Re: measure frequency of an LED?
Salmon Egg
dave <da...@127.0.0.1> wrote:
> Is there an way to measure the spread wavelength emitted by an LED?
> (without looking at manufacturers data sheet or guesstimating from
> it's colour) :-)
Look at its coherence length. I have never tried that on an LED.
If you place two pieces of glass plate on top of each other, would
expect to see interference fringes, even for LED's. The coherence length
is measured by how far apart the glass plates can be and still see
fringes. You could also try using a DVD as a diffraction grating.
Unfortunately, a certain amount of knowledge is required to interpret
what you see.
Bill
--
An old man would be better off never having been born.
Louis Boyd
> Is there an way to measure the spread wavelength emitted by an LED?
> (without looking at manufacturers data sheet or guesstimating from
> it's colour) :-)
It's easy with an optical spectrum analyzer. They're mostly used by
the fiber optics telecommunications industry. There are usually a few
for sale on ebay.
You could also use one of the Ocean Optics spectrometers which plug
into a PC USB port. That would be calibrated for relative intensity and
wavelength.
You could just look at the led with a black&white ccd webcam and a PC
(USB or firewire) though a replica grating. Those are fairly cheap. It
wouldn't be calibrated but you could use it to compare LEDs. It could be
calibrated with a spectral lamp but not not likely worth the effort.
Samuel M. Goldwasser
> Thanks for the replies.
> I had tried measuing a laser (laser pointer) wavelength with a plain
> DVD and that does agree with the expected wavelength(s) (+/-). But
> doesn't work with a "normal" LED.
The beam needs to be collimated to reflect it from the DVD or CD and
that can't be done well with a typical LED.
But you can view the reflection of the LED at a dustance (so it is
close to a point source) in the DVD or CD.
> I have a web cam which switches to B&W in darkness and illuminates the
> scene witha row of IR leds. So I could try your suggestion - but don't
> know what a replica grating is (I'm off to google) :-)
Cheap compared to a high quality spectrascopic grating. :)
MAde by a process similar to the one used for CDs, etc. - stamping from
a master grating.
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Louis Boyd
> dave <da...@127.0.0.1> writes:
>
>
>>On Sat, 02 Jan 2010 10:03:10 -0700, Louis Boyd
>>scene witha row of IR leds. So I could try your suggestion - but don't
>>know what a replica grating is (I'm off to google) :-)
>
>
> Cheap compared to a high quality spectrascopic grating. :)
>
> MAde by a process similar to the one used for CDs, etc. - stamping from
> a master grating.
This was what I had in mind by a cheap grating:
http://scientificsonline.com/product.asp_Q_pn_E_3001307
These work in transmission rather than by reflection and their main
advantage over using a CD is the lines are straight, not curved as on a
CD. They're made photographically. They are not high efficiency but
LEDs are bright and CCD cameras are fairly sensitive.
CCD cameras which operate both in color and in infrared still have a
color mask present. They just shut down the color information
electronically and use whatever signal is left as a black and white
output. They have poor pixel to pixel uniformity as only every third
pixel is sensitive to IR light. You can see a spectrum with them but it
makes it more difficult to try to make brightness measurements than with
as true black and white camera which has no color mask. Uncompressed
digital cameras (USB2.0 or firewire) will generally give better results
than analog cameras for scientific measurements because they handle each
pixel separately and don't use compression in the camera like most
USB1.1 cameras do. Analog cameras tend to smear the signal from pixel
to pixel in the horizontal axis. Also when trying to make scientific
measurements it's best not to use jpeg (or any other) compression.
Use one of the uncompressed video formats. Also shut off gamma
correction if your camera or video software has that feature. It makes
the intensity information non-linear.
I shouldn't have used the term replica grating. It's usually used to
refer to much higher quality reflection ratings which are made in
contact with machine cut master plates then coated with aluminum in a a
vacuum chamber. Those are still fairly expensive though cheap
compared to an original cut grating. High quality gratings have a
"sawtooth" surface shape which is called "blazing" which improves
their efficiency. High quality grating (original or replica) have very
uniform line spacing and are made on low expansion substrates. Plastic
films are not low expansion, nor are CDs. That won't matter though for
measuring the spectral shape of an LED.
Salmon Egg
s...@repairfaq.org (Samuel M. Goldwasser) wrote:
> The beam needs to be collimated to reflect it from the DVD or CD and
> that can't be done well with a typical LED.
>
> But you can view the reflection of the LED at a dustance (so it is
> close to a point source) in the DVD or CD.
If you can get hold of a shaving (concave) mirror, that may be able to
collimate an LED well enough if you have a relatively dim one.
Although I have not tried to do the spectroscopy, it is relatively
obvious to me that the DVD will have more dispersion than the CD. In
essence, the DVD has a higher density of spots. That is equivalent to
have a grating with a greater number of lines.
Bob May
standards. For general imaging like that of a pretty girl or quiet
mountain lake scene, usee JPG format as it works well with such stuff.
For lossless image compression, use the GIF format as it will faithfully
reproduce the image without loss of info. For someething likee a drawing,
the B&W drawing, the 1 bit per pixel B&W mode is best while for other
drawings, there are other formts in the seeries that will do well.
JPG compreesses by doing a Transform on the image and then dropping
unimportant parts of that data.
--
Bob May
rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net
Samuel M. Goldwasser
The problem is that it becomes too good as the wavelength approaches near-IR.
A deep red wavelength will be too long to even produce a first order beam.
The track pitch on a DVD is 0.74 um while a CD is 1.6 um.