Can a phone app measure PAR? Here is what I found

[Seattle_Aquarist]

Hi All,

First of all lumens and PAR are not the same.  Lumens measure the degree of light (brightness) in the visual range of human beings.  PAR (photoactive radiation) deals with the amount of light that hits a given surface in the range that plants can utilize.  Our smartphones contain cameras with a sensor (CCD and CMOS) that measure the lumen (brightness) of the light entering the lens and adjust the aperture of the lens and/or the speed of the opening to achieve a good (hopefully) picture.  While a sensor for a PAR meter measures the quantity of light entering the sensor.

That said, there are several apps out there for smartphones that portend to provide a PAR reading and to some degree they do but unfortunately they are not very accurate typically reading 15% to 20% higher than the actual PAR level.  Keeping in mind that even PAR meters are +/-5% accurate then the phone app readings could be as much as 25% off. 



I have a 1 year old Samsung S20FE phone and an Apogee SQ510 PAR meter that was just returned from calibration at Apogee Instruments on 2/10/22 so I decided to see for myself.  I downloaded the 'Light Sensor to PPFD' app and compared the readings in 'free air' (meaning not underwater) under 60 watt LED bulb.  The reading of the PAR meter was 47 and the reading for the app on the phone was 54 so the app was 14.9% high.

I also had a extra LED reflector I could use so I measured the PAR in 'free air' with the Apogee PAR meter and the phone app.  The PAR meter reading was 131 and the phone app reading was 150 so again the app read higher by about 15%.  So my testing pretty much verifies the findings in the table above.

In conclusion if you would like to 'guesstimate' the PAR output level of your fixture, then using the app I used will give you an approximate reading that is 15% - 20% higher than the reading a PAR meter might give you.  Remember that 'free air' readings will always be higher than the actual PAR readings taken under water due to light reflection at the surface, surface waves from filters, and the fact that water is not 100% clear.  Also the PAR levels drop off substantially as the distance increases from the light source to the sensor due to the inverse square law (The intensity of light is inversely proportional to the square of the distance between light source and measuring point. so if the distance is 4 times as far the PAR level is 1/16 as great).
Hope this helps! -Roy

[Zenzo]

Very cool. What would be even more cool is if that old Samsung was water proof, and you tested it underwater. Is this a thing (like, would the app function underwater the same)?

[Seattle_Aquarist]

On 2/14/2022 at 4:56 PM, Zenzo said:

Very cool. What would be even more cool is if that old Samsung was water proof, and you tested it underwater. Is this a thing (like, would the app function underwater the same)?

Hi Zenzo,  I'm not inclined to put my phone into my tank.  However I would assume the same 15% - 20% variation would be the same under water.

[Streetwise]

If you don’t trust a phone underwater, you could setup a test rimmed tank with no substrate, and with the right stand, place the phone underneath. Fill with appropriate test water.

[Struggle]

On 2/14/2022 at 8:31 PM, Seattle_Aquarist said:

Hi Zenzo,  I'm not inclined to put my phone into my tank.  However I would assume the same 15% - 20% variation would be the same under water.

I’m not putting my phone in my tank or underwater either, regardless of what they say the warranty is. 

[Zenzo]

On 2/14/2022 at 6:31 PM, Seattle_Aquarist said:

Hi Zenzo,  I'm not inclined to put my phone into my tank.  However I would assume the same 15% - 20% variation would be the same under water.

Totally understand. I was halfway teasing. I do like the suggestion from Streetwise, but the 15-20% test was great. Thanks for sharing! 

[Torrey]

My phone is water proof and takes decent photos underwater... but doesn't have enough available memory to download the app.🤦‍♂️

[CT_]

phone camera firmware does crazy stuff under the hood and its not easy (or sometimes possible) to tell everything that its doing.  A typical CCD or CMOS sensor also dosn't usually have enough dynamic range to not saturate when pointed at a light source.  Because of that they make lousy photometers.  However every phone I've owned has a lux meter that they use to adjust screen brightness.  I think that's what these apps are using. 

 

I've been using this website to convert my phone lux to a guesstimate of par/ppfd

https://www.waveformlighting.com/horticulture/convert-lux-to-ppfd-online-calculator

There's a drop down where you can tell it what kind of light you have (broadly speaking) and it'll use that to guesstimate the spectrum for the conversion.

[dmurray407]

On 2/14/2022 at 11:01 PM, CT_ said:

I've been using this website to convert my phone lux to a guesstimate of par/ppfd

Have you compared the results with an actual PAR meter?

[anewbie]

On 2/14/2022 at 8:31 PM, Seattle_Aquarist said:

Hi Zenzo,  I'm not inclined to put my phone into my tank.  However I would assume the same 15% - 20% variation would be the same under water.

I would not make that assumption because of the impact of water on different wavelengths.

On 2/15/2022 at 11:23 AM, dmurray407 said:

Have you compared the results with an actual PAR meter?

The first column is an actual par meter.

[CT_]

On 2/15/2022 at 9:23 AM, dmurray407 said:

Have you compared the results with an actual PAR meter?

Sadly no.  I don't have any equipment that does absolute counting of photons.  I have some colorimeter ICs and a spectrometer but I don't have anything to calibrate those against for absolute photon counts.

 

For me +/- 25% is okay since I'm just trying to judge low/med/high -ish.  For example the light that I just installed was putting out 5000 at full and I wanted about 1500 which would put me near 20par.  by eye I can't really tell 5000 vs 1500 vs 300.  1500 looked dimmer to me than it does in my other tank for example.

 

Edited February 15, 2022 by CT_

[dmurray407]

On 2/15/2022 at 1:29 PM, CT_ said:

For me +/- 25% is okay since I'm just trying to judge low/med/high -ish

True-I have to remember it's not an exact science

🙂

[CT_]

On 2/15/2022 at 12:29 PM, dmurray407 said:

True-I have to remember it's not an exact science

🙂

If only it was.  I'd love for there to be aquarium "plans".  "This PAR, these plants in these places, RO + these salts = perfect aquarium no algae ever" 😉

[CT_]

@Seattle_Aquarist  I took a look at the "PPFD Meter" I noticed that you can also tell it your light source, so I bet it uses the same or similar conversion constants as that website I linked!  Nice to have it all in one app.

Another thing I noticed after taking a second look at your table is that the phone readings seem to saturate.  I'm guessing you're maxing out your phone sensor's dynamic range.  I'd expect the phone to be off by a constant factor (for the same light) across all intensities. 

 

Perhaps another tip for users of these apps is to hold it close to your light and see what your specific phone maxes out at.  That way you won't get caught out thinking you have one PPFD, but instead have something way brighter.

 

 

[Cory]

This is a neat experiment. If I find the time, I'll try my PAR meter vs iphone under water. I don't believe my PAR meter has the latest firmware though. But would be accurate enough for a comparison within the scope of a project like this.

[CT_]

@Cory  Since you have a PAR meter it would be cool to measure the upcoming co-op light with both the par meter and a lux weighted meter and give out a conversion factor.  No other light maker does that!

[Cory]

I didn't find the answer easily. What would a conversion factor be used for?

[CT_]

It would be so us plebeians with only phones and cheap lux meters could measure lux and compute PAR.  Since they're both weighted averages there is a scalar (single number) conversion factor between the two for a given light source.

 

ie what I'm asking for is to take the co-op light and a lux meter and place them next to each other and measure the co-op light (and any others if you want to help competitors).  The ratio of PPFD to lux should give a conversion factor that anyone can use, and would be accurate down to the accuracy of the original meters and the one they're using.

[Cory]

@CT_is there a go to lux meter in the planted community? if I'm gonna do it, may as well make it as easy as pobbile. Apogee was the go too PAR meter.

[CT_]

On 2/15/2022 at 4:41 PM, Cory said:

@CT_is there a go to lux meter in the planted community? if I'm gonna do it, may as well make it as easy as pobbile. Apogee was the go too PAR meter.

I have no clue I'm not really one of them, more of a wannabe or plant nerd groupie.  I think the "true believers" all buy or borrow PAR meters.  All the phone ambient light sensor's I've worked with are lux weighted, how correct their weighting is us up to debate, most are "close" by eyeballing the datasheets I've read.  I think the most popular lux meter out there is probably what ever the most popular phone is, though it sounds like apple won't let app devs read the ambient light 😞 (There's some odd work arounds but they may not be accurate at all.).

 

 

[dmurray407]

On 2/15/2022 at 6:16 PM, Cory said:

I'll try my PAR meter vs iphone under water

That would great!!!!

[Torrey]

On 2/15/2022 at 5:41 PM, Cory said:

@CT_is there a go to lux meter in the planted community? if I'm gonna do it, may as well make it as easy as pobbile. Apogee was the go too PAR meter.

I've been out of the light business long enough the company no longer exists.

Film industry here in NM may have the information on the most reputable meters. I just sent a text to one of my kids in the film industry to see what answer I get.

[NanoFishNut]

Sorry to break taboo and open up a 2-year old thread, but this still comes up frequently in my search results when I google comparisons between PAR meters v apps.

I wonder how Photone rates nowadays. It's been under development for about 3 years now and consistently blows all other apps out of the water in regards to other tests that I've seen. For iPhone users, they usually need to make a paper diffuser using 80mg/m² (or 22 lb) paper. The paper type matters, but generally, Android users don't seem to need to fuss about with that.

I was also wondering about the Apogee unit you used. Did you use the SQ-520 sensor directly plugged into your computer, or did you use a reader (SQ-500, SQ-510). The reader makes a significant difference, since the SQ-510 that we usually get for aquariums applies a 1.25 correction factor to account for increased backscattering from the higher refractive index of the water. So less light makes the transition from the water to the sensor, so it reads low. As a result, it reads 25% high when used *out of water*, since it's still applying that 1.25 conversion factor. So, when taking measurements in air, we need to divide the result by 1.25 to get an accurate reading, which would mean those apps from 2 years ago had an even higher error than you thought. We can also use the SQ-500, we just have to apply the 1.25 conversion factor ourselves when measuring underwater.

Temperature has an impact too, but it generally falls <1%

I have access to an SQ-510 unit, so I'm planning on making some comparisons myself in a bit