Physics in a nutshell

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The term Radiation refers to energy that is transported by means of photons/electromagnetic waves. Each photon carries a specific amount of energy $Q_\text{ph} = h \cdot \nu_\text{ph}$ that depends on the photon's frequency $\nu_\text{ph}$ or equivalently its wavelength $\lambda = \frac{c}{\nu}_\text{ph}$ with $c$ being the propagation velocity of EM waves (speed of light) and $h$ being Planck's constant.

The branch of physics that deals with the process of measuring radiation by means of physical devices is called radiometry as opposed to photometry which deals with how radiation is perceived by the human's eye.

These two areas differ due to the fact that physical measurement devices are expected to be equally sensitive to all wavelengths of radiation which is however not the case for the human eye: It's perception of light depends strongly on the wavelength.

## Issues in the Terminology

The concept of radiation is important for a couple of completely different branches of physics, all of them having their own terminology.

This gives rise to the fact that different terms denote the same quantity or that - even worse - a specific term refers to different physical quantities. The latter can be very confusing and one of the most outstanding examples might be the term intensity in the context of radiation: Some people use it for what I call flux density, others for what I call radiance, still others for the strength of the electromagnetic field while from a radiometric point of view one would might refer to what I call intensity.[1] Therefore, this chapter aims at establishing more clarity in the radiometric terminology!

## References

 [1] W. L. Wolfe Introduction to Radiometry SPIE Press 1998 (ch. 2.3)

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