Hello Vanessa,
W/m2/sr is a function of camera sensor. It will need a known transfer function/gauge/lookup table to convert grayscale values to radiance for this particular camera sensor. Basically, a function of the camera's sensitivity for each color channel (if it's a color sensor) and then you'd have to also account for the grayscale conversion (red is ~.4, green is something close to that and blue is ~.15)
Hope this helps
