Meaning of two ADC channels in an LTR-559ALS-01 optical sensor
The two channels are connected to two different light sensing diodes:

One is sensitive to IR only
The other is sensitive to IR + visible light

This gives you more flexibility in choosing what spectral range you are interested in. If you are only interested in visible light intensity you can simply subtract Ch1 from Ch0.
To be honest I'm not sure what this &quot;ALS ratio&quot; is supposed to to tell us, it will be different for different light sources. They specify that value for a &quot;white LED&quot;, whatever that says about the spectrum...