Hello all.
I would like to convert the readings from a light sensor to units (lux, candelas, or lumen - don't know which is best). I want to know how much light is present in a room but with less abstract numbers (such as voltage readings).
Thanks
Check - Arduino Playground - TSL235R - great sensor
You probably need to create a lookup table for conversion
Thanks for the suggestion.
However, I am interested in converting voltage to lux, lumen, or candelas, as I would like to know the luminance of the room. Frequencies do not help much with that.
So... how can I convert voltages to this? How can I proceed?
Why do you need the sensor measurement converted into standard units? The arduino code can work with the voltage just as well. If you need to display the reading of course, that's a different matter.
The only way I can see to do it is to use a light meter that does measure lumens and use it to calibrate the sensor, ending up with either a lookup table as suggested or a formula that approximates the mapping.
What range do you want ?
Depending on accuracy you will probably need to range your detector.
0 to 5 v will give a very coarse reading as light intensity varies greatly.
For absolute readings in lumens you will need a calibration scource.
Voltage will depend on the type of sensor you are using as well, different sensors give different outputs for the same light level and may not be linear anyway.
As I don't have any specialized equipment at the moment for the calibration, I might as well use an Android app that I found to do this. The app uses the device's sensor to get light indications, and then convert it to a unit. I will try and calibrate my sensor with this approach, although I am not very certain about the detail of the readings - although this will be a very rough approximation and not an accurate measurement.
So, after some search I found this article (Lux – Arduino, ESP8266, ESP32 & Raspberry Pi stuff) that converts the voltage of the sensor in lux units.
I am using this sensor (http://www.fezhydra.com/GHI-Light-Sensor-3-Pin-eblock/dp/B00614OY6K) with an Arduino Uno, and an Android application as a reference to check the values (https://play.google.com/store/apps/details?id=com.bti.lightMeter&hl=en).
However I get different indications - especially when there is a lot of brightness.
Anyone has a tip on how to calibrate it correctly?
As others have suggested, the only way to calibrate your device is to compare the results you get with those of a known accurate lightmeter, over the entire range of expected light levels. You may need a lookup table or nonlinear correction curve to map the two together.
I don't think approximation:
Rldr=500/Lux, or
Lux=500/Rldr (in kOhm)
is accurate, I would expect exponential , exp(x) ^-1 dependence, but anyway, don't try to calculate your lux level, it's really unlikely you would get meaningful results with a sensors linked.
There are two option, buy specially made sensor like light-frequency, as arduino is much better to measure freq., than voltage.
Or create a table, where you enter measured values vs values you get from other device, and after you may ask arduino to calculate polynomial approximation.
...you might find this useful - Overview | TSL2561 Luminosity Sensor | Adafruit Learning System 
non linear mapping is easy with multiMap() - Arduino Playground - MultiMap -
Whole lots easier to just pick the right sensor... LUX expresses light intensity as referenced by human vision... therefore, it is spectrum-centric.
SFH 3710
Ambient light sensor with Vlambda Characteristics in SMT Chipled Package
http://catalog.osram-os.com/catalogue/catalogue.do?act=showBookmark&favOid=00000001000010480002003a
These units output a linear current with incident light which is already corrected LUX.
Brightness
Brightness is a term that describes how intense a light source is perceived by the human eye. Brightness is measured in units called “LUX”. Light sources with the same LUX level appear at the same brightness to the human eye. Table 1 shows the bright- ness (LUX measurement) of some everyday light sources. The technical term for brightness is illuminance.
Similar devices in clear "LED like" packaging have been available from time to time in the surplus sites.
Ray
General_appnote_for_ambient_light_sensors_Version4.pdf (399 KB)
