Class IV lasers are lasers with an optical power higher than 500mW, and for these lasers "even diffuse reflections can cause permanent eye damage". Well, I would like to understand how this is possible exactly.
Let's imagine I have a 1W laser pointer and I shine it on a white wall. If the wall has a rough surface, then the subsequent scattering of the light should appear approximately isotropic. Let's say it is, for simplicity, and let's imagine that no light is absorbed by the wall.
In this case, then, 1W of light will be emitted from the wall in a semisphere around it.
Let's now say that I am at 1m distance from the wall and I look at the reflection. Let's say that my iris is 10mm in diameter (this is more than normal for a human), which means that I should intercept about [π(0.01)²]/(2π)=50 microwatt of optical power... How can this be dangerous for the eye?
Am I using the wrong assumptions or calculations?