# Converting radiometric units (W/m^2) to photometric units (Lux)

1. Nov 6, 2011

### cs4809

1. I am doing a lab about solar radiation. We collected data using a lightmeter which measures illuminance in lux. Now to calculate the optical depth of the atmosphere I either need to find an expression for the solar constant in lux or be able to convert illuminance values in lux to W/m². The conversion is straightforward for monochrome radiation but for a range of frequencies it becomes much more complicated because each wavelength has a different conversion factor. I've been told that integrating a blackbody intensity curve is a good place to start but any approximate methods for this conversion would also be welcome.

2. Relevant equations
I0 is the intensity of radiation at the source (Solar constant) and I is the observed intensity after a given path, then optical depth τ is defined by the following equation