On Thu, 27 May 2021 22:45:22 -0400, Paul
wrote:

snip

[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)."

My DG is 30+ years old so I'm not sure how fancy it was in those days,
however, I've been thinking ...

Your device probably doesn't need something that exotic.

No, I don't suppose it does. ;-)

The question would be more one of "what fitting is on your project",
like a C mount or whatever.

None Paul. It's a 'bare bones' opto-sensor device mounted on a postage
stamp sized PCB with a few other surface mount components and aimed at
hobbyists and to work with an Arduino or (in my case), an ESP32 WiFi
microcontroller into my Home Assistant HA package.

https://esphome.io/components/sensor...?highlight=tsl

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).

See above ... but yes, maybe a camera addon lens might provide the
sort of window I need for this.

Looking at the spec it suggests the light incidence angle is relevant
to the sensitivity and I have had a think re what I want to use this
for. ATM I have the sensor standing on a shelf just inside the
curtains generally looking at the sky though the South facing window.
If you look at the graph from Home Assistant you can see the light
levels going up and down pretty deep/quickly as clouds pass overhead.

Whilst the 'best case' sunlight levels available might be of interest
from a weather POV, for lighting automation I think I need a more
'ambient' light level indication from say inside the window but not
facing the sky at all, just looking horizontally at the general
reflected light levels?

For exotic windows, be aware that some are water soluble,
some have ammonia or alcohol sensitive surfaces.

Noted.

Using
a less performant material might give a more environmentally
robust solution (can be wiped with a damp cloth).

That was my thought. Long term stability against UV and easily
dustable. If the sensor window was vertical so the sensor can look
horizontally that might also help stop the window gather dust. ;-)

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").

I think anything that lets the widest spectrum of light though would
be good but maybe a UV filter might help against damage from UV to the
sensor or other components (whilst not impacting the 'light
readings')?

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

Stuck vertically on the widow reveal behind the curtains may well give
me the result I want.

Maybe a little glass box (jewellery display case top) might work and I
can just print the base to hold the PCB and provide a wall /
windowsill mounting?

Cheers, T i m