2
$\begingroup$

I am currently working on a UV detector meant to be mounted on a CubeSat satellite. To select the bandwidth, I consider using an interference filter placed between the detector and the source. However, I have been told that the incoming solar radiation will damage the filter, causing it to transmit less light in the selected bandwith, and potentially modifying the passing band. I have two interrogations about this:
-What exactly causes this kind of damage? Is this just the overheating, or maybe a chemical reaction darkening the filter?
-Are there any numbers/formulas available to evaluate the impact of solar radiation on the filter? I did not manage to find anything more specific than "EM waves can damage filters", and I really need to find out what the lifespan of my filter will be and how fast its properties will evolve.

The passing band is 200-240nm.

$\endgroup$
  • 2
    $\begingroup$ UV light is a form of ionizing radiation. Meaning, it can break chemical bonds and/or liberate electrons from atoms/molecules. Generally, the damage is done by breaking down the substance (i.e., breaking chemical bonds) used in the filter, which produces new "impurities" that have different transmission coefficients in a given energy/frequency range. $\endgroup$ – honeste_vivere Apr 4 '16 at 15:21
  • $\begingroup$ @honeste_vivere That sounds like an answer, to me. $\endgroup$ – garyp Apr 4 '16 at 15:23
  • $\begingroup$ @garyp - Perhaps, but I feel apprehensive about such short answers. I have written similarly short answers in the past and was then blasted by other users for being too brief... $\endgroup$ – honeste_vivere Apr 4 '16 at 15:25
  • $\begingroup$ @honeste_vivere If you added a credible source I would upvote that $\endgroup$ – pentane Apr 4 '16 at 15:30
5
$\begingroup$

What exactly causes this kind of damage? Is this just the overheating, or maybe a chemical reaction darkening the filter?

UV light is a form of ionizing radiation. Meaning, it can break chemical bonds and/or liberate electrons from atoms/molecules. Generally, the damage is done by breaking down the substance (i.e., breaking chemical bonds) used in the filter, which produces new "impurities" that have different transmission coefficients in a given energy/frequency range. A similar effect occurs on the conductive glasses used to cover solar panels on spacecraft, where over time they become effectively black due to UV damage. There are some notes on this in the 2015 Senior Review (PDF) proposal for the Wind spacecraft (e.g., see Figure 15 in review showing the degradation of the solar panel current output). The change in slope in Figure 15 is due to the weaker solar cycle (i.e., lower high energy photon fluxes). A more explicit set of results discussion solar cell damage on spacecraft can be found in this NASA Technical Report Study (PDF).

Are there any numbers/formulas available to evaluate the impact of solar radiation on the filter? I did not manage to find anything more specific than "EM waves can damage filters", and I really need to find out what the lifespan of my filter will be and how fast its properties will evolve.

You could do something like determine the work function and/or bond energy of each atom/molecule in the filter substance. Then using known average UV fluxes/intensities, you could estimate how long a given volume of material would last. You can look up ionizing energy tables where the values are given in useful units of kJ/mol, which you can easily convert if you have a known volume of a substance. Figure out the molecular weight of the filter substance, then determine the total number of moles in a given volume, then you can determine the total deposited energy necessary to destroy the entire filter (as a sort of longest duration upper limit, where the true value will actually be shorter).

There is a very detailed explanation of effects and analytical approximations for ionizing radiation damage to solar cells from the Solar Cell Radiation Handbook. The details are mostly about the physical damage to solar cells from high energy electron, proton, and neutron radiation which alters the semi-conductor structure.

| cite | improve this answer | |
$\endgroup$
  • $\begingroup$ Could you point out where in the Wind review they talk about this effect? Couldn't find it after a brief skim $\endgroup$ – pentane Apr 4 '16 at 18:49
  • $\begingroup$ @pentane - done $\endgroup$ – honeste_vivere Apr 4 '16 at 19:08
  • $\begingroup$ Where do they mention the degradation being due to ionizing radiation? I still don't see it $\endgroup$ – pentane Apr 4 '16 at 19:15
  • $\begingroup$ @pentane - The decrease in output current vs. time of the solar arrays is due to UV radiation damaging the conductive glass over the solar panels. That is shown in Figure 15 of the review. $\endgroup$ – honeste_vivere Apr 4 '16 at 19:18
  • $\begingroup$ How do you know that is the cause of the decrease? Is it stated somewhere in the report? That is the source I am looking for $\endgroup$ – pentane Apr 4 '16 at 19:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.