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Answer(s) to Did Valentin Lebedev or any another Salut cosmonaut actually get suntanned for fun and/or for science? explain that at least a few Salut cosmonauts "experimented" with getting very rapid (few minute) local tans at one of the few fused silica windows that allowed the full visible plus UV spectrum of the Sun to enter the station when not covered.

These days I'd assumed that all space windows for crew viewing are built to ensure crew are not exposed to dangerous UV, though there is this one.

How is this done? is it a certain formulation of glass that has the thermal, mechanical and optical properties to qualify as a space window and at the same time filters dangerous UV but is transparent in the visible? Or are coatings and/or filters used as well.

The biggest windows I can think of that can expose crew to a good dose of sunlight are the ISS' cupola, so those might be a good starting point.

Andre Kuipers, ISS Expedition 30

Andre Kuipers, ISS Expedition 30

above: TOP: Andre Kuipers, Exp. 30, BOTTOM: Paolo Nespoli, cropped/enlarged from image below.

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    $\begingroup$ You usually have to try hard to make sure your optics will pass UV. Fused silica (basically pure SiO2) will to a point, normal window glass not so much. $\endgroup$
    – Jon Custer
    Apr 30, 2021 at 20:20
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    $\begingroup$ Related: space.stackexchange.com/a/31025/6944 $\endgroup$
    – Organic Marble
    Apr 30, 2021 at 20:23
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    $\begingroup$ Is an Apollo answer allowed? $\endgroup$
    – DrSheldon
    Apr 30, 2021 at 21:06
  • $\begingroup$ @DrSheldon I'm primarily interested in what's done in recent times to see if there's been a convergence on some particular formulation of glass, but if there's an interesting answer about Apollo I don't think the internet will break if you post it with a "Not modern, but..." preamble. $\endgroup$
    – uhoh
    May 1, 2021 at 1:32
  • $\begingroup$ @JonCuster there's not just one kind of "ultraviolet"; different types of glasses provide different amounts of attenuation at different wavelengths, and different wavelengths have different effects on tanning versus damaging DNA as well has have differing levels of suitability for spacecraft windows. We don't want the UV or cosmic rays to produce color centers or solarize the glass, we don't want the thermal expansion/contraction of the glass to fatigue the seal, etc. I think there has been significant attention to what types of glass are most suitable. $\endgroup$
    – uhoh
    May 1, 2021 at 4:56

3 Answers 3

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This article from Air Space Mag (https://www.airspacemag.com/space/window-on-the-world-1142261/) says that the window for the Destiny module was designed as such (emphasis my own)

The window’s glass is a special stock of high-purity, colorless, synthetic fused silicon dioxide—a material chosen for its resistance to the effects of thermal variations and its exceptional light transmission. Corning took extra care in producing the glass, making sure that tiny imperfections such as bubbles and other blemishes were minimized. The homogeneity of the window is very high—light waves pass through the glass with barely any distortion. A polishing job by Zygo Corporation, a high-performance testing, manufacturing, and measurement firm in Middlefield, Connecticut, made the glass almost perfectly smooth, and coatings by Optical Coating Laboratory, Inc., of Santa Rosa, California, will block out harmful ultraviolet radiation and enhance light transmission to give the space window its unequalled optical rating. “For the astronauts to be able to see without the distortion is going to be amazing,” Sutton says.

So it looks like historically (2001) this has been utilised.

There are some proposed new technologies as well. This article describes a paper with a new technique for producing a "composite glass-based UV absorber". This is yet to see any use but they do mention its applications in spaceflight.

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    $\begingroup$ Interesting! it's done with coatings rather than the glass formulation. Thanks! $\endgroup$
    – uhoh
    May 2, 2021 at 14:22
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Not modern, but Apollo used coatings on the windows of the command module and lunar module. Source: Apollo Experience Report: Spacecraft Structural Windows, NASA Tech Note D-7439.

The CM windows were triple-pane (except for the telescope optics). The outer-most pane was made of fused amorphous silica and protected the cabin from micrometeroids, radiation, and re-entry heat. Because the outer-most pane was not air-tight, the space under it voided to vacuum once in space. The two inner panes were made of thermally-tempered aluminosilicate glass, provided an air-tight seal, and filled in between with 7.0 psia dry nitrogen.

From outside to inside, the panes of the CM were coated as thus:

  • A coating of magnesium fluoride on the outer surface of the window.
  • A blue/red (BR) coating on the inner surface of the outer pane.
  • A high-efficiency anti-reflective (HEA) coating on all four surfaces of the two inner panes.

CM window

The BR coating was removed from the hatch window on Apollo 13, 14, and 15 to allow a mulitspectral photography experiment.

All LM windows (except for sextant optics) were double-pane. The outer pane protected the cabin from micrometeroids and radiation, and were made of annealed Vycor 7913 glass. The inner pane provided an air-tight seal and was made of chemically-tempered Chemcor 0312 glass. The space between the panes was vented to space.

From outside to inside, the panes of the LM had coatings:

  • "A final BR [blue/red] coating was applied to the outer surface of each outer pane to restrict the amount of infrared and ultraviolet light to the cabin."
  • An high-efficiency anti-reflective (HEA) coating on the inner surface of the outer pane.
  • An electrically conductive coating (ECC) to defog the window by electrical heating. " After ECC application, light transmission was reduced to approximately 76 percent."
  • Another HEA coating on the inner surface of the inner pane. "To increase the light transmission and decrease the reflection caused by the ECC, an HEA coating was applied to the in-board surface of each inner and outer pane. When a pane was completely coated and a black edge (black velvet paint) was applied to the periphery, the light transmission was increased to approximately 82 percent and the reflection was reduced from approximately 14 percent to 5 percent. "

LM window

I wish the report gave more details about the chemical identity and absorption spectra of the coatings, but they are not provided.

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  • $\begingroup$ Wow, those are some very elaborate windows! I don't know what the UV properties of aluminosilicate glasses are but again they have a blue-absorbing layer applied. $\endgroup$
    – uhoh
    May 2, 2021 at 21:32
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The biggest windows I can think of that can expose crew to a good dose of sunlight are the ISS' cupola, so those might be a good starting point.

Like all the other US spacecraft windows I know of (except for the shuttle side hatch window) the cupola windows have coatings to keep out the UV.

On each pane there is a coating to prevent reflection when the panes are stacked together. The coatings also filter out ultraviolet and infrared light but do not affect the physical clarity of the glass.

Boeing provides panes, attaching mechanism for observatory on ISS

So does the wonderfully named ISS WORF.

enter image description here

Windows on the World

The US Lab window (behind the WORF rack) has a transmittance graph here

enter image description here

as well as a cross-section.

enter image description here

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