Stacking ND Filters: The Right Way and the Wrong Way

ND filters are a great way to get your laser down to a manageable power for beam profiling.

But what’s the best way to physically attach the filters to your laser beam profiler?

Stackable ND Filters

Ophir ND filters are C-mount threads, which means you can screw one onto the profiler and another onto the first, and so on.  This is handy if you need extra attenuation.

But – there’s a right way and a wrong way to do this:

Always put the strongest attenuator (highest ND value) closest to the camera.

Why are we making extra rules for you?

Let’s imagine what happens as the laser hits the ND filter and begins to penetrate it.

A significant part of the laser energy is absorbed within the filter.  But how far does it get?  The answer is that the denser the filter, the less it allows the laser to penetrate, so more energy is absorbed closer to the front of the lens.  A weaker (less dense) filter allows the absorbed part of the laser energy to be spread out through more of the filter volume.

All this leads to the result that the stronger an ND filter is, the lower its damage threshold will be.

Which is why I urge you to always place the lower ND filters in front of the higher ones, so the higher filters only “meet” the beam once it has been attenuated a bit.

And for those visual learners among us, here’s an example:

Correct ND Filter Order

Incorrect ND Filter Order

Notice that the different order doesn’t affect the energy of the laser hitting the CCD.  But it does change how much the stronger filter (ND1, in our case) absorbs.  In the first case, it absorbs less than half of the laser power and turns it into heat.  In the second case on the other hand, the stronger filter must dissipate 90% of the laser.  Since it’s safe to assume that ND0.3 has a higher damage threshold, it makes sense to let it absorb the lion’s share of the laser energy.

P.S.

Interested in hearing more about laser beam profiler attenuation?  Stay tuned.

Interested in something else?  Let me know in the comments.