# New Budget Laser Power Meter in the Works - Future GB material. 1-800mW



## dr_lava (Feb 1, 2007)

After looking around at some of the laser power meters out there - 

The Sper scientific pocket laser meter costs $190, has a 40mW range, and a hand-calculator is necessary for wavelength adjustments, as the display is only calibrated for 1 wavelength.

The coherent lasercheck costs $300-400, range 1-1000mW, and wavelength is entered for automatic reading adjustment.

Ted_park mentioned an Andover Holography unit that was made for holography primarily, but could work.

I have been talking with 'Andover holography' about tuning this unit for our purposes. So far, this is a preliminary spec:

· Autoranging measurement of light power in 4 ranges from ~100 uW to 100mW 
· At least 5% accuracy over native range
· Range extends to 800mW with filter in 'ND8' mode (calibrated)
· Multiple sensor calibrations available for different wavelengths :488,532,635,660, selectable in a menu.
· 16 character by 2 line alphanumeric display
· 3 Pushbuttons provide control of measurements & configuration using a simple menu system. 
· Automatic 5-minute power down to extend battery life 
· RS232 datalogging
· Peak hold remembers the peak reading in a series
· 2Hz (2 samples/sec) update and logging rate, 400Hz internal sample rate


Available via a group buy soon at less than the Sper Scientific cost.

Here's a pic of the holography meter. it has an external sensor. The laser power meter version will have a built-in sensor like the lasercheck, and no backlight.







What do you think?
Would you be interested in a unit like this?


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## SenKat (Feb 1, 2007)

Bring it on, man ! Count me in on that sucker !


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## ted_park (Feb 1, 2007)

I have one of the holography units, and it works well. I don't typically use it for green laser power measurement, but the results I have got are consistent with the sper meter. 

The stock unit reads for 532 and 633 nm, so that should cover green laser enthusiasts. Perhaps it might be nice if sensors that could read >100mw are available - the standard unit reads to 100mw, which is enough for the holography work that I do. I think the meters I have already should be enough though.

--Ted.


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## shawndoe (Feb 1, 2007)

Hi,

Sounds like an affordable unit and I really like the logging option. I could be interested depending on how much they want for it. 

Have a good one.
Shawndoe


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## zigziggityzoo (Feb 1, 2007)

Yeah, sounds awesome. It does depend on the price for me... after all, my hobby(habit?) has a budget!


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## dr_lava (Feb 1, 2007)

The estimated unit price for a 10-unit group-buy is $150/unit. First come, first served.


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## Hemlock Mike (Feb 1, 2007)

Dr_Lava --


I'm in for $150.00  Let me know when to send $. 
I still have your address from the diodes GB.

Mike


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## PhotonWrangler (Feb 1, 2007)

dr_lava said:


> The estimated unit price for a 10-unit group-buy is $150/unit. First come, first served.



This looks interesting to me also. I'm in for a group buy.


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## RVRM (Feb 1, 2007)

in


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## LED123 (Feb 2, 2007)

Interested in one. Does it come with the filter for the 800mw measurement.


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## dr_lava (Feb 2, 2007)

Yes it does. 

The purpose of this thread was to solicit feedback and comments on the design, not really to start a group buy (that belongs in the GB area) If you want to get in line this early, please PM me only. thank you.


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## dr_lava (Feb 2, 2007)

Updated the spec to include sample rate.


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## Kiessling (Feb 2, 2007)

dr_lava said:


> The purpose of this thread was to solicit feedback and comments on the design, not really to start a group buy (that belongs in the GB area) If you want to get in line this early, please PM me only. thank you.



Good call. 
Please discuss the design only and wait for the gb to actually start (in the gb forum then). A link pointing there in this thread would be a good idea then 
Thanx.
bernhard


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## jkaiser3000 (Feb 2, 2007)

probably include a calibration function so it could be used with other wavelengths (405nm, 780nm, etc.)


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## dr_lava (Feb 2, 2007)

jkaiser3000 said:


> probably include a calibration function so it could be used with other wavelengths (405nm, 780nm, etc.)



I've talked with the designer about exactly that and he feels that it could cause corruption in the other cal values to enable eeprom writes to add new values - if there were to be power glitches while inserting and removing the batteries. 


This being the case he has said he will do updates and re-cals for free, just mail the unit to him.


If this is a big issue to you or anyone considering buying the unit, please let me know and I will push the issue (cause I'd like to see it, too).


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## Hemlock Mike (Feb 2, 2007)

I think for a ~ $150 unit, we can't expect too much. That would be a nice feature however but would require the user to have several known units to establish a power curve.


Mike


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## Hemlock Mike (Feb 3, 2007)

Dr_Lava-

What's the OD of the cases you are sending ?? I'm on my lathe tonight !!! I'll get it close.

Mike


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## jkaiser3000 (Feb 4, 2007)

Regarding the calibration curve, I was actually thinking about taking a realtive reading from an already available wavelength. Say you want to measure 405 nm, make the reading at the 532nm setting and then apply a relationship to get the true output at 405 (like 405nm is half 532nm output reading). Sort of like that.

However, I only propose it because of the niftinness, but I could live without that feature. After all, 405nm is not very common yet. On the other hand, 780nm and 808nm are quite common. 

Oh well, guess we can't win'em all


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## Ragnarok (Feb 4, 2007)

Yes, if we knew the sensitivity curve of the sensor, measurements of "in-between" wavelengths could at least be interpolated with a fair degree of accuracy. If the units are only capable of a limited number of values "built in", could/would they supply a written list of multipliers for different wavelengths?

I guess that depends on the unit-to-unit consistency of the detector, though.

I would be interested in purchasing one of these meters if the project moves forward.


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## The_LED_Museum (Feb 4, 2007)

The Sper Scientific has wavelength multipliers printed on the inside of its lid; to wit:






I have not found this to be a pain in the toliet muscle at all; as long as you have ready access to a calculator (Windows has one built-in), it is easy.


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## dr_lava (Feb 4, 2007)

The sensitivity is calibrated for each sensor, but probably the wavelength curve shape remains the same between sensors. The unit calculates the power automatically, any wavelengths you want can be put into the units memory, provided he has access or can gain access to that laser type, or you can make measurements, record the meter's output, and give that information to andover holography, and the calibration can be entered. 

I'll see if a general wavelength sensitivity curve can be found for the silicon sensors that are used.


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## stevetexas (Feb 5, 2007)

I can't wait!!! This sounds like exactly what I've been looking for!!! 

Will there be different sensors available at other wavelengths if we need one in the future? This may be more accurate than extrapolating a curve at the extremes of this sensors capabilities... 

Any way you slice it, I think we've all been looking for something like this!!
:goodjob: 
Steve


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## stevetexas (Feb 5, 2007)

Another thought. If we needed to measure power beyond spec for the meter (say 250 mW...), we could use a "cheap" (around $30 USD) neutral density filter that blocks a known percentage of the transmission at a given wavelength. That would give us a fairly accurate measure of power after a few simple calculations... Any thoughts? :thinking: 

Steve

See the graph and link below from Edmund Optics:






http://www.edmundoptics.com/onlinecatalog/DisplayProduct.cfm?productid=1523


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## dr_lava (Feb 5, 2007)

That's what the ND8 filter that extends its measurements to 800mW is all about. It will be already included and calibrated for the wavelengths.


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## jkaiser3000 (Feb 5, 2007)

I can't wait for this to happen :rock:. Any idea of the time table?


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## dr_lava (Feb 5, 2007)

Yes, actually, Andover Holography said that the prototype would be finished in about a week, then mailed to me for testing. At that time I'll test and review it, and if it's up to snuff (I have little doubt it will be) we'll start the GB.

If anyone has a low-ish power (20mW or so) blue laser they would be willing to mail in for calibration, it would help us a lot. Its output will be measured by a calibrated 'pro' unit, and then the LPM-1 will be calibrated from that. The same goes for other unusual wavelengths that you would like dialed in to this unit!


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## The_LED_Museum (Feb 5, 2007)

I have two blue DPSS lasers; but one measures <2mW and the other measures >120mW. :shakehead:


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## ajohnson (Feb 12, 2007)

Actually sounds like the ND8 is very similar to the edmund ND-13 if you consider the numbers. At any rate, I can't wait for your review. I'm curious as to the dimensions of the final unit. The data logging is a nice feature too. Dump it into a computer and graph your lasers output over time.


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## Hemlock Mike (Feb 12, 2007)

I'm still in. eMail me when ready.
Thanks - Mike


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## stevetexas (Feb 14, 2007)

me too!! I'm anxious to see what we'll be working with.


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## dr_lava (Feb 14, 2007)

OK, the review is done and the Group Buy thread is posted! 

I have some good news, this meter seems to have turned out very nicely. A big thanks to Tony from Andover Holography for working with me concerning the specifications.

The LPM-1





It's a nice looking unit with an easy to use menu and silver buttons, but who cares how it looks? How does it WORK?

First, you select the wavelength you want to measure from the menu, including 473, 532, 635, 660, and 808nm options. Once selected, the display is continuously updated at 2Hz with the measured value. The peak value is recorded and can be reset with the push of a button.

The really slick feature for us experimenters that none of the other portable meters have is the serial datalogging option. Automatic power-down can be turned off in the menu for long datalogging sessions.

Example datalogging plot made using the meter and excel:





It's a very easy feature to use and enables the tweaker/experimenter to monitor the output of their laser over time. The datarate is 57600-8-n-1. Samples transmitted every 0.5 seconds.



Now, I was skeptical about the accuracy claims at first when I read about this meter, is uses a monocrystalline solar cell as the sensor, and relies on I->V conversion for the measurement. After I got the temporary test unit I tested the linearity and accuracy by comparing its reading with that of my Scientech calorimeter laser meter. (I used a red diode from our group buy for the test) The results were impressive:


Linearity test comparing calorimeter laser meter reading with the LPM-1:





result: less than 2% error (within the accuracy of my control standard) within the specified range. So there's no problem there.


Here's a list of features:
· Autoranging measurement of light power in 4 ranges from ~100 uW to 100mW (800mW with ND8 option)
· At least 5% accuracy over native range
· Multiple sensor calibrations for different wavelengths :473,532,635,660, and 808nm selectable in a menu.
· 16 character by 2 line alphanumeric display
· 3 Pushbuttons provide control of measurements & configuration using a simple menu system.
· Automatic 5-minute power down to extend battery life (can be turned off for datalogging)
· RS232 datalogging capability
· Peak hold remembers the peak reading in a series
· 2Hz (2 samples/sec) update and logging rate, 400Hz internal sample rate
· Sensor at top of meter for easy, safe readings
· Rugged surface-mount component construction
· Operated on a 9V battery
Optional features:
· Range extending ND8 neutral density filter included and calibrated
· RS232 datalogging pre-built cable OR
· RS232 datalogging DIY kit
· Backlit display

The kit schematics for the RS232 interface will be posted free for those who can build their own.


So, check out the group buy thread if you're interested.


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## erckgillis (Feb 15, 2007)

Nice meter better than my DIY. I'll take one from GB!

.You can make your own for free...most optical mice have sensitive laser photodiodes. Techniques for calibration for various wavelengths and conversion factors exist.

see

http://www.cat.ernet.in/others/lasinfo/p-meter.html

make you own with a diode of the same laser color and use the PNP junction effect to measure resistance to current changes in diode as power changes.

basic physics

Good enough for free DIY'ers

Someone can baseline a known "Lab certified" Laser and then measure on these rigs. From that or several other known lasers we can get fixed measures for conversion factors.

E


Here are some comments on these approachs: 

(From: Bill Sloman ([email protected])). 

The important thing to note is that a photo-diode actually detects photons, not power. Up to about 850nm, each photon actually reaching the diode junction generates one pair of charge carriers. A 425nm photon, carrying twice the energy of an 850nm photon generates the same pair of charge carriers, so the same current represents the absorption of twice the power. 

Since the 425 nm photon has rather less chance than the 850 nm photon of actually surviving the trip down to the diode junction, so the actual ratio is closer to 2.5:1. 

Above 850 nm, the photons haven't got quite enough energy to separate a pair of charge carriers, and can only separate those that are already somewhat excited. The proportion that are sufficiently excited depends on temperature. A electric field also helps, so biasing the diode increases it sensitivity to long wavelength photons. As the wavelength rises above 850nm the extra energy required to separate the charge carriers also rises, so the proportion of 'sufficiently excited' carriers declines quite rapidly. 

In principle one could build a wavelength correction into the power meter, but you would need to add a wavelength sensor to the power meter to make it a useful feature. 

The Centronics data book gives a typical spectral response for the 5T series diodes, which effectively gives you the inverse of the wavelength correction function, albeit with rather low precision. The alternative approach is to use a sensor which responds to the heating effect of the laser beam. These exist, but what you win on wavelength independent calibration, you lose on sensitivity and zero stability - in effect you have built a thermometer to measure the heating effect of your laser beam on a more or less thermally insulated target. Unless someone has done something very neat in this line, it doesn't strike me as a practical proposition for your application, granting your limited budget.


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