# Is there a relationship between the driving current and the optical power emitted by a light source?

If we have an LED light source which is driven by 0.5 Ampere can we infer what is the optical power (aka radiant flux, radiant power) emitted by that light source?

Furthermore is there a way to convert Illuminance (measured in lux) to optical power (measured in Watt)?

Short answer - no, the current is not sufficient.

At the very least, you need both the driving current and the voltage (which depends on the band gap, and thus the wavelength) to know the input power. But the actual efficiency of a light source (LED, or any other light) is a function of many factors. The output of most LEDs is a function of temperature, and is not even linear with current (because of certain saturating self-absorption phenomena).

Conversion of illuminance (lux) to power requires you to know the "luminous efficacy per Watt" (which is a function of the wavelength distribution). It's complicated. This is coverend in this answer

You want to convert from photometric luminous power to radiometric power.

Radiometric power, measured in Watts, emitting from an LED is a measurement total amount of photons emitted.

lm is the photometric equivalent of the radiant power which is the luminous flux or luminous power measured in lumens.

Photometrics is how the human eye perceives the photons emitted from a light source.

In addition to power there is the intensity which is the measure of photon at a single point at a particular angle.

There is also the concentration of photons within a given area within a cone where the cone has a given angle.

Radiometric units                              Photometric units
Radiant power Watts                            Luminous flux Lumens (lm)