# Luminostiy, Flux, Intensity definitions and meaning

I am taking astrophysics course and we are constantly talking about Luminosity,Brightness,Intensity,Flux etc. I thought that I understand the idea properly but I see now that I dont.

Intensity is the light passing through a steradian per second

$$I=E/dtd\Omega$$

Luminosity is intensity per area

$$L=E/dtdA\Omega=I/dA$$

And Flux is the Luminosity per area ?

$$F= L/4\pi r^2$$

And brightness = Flux

Are these statements true ?

I am confusing these definitions and their meanings constantly. Can someone help me to understand it better.

## 1 Answer

There are 6 words that are alike enough to be confusing. Some are used for the object emitting the light, and others for the illuminated object.

And some are used for an instrument that detects the energy in light. And others for measuring light by looking at it. This is much the same, but your eye is more sensitive to green light than light that is almost infrared.

Here is a link that does a pretty good job of explaining.