"Good-enough" white light source - Page 1
Forum section: electronics / general

author: MarkMLl
author_rank: Frequent Contributor
timestamp: February 12, 2021, 09:00:33 pm
content: Allowing that spectral sensors of various types (https://ams.com/as7341https://www.hamamatsu.com/eu/en/product/type/C12666MA/index.html) are now available fairly cheaply, does anybody have any suggestions for comparably cheap sources of white light?I've seen suggestion that a small filament lamp (e.g. E10 torch bulb) driven fairly aggressively can be useful, but by today's standards it seems rather crude.MarkMLl

author: Kleinstein
author_rank: Super Contributor
timestamp: February 12, 2021, 09:11:51 pm
content: A classical filamant lamp is OK. If more intensity is needed a 12 V halogen lamp (standard clear quarz, no extra fancy filtering) is also a good option. For spectroscopy in the blue to near UV part it is common to overdrive quite a bit. Compared to a classical filament there is less deposition of metal on the glass and thus less drift in intensity and spectrum.

author: MarkMLl
author_rank: Frequent Contributor
timestamp: February 12, 2021, 11:21:20 pm
content: Quote from: Kleinstein on February 12, 2021, 09:11:51 pmThanks for that. I suppose that a halogen bulb still approximates a standard black-body source, unlike anything that has an arc or a phosphor.I was always amused by the halogen bulb I had in a "pigeon roaster" torch: when turned off the envelope glowed blue for a couple of seconds.MarkMLl

author: penfold
author_rank: Frequent Contributor
timestamp: February 13, 2021, 02:01:10 am
content: How white is white? I was informed by an opto-electronics researcher that when he contacted an optical standards supplier, and informed them he only had about £150 funding left but needed an "approximately 1cd source of low-ish colour temperature continuous spectrum light", that they recommended he use a birthday candle, they are apparently surprisingly standard... considering the availability of accurately controlled filament lamps at relatively low cost, I can only imagine there must have been some spectral benefit

author: MarkMLl
author_rank: Frequent Contributor
timestamp: February 13, 2021, 08:45:00 am
content: Quote from: penfold on February 13, 2021, 02:01:10 amTFT, interesting thought. I suppose that what that boils down to is that the wax has a fairly well-understood composition, and isn't loaded with metals etc. that will give distinct spectral lines.MarkMLl

author: penfold
author_rank: Frequent Contributor
timestamp: February 13, 2021, 02:54:03 pm
content: Quote from: MarkMLl on February 13, 2021, 08:45:00 amI think you're right, so also the two "emissive" elements being the wax vapour or hot carbonised wick are also considered black-bodies and the temperature is still low enough that the contribution of any residual metals is pretty minimal... I'm only speculating because I don't recall hearing anything about the results he got

author: Kleinstein
author_rank: Super Contributor
timestamp: February 13, 2021, 05:14:37 pm
content: In the old days the standard for 1 Cd thermal light was actuall a well defined type of candle. One still needed to control the speed it burned and maybe correct for the speed as it could change with temperature and air pressure. Still the flame can be quite complicated with different temperatures at different points and maybe too small to be fully absorbing.The more modern reference lamps use a tungsten band as emitter and thus nead a rather high current to operate. I have seen such a lamp in operation once and mainly remember that it was not so easy to get the get a suitable supply.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: February 13, 2021, 05:53:22 pm
content: Given that your only definition of "good enough" is a low cost light sensor can I suggest using any available quartz envelope bulb and using your "good enough" sensor to measure its output. You can adjust current to get what you want.If perhaps your goal is to calibrate your "good enough" sensor you will have to expend a lot of thought to do it on the cheap. As a hobby it is doable, but I strongly suspect any business would profit from paying what it takes to get a decent calibration source, or to get your inexpensive sensor calibrated by a lab.

author: MarkMLl
author_rank: Frequent Contributor
timestamp: February 14, 2021, 09:44:10 am
content: Quote from: CatalinaWOW on February 13, 2021, 05:53:22 pmNo particular application in mind for this, I was just trying to "close the loop" as it were for situations where there isn't ambient sunlight.Allowing that a couple of people have mentioned using fairly standard lightbulbs for this, I take it that the properties (absorptive/emissive) of the glass envelope are considered to be negligible. However since this is basically a filter it occurs to me that selecting a bulb type that keeps this thin might be prudent.I note that nobody has said anything about LED+phosphor sources... I've always assumed that spectrally they're pretty grotty.MarkMLl

author: Kleinstein
author_rank: Super Contributor
timestamp: February 14, 2021, 10:14:26 am
content: Even the high CRI LEDs are quite a bit away from a smooth spectrum. Usually there is some of the original deep blue coming through, way higher than in a thermal spectrum.As a reference in the wavelength domain a compact fluorecent lamp can be usefull - they usually give a nice set of discrete limes with well defiened waveleghts.Unless one is looking at active light sources, the refererence may not be a white light, but more like a white "reflector".For the spectrometer one still needs a little optics at the input side to deliver the light in the correct way (e.g. get even illumination). Here is helps if the light source has a defined size and is not too small. So it is not just about spectrum, but also about simple dimensions. In this aspect lamps made to be focussed can have an advantage.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: February 14, 2021, 02:42:32 pm
content: Again, your purpose will impact your decision. For visible white light a glass bulb will do. If you need deep violet/near UV, or anything that concerns sun tans or material aging you need quartz or something like it. It is fairly easy to google the transmission of the common bulb envelopes and see if they meet your needs.