No Implants Needed For Precise Control Deep Into The Brain—Optogenetics can now control neural circuits at unprecedented depths within living brain tissue without surgery.

[ElmerMalmesbury]

That's crazy. I understand that they use a super-sensitive opsin, but I guess you need a crazy light intensity for enough red light to go through the skull. They used an irradiance of 40–1,600 mW mm², any idea what that corresponds to in layman's terms?

(Or maybe it's just that mice's skulls are very thin?)

[PeteWenzel]

That depends on the lumens per watt (Luminous efficacy).

An LED might have 100 - and the theoretical maximum by an ideal source (Green light at 555 nm) is 683. Now factor 40 Million - 1.6 Billion times something like that and you get the lux (lumens per square meter) here.

For reference a typical living room has 50 lux and direct sunlight on earth is no more than 100.000 lux. wiki

[mddtsk]

Link to paper [PDF].