What is intensity?
Radiant intensity refers to radiant power (flux) in a particular, specified direction. Formally, it is the rate at which radiant energy is transferred, per unit solid angle. Radiant intensity is expressed in watts per steradian (W · sr-1). It is what I call a linear-light measure. Radiant intensity potentially has a large spatial extent, but imaging systems use pixels with small area: It is inappropriate to use intensity as a metric for image data. A more suitable quantity is radiance, defined as radiant intensity per unit projected area. It is expressed in watts per steradian per meter squared. Image data stored in a file – such as TIFF or PPM – may be proportional to radiance; however, as I have mentioned, pixel values are usually subject to a nonlinear transfer function.
Intensity is a measure over some interval of the electromagnetic spectrum of the flow of power that is radiated from, or incident on, a surface. Intensity is what I call a linear-light measure, expressed in units such as watts per square meter. The voltages presented to a CRT monitor control the intensities of the colour components, but in a nonlinear manner. CRT voltages are not proportional to intensity. Image data stored in a file (TIFF, JFIF, PPM, etc.) may or may not represent intensity, even if it is so described.
