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.
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.
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:)
So there is a fan to blow air into the enclosure but where does i exit?