There is an Ambient Light Sensor with Microsoft Band, and some apps in the Windows Store show the ALS value in Lux, But I can't find a way in the Band SDK to read the ALS Lux value.
How can I access the ALS?
As mentioned, that sensor is not exposed by the public SDK - but it is apparently possible to get the information (and a heap of other stuff) by using the raw Bluetooth interface - the public API is a kind of wrapper around the raw Bluetooth protocol.
So, if you're not afraid of reverse-engineering and fiddling around with raw bytes you may be able to figure out how to decode the sensor data. You can use the Windows.Devices.Bluetooth.Rfcomm library - there's a code sample from Microsoft that shows how to setup basic Bluetooth rfccomm communication: https://code.msdn.microsoft.com/windowsapps/Bluetooth-Rfcomm-Chat-afcee559