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?