How reliable are smartphone light meter apps?

[lepetitmartien]

Erh… LOL

 

[ALToronto]

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.

 

[PaphMadMan]

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.

 

[gonewild]

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.

 

[gonewild]

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.

 

[PaphMadMan]

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.

 

[PaphMadMan]

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?

 

[gonewild]

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.

 

[gonewild]

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.

 

[ALToronto]

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?

 

[naoki]

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.

 

[PaphMadMan]

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.

 

[Leo Schordje]

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.

 

[Ray]

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


Ray Barkalow
firstrays.com

 

[ALToronto]

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?

 

[lepetitmartien]

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.

 

[gonewild]

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.