Question

What is brilliance ?
I mean the term which has the unit of
photons/(s . mm2 . mrad2 . 0.1%BW)
Please describe the unit also .

Answer 1

I think you must be talking about photometric brilliance or Luminance . This the light energy per second per square meter per steradian hitting a surface. The photometric unit would be lumens/m^2/steradian. I do not know what the 0.1%BW means above although I suspect it is a condition of measurement.

The energy can be expressed as number of photons. but the official term would be joules.

The official units of Luminance is lumen/meter^2 or the NIT!, where the lumen is the total energy flux/second emitted by the light source.

A one lumen source produces an Luminous intensity of one candela or approximately the brightness of a candle. The term brightness is rarely used because it is subjective.

This probably sound confusing but I hope it is helpful.