# How reliable are smartphone light meter apps?



## ALToronto (Sep 27, 2014)

I have two different apps on my phone, and they correspond closely. But that's not surprising - both apps use the same sensor. What's worse, it's the sensor for the front facing camera, which is always inferior to the rear facing camera.

Has anyone with a serious light meter tested the meter against an app? Is the error random, or do smartphones consistently measure higher or lower? I'm interested in anything but Apple devices - Apple is the worst for colour accuracy, so I wouldn't trust it.

What's prompting this question is a recent presentation I attended by Glenn Decker of Piping Rock Orchids, where he said that most paphs require about 1200 foot-candles of light, or 'low' light level. Well, according to my phone, 1200 fc is pretty bright - I can't get close to it without some direct sunlight. And my living wall with 70 W total of LED's, can barely reach 450 fc in the areas closest to the lights, where my cattleyas are getting hyperpigmentation. 

So is it 1200 fc or 1200 lux? And if it's fc, just how far off is my phone?


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## gonewild (Sep 27, 2014)

Most cell phones cant measure over 1000 fc. really the apps are made by people that measure light sitting at their computer.
I tried to find one that worked on my HTC but none gave the same reading on the same spot twice. Maybe at fc under 500 they work?
I just ordered a cheap light meter from Amazon.


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## gonewild (Sep 27, 2014)

1200 lux is only about 120 fc.

1200 fc is about 88% shade


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## PaphMadMan (Sep 27, 2014)

I hope we get some good answers because I have the same questions. 

At least one app I have looked at can be set to read in both fc and lux, and the relative readings of fc and lux seem correct on it. (~10.76 lux per 1 fc) It also has pre-set calibrations for various devices, or an option to let you calibrate it to match another meter or a known lux level. I haven't tried that. 

The 1200 fc recommendation should be about right for Paphs. I have no question that he did mean fc. But I have wondered about the applicability af any light meter to LEDs or any other lighting that isn't a full spectrum similar to sunlight. You could have all the wavelengths of light your plants need at the right intensities, but the fc from a meter calibrated for full spectrum ought to read lower than the recomemnded fc for full spectrum since it is receiving much less total light energy, but much of it from wavelengths not used by plants.


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## ALToronto (Sep 27, 2014)

gonewild said:


> Most cell phones cant measure over 1000 fc. really the apps are made by people that measure light sitting at their computer.
> I tried to find one that worked on my HTC but none gave the same reading on the same spot twice. Maybe at fc under 500 they work?
> I just ordered a cheap light meter from Amazon.



My phone gives very consistent readings in the same spots. One app samples and holds, so I don't like using it. The other app changes continuously, so I can aim the phone until I get the highest reading, and that's the value I use. I can go from the living wall to the windowsill and back, and the readings will be within a few %. So if there is an error in the sensor, it's non-random.

My phone can certainly measure over 1000 fc. Full sunlight is 65535 lux, which is 2^16-1 (2 to the 16th power, minus 1). Must be an arbitrary limit. This corresponds to 6088 fc. Dappled sunlight is around 3000 fc, but full shade in an otherwise bright window is maybe 500 fc. Does this correspond to readings from real light meters?


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## gonewild (Sep 27, 2014)

I have an app called "Light Meter" sitting on my desk pointing up at 2 t8 tubes it reads 18fc. Right beside it an old photo light meter reads 46fc.
Switch apps to "BeeCam" it reads 160 lux which is about 16fc.
so both apps read about the same but only measure about have as much light as the photo meter.

maybe someone has an answer which is correct if any.


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## RNCollins (Sep 27, 2014)

I don't know how accurate it is, but I use the Mobility Quotient Green Thumb light meter on my iPad. It measures in foot candles or lux. 

http://mobilityquotient.com/press/plant-lovers-love-mqgreenthumb


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## lepetitmartien (Sep 27, 2014)

If you have a camera (digital or film) you can use its lightmeter to calculate the incoming light and therefore verify if the apps are close to it.

Note that it's not important to have a perfect measure but an order of magnitude of what's going on.


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## troy (Sep 27, 2014)

I use lux meter level pro free app it reads from the face of my phone which kind of sucks but it reads up to over 30,000 lux


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## ALToronto (Sep 27, 2014)

lepetitmartien said:


> If you have a camera (digital or film) you can use its lightmeter to calculate the incoming light and therefore verify if the apps are close to it.
> 
> Note that it's not important to have a perfect measure but an order of magnitude of what's going on.



I haven't used a real camera since 2008, when I bought a phone with a 5 MP camera. But how accurate is a camera's light meter? Most professional photographers I know use separate devices, and some are worth hundreds of $$.

And this is another question: how do analog light meters compare to each other? Is a cheap one less accurate because it has a higher random error, or is it more like a smartphone sensor that simply measures less light, or ignores some wavelengths altogether?

I'm not surprised by Lance's result - a factor of almost 3. So this means that my living wall catts are probably not living under a rock, but are actually getting enough light to eventually bloom. Maybe I should just trust my eyes a little more.


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## lepetitmartien (Sep 28, 2014)

Metrology even on the scientific level is no pleasure. That's why I gave you the advice to search for the general idea and not be fixed over the numbers. Camera meters are reliable enough to use accurately films of defined sensibilities. Pro light meters are not only accurate but have other functions like measuring the different types of light in the area (incident, spot level on the subject etc. and flash lights), you only need to know whats going on.

Btw you can find real cheap luxmeters made in China that are reliable enough for our job…

Now if you have results in the same league measured from different means (phone, camera, cheap luxmeter) you can bet the numbers may not be accurate but the general idea is. It's like weighting one thing with a bad balance, you have to make multiple measures to get things right.

I've got to compare my luxmeter and my EOS 7D, I'll let you know.


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## ALToronto (Sep 29, 2014)

Thanks lpm.

I'm glad you brought up metrology. The big challenge of metrology is separating and calculating random and non-random errors. From my own observations, the random error of my phone sensor is very small. However, the non-random error is probably significant enough that it's worth finding out by how much and possibly plotting a calibration curve.

I'll try to borrow a professional light meter.


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## lepetitmartien (Sep 29, 2014)

Someone had to bring up the bad words! LOL I know how it can s…k to figure out errors and compensate them now I think it's more important to know if we have about 3000 lm or 30000 lm than 2997 and 34485. 

For those who want to calculate using a camera light meter:

- switch to 100 ISO
- if you can, select Av priority (aperture), to f/8
- if totally automatic, just go on
- place a white paper on the zone you want to test, it's your target.
- measure, note the aperture (F/8 or the one decided by the camera) and the exposure time.
- Calculate!

the formula is:

Light intensity in lux = (70 x f^2)/(exposure time x ISO)

For 100 ISO, f=8 and 1/1000th of second (0,001") it'll give 44800 Lux

Else, a french calculator: http://orchideenord.free.fr/index.php?section=Lux.html (in lux of fc)


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## Ray (Sep 29, 2014)

RNCollins said:


> I don't know how accurate it is, but I use the Mobility Quotient Green Thumb light meter on my iPad. It measures in foot candles or lux.
> 
> http://mobilityquotient.com/press/plant-lovers-love-mqgreenthumb



As someone pretty well known to test, assess, and publicly feed back on new stuff, they gave me a free copy to test.

It seems pretty good, but needs a paper diffuser over the phone lens in order to work, and the "weight" (thickness) of the paper, and it's "brightness" (degree of coating) both affect paper's light transmission, so it seems a bit "iffy" to me.

Of course there is a way to "calibrate" it using full noontime sun, so maybe the paper quality isn't that big of a difference....


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## Clark (Sep 29, 2014)

lepetitmartien said:


> For those who want to calculate using a camera light meter:
> 
> - switch to 100 ISO
> - if you can, select Av priority (aperture), to f/8
> ...



Thanks!


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## lepetitmartien (Sep 30, 2014)

I was sure it'd be of help for some   :rollhappy:


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## lepetitmartien (Sep 30, 2014)

My ridiculously cheap luxmeter from evil China gives me about 1580-1600 lux

And at the same time, my eos 7D :

(70 x 8^2)/(0,01 x 200) = 2240 lux

Another measure, respectively: 5800 (China) and 5600 (7D by hand) 5480 (7D using the calculator)

Note there's big variations of light due to clouds today, but it's better than yesterday, and I did that near windows at home. I'll repeat outside with full sun when I can. Seemingly, my luxmeter is not totally out in space though I would never work for a Nobel with it. :evil:


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## gonewild (Oct 2, 2014)

i just got an inexpensive digital light meter.
Here is a comparison of readings...

Dr. Meter = 86fc
Old GE photo meter = 65fc
Light Meter app on HTC phone = 18fc

The old meter and new digital meter are at least in the same general range but the phone app is no where close.


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## PaphMadMan (Oct 2, 2014)

gonewild said:


> i just got an inexpensive digital light meter.
> Here is a comparison of readings...
> 
> Dr. Meter = 86fc
> ...




Does the phone app allow for any kind of calibration setting?


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## gonewild (Oct 2, 2014)

PaphMadMan said:


> Does the phone app allow for any kind of calibration setting?



Yes it has several calibration options. I have not tried to adjust. Calibration requires use of a normal light meter and I just got the new one. But if you have a meter already who needs the phone app?


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## lepetitmartien (Oct 3, 2014)

Erh… LOL


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## ALToronto (Oct 3, 2014)

Thanks Lance, that's pretty ummm... enlightening...

I'll check if my apps allow for calibration, and then borrow a $1000 light meter from one of my professional photographer acquaintances.


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## PaphMadMan (Oct 3, 2014)

gonewild said:


> Yes it has several calibration options. I have not tried to adjust. Calibration requires use of a normal light meter and I just got the new one. But if you have a meter already who needs the phone app?



I have the same app on my phone. I know it makes a big difference having the correct pre-set calibration for the phone selected. Not fair to judge it if you haven't at least done that.


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## gonewild (Oct 3, 2014)

PaphMadMan said:


> I have the same app on my phone. I know it makes a big difference having the correct pre-set calibration for the phone selected. Not fair to judge it if you haven't at least done that.



I'm not judging it or even trying to criticize it, I'm just showing the results of the comparison. I will calibrate it and see if it then gives uniform readings.

I agree completely about the fairness to evaluate the potential of the app. But when I downloaded the app I was under the impression it would measure the light fairly accurately and had no idea it would need to be calibrated. If an orchid grower gets the app and believes it at face value they may be putting their plants in an extreme amount of light depending on what phone the have.


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## gonewild (Oct 3, 2014)

OK I calibrated the app Light Meter.

Not easy to do by following the instructions exactly.
Instructions say adjust room light level to 100 lux using a "REAL" light meter. ("real" is their choice of words)...Not possible to adjust room light to exactly 100 as I don't have a dimmer switch and 100 lux is pretty low light.
So I just used room light at 790 lux and adjusted the app to read exactly as the real light meter. 

Walking around inside the house both the app and the light meter give similar readings. not bad.
Step outside and measure filtered light under a patio roof and foliage and the readings are not close. The real meter reads fairly consistent as it is moved around. The phone app gives readings with extreme variations (towards lower level) depending on how the phone is held. 
There is obviously a reason that "real" light meters have a domed sensor and not a flat screen. 

Now I have an opinion about this particular app... it works great to determine the presence of light but is worthless to measure light for horticulture. The cheap "real" light meter is very handy.


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## PaphMadMan (Oct 3, 2014)

No criticism intended here, but the app has pre-set calibrations that you can select for various models of phone. That's what I was trying to ask if you had used, not suggesting you attempt an actual calibration, which introduces other sources of error. Regardless, your observation that the phone readings are variable is valid, and likely at least partly due to the directionality of the sensor. It is also possible that the phone app just can't read brighter light as precisely, or might perform better in bright light if you could calibrate in bright light. Or the limitations may be in the phone, dependent on model, not in the app. I'm not ready to give up on the possibility of some usefulness of a phone app light meter because I'm not likely to ever start carrying a light meter with me everywhere I go.


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## PaphMadMan (Oct 3, 2014)

On another aspect of the topic... No one picked up on my question about full spectrum light versus selected wavelengths, so I'll ask again.

For example, a properly calibrated light meter is reading 1200fc in filtered sunlight (full spectrum). An LED light panel supplies mostly just selected wavelengths needed for plant grown and development. Even if the LED is putting out just as much energy at those specific wavelengths as the 1200fc sunlight supplies, the light meter will have a lower fc reading, right? There is much less total light energy but plants happy in 1200fc sunlight could still be getting all they need. And it would be pretty hard to calculate what fc reading that should be. Or am I missing something?


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## gonewild (Oct 3, 2014)

PaphMadMan said:


> No criticism intended here, but the app has pre-set calibrations that you can select for various models of phone. That's what I was trying to ask if you had used, not suggesting you attempt an actual calibration, which introduces other sources of error. Regardless, your observation that the phone readings are variable is valid, and likely at least partly due to the directionality of the sensor. It is also possible that the phone app just can't read brighter light as precisely, or might perform better in bright light if you could calibrate in bright light. Or the limitations may be in the phone, dependent on model, not in the app. I'm not ready to give up on the possibility of some usefulness of a phone app light meter because I'm not likely to ever start carrying a light meter with me everywhere I go.



Sorry I did not connect your point about the preset calibration list. I did use the list. My phone is an HTC Inspire. It is not on the list so I chose the almost identical HTC Desire model. Maybe the light sensor is the same or completely different. In the info with the app it does say that there can be differences even between different phones of the same model.


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## gonewild (Oct 3, 2014)

PaphMadMan said:


> On another aspect of the topic... No one picked up on my question about full spectrum light versus selected wavelengths, so I'll ask again.
> 
> For example, a properly calibrated light meter is reading 1200fc in filtered sunlight (full spectrum). An LED light panel supplies mostly just selected wavelengths needed for plant grown and development. Even if the LED is putting out just as much energy at those specific wavelengths as the 1200fc sunlight supplies, the light meter will have a lower fc reading, right? There is much less total light energy but plants happy in 1200fc sunlight could still be getting all they need. And it would be pretty hard to calculate what fc reading that should be. Or am I missing something?



I think all of our current knowledge about light requirements for orchids is based on a "Natural sunlight spectrum".
So you are not missing something... probably using lux or footcandles to set the intensity of LED lights is not of much accuracy.
If you remove some part of the light spectrum it could and probably does change the plants response to the light. Just as an example, and not represented as fact... if you filter out all UV light a Phal may grow well in otherwise full sunlight.
So in reality with all the new light sources that are now available require trial and error. How the new light spectrums are read by a lux meter remains to be learned.


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## ALToronto (Oct 3, 2014)

What does an analog light meter actually measure? Is it the electrical potential in meV? So light would be captured by a photovoltaic cell and generate a small amount of voltage? If not, how does it work, and how does a digital light sensor in a camera work?


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## naoki (Oct 3, 2014)

PaphMadMan said:


> On another aspect of the topic... No one picked up on my question about full spectrum light versus selected wavelengths, so I'll ask again.
> 
> For example, a properly calibrated light meter is reading 1200fc in filtered sunlight (full spectrum). An LED light panel supplies mostly just selected wavelengths needed for plant grown and development. Even if the LED is putting out just as much energy at those specific wavelengths as the 1200fc sunlight supplies, the light meter will have a lower fc reading, right? There is much less total light energy but plants happy in 1200fc sunlight could still be getting all they need. And it would be pretty hard to calculate what fc reading that should be. Or am I missing something?



You have it correctly, Kirk. That's why lux/fc meter is good enough to compare the light intensity of same type of light, but if you want to compare LED vs fluoro vs sunlight, you need quantum PAR meter. 
http://www.gpnmag.com/sites/default/files/16_TechnicallySpeaking_GPN0913 FINAL.pdf

PAR (measured in PPFD) is not a perfect measurement to gauge the intensity of light relevant to photosynthesis, but much better than fc/lux, and it is a standard we scientists use to report the light intensity. It counts the number of photons between 400-700nm. Different light source has different amount of light relevant for photosynthesis per given fc. For example, if you take a look at "Ratios important for photosynthesis" column of this table:
http://cpl.usu.edu/files/publications/poster/pub__6740181.pdf
PPF/lux of sunlight is 16.2, and red+blue LED is 29.3 (actually YPF/lux (=YPF/PPF * PPF/lux) is better, but YPF is a bit more complex measurement than PPF). So this means that 1000 fc of RB LED is equivalent to 1808.6 fc of sun light.

However, even the cheapest PAR meter costs you >$100. The cheap PAR meter doesn't have a perfect spectral response (i.e. it's not so sensitive to light close to 700nm, so it's not perfect for LED which uses 660nm diodes). (see DavidCampen's comment here)
http://www.orchidboard.com/community/growing-under-lights/71306-par-meter.html

Alla, I posted this before. But most consumer-level doesn't seem to follow CIE luminosity function. If you can borrow a lux meter (e.g. Li-Cor) designed for science (not for photographer), it is a better calibration point.

Big diff between meters, and different color temp influence the magnitude of discrepancy.


```
fc1 fc2  fc1/fc2
WW 730 1120 1.5
CW 630 1293 2.1
```

fc1: foot-candle measured by Gossen Ultra-Pro calibrated recently.
fc2: foot-candle measured by Dr.Meter LX1330B, which is recently bought.

The sensitivity shape of LX1330B (same as Lance's) seems to be very good in the user manual, but I don't know if it is the real spec.

It's probably more details than most people care about light, though.


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## PaphMadMan (Oct 3, 2014)

Thank you for such a thorough answer, naoki. That's a lot of information to digest, but I can see the first 2 links are quite helpful to start. I, for one, appreciate all the details.


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## Leo Schordje (Oct 7, 2014)

I am retired from a "sometimes miserable" job in the chemical industries. I have learned to hate metrology, and don't even get me started on pH. 

"Exact" results I feel are pointless. The real question, what is "good enough" to get your plants healthy and blooming? 

You can "calibrate" your light meter in a relative way. If your Cattleya are blooming and growing to your satisfaction, measure the light there. Maudiae type Paphs will do well with about half that amount of light. Most, if not all Paphs will do well between half of what a Cattley will do well with, and what a Maudiae will do well with. The exact number is trivial, it is the results that count. 

I have a couple bromeliads - Crypanthus types. Over the years I have gotten familiar with how light effects the red pigment of their foliage. When they have some red flush to their leaves, the light is good enough for the majority of Paphs, when they are very red, it is good enough for Cattleya. - Living light meters. And not bad to took at either. There are many quicker growing house plants that you can use this way, because of their fast growth, they are more responsive to light conditions, easier to gauge where you are at without having to wait for the Cattleya or Maudiae to bloom. 

Just my 2 cents.


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## Ray (Oct 13, 2014)

That two cents would buy you a beer (or glass of wine) in my house!


Ray Barkalow
firstrays.com


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## ALToronto (Oct 13, 2014)

I agree completely with those who say that precise measurements are meaningless. However, we're dealing with a difference of a factor of 3 or 4, and that's far beyond a practical confidence interval. I wish I could afford the time to 'calibrate' my phone app by whether or not certain plants will bloom under current conditions. But that's not a realistic time frame. And I can't compare light levels from LED's to natural light levels that my windowsill dwellers see - the difference between sunlight and shade is dramatic. So what to do?


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## lepetitmartien (Oct 13, 2014)

Trouble is, if there's a calibration procedure for the app, you'd better to it…  The point that the exact values are meaningless doesn't mean you don't have to calibrate or try to aim at better accuracy. Aiming at accuracy (that you won't achieve anyway) means reliability and reproducibility of measures.

At worse, use the light cell of a camera, it'll be better than nothing. Any camera you can read the values can do, a white paper, a pen to write down values, and off you go to the calculator script or the formula.

I'm still waiting for a decent day (and free time) to have a try again in full light with the 7D and the evil cheap chinese light meter…

Measuring LED lights will only be meaningful with other measures of other LEDs, as long as the references are or the same type (color temperature or spectrum) as the LEDs you want to measure, but anyway, a spectrophotometer would be better anyway. We are trying to set up a lab afternoon at my orchid association, with a SPM so as to test the lamps (including LEDs) we are using, and plotting their spectrum. I'll keep you informed.


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## gonewild (Oct 13, 2014)

I bought a light meter for one reason. It's not to determine a number value of the light.

I have a growing area where I have layers of shade.
1st layer is 80% shade cloth.
2nd layer is rigid white acrylic plastic (40%?)
3rd layer is 50% shade cloth.

By using the given percentages and math the light level should be 600 foot candles when the sun is full.
But when I look at the light intensity it seems very bright to me.
I am growing Phals under it and their foliage wont turn dark green even when I force them with nitrogen.
So I suspect the percentages the shade cloth claims my not be correct.

I tried the phone app and it would not give consistent readings at all, nor was the reading close to 600 foot candles. So I bought the light meter and will test when I get back to the plants next week.

Other than this I have always relied on my own eyes to tell me the light quality and also let the appearance of the plants show me.

If in doubt about your light intensity it seems like a good idea to spend $35 on a light meter....it seems to work great around the house.


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