I happened upon some Solar radiation unit conversion equations from the USDA at http://www.nrcs.usda.gov/wps/portal/nrcs/detailfull/national/home/?cid=stelprdb1043619. I am very grateful that someone decided that it was in the best interests of the country to provide this information. Anyway, now I have some basis to get values from the TSL2561 luminosity sensor. Other conversion factors are at http://www.egc.com/useful_info_lighting.php. According to the lbl.gov site at http://bccp.lbl.gov/Academy/workshop08/08%20PDFs/Inv_Square_Law.pdf, when converting Lux to Watts/m^2:
Converting Lux to W/m2
• There is no simple conversion….it depends on
the wavelength or color of the light.
• However, for the SUN, there is an approximate
conversion of 0.0079 W/m2 per Lux.
• Example: We read 75,000 Lux on our light
sensor. We can convert that reading to W/m2.
75,000 x 0.0079 = 590 W/m2
But now I have a check on the USDA information. The primary value which is output from the TSL2561 is in Lux. The Merriam-Webster definition of Lux is "a unit of illumination equal to the direct illumination on a surface that is everywhere one meter from a uniform point source of one candle intensity or equal to one lumen per square meter."
So from the above equations I take the output of the TSL2561 sensor in Lux and convert to W/m2 by multiplying by 0.0079.
This is a blog mostly about techie things, what I am doing to my apartment network on the cheap, IOT, 3D Printing, Raspberry Pis, Arduinos, ESP32, ESP8266, Home Automation, Personal Weather Stations, Things That Go Bump in the Night, and some side issues that need discussing. Remember, sometimes the journey to an end is as much fun as the goal achieved!
