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I wasn't suggesting to use immersion oil on the power meter sensor, and I don't know how those things are engineered… although I'm guessing a coevrslip-like protective window in front of the sensor could take care of that problem.
However, measuring the power at the nosepiece seems more complicated to me, and then I would have no way to know which fraction of that power actually enters the objective…. For instance, we have two objectives from the same vendor with the same specs (63/1.4 Plan Apo) which have back apertures of different sizes (and one therefore delivers more light to the focal plane, all other thing being equal).
On the other hand, if the power meter sensor is fairly large and is held pretty close to the front element of the lens, I suspect the error due to rays that miss the sensor (i.e rays at close to 180 degrees, would be pretty small… certainly smaller in my opinion to the error in estimating how much light coming out of the nosepiece actually enters the objective.
Julio.
----- Original Message -----
From: "John Oreopoulos" <[log in to unmask]>
To: [log in to unmask]
Sent: Tuesday, March 12, 2013 10:04:08 AM
Subject: Re: Laser power meter and sensor
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Julio, on the specifications tab of the weblink you sent, it says regarding objective compatibility:
4X-63X; air coupled, with FOV diameters less than 10mm
Just to re-iterate, I don't think you can use an immersion objective with a power meter like this (someone from Lumen Dynamics can confirm this). Not only would it require you to place the optical sensor in contact with the immersion fluid of the objective (which might damage it), but even if you could do that, it still wouldn't work because these devices have a low acceptance angle for light, that is to say, these devices won't capture/register the light that approaches the sensor at very oblique angles which are typically present when using high NA immersion objectives.
The absence of the immersion medium is also not an option because then many of the oblique rays will be lost at the front lens due to total internal reflection at the glass-air interface.
John Oreopoulos
On 2013-03-12, at 12:38 PM, Vazquez Lopez, Julio wrote:
> *****
> To join, leave or search the confocal microscopy listserv, go to:
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> *****
>
> You can check this:
>
> http://www.ldgi-xcite.com/products-xr2100-xp750.php
>
> We use a similar meter from Coherent, but i'm not in the lab now and can't tell you the model #. We routinely use power at the focal plane with up to 20x objectives. Not sure how accurate it would be with high NA… depends on how close you can get to the objective…
>
> Julio Vazquez.
>
>
> ----- Original Message -----
> From: "John Oreopoulos" <[log in to unmask]>
> To: [log in to unmask]
> Sent: Tuesday, March 12, 2013 9:09:35 AM
> Subject: Re: Laser power meter and sensor
>
> *****
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> Hi Martin, it regards to question 2, I don't think such a power meter exists (although if someone else knows otherwise, I'd love to know as well). You're better off measuring the power at the back of the objective (ie: unscrew the objective and measure the power at the empty nosepiece), and then use the wavelength transmission curve of the objective (usually available from the manufacturer) to calculate the percentage of light that gets through to the front end of the objective - some error is introduced here as there is some variability in the transmission for each identical objective, but at least you get a good estimate of the actual power on the other side of the objective.
>
> You might also consider over what area that power that gets to the other side of the objective is projected to determine the power density (power per unit area), which is is a more relevant parameter. There is a nice Nature Protocols paper from 2008 that talks about these kinds of measurements, and there is mention about choice of power meter I think:
>
> Grunwald, D., et al., Calibrating excitation light fluxes for quantitative light microscopy in cell biology. Nature Protocols, 2008. 3(11): p. 1809-1814.
>
> Cheers,
>
> John Oreopoulos
> Staff Scientist
> Spectral Applied Research
> Richmond Hill, Ontario
> Canada
> www.spectral.ca
>
>
> On 2013-03-12, at 11:17 AM, Martin Seem wrote:
>
>> *****
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>> A couple of questions for the list:
>>
>> 1, We are currently in the process of buying a new laser power meter that will
>> primarily be used to check the general stability and power output of the laser
>> lines of our CLSMs. Have anybody recently bought a power meter after
>> considering some of currently offered products (like the Coherent LabMax
>> TOP, Newport 1918-R, GigaHertz PT-9610 and Ophir Vega). Which one did
>> you choose and why?
>>
>> 2, Are there any commercially available laser meter sensors that will allow us
>> to measure the actual laser light output from a high NA (NA>1) water or oil
>> immersion objective?
>>
>> Martin
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