Luminosity and irradiance sensors

Research: What light sensor is good to measure sunlight?

  • TSL2561
  • BH1750

overview and comparison table:

My preference is for the Rohm sensor as it does not saturate like the TSL2561 does if pointed directly at the sun.

The TSL2561 also measures infra red. It has two diodes, one for visible and infra red light and one for infra red light only.
To my opinion, BH1750 fits most requirements, whereas the TSL2561 offers a wider spectrum, more sensitivity and more accuracy.

May I put the question itself in question: Want do we want do measure? Is it really luminosity? To my understand those regular low-cost sensors report the illuminance (luminosity/brightness) in Lux.

From the meteorological point of view the irradiance (the transferred energy) would be more interesting, stated in Watts per area. This parameter is used to determine if there is “sunshine” (>120W/m² in upright position as of WMO). One can’t convert Lux in W/m².

But there is a cool DIY-approach! What might be a sensor for “getting the watts per area sent by the sun”? Of course the short-circuit current of a solar-panel! Brilliant:

I still don’t see an approach to compensate the “upright-position”-problem. May be a compensation taking the current position of the sun relative to sensor into account could be an approach. Or the usage of a big fish-eye lens. Or using multiple cells.


Good to have asked you @wtf! :-) My question behind is: What light conditions are good / bad for plants or for bees. So irradiation could be a more fitting value than luminance.

Some thoughts following up …

as always, a good start for reading (in german) about meteorological measurements is DWD’s guideline #6 on ground sensors, chapter 14 on solar-radiance, starting on page 141. Some renewal of the understanding of direct, diffuse and global radiation helped me a bit.

Yep, page 142 says: Lambert’s cosine law solved this in 1760.

Another experiment using a solar panel as sensor, but taking the open-circuit current into account, too:
Design of a low cost irradiance meter using a photovoltaic panel

Last year a bachelor thesis was published at the university of Twente: Designing a low-cost autonomous pyranometer (PDF). The autor did some comparsion of low-cost sensors with a Davis instrument (spoiler: in the end it turned out this reference was a bit faulty), and decided a path using the combination of two low-cost sensors, Si1145 and ML8511.