Solar Intensity: Looks Can Be Deceiving

Posted on 12/29/2017 at 12:00 by Daniel Stieler, Phd


Solar Intensity Looks Can Be Deceiving Title Graphic


It looks bright. Why doesn’t my solar panel produce more power?

This is a fantastic question and one that we get a lot.


The root of this issue is our eyes.

They say seeing is believing, but the human eye is easily deceived.

Humans are capable of seeing a broad range of light intensity.  

For example, indoors in a typical office environment, the light level is around 400 lux, whereas direct sunlight measures about 100,000 lux.

Have you ever wondered why reading your cell phone outside can be difficult?

At any given time, the human eye can only see a dynamic contrast range of about 1000:1 even though it can see a total range of 1,000,000,000:1.


If you are outside on a sunny day, your eyes can perceive between 10,000 and 100,000 lux.

This is why indoors, your eyes could perceive 10 to 10,000 lux, making reading a computer or phone easy.

In both cases, your eye will perceive that the upper end of the range is bright and the lower is dark. This changing dynamic contrast range of the human eye makes it a poor judge of absolute light intensity.  


During the recent solar eclipse, many people purchased eclipse glasses to protect their eyes while viewing it (some better than others).

If you have a pair of eclipse glasses, you can use them to view the sun and then take them inside and use them to view the lights indoors. You will find dramatically different results. This is because indoor light sources have much less intensity than the sun.

The intensity of the visual spectrum of indoor light sources alone is <1% of sunlight. Not to mention that the IR spectrum of light produced by high-efficiency indoor lighting is extremely low compared to sunlight.

What does all this mean?

Careful intensity measurements are crucial, especially when solar is used for applications that will not be receiving direct sunlight.


Do you need help taking your system off-grid and thinking through the details of your specific application?

We would love to speak to you!

Contact us or leave a comment below.


Categories: Solar Education