Luxi iPhone Light Meter Tested

Adding a free app to your smartphone to measure light levels sounds great but our recent test showed how poorly they performed. However, a new iPhone accessory, Luxi, claims to deliver far more accurate results at a fraction of the cost (online for $29.95 USD) of professional lux meters ($100+).

Here's what it looks like attached to an iPhone:

In this report, we:

• Test Luxi's performance at default and calibrated levels against a professional Extech light meter [link no longer available], which acted as our baseline.
• Compare lux measurements in a variety of scenes.
• Offer feedback on usability and the importance of calibration.

Here are our key findings from this test:

• Default settings are highly inaccurate, and Luxi should be calibrated with a professional light meter.
• A separate calibration is necessary for particularly low light scenes (less than 1 lux.), and only low light scenes.
• When properly calibrated, Luxi's measurements are exact or off ~1-2 lux.

Comparisons

Below is an image compiled with scenes used for the test. We tested in these scenes because they offered us a wide variance in lux measurements. The numbers displayed in each scene is the Extech Light Meter measurement.



Below is a spreadsheet containing our findings. Default calibration was highly inaccurate, but improved when calibrated. We cannot insure that the calibration metrics we used would work as well for users. Calibration metrics tested were:

• Default: Exposure Correction: 0, Lux Calibration Constant: 2.5
• Nominal Calibration: Exposure Correction: -2, Lux Calibration Constant: 2.5
• Low Light Calibration: Exposure Correction: -2, Lux Calibration Constant: .477



 Below is an image of the Luxi add-on and app next to our Extech light meter.



Usability

It was simple obtaining measurements with Luxi since it obtains lux measurements automatically. Luxi uses the front or "face" camera to acquire different measurements, and it is important that the user does not get in the way of the camera. Attaching the add-on is simple, as it slides onto the top of the phone over the front camera. Just be careful, since the add-on could slide off since it does not clip on or attach to the phone.

Below is a Luxi overview video.

 

Recommendations

Since Luxi requires calibration against a professional light meter to be accurate, we do not recommend it as a sole source of lux readings. Users should purchase a professional meter instead. However, larger organizations that want to have many techs to take light measurements might find Luxi as an inexpensive low cost compliment to a single professional lux meter for general use.