Regarding recent questions of LPM accuracy.

[MarioMaster]

It's strange that the results are so uneven, did you make sure to position the beam in the exact center of the sensor? I find this to be quite important in regards to getting the best accuracy since the Ophir heads do seem to vary quite a bit in readings if the beam is not placed in the same spot.

My head seems to be pretty much spot on :shrug:

 

[Trevor]

My data included repeated tests each at center, 12:00, 3:00, 6:00, and 9:00.

Trevor

 

[lasersbee]

It's strange that the results are so uneven, did you make sure to position the beam in the exact center of the sensor? I find this to be quite important in regards to getting the best accuracy since the Ophir heads do seem to vary quite a bit in readings if the beam is not placed in the same spot.

My head seems to be pretty much spot on :shrug:

Yeah... If you remember... I was the one that brought up the reading
discrepancies of the Surplus OPHIR heads when not exactly in the
center when the Kenometer PRO and USB were first being offered.

I even posted a pic of the different readings at different areas of
the Active Area of the Sensor using the same laser and power.

Your OPHIR head does seem spot on... Perhaps you have a pristine
untouched one like the one I used for the tests I did above...

My data included repeated tests each at center, 12:00, 3:00, 6:00, and 9:00.

Trevor

Are you saying that you got the same readings at center, 12:00, 3:00,
6:00, and 9:00 on the sensor's Active Area...:thinking:


Jerry

 

[daguin]

I thought that the most effective way to utilize a thermopile was to de-focus the beam and spread it across the sensor as widely as possible :thinking:

Peace,
dave

 

[lasersbee]

I thought that the most effective way to utilize a thermopile was to de-focus the beam and spread it across the sensor as widely as possible

Peace,
dave

It is the best way for radial Thermopile Sensors... but if your laser
has a smaller beam such as a DPSS green that does not have
adjustable focus then the best spot on these OPHIR Heads is a the
exact center if the Active Area.

By shining a Laser at the exact center it allows all the thermocouples
that are locate around the perimeter (radially) of the Active Area of the
sensor to receive the heat generated by the Laser's beam at the same
time and equal intensity...


Jerry

 

[Trevor]

Are you saying that you got the same readings at center, 12:00, 3:00,
6:00, and 9:00 on the sensor's Active Area...:thinking:


Jerry

Rather, I took readings from those locations and reported the ones in the center.

Trevor

 

[lasersbee]

Rather, I took readings from those locations and reported the ones in the center.

Trevor

Gotcha.... that makes more sense...

Jerry

 

[lasersbee]

Well I finally found a bit if time today to test the few heads we
had left.
They were tested with the same simple dual 9Volt battery split
supply as in the tests above with freshly charged batteries.

Heads #1-#3 are from the eBay Seller of this summer and #4
was from a Kenometer Lite we bought last year and the fifth
was the one that came with the damaged Kenometer PRO we
bought from CDBeam..

Startup acclimation results for 15 minutes...

Head ----- #1 ----- #2 ----- #3 ----- #4 ----- PRO

0 min ------ 0mV --- 0mV --- 0mV -- -1mV --- 0mV
5 min ------ 2mV --- 1mV --- 0mV --- 0mV --- 6mV
10min ----- 3mV --- 1mV --- 1mV --- 1mV --- 7mV
15min ----- 3mV --- 1mV --- 1mV --- 1mV --- 7mV

We then tested each head after calibration at only one power
level of 500mW. For the 1000mW test you can extrapolate the
numbers. The OPHIR head's output is quite linear.

Head ----- #1 ----- #2 ----- #3 ----- #4 ----- PRO

500mW -- 506 ---- 499 ---- 501 ---- 519 --- 518

By the data collected I wonder if the recent eBay seller's
OPHIR heads were from the same batch as the Heads
supplied with the Kenometers we bought..

It is to be noted that I'm only able to test the few OPHIR
20C heads that we have in the shop at this time and these
tests are not an indication of other OPHIR heads.
It doesn't mean that your OPHIR 20C head is off calibration.

I would suggest as mark-in-dallas suggested that if you
have reason to doubt the accuracy of your particular OPHIR
20C head have it checked by someone with an accurate
Laser Power Meter.



Jerry

 

[snoutke]

Didn't read the entire thread.

I have both Jerry's LaserBee USB LPM and MarioMaster's Radiant Alpha LPM.

If it helps this discussion, I will just say that:

Testing all my lasers, low power and high power as well, with both LPMs, in the same conditions, i.e.: 15cm distance to sensor, pointing the laser to the center of the sensor, laser in the same focus while tested with both LPMs and same temperature and hour of the day, I can report that there is a 5% difference between both LPMs readings, while the LaserBee USB readings are HIGHER.

I did this experiment (testing all my lasers with both LPMs in the same conditions) few times, and therefore my conclusion is rather established.

 

[lasersbee]

Didn't read the entire thread.

I have both Jerry's LaserBee USB LPM and MarioMaster's Radiant Alpha LPM.

If it helps this discussion, I will just say that:

Testing all my lasers, low power and high power as well, with both LPMs, in the same conditions, i.e.: 15cm distance to sensor, pointing the laser to the center of the sensor, laser in the same focus while tested with both LPMs and same temperature and hour of the day, I can report that there is a 5% difference between both LPMs readings, while the LaserBee USB readings are HIGHER.

I did this experiment (testing all my lasers with both LPMs in the same conditions) few times, and therefore my conclusion is rather established.

Or you could say the Apha measured LOWER....

Pot(ey)to.... Pot(ah)to...

They would both need to be tested against a known
accurate LPM to see where the 5% difference lies..:beer:

Jerry

 

[lasersbee]

I've been working on a research project here at Virginia Tech - I met with a Coherent representative earlier this semester. They donated a 641nm 100mW-rated CUBE laser to me. It is brand new, perfectly calibrated, and is computer controllable.

A few weeks ago a user came to me when he was experiencing vastly different readings on his Kenomter USB vs. his Laserbee 2.5W - lasers were reading around 15% higher on the Kenometer USB. I sent him the Kenometer USB firmware update and gave him a new version of Luminosity. The issue persisted.

For the sake of science, I decided to test my four Ophir sensors. I set my CUBE to output 80mW. The results were as follows:

• 10.7% LOW
• 5.4% LOW
• 4.1% LOW
• 8.3% LOW

I was rather dismayed. This opened up the possibility of the LaserBee 2.5W LPM's to be incorrect. If we thought the Ophir sensors were reading high but they are actually reading low, then it means the LaserBees are probably reading very low.

This was rather distressing. Jerry calibrates his LPM's against a Newport LPM. They should be accurate and we assume them to be accurate. I went back and found a graph on PL that showed them to be accurate. If they are not accurate, it could mean that a lot of our benchmarks and fundamental assumptions regarding laser diodes (output in millwatts being less than input in milliamps, efficiency, etc).

However, I had not tested the user's Ophir sensor. So he shipped it to me. I once again set my CUBE to output 80mW. I fired up my Apex LPM with his Ophir sensor attached. It read 3.5mW with no power input. I moved the laser over to the sensor surface, expecting to see a high reading.

83.5mW. 3.5mW + 80mW = 83.5mW. That was NOT supposed to happen. This is BAD.

I shipped the Ophir sensor back to the user, intending to test his LaserBee next.

Last Thursday (after I'd been covering the shooting for the student paper), I got a call from him. He'd gone to test some of his lasers at a nearby university and had gotten numbers that were effectively the same as the LaserBee's.

Trevor

Just an update to this "questions of LPM accuracy" Thread....

I recently received information that Trevor had tested his Cube
Laser against a lab LPM at his Campus and was kind enough
to let me know that his Brand New CUBE laser set to 80mW
actually only puts out 73mW...

Since he used his CUBE Laser set at 80mW (73mW or 9% Lower)
to do his test...the test he did would be skewed by 7mW...

EDIT
Removed incorrect Chart

If your tests with the original high reading OPHIR Head using
your Apex LPM read 3.5mW + 80mW = 83.5mW
(Which now would be 3.5mW + 73mW = 76.5mW)
and I measured that head at 9% higher then something is
questionable...
With new data I would suspect one of the following of being
off by a significant margin...

1) my recently calibrated NewPort 1825C
2) your CUBE Laser
3) your Campus LPM
4) your Apex LPM
5) your OPHIR Heads


Jerry

 

[Trevor]

The confirmation that my CUBE was outputting low confirms that Ophir sensors that I had on hand were accurate, and that Mark's was reading significantly high.

If I expected the sensors to read 80mW, but my CUBE was actually putting out ~73mW, it would appear the sensors were reading low, but were in fact accurate.

If my CUBE were outputting higher than 80mW, yes, the error would be even larger.

So, you've said:

I set my CUBE to output 80mW. The results were as follows:

    10.7% LOW would now read 17.7% LOW
    5.4%  LOW would now read 12.4% LOW
    4.1%  LOW would now read 11.1% LOW
    8.3%  LOW would now read 15.3% LOW"

But see, I was getting low readings, when I was assuming the CUBE was outputting 80mW. In reality, it wasn't. Here were my original results, noted as straight power readings:

I set my CUBE to output 80mW. The results were as follows:

    10.7% LOW ( 71.9mW )
    5.4%  LOW ( 75.5mW )
    4.1%  LOW ( 76.5mW )
    8.3%  LOW ( 73.5mW )

...which, as it turns out, matches my CUBE outputting ~73mW. Make sense?

Trevor

 

[rhd]

As Trevor said, doesn't this put the Ophir readings right no target?

I set my CUBE to output 80mW. The results were as follows:

10.7% LOW ( 71.9mW )
5.4% LOW ( 75.5mW )
4.1% LOW ( 76.5mW )
8.3% LOW ( 73.5mW )

By my calcs:

He said: Which is: Should be: Actually Off:
80 -10.70% 71.44 73 -2.1%
80 -5.40% 75.68 73 3.7%
80 -4.10% 76.72 73 5.1%
80 -8.30% 73.36 73 0.5%

 

[Trevor]

Here's the original raw data. The "Center" reading is what you want to pay attention to:

Somewhere buried in the papers in my room I have the report from the lab here. I'll see if I can dig it up.

I later tested the sensors in the lab at 1064nm (the wavelength they're calibrated for) and found they were all within spec.

Trevor

 

[lasersbee]

After ingesting a few more cups of coffee... You guys are
obviously correct.
My Bad... (I blame it on a caffeine low brain fart..)

If the Cube Laser was putting out only 73mW instead of the
adjusted 80mW then the readings should reflect being ~9% low..

Taking that into account and adding ~9% to your original low
readings would obviously increase the readings and not decrease
them...

I set my CUBE to output 80mW. The results were as follows:

• 10.7% LOW would now read ~1.7% LOW
• 5.4% LOW would now read ~3.6% HIGH
• 4.1% LOW would now read ~4.9% HIGH
• 8.3% LOW would now read ~0.7% HIGH"

Those figures show that your heads were better than 5% accurate
with one almost right on...


Jerry

 

[ARG]

I know this is an old thread, but I thought I should add my own results.

The set up with my stable 808nm laser. (The same wavelength used to calibrate the laserbee's IIRC)

The Ophir reading (Minus 7 mV for zeroing)

The laserbee II reading:

I know that my Ophir is reading correct because I sent it off to MarioMaster who checked it against his Coherent Fieldmax, it was reading the same as a fieldmax.

The laser bee II (in my case) is around 10% off.
There has been no damage to the Laser Bee since I bought it.



Now, I also received a used HLPM recently.
This test was done with a pen laser as I don't have a lab style 405nm laser.
Ophir reading: 45mV (minus 7 for zeroing) so 38mW.
The Laserbee II read 35mW.
The HLPM read 4.4mV, and according to the correction factor chart it should be multiplied by 23.3. 102.52mW That's 70% off!
When reading in the red spectrum it's fine, but I think the wavelength chart may be a little off.