Analog value to lux?

Projects General Guidance
1

We're supposed to set the brightness of our light bulb to a certain value that is equivalent to a certain value of lux.

Suppose we're in a laboratory, and this laboratory has to have 300 lux, so to satisfy the 300 lux we're suppposed to set a brightness value to the light bulb that is equivalent to 300 lux in terms of analog values from 0 to 255. Does anyone know the relationship between them?

Thank you.

2

Does anyone know the relationship between them?

No... You'll have to get a lightmeter and do the calibration yourself.

The brightness depends on the bulb (it's wattage rating and efficiency), the average/RMS voltage* applied to it the light fixture, the distance from the bulb, the reflectivity of the room, and windows or other sources of light, etc.

  • An incandescent light is [u]dimmed[/u] dimmed by turning-off the power for a part of the AC cycle (similar to PWM dimming). But depending on the software, there is no guarantee that a value of 126 corresponds to half-voltage or half-power (not the same thing), and in fact it may not be controlled by a 0-255 value in software.
3

DVDdoug is absolutely right.

4

I have done som calculations for the LDR GL5537 (R=20k..30k @ 10 Lux)
Gamma is 0.7 (the graf shown in datasheet says 0.682, found with excel)

I connected this sensor in series with a 10k resistor (LDR connects to 5V, Resistor to GND)
Excel regresion gave me:

float lux = exp(0.007251*analogRead(Apin));

('hits the roof' @ 1600 Lux. More light ? replace R with <1k)

ldr.png
ldr.png801×542 120 KB

5

DVDdoug:
No... You'll have to get a lightmeter and do the calibration yourself.

The brightness depends on the bulb (it's wattage rating and efficiency), the average/RMS voltage* applied to it the light fixture, the distance from the bulb, the reflectivity of the room, and windows or other sources of light, etc.

  • An incandescent light is [u]dimmed[/u] dimmed by turning-off the power for a part of the AC cycle (similar to PWM dimming). But depending on the software, there is no guarantee that a value of 126 corresponds to half-voltage or half-power (not the same thing), and in fact it may not be controlled by a 0-255 value in software.

I forgot to indicate that I have a luxmeter.
If I do the calibration myself, do you mind to elaborate? I'm a beginner and I haven't the slightest idea what to do. Thank you very much, sir.