Failed! Lux Meter Apps

Author: Derek Ward, Published on Sep 10, 2013

Manufacturer illumination ratings cannot be trusted, so you should use a lux meter and test yourself. Some people, though, are too lazy to get their own lux meter or want to use an application that can be loaded on their smartphone. However, this begs the question - How well do these applications work?

We tested 2 iOS and 2 Android mobile device apps named "Lux Meter" and "Light Meter". You can find these apps on either the Android Play or iOS App stores respectively. 

The only paid app was the iOS Light Meter for $1.99 USD. All other apps were free.

In this report, we:

  • Test the performance of 4 mobile light meter apps against a professional Extech light meter, which acted as our baseline.
  • Compare lux measurements in a variety of scenes, highlighting the pros and cons of each app tested.
  • Offer feedback on device and app usability.

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Comments (16)

How and when did you calibrate you "professional" Lux meter ?

Thanks

Alex,

The Extech LT300 Light Meter is factory calibrated and ready for use right out of the box (which is stated in the companies website FAQ and user manual).

I was just curious. I am sure meter is much better then Apps. by the way I have the same meter :)

But after few years would be nice to calibrate as they suggest

The Extech in the test is only about 8 months old, actually. I do plan to have it recalibrated in time, but it shouldn't require it yet.

"Some people, though, are too lazy to get their own lux meter"

Please, don't beat around the bush. Say what you think.

"the iOS apps are stronger in terms of usability and overall indoor measurements. "

I am not sure we can conclude that from the informations given. It could also be because of the phones sensor version. For example it may be very plausible that devices running either Android or IOS have different sensors depending on the model or year of manufacture. Thus, a Galaxy S4 could be even more accurate than an Iphone.

I understand that the pro LT300 meter is the more accurate device, but an app that could be calibrated with a decent sensor would be much acessible to many professionals

I think the key takeaway, though, is how do you know your smartphone has a decent sensor? How much experimentation would you be willing to go through (different apps and phones) in order to find one that works?

It wasn't part of Derek's test, but I've tried calibrating these apps using a light meter on other phones, and even then they're still inaccurate.

For Android, there is an app called "Android Sensor" where you can check the harware version and resolution. I hope that one day we will get better sensors in order to decrease the gap between apps and pro devices.

Andre, there may very well come a time 1, 2, 3, 5, years into the future where the sensors / cameras are sophisticated enough that they can match on their own. Today, though...

One option that might be worth considering today is add on sensors for phones, like Luxi, which evidently is now shipping and costs $29.95. Thoughts?

That is great! And the price seems fair. I think that could fit our market need perfectly. Has anyone tried it yet?

I didn't see listed anywhere what Android phone you used... some actually have a light sensor (for automatic brightness adjustment of the screen) rather than using the camera, and in addition to "lux meter" apps, there are several Android apps that will simply display the output of the various sensors in the phone (lux, temp, pressure, accelerometers, gravity sensors, magnetic sensor/compass, etc.)

Now that said, I don't think the light sensors tend to have very good resolution (ie. very rough measurements), and I don't know that they're generally calibrated in lux as all they need to do is match the screen brightness to a pre-defined sensor output. And on my HTC, it doesn't output any numbers below 10 lux, so not really useful for this application...

Matt,

The Android phone used in the test was an HTC One X, Android version 4.1.1. Also, thank you for the added information! Do you know what android apps would display phone sensors?

You can find this app that display android sensor information + harware version with resolution.

on my device de resolution is 1 Lux

I've used SensorBox, also played with Sensor Kinetics, but prefer Sensor Readout. There are tons more, though.

Here are a couple screenshots of Sensor Readout on my HTC Desire HD's light sensor:

Here's my experience from over 30 years in this industry, not all in the camera side....

Rule 1 There is always time and money to replace cameras that are not providing a suitable image. Not enough time and money to investigate in advance.

Rule 2 It always works in conference room demos and PowerPoint presentations.

Rule 3 The hardest thing to manage is a customers expectations after the sale when not managed before the sale.

Good article: DON’T ‘PHONE IT IN’: WHY SMARTPHONES ARE POOR SUBSTITUTES FOR LIGHT METERS, key quotes:

Light meters generally include three key photometric (i.e. light measurement) aspects: a photodiode, a luminosity function filter, and a cosine correcting lens.

  • The photodiode converts the whole wide range of visible colors and invisible ultraviolet (UV) and infrared (IR) wavelengths into an analog signal for the light meter to measure and report.
  • The luminosity function filter moderates or blocks wavelengths based on how important they are to human photopic vision.
  • The cosine correcting lens further moderates the light received by the sensor based on how humans perceive diffuse (i.e. Lambertian) surfaces.

To render images well, the camera minimizes any barriers between the environment and the image sensor. As a rule, cellphones prefer transparent screens. The glass screen minimally distorts the light coming-in, but it does not offer a cosine correction. Some light meter app proponents have tried to solve the problem with a snap-on cosine correcting lens; however, the exact photometric performance is hard to verify.

The glass on cameras tends to have a specular surface, which generally redirects low angle light, while high angle light tends to pass through to the sensor easily. Even though we wish to discount low-angle light sources, we cannot ignore it all together. Furthermore, any low angle light that does pass through the glass, may not even reach the camera due to the relatively small camera angle of view (generally 50° to 70°) typical of cell phones.

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