Others have made similar posts that contradict your info. I’m not siding with them or siding with you. Just taking a position that as long as it appears you’re all posting information in good faith and you believe it to be accurate, then I’ll allow it to stay. Doesn’t mean someone with same of differing thoughts is posting false information.
2 Likes
Have you compared readings with the diffuser vs. 22-lb paper? It would be interesting to see how much better the diffuser is.
1 Like
I had a paper diffuser i made but it was 92 brightness. If i get ahold of some 22 i will post my findings. The diffuser will be better regardless because of the way it has to center over the front camera lens. I have to take my phone out of its case to get even contact over the lens. Then theres a sheilding around the eye to direct the light into the camera lens. You would have to get a diffuser for 20 bucs and experiment around with it to understand. It reads the same readings my actual Quantum Par Meter does. I don’t think thats coincidence.
@dbrn32
Believe me, i understand what your saying. I am just trying to figure this all out. I am pretty confident in my findings. Things weren’t adding up until they’re recent update. Now everything matches.
1 Like
I’m not surprised that a cosine receptor (i.e., the diffuser) works well, but would like to learn how much better the resulting accuracy and repeatability is, compared with the 22-lb paper option.
1 Like
I hear you… i am just trying to keep it simple without all the scientific tests and analysis. Its just my personal findings.
2 Likes
@dbrn32
Come to think about it, this is why i actually bought the diffuser and ran the test myself. Because i don’t trust anyone else also and had to find out for myself. How do i even know these 2 devices are correct? They were made by someone else also. They could both be wrong even though they match up to eachother perfectly. Whos to say the 500 dollar Apogees are correct even? They could be off. Nothing is perfect in this world.
What i have learned from creating this post is, that its best to take multiple readings with multiple devices and average up all the readings and take that number and hope for the best. So far I’ve used 2 devices and came up with the same readings. I guess it will need a new 3rd device to take a 3rd reading. Even then, the 3rd one could be wrong, then maybe 5 seperate par meters from 5 different manufacturers might be enough to spot the defects in marketing and find the true par readings.
2 Likes
That’s very improbable.
Averaging multiple readings is a good idea when taking measurements that depend on the distance from the light source and the sensor’s orientation WRT the source, as PAR measurements do. Averaging tends to compensate for unintentional variations in those two parameters that are caused by the user.
If the sensor is fixed in place, though, averaging rarely improves accuracy because the repeatability of solid-state photodetectors is quite good. PhotoBio, for example, quotes a repeatability of ±1 umol/m2/s. (Repeatability describes the instrument’s ability to produce the same reading in response to the same input.)
Averaging over multiple instruments, though, makes no sense. If one instrument is more accurate than the other(s), averaging reduces accuracy. If their accuracy is the same, you’re wasting your time.
It’s best to have one instrument you trust and use it exclusively. In your case, I’d go with the PhotoBio. The Photone’s cosine receptor improves its repeatability, but Photone has no control over sensor variability, as PhotoBio does.
2 Likes
You said it better than i could. There are alot of variables when it comes to this end of Horticulture. I have the AMTAST Quantum Par Meter. It looks like its the samething as the Photobio meter with different lettering/lable. I do just use the AMTAST and rarely double check it with the Photone app. The Photone app got me messed up once before. It was way off before the update and Cosion diffuser. It seems better now. I caused my plants to foxtail and produce nanners. I even dried up a budtop using the Photone app and being negligent. Now i have better control over .y pat readings and can trust them. Before, i didn’t trust what the meter or the App was reading. I just went ahead and cranked my lights to 100% and let them ride. That was a bad idea, even though everything turned out alright but it was not needed.
1 Like
I agree to an extent. I haven’t looked at any of these in years, but when I was interested Apogee was clearly the industry leader. My take on the reasoning for this was they disclosed the shortcomings of their sensors by way of telling users exactly what their equipment would/wouldn’t do and provided calibration data. They were the only quantum par sensor doing it at the time and to get more widely accepted results you would need an integrated sphere or goniophotometer. Even if you were to send your fixture out for this testing you would still be trusting the certifications and process of test center. So it’s all relevant to some level of acceptance on our part. As an individual you have to qualify the value of the tool and/or data you’re seeking. I don’t have a single device to measure light intensity. I have a general idea on the overall light output of my fixtures and I can do everything I need from here. Someone else may very easily justify spending $500+ for a tool to record this data that I essentially don’t care anything about. I don’t think either or anywhere in between is right or wrong. If you feel good about what you’re doing should be all you need.
3 Likes
Thank you, i agree. If one is getting good results with what you have and its working theres no sense in trying to make it complicated as long as you can repeat the same results time after time.
3 Likes
No measuring device – not even NIST’s equipment – is perfectly accurate. The best a manufacturer can do is make the error as small as possible, given the price point they’re aiming for.
The design and execution of an instrument sets a limit on its potential accuracy. Apogee, for example, uses a more sophisticated design than PhotoBio (or Amtast); hence, it’s higher cost.
The second critical factor is calibration. I’d bet that Apogee uses a “standard light source” that’s calibrated regularly by NIST, which is the best any manufacturer can do. @dbrn32 noted that Apogee provides calibration data for each instrument – that’s an earmark of a lab-grade instrument.
A standard light source is one that’s designed to be especially stable and repeatable so it can be used to calibrate light-measuring instruments. It must be calibrated regularly and, in the US anyway, NIST is the best calibration service available.
2 Likes
Thank you for the information. How does one use NISTs calibration? I had to calibrate the AMTAST par meter. It says to put the diffuser in complete darkness to calibrate. Is there a way i could use a 2 point calibration through NIST?
1 Like
You send your device to them and pay to have it calibrated. Too expensive for most of us and silly for a consumer-grade instrument. It would be like sending them a $3 tire pressure gauge to calibrate.
For instruments like the PhotoBio and Amast, we have to trust that the manufacturer calibrated it before shipping it. Ideally, they use a NIST-calibrated standard light source for that purpose.
Zeroing the instrument in the dark doesn’t really calibrate it, despite what a manufacturer may call this step. Instead, it causes the instrument to take a reading that gets subtracted from subsequent measurements. Photodetectors generate a tiny signal (called “dark current”), even in total darkness, due to thermal noise, so subtracting it from subsequent measurements improves their accuracy.
2 Likes
So calibrating the Photobio/AMTAST par meters in total darkness improves their accuracy? I understand these are cheap units and sending it for NIST for calibration is silly.
1 Like
Yes, although that process is termed “zeroing” the meter, rather than “calibrating.” I don’t know why some manufacturers are sloppy with their terminology.
1 Like
This is what i did as soon as i got it. I took a reading with my lights at 100% at 12 inches away and took a reading. Then i put it in complete darkness and ran the “Zeroing” calibration and got a 75% drop from my first measurement. If my memory serves me correctly. It does seem accurate after all the tests I ran it through. I was even outside measuring the Full Solar Eclipse we had earlier this year…LOL !! It was interesting to watch the numbers drop…
I’m just relieved and happy to know i at least am in the ballpark for positive DLI/PPFD Umols when i take my measurements. Things like this bug the heck out of me.
1 Like
Wow. 75% means you probably triggered the zeroing function by accident at some point, when the sensor was well illuminated.
That button should be recessed, to reduce the likelihood of that pitfall. Meanwhile, you might glue a small tack to it. 
= 
1 Like
Well, maybe i meant 75 points. Its been awhile and my memory is dull but if it would’ve read 275 ppfd, then, after the calibration it went to 220ish or something. Thats what i remember happened after the calibration. I didn’t hit anything by accident as i was being very adamant about performing the calibration correctly. I know i didn’t hit anything by mistake.
It wasnt that drastic of a change after the Zeroing. It might have even been 50 points, it wasn’t to different than the original reading before the Zeroing calibration. This was how i knew i did it correctly. I tried keeping the change in the back of my head in case i started getting funny readings and wanted to remember the factory calibration readings. But, it wasn’t way off. Like 50 or 75 points. Not percentage, sorry. That would be a big difference.
1 Like
Apologies – I didn’t mean to poke fun at you or suggest that you’re careless. I meant to point out a design weakness in those meters that makes it possible for anyone to cause errors in their readings by making a forseeable mistake.
2 Likes
None taken. Totally
2 Likes