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 Post subject: refractometers
PostPosted: Sun Jan 24, 2016 4:25 pm 
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So here is a potentially dumb question. Why are cheap lasers not used with refractometers? With a light diffuser I expect the beam could be broadened out. it would give a monochromatic light source. It might make it easier to make a ccd read the result instead of having to look at it and it might be a lot brighter. I know they were invented a long time ago but cheap lasers have been in dollar stores for years now.


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 Post subject: Re: refractometers
PostPosted: Sun Jan 24, 2016 11:14 pm 
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Well you'd need yellow 590 nm laser light, and that color isn't cheap. But a more fundamental problem is that laser light produces a speckle pattern of bright and dark spots, not a nice uniform illumination.


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 Post subject: Re: refractometers
PostPosted: Sun Jan 24, 2016 11:33 pm 
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Couldn't things be recalibrated for a different wavelength?


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 Post subject: Re: refractometers
PostPosted: Sun Jan 24, 2016 11:37 pm 
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Why would laser lights be more desirable?


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 Post subject: Re: refractometers
PostPosted: Sun Jan 24, 2016 11:50 pm 
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I was thinking the monochromatic light might be desirable. I think I might just try it and see what happens.


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 Post subject: Re: refractometers
PostPosted: Mon Jan 25, 2016 12:06 am 
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The preferred 590nm sodium light is monochromatic, no?


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 Post subject: Re: refractometers
PostPosted: Mon Jan 25, 2016 5:29 pm 
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dchallener wrote:
I was thinking the monochromatic light might be desirable. I think I might just try it and see what happens.

Been there, done that. As did B.W. Anderson some 60 years ago and wrote about it.
Consequently, I have a filter called "Measuring Birefringence using White Light"


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 Post subject: Re: refractometers
PostPosted: Mon Jan 25, 2016 6:13 pm 
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Hmm.. lasers weren't invented 60 years ago, unless you count the letter von Neumann never sent.


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 Post subject: Re: refractometers
PostPosted: Tue Jan 26, 2016 11:50 am 
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Quote:
Consequently, I have a filter called "Measuring Birefringence using White Light"

Might that be a 590nm filter?


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 Post subject: Re: refractometers
PostPosted: Wed Jan 27, 2016 11:19 am 
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Aren't most modern refractometers equipped with a yellow filter at the rear part ?


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 Post subject: Re: refractometers
PostPosted: Wed Jan 27, 2016 11:51 am 
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The good ones are.


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 Post subject: Re: refractometers
PostPosted: Wed Jan 27, 2016 5:27 pm 
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Brian wrote:
Well you'd need yellow 590 nm laser light, and that color isn't cheap. But a more fundamental problem is that laser light produces a speckle pattern of bright and dark spots, not a nice uniform illumination.



Things may have changed. at least on the price front:

for led's only.
Single led: about $4
120 degree beam
Output up to 77 lm
Wave length 584 – 594 with 591 typical

http://www.luxeonstar.com/lxml-pl01-004 ... l-led-77lm

do not want to fool with a diffuser, how about a package that is 2 inches square, should give the coverage, for $8
http://www.dx.com/p/20w-1800lm-585-590n ... qkvkiorLIU

can get them as low as .05 but at this level the outut may be to low.
But for that rice, you could cluster several….

160 viewing angle as low as .05
As a ref:

The maglight output: (not sure I am looking at the right ones)
The mini key chain ones are 2 lumne
incidecent minis are 14 lumen.
LEDs range from minimum of 40 lumen, many in the 100-200 range even for the minis.


This only addresses the price discussion,




The issue with speckel and dark spots is something I have not looked at.


Does it have other problems..... not looked at.

But for a few bucks you can not test them.



I remember t=in previous discussions that the sodium light actually had 2 close bands.

found this:

Image

Bill and others,

from a purely technical stand point would a 585nm vs 595nm vs 590nm make a difernece in the readings?


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 Post subject: Re: refractometers
PostPosted: Thu Jan 28, 2016 12:39 am 
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Sorry, I'm still confused. Why would one use a laser to illuminate a refractometer?
Why would someone risk photochemical damage to the retina or a retinal burn shining this light in the direction of the eye?

What are we trying to improve with the use of a laser?


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 Post subject: Re: refractometers
PostPosted: Thu Jan 28, 2016 12:57 am 
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Sodium spectral lamp (low pressure sodium) has two extremely narrow lines separated by 0.5 nm. For RI measurements, that small separation is indistinguishably monochromatic. Your spectral graphs were recorded at low resolution, so that you cannot even see the separate lines... the wavelength spread you see in the picture is due to the poor resolution spectrometer, not the actual line source.

Sodium spectral lamp's other useful feature is that it is extremely bright, the most efficient visible (near-)monochrome light source available (the only monochrome light source with greater efficiency is mercury, but that light is short wave UV). So that is why RI measurements have been made with a sodium spectral lamp for over a century; it is the standard against which all other light sources pale.

With an LED, a peak value in range 584 - 594 nm may not seem so bad... A +/-5 nm shift in wavelength isn't going to affect RI noticeably. But just as important, you need to know how broadly is the range over which the LED produces light. Of course the brightest light will be 591 nm, so the light from the LED looks yellow. But the LED produces light from red to blue (take a look at a yellow LED through a spectroscope and be prepared to be disappointed) and so it is only slightly better than using a white light source for RI measurements.

Who cares about diffuser with LED light? The LED already produces a large angle cone of light with a relatively even spread. Diffuser is overkill.

Yellow filters are more useful than LEDs since they filter white light down to a narrower, near monochromatic band of light. But that filtering throws away a lot of light, and so you better have a very bright white light source to begin with. Still can't beat a sodium spectral lamp for brightness, nor for monochromacity.

Lasers are bright and monochromatic (but not very efficient). But also lasers are coherent light sources (whereas sodium spectral lamps are incoherent sources), and coherent light has the nasty little side effect of producing speckle pattern. I've had a lot of experience expanding laser beams, sometimes to diameters as large as six inches. And when you've seen enough laser speckle, you'll understand what a problem it will cause when you are trying to find a line that separates light from dark band. And you cannot get rid of speckle pattern with a diffuser, it only rearranges the pattern of bright and dark spots.


Last edited by Brian on Fri Jan 29, 2016 1:16 am, edited 1 time in total.

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 Post subject: Re: refractometers
PostPosted: Thu Jan 28, 2016 2:15 am 
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Thanks for shedding light on this Brian :)

I use a Wratten #23A Filter which blocks anything below 550nm, a yellow filter would block somwhere in the range of 470 nm. Why do you think that's superior? Is it brighter?

Another question regarding SOX lamps: How much power would be reasonable?
I think I'll give em a try, but I am not sure what to buy - I can get SOX lamps anywhere from 18 to 180 watts.


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