light meter image

  Light Meter
      from KEUWLSOFT

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This app uses the light sensor to measure illuminance in lumens per square meter (lx) or lumens per square foot (foot candles). FOR INDICATION ONLY. Results depend on your device and its hardware.

Features of this SPL meter include:

• Averaging.
• Analogue dial to show current Lux value.
• lx or ft-c units.
• Maximum and minimum values.
• Time graph of the Lux value.
• Auto-scale or manual (pinch & pan) Y-Axis.
• Calibrate option – if you have a calibrated illuminance meter or known light source, you can use this option to calibrate the meter.

light meter screenshot
Light Meter
  light meter screenshot
Settings Menu

Using the App

Once loaded the app will immediately start showing the lux value in the dial, initiate averaging and reset the maximum and minium values. The minimum and maximum values are shown by blue needles. Below the dial, the current, minimum, maximum and averaged values are shown. The time in which the averaging has taken place is also shown beside the averaged value.

Below the dial are 3 buttons:

Clear - This clears the minimum, maximum and averaging values.

Info - This brings up a help screen with a brief explanation, sensor info & a link back to this web page & other relevant info.

Pause - This pauses the measurement. Press again to unpause.

Below the first 3 buttons is a digital display whose content will vary depending on the bottom 3 buttons:

Settings - This opens up a settings menu from which some options can be changed.

Graph - A time series graph of the Lux values. There are 3 speed options for the graph as selected in the settings menu.

CAL - This gives the option to calibrate for better results.

If in manual y-axis mode then panning or pinching the y-axis will scale or shift the y-axis accordingly. The manual or auto-scale y-axis option is found in the settings menu.


This app reads the value of lux from the light sensor. Lux is a measure of illuminance and is measured in lumens per square meter. 1 lux = 1 lumen per square meter. For those of you that work in feet, 1 foot candle = 1 lumen per square foot. foot candles are abbreviated ft-c in the app.

1 lux = 10.76 ft-c.

Time Constant

The time constant can be set to 0.5 seconds, 1 second, 2 seconds or 5 seconds. For rapidly changing light levels a lower time constant is best. The app will average the data over the time constant weighting it with a linear fall off over the length of the time constant.

The update speed is how often a new measurement is displayed. The default is 100 ms, but it can be changed to 350 ms or 1 s if slower updates are prefered.


To calibrate, first press the CAL button. The CAL display lets you measure up to three calibration points and enter the actual value of them. The actual value can be determined from a calibrated illuminance meter a light source of known volume. If not available, the CAL can still be use to adjust the reading up or down if you believe it is incorrect on your device. Any cal values are stored on exiting and restarting the app. To clear any calibration values, tap clear CAL. Tapping cancel exits the calibration mode. Tapping Set CAL will set the new calibration based on the data in the table. At least one data point and corresponding actual value is required before a new calibration can be set.

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Calibration Screen

Tapping the measurement box, will cause the Lux to be measured. The duration of the measurement can be set in the settings menu as 1s, 4s, 12s or 30s. A blue progress ring appears and counts down the measurement time. If a delay is chosen, again in the settings menu (0s, 2s, 10s or 30s), then a red countdown will first appear and countdown until the measurement starts. After the measurement the measured value in lux or ft-c is shown depending on the units chosen.

Tapping the enter actual box will allow you to type in the value which is measured value should have been (e.g. based on the calibrated illuminance meter).

Depending on the number of data points entered, the calibration will be an offset, a linear, or a polynomial fit to log10 of the data points from which future Lux values will be calculated. The calibration only accepts values between 0.1 and 100000.

Calibration points should be chosen to reflect the measurement range being tested. If one point is used, it should be as close as possible to the target illuminance level. If two points are used, they should be at each end of the range being used. If a third calibration point is used, it should be midway between the first two.

Note that even after calibration the app should still be classified "for indication only". Use a calibrated illuminance meter where it is required.