Optically transparent covering?

I have not read the spec on the sensor, but normal glass is opaque to some wavelengths of IR, so that may account for the reduction in level.

(for thermal imaging cameras they often have to use lenses machined from germanium).

In addition to John's point about IR bear in mind that reflection from the 2 surfaces mean the light transmitted will be less than the light incident - about 8 per cent less for normal incidence and glass with RI=1.5

Clear cast acrylic sheet is what I would try - inherently UV resistant and with up to 92% visible light transmission - highest when the light strikes perpendicular to the surface. I can't find values for its transmission in the IR range though.

I think, go with whatever is to hand and re-calibrate the thresholds expected in the software to allow for the chosen window material.

No idea.

Have you considered an old camera lens glass for the cover? If you don't have one, a camera shop selling secondhand film cameras might have some old partially-broken lenses which could serve as a source, or even an old UV filter. However, depending on what spectral range you are trying to measure the light level of, you might want to have a look at this first:

Thanks for all the suggestions so far guys. A few things to try there.

No, I hadn't (so thanks). I had thought of similar in the form of a microscope slide but I can remember buying 'daylight filters' for my SLR cameras to (sacrificially) protect the lens from physical damage.

Our last camera shop went a few years ago but we do have a 'Cash Converters' that may have a bits n bobs bin.

According to the spec I believe it covers the range of 300-1000 nm (even though I understand we can only see around 380 to 750 nm etc) but what I would describe as 'daylight' is all I'm really interested in re managing general light levels indoors. Given this sensor seems to be designed to 'see' the sort of thing we see, I think that as long as whatever light it gets isn't filtered at all, it should be ok.

Interesting.

Whilst I had a few film cameras along the way I never really used them much because of the time delay between taking the / 'a' picture and getting the result. Ignoring my time with Kodak (microfilm / fiche) and the Polaroid and a scanner on my PC, my first digital (A Fuji DX-5) revolutionised it all for me and the fact that it was only

I hadn't thought of a round 'window' but one makes sense and one I could easily incorporate in a 3D printed box.

I'm not trying to actually 'measure' anything (although the sorts of levels are interesting if they are correct and when compared with the stated light output levels of domestic lighting) but to just get the comparable levels that could be useful to automate what might equal 'it's a bit dark outside, better turn the light on', or enable the 'turning on a light on in the hall automatically with a PIR during an overcast day and before sunset' ... sort of thing. ;-)

Cheers, T i m

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

Is there any reason why it cannot be fitted through a hole in the case? One could bond it in with epoxy or something, just a thought. Brian

Erm, yes, it probably could Brian, but I was thinking it might make it more vulnerable to dust than if it was behind a bigger window as such?

The tiny sensor is on the surface of a postage stamp sized PCB with a few other SM components so I'd have to print a funnel / collar shaped hole in the case to present the actual sensor and not everything else.

Cheers, T i m

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

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

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.

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?

Noted.

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. ;-)

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')?

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

...

You do, however, need to be sure that, whatever you use, is not degraded by UV. That would lead to a progressive loss of transmission over time. That is why I like acrylic sheet - it is completely UV resistant without the need for any coating.

It is also likely to be what I have to hand :-)

bog standard UV filters might work - use em as lens protectors

random example gave me a clear glass protector

Yup acrylic and proper glass should be fine. Polycarbonate might not be so good in this case. Having said that it can still get dirty outside regardless of material - so a bit of slack built into the thresholds is not a bad idea.

The sensor you've chosen can see out to 1000nm. Most glasses and plastics will transit light to near IR

The advantage of using a UV filter is the surfaces will be optically coated to minimise reflection and maximise transmission over the wavelengths of interest.

Interesting. Given everything is relative and open to calibration, crude or otherwise, I would say any old piece of glass or plastic window would be fine.

For that purpose I think you can safely forget about losses from anything that starts off looking clear and is stable. The odd 20% is neither here nor there. The difference in illuminance between direct sunlight and an overcast day is around a factor 100. It's another factor 10 or so down to a typical living room at night.

JOOI does the home automation software have built-in facility to ignore short-term changes so e.g. the headlights of a car arriving don't turn off the lights on the stairs?

A friend of mine related a story of a friend of his who had installed a motorised curtain system in his lounge activated by it going dark. He had my friend round along with some other mates to witness the system working. Sure enough as it fell dark the curtains closed only for a few minutes later for them to suddenly start to open and then close again this continued all night every time a car went past.

Fortunately he had not installed the system in the bedroom, I can only imagine the embarrassment being caught ?in flagantre?

Richard

The bit of very clear blister pack seemed to have quite an impact so I'll run some tests and compare. ;-)

True, if I'm only interested in the difference in levels etc.

Yeah, So I've already observed, or even a cloud going over.

Noted.

Hehe ... erm, I'll say 'yes' in as much as you can set thresholds and delays plus conditions where once triggered (because it was much darker than usual say 1hr before sunset) then it would trigger the lights on but wouldn't allow the lights to be turned back off till say midnight or sunrise etc.

Within each Automation you can have multiple triggers, multiple conditions and multiple actions (turning on several lights at specific brightness etc).

Cheers, T i m

As your window glass will have attenuated the light a lot, the best you could do, short of putting it outside with some proper covering, would be to stick it to the wundow without any extra covering, sealed so that dust can't get in.

Ah, now there is a bit of out of the box thinking. ;-)

Yes, it would be easy to print a small box with a reasonable (~5mm) flange and simply stick it to the inside of the window (facing out etc) with some double sided tape. If I leave a some ventilation underneath it shouldn't fog etc.

I could have two, one 'looking' outside and one monitoring the ambient light levels in say the hall.

Cheers, T i m