly.. Long arc xenon lamps are also in kW range and produced mostly in USSR for lagre area lighting. None of this two types was ever used as bulbs for car headlights.
Low power xenon lamps are flash lamps – well-known U-shaped things. But they can’t give continous light at all, only short flashes.
There is just no type of xenon lamp that could be used as headlight bulb.
Xenon lamps give bright light similar to sunlight and had an image of a very good and advanced light source at the time of incandenced illumination, car bulb manufacturers just used this image of Great Famous Noble Xenon Lamp to name their metal-halide bulbs. Some even add some timy amount of xenon instead of argon to them to not be sued for , but this xenon had nothing to do with light of metal-halide. Also, “halide” or “halogen” have an image of something poisonous in mass perception.
Yeah, as MM points out, those aspheric lenses definitely aren’t pointed in the right direction. I have to assume Markus tested it and found this gave him a nicer-looking beam shape (or some other criteria), but he’s certainly leaving a lot of efficiency on the table. Same with that light blocker on the back end — it’s certainly not operating as a reflector.
But more curious to me is: Where is the air exhaust? He goes through a lot of trouble to fit a fan with a light-blocking vent, to blow air into the enclosure, but there is no exhaust light blocker, and no obvious exhaust air path at all: No airflow through the enclosure!
The two plano convex lenses forming a biconvex converging lens. I tried out various configurations and this one had the best result. Thorlabs uses the same configuration in some of their light sources, if I remember correctly. The lenses are made of borosilicate glass. Quartz would be better, but they are very expensive. The fan is a static pressure fan, recommended when using a filter (light trap). As I don’t own a CNC mill or lathe, I’m dependent on semi-finished products. I know that a mirror made from a cut aluminum tube is not ideal:)
How did you cut up and drill all the aluminum so nicely? Do you have a blog for that part of the project? If so I’d love to read it.
I also don’t have a CNC and my mortal enemy is project enclosures.ive been considering getting a laser cutter to help that.
File to the scribe line, fretsaw with metal saw blade and a cheap drill press with a lot of runout. I don’t have a blog (anymore), just YT and Patreon, where I post daily:)
Check out the Thorlabs guides to condenser applications for aspheric lenses:
https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=3835
thank you Paul. The reason is spherical aberration which grows with a higher power of the angle of incidence of the light rays. This can be reduced by ‘balancing’ the curvatures of the lens on both sides, resulting in pointing the surface with higher curvature to the lamp, one of the simplest measure in lens design. In addition the surface with curvature in such condensers has a quite aspheric shape (which, may be, makes the thin gs worse when the lens is put in in the wrong way). – Putting in the lens in the wrong way leads to a reasonable defined spot with a strong halo (see video after 2:28). And when we talk about efficiency we have to take into account the numerical aperture of the fiber the light ist sent through. It would be interesting to compare the light intensity at the end of the fiber for the two lens configurations ‘flat to flat’ and ‘convex to convex’.
Or actually spelled out in detail by Edmund:
https://www.edmundoptics.com/knowledge-center/application-notes/optics/optics-application-examples/
So there is a fan to blow air into the enclosure but where does i exit?
Yep, still wondering this myself.
A common mistake. I used to have to educate some of our MEs that cooling requires air flow, and that a heat sink by itself does not remove heat.
“Hot Air Rises and Heat Sinks” is an excellent book on the subject.
The cooling fins are located on the outside of the housing, and we usually don’t live in space but in rooms with sufficient air flow. Now, imagine a cup of hot coffee. You blow into it to cool it down. What happens? You feel warm air. A fan is not a closed system like a valve. Enough warm air escapes to keep the whole thing at a constant temperature, which never exceeded 30 degrees Celsius even after half an hour of continuous operation. So where is the problem? Another opening would just introduce again stray light into the system.
Btw., the xenon bulbs are mounted in plastic housings (headlights) on the car without any cooling and nearly no air flow, because they have to be splash water proof.
In the theme of component abuse, it’s possible to use a fluorescent light glow starter as a calibration source: https://www.ursusmajor.ch/downloads/multi-spectral-calibration-lamp.pdf
They all have different mixes of gasses, but you can usually figure out the mix based on a rough spectrum, then use that knowledge for precise calibration
This!
I’m using this exact setup, and the relco sc480 starters are amazing for it.