Fwd: RE: objective lens laser damage--Commercial Response

[Martin Wessendorf-2]

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 From Dr. Asanov:
-----------------------
Dear Joshua,

Theoretically, the laser damage threshold for the microscope objective
can be as low as 1-5 mW, which is close to the eye-damaging power. In
the case of eye exposure, our reflexes make us blink or turn away from
bright light.  In the case of a microscope objective, continuous
exposure potentially can cause damage to the glue and other organic and
inorganic compounds in the objective.

The damaging factor is the intensity - optical power per unit surface
area. As we know, 100-1,000 Watts/cm2 causes rapid photobleaching of
organic fluorophores, while >10,000 Watts/cm2 initiates burning of
organic materials. The intensity, which can be generated by a laser,
depends on the radiance, which is defined as optical power per unit
surface area per unit sterical angle. The radiance of a 1 mW laser with
1 mm beam diameter and a typical divergence of 1 milli-radian is ~1.6 x
100,000 Watts/cm2-steradian. In comparison, the Sun’s radiance is only
~100 Watts/cm2-steradian. Due to this unique feature, 1 mW of such laser
focused into a 1-micron produces the intensity ~100,000 Watts/cm2, which
is sufficient for burning. Thus, the damage threshold depends on the
radiance, which is the measure of the “focus-ability” of the laser.
Naturally, for fiber-coupled lasers the radiance is different from that
before coupling.

One more important factor is related to foreign inclusions and other
imperfections at the surface and in the bulk of optical materials of the
objective, including lens glue. These imperfections provoke the
“nucleation” of burning. It is not the question of “whether”, but rather
“how much” of imperfections are present in your objective. Dr. Sripad
Ram mentioned about a dry objective with large inclusion he dealt with.
Large inclusions are rare. Microscopic imperfections are common.

We in TIRF Labs tested eleven high NA objectives made by Olympus and
Nikon. Those lenses did not contain visually detectable imperfections,
but produced a significant amount of stray light – from 15% to 45% of
the evanescent wave at the surface. Surprisingly, the same model
objectives, from the same manufacturer yielded different amounts of stay
light. The articles cited at TIRF Labs website analyze the sources of
stray light in TIRF experiments. I remember, Joshua, you mentioned that
the amount of stray light is not important for your STORM/PALM
experiments. Nevertheless, if you are concerned about the laser damage,
looking at the amount of stray light could help you to rank the
objectives by their resistance to the laser damage.

Best regards,

Alexander N. Asanov, Ph.D.

President, TIRF Labs

Cary, NC 27519

[hidden email] <mailto:[hidden email]>

www.tirf-labs.com <http://www.tirf-labs.com/> www.TIRFmicroscopy.com
<http://www.tirfmicroscopy.com/>

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Thanks for several thoughtful replies. I have a couple of follow up points.

Is anyone aware of damage threshold data for objective lenses, from
Nikon or other manufacturers? It seems many people suspect glue damage
for my lens. I asked my local Nikon sales rep but he was unable to get
data for me.

Does anybody have a favorite objective lens for STORM/PALM among the
following Nikon lenses? I'm intrigued by #4, below, which is rated for
high power (Seamus mentioned).

1. CFI Plan Apo Lambda 100X Oil 1.45 NA (current lens) 2. CFI Plan Apo
Lambda DM 100X Oil 1.45 NA 3. CFI Apo SR TIRF 100X Oil 1.49 NA 4. CFI HP
Apochromat TIRF 100x oil 1.49 NA

To John Oreopoulos... No, we don't need 1W lasers for imaging. We
generally work with up to 50-100 mW for the 488/561/647/750 nm lines and
use the rest of the laser power for other stuff including spectroscopy
experiments. For the 405 nm line, it is typically below 1 mW, but
considering our test which showed 405 nm is likely the problem, I wonder
if a user may have blasted with 405 nm at some point leading to damage
that got worse with repeated use even at lower power.

Joshua C. Vaughan

Assistant Professor

Department of Chemistry

Box 351700

University of Washington

Seattle, WA 98195


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