T i m wrote:
I have added an ambient light level sensor to my Home Assistant setup
and it seems to work very well (snapshot taken just now).

https://ibb.co/QHfKhv9

I've currently got it on an internal South facing windowsill so it
will already be looking though the double glazing [1] but I want to
print a case for it with it's own 'window' to keep the dust out I just
tried putting some plastic cut from a blister pack over it to see if
it would have any impact to the reading. It did, it went from about
11,000 lux to about 9,000 so I'm guessing it's only 'translucent' and
whilst I'm really only interested in the change of lighting levels
(for general lighting automation) I'd rather any such window didn't
impact the reading more than necessary.

The actual sensor is a TSL2561:
https://cdn-shop.adafruit.com/datasheets/TSL2561.pdf

And it suggests it combines the output of two sensors ...

"Each device combines one broadband photodiode (visible plus
infrared) and one infrared-responding photodiode on a single CMOS
integrated circuit capable of providing a near-photopic response
over an effective 20-bit dynamic range (16-bit resolution)."

So I was wondering if anyone could recommend a source of a small piece
of something as a 'window' that would be transparent for this role?

Failing something that one might have kicking about, I wonder if the
local glazing shop would be able to cut a bit of picture frame glass
and if that is likely to be transparent for this role?

Cheers, T i m

[1] How likely is it that the fairly old DG has some sort of 'Solar
glass' in it and is already impacting the readings somewhat?

https://www.flir.com/products/ir-windows/

"crystal window" but it wouldn't say what it was made of

https://flir.custhelp.com/app/answer...9-transmission

"The material of the FLIR IR Windows is CaF2 (Calcium Fluoride Crystal)."

Your device probably doesn't need something that exotic.
The question would be more one of "what fitting is on your project",
like a C mount or whatever. If there was a standard mounting, you
could go to a photography shop and look at their filters. Maybe
there are filters they use for spacing or something, elements
that aren't supposed to have optical effect, and have an AR
coating as well. The AR coating might be MgF2
on some sort of glass (crown glass).

For exotic windows, be aware that some are water soluble,
some have ammonia or alcohol sensitive surfaces. Using
a less performant material might give a more environmentally
robust solution (can be wiped with a damp cloth).

Just a flat glass with an AR coating might be enough.
And the AR coating is just to avoid surprises if possible
("can't figure out what it sees").

You can also mount the material at an angle, like a Brewster window.

Paul