ECE532 Biomedical Optics © 1998 Steven L. Jacques, Scott A. Prahl Oregon Graduate Institute

Radiant Power

The power output of a source is described by its radiant power P [W]. If one could enclose a source within a box and capture all the energy radiating from the source, the rate of increase of energy, joules per second [J/s], in the box would equal the source's radiant power [W].

A source with a power output P [W] which is turned on for a time duration t [s] will yield a radiant energy Q [J]:

Some examples of radiant sources:

Let us pretend that we are the pseudo National Institute of Standards and Testing (pNIST) and we must calibrate some light sources. Let's use a cup of black water, water + ink, as our "box" and capture all the energy emitted by our unknown sources. The energy absorbed by the ink (carbon particles) will warm the water. Measuring the rate of temperature increase of the water caused by a source will allow calculation of the radiant power of the source. We consider two example sources:

• Experiment: light bulb immersed in black water
• Experiment: laser aimed into black water

Now that we have a couple calibrated sources, we can use these sources in subsequent experiments to calibrate other radiant power detectors. We have bootstrapped ourselves into generating pNIST-traceable calibrated detectors for our laboratory!