A word about calibration...

[lasersbee]

Exactly....

Jerry

 

[Greenmechanic]

I have a question when it comes to thermopile heads.

Why do the readings change according to relative angle of the beam?
The reading is lower when shining the laser at a 90 degree angle,rises by 10-15mW if I angle the beam (20-30 degrees) and shine it toward the edge of the sensor?

I just received my first thermo. LPM and noticed this.
All my other LPM's are photodiode, which does not have this effect.
I read something about this effect somewhere on here, but just did a search and could not find it.

EDIT: I assume this is not caused by body heat, as Jerry stated above, as this also occurs standing 10' away from the LPM.

 

[Kenom]

You Kenometer is designed to shine only on the center of teh thermopile at a 90 degree angle. If your getting a higher reading at a different place on the thermopile it's not accurate.

Here is a picture of the inside of your Thermopile.

As you can see the junctions are arrayed around the perimeter. This is designed to have the beam in the center and have the heat radiate in the thermopile equally to all the junctions. If it's not centered its going to be skewed a little.


As to Ben's statement in the OP, I agree that having a calibrated LPM is desirable, however this is one of the major differences between having a $1500 LPM and a $200-$400 LPM. You PAY for the nist tracking and to have the thing, to quote the movie "My cousin Vinnie" "Dead on balls accurate" This just isn't feasible for the lower end laser power meters.

 

[Greenmechanic]

Thanks Ken.
That answers that question. As I stated, I knew I had read the reason on here somewhere, but couldn't remember (or find) it.:thanks:

 

[lasersbee]

There are basically 2 types of Thermopile Sensors...

The Disk Thermopile

"The disk is made of one set of junctions laid out radially. One set of junctions is
arranged under the aperture while the alternate set is near the edge of the disk
which is attached to a massive heat sink."

Like Kenom posted....For accurate measurements of this Disk type just aim your
laser's beam onto the center of the Active surface...

The other is....
The Wafer-Type Thermopile

"The second one resembles a wafer, or sandwich, with thermocouples running between
the two sides. One rectangular face of the wafer thermopile receives the heat.
That creates a large temperature gradient across the small distance to the other
face that is in contact with the massive heat sink. The array of solid state
thermocouples in the thermopile generates a voltage proportional to this gradient
just like in the disk thermopile. Because of the close spacing of the thermocouples
to each other, the resulting large number of thermocouples in the wafer, and the
large temperature gradient across the two surfaces, the output voltage of this
thermopile is the most sensitive to laser power and the least sensitive to beam
position and size. "


These Wafer types do not exhibit the non-linear response across it's surface
associated with the Disk type.
This is the type used in the LaserBee Thermopile heads...

BTW...they are both accurate Thermopile Heads when used properly..


Jerry

 

[JLSE]

So what happens with the disk type thermopiles if you dont hit the center?
Does it not read or just not read correctly?

I was looking at a used meter on ebay made by Ophir which was disk type,
maybe a good thing they didnt accept my first offer?

There are basically 2 types of Thermopile Sensors...

The Disk Thermopile

"The disk is made of one set of junctions laid out radially. One set of junctions is
arranged under the aperture while the alternate set is near the edge of the disk
which is attached to a massive heat sink."

Like Kenom posted....For accurate measurements of this Disk type just aim your
laser's beam onto the center of the Active surface...

The other is....
The Wafer-Type Thermopile

"The second one resembles a wafer, or sandwich, with thermocouples running between
the two sides. One rectangular face of the wafer thermopile receives the heat.
That creates a large temperature gradient across the small distance to the other
face that is in contact with the massive heat sink. The array of solid state
thermocouples in the thermopile generates a voltage proportional to this gradient
just like in the disk thermopile. Because of the close spacing of the thermocouples
to each other, the resulting large number of thermocouples in the wafer, and the
large temperature gradient across the two surfaces, the output voltage of this
thermopile is the most sensitive to laser power and the least sensitive to beam
position and size. "


These Wafer types do not exhibit the non-linear response across it's surface
associated with the Disk type.
This is the type used in the LaserBee Thermopile heads...

BTW...they are both accurate Thermopile Heads when used properly..


Jerry

 

[xStatiCa]

What is the best way to measure the power for each head type as far as beam width?

For example.
- Take a perfectly calibrated laser that outputs precisely 100mw.
- Take a NIST calibrated high end meter with the two types of thermopiles
- Adjust beam size to 2mm (perfectly centered on the active sensor at 90 degrees)
- Shine it on each thermopile and note the power
- Adjust beam size to 10mm (perfectly centered on the active sensor at 90 degrees)
- Shine it on each thermopile and note the power

Which thermopile head would be more accurate over the range of beam sizes? Is there an optimal beam size when measuring?

I don't need NIST accuracy as long as they are somewhat close. I do hope to find something to detect if the measurements are drifting though over time. Maybe some kind of heat source that is very consistent or something? This sounds like a good project for those with LPMs to at least know if their meter is getting too far out of calibration. You would at least know if the readings are changing over time.

 

[lasersbee]

So what happens with the disk type thermopiles if you dont hit the center?
Does it not read or just not read correctly?

I was looking at a used meter on ebay made by Ophir which was disk type,
maybe a good thing they didnt accept my first offer?

The Disk type Thermopiles are the most accurate at the center of the Disk...
if the Sensor's Active surface is small... veering off center will produce
a differential output from thermocoulples around the circumference of
the sensor area..
My Kenometers using the Large Coherent thermopile heads do not exhibit
a large variation swing... it is negligible while the smaller heads exhibit
a slightly larger variation..
(I know this because I spent 3 days testing all of my LPM heads. That
includes Coherent... Ophir... Scientech... Newport and LaserBee I..)

Unless a Disk type Thermopile head is physically or electrically damaged
they will work just fine..

@ xStatiCa...

The best way is just off of 90 degrees to not get reflections back
into the Laser...

A good Thermopile Sensor will read a 100mW Laser with a 2mm diameter
beam the same as that same 100mW laser with the total beam covering
the whole sensor... For the Disk Type... due to the above variations...
There is usually a plate with a proper size hole to keep the beam in
between the inner thermocouplles..

With a Wafer type you can place your beam anywhwere on it's surface
and the reading will be the same... with the beam at 2mm or the beam
large enough to cover the entire Wafer Thermopile's surface...

We do our testing with beams no smaller that 2mm and as large as 8mm
diameter on the LaserBee Thermopile.

The case of the LaserBee I... the problem of beam size is only due to
the max Power density/CM2 allowable on the Thermopile's coating to not
damage it.

We prefer the Wafer Type... that's why we chose going that route..

As to drift... If the Thermopile's Hearsink is at ambient temperature
and you are not pumping watts of Laser energy into the head heating
it up way over ambient... then there should be no drift.

We have used the same Shop LaserBee I for over a year and it is still
spot on to the calibration we did back then... But we take care not to
touch or damage/burn the Thermopile's surface and store it in a dust
free drawer when not in use..

Like I said... we have both types in the shop... If they were no good..
we wouldn't have them...

Jerry

 

[ninja_tux]

Jerry, your explanations (especially concerning LPMs) are always very informative and a pleasure to read. That cleared up a lot of the questions I had concerning the difference in sensors...and I didn't even ask the question! I would rep you but apparently I have to "spread it around" first /

 

[Benm]

I imagine for the builders and DIY folks that calibration is important, in order to have some credibility in a market full of exaggerated claims.

For DIY i don't see the purpose.

For hobbyists is mostly a matter of roughly asserting some manufacturers claim, or tuning some system for maximum power output, regardless of what that maximum is exactly. Often, it doesnt really matter all that much even if the value is off by as much as 10% in either direction.

If you actually sell lasers with some guaranteed output power level, a calibrated meter can be important, or you should at least know how far its off. If you know it cannot be off more than 10% and want to sell 100 mW+ lasers, you can decide to put the QC pass at 111 mW, so that no unit will be under spec. This could be cheaper than recalibration under some circumstances.

 

[lasersbee]

That is fine if you know that you are off by 10%... but how do you know..:thinking:
I've bought LPMs + heads that were 40% out of calibration and some that
were spot on... You never know what the previous owner has done to them..

Once you know your LPM is calibrated... and you take care of it... then I can
see that even after years of use it shouldn't be off by more that 10%..

BTW... DIYers want bragging rights "Hey look what I built" and with that goes
"and it puts out XXX mW of Laser Goodness"... to do that honestly without
exaggeration... a good LPM is an asset.. IMO


Jerry

 

[Benm]

Well, thats why i like to design things with internal thermal calibration built in. They arent the most accurate meters in the world by far, but they can get you 10% or so accuracy with no need for further verification against known-good lasers or other meters.

These are entirely different leagues though: For hobby use, i'd say something that is 10% accurate and represents trend (ie higher readout = actually more) well is usable. If you want to make % specific statements and measurements, you will need calibration against known sources.

40% off is mad if you ask me. Anyone can do much better than that DIY using available materials and a bit of care.

 

[Hemlock_Mike]

Lbee ---

Considering the Scientech disk heads (360001), If I input a VxI input to the calibration coil of say, 500 mW, should I see ~ 500 mW on the meter?
Loss in the cables should be minimal compared to the R of the heater between 50 and 40 ohms. A correction factor of 1.005 is almost nothing.

HMike

 

[lasersbee]

I agree with you H_M that some heads like the Scientechs can be
calibrated quite close using their built in Heaters... but not all Heads
have heaters for calibration.... and those are the ones that would
require a known Laser Power Source or good LPM to see if they are
at least in that 10% accuracy range... (that is if someone is so inclined)
Otherwise... we really don't know and are just guessing... IMO

I may be a bit biased... not because we sell an LPM... but probably
because since we've purchased our first LPM (and had calibrated)...
we will never guess again...


Jerry

 

[HIMNL9]

...... You never know what the previous owner has done to them.. ........
Jerry

You don't know how much your words are right, in some cases ..... :na:

I had to change a coherent head some times ago, from a machine ..... can't say names, ofcourse, but someone before me, for repair it, thinking that the "black cover in the hole" was an inspection cover, poked it with a screwdriver for try to pop it out :crackup: :crackup: :crackup:

The owner of the machine kept the broken head, don't know for sure but probably wanted to ask that guy for a refund ..... but i took a pic with my cellphone, before left it to him, cause it was the first time that i've seen a coherent thermopile destroyed in this way ..... ( bad quality from the cellphone, but just in case you're wondering, yes, it's a hole :crackup: )

 

[lasersbee]

Wow...
that is one of the problems when wanting to buy from an unknown seller
who may have no clue as to what a Thermopile is or how to handle it...

Jerry