In the previous installment we talked about what lumens are and why they are not the right rating to consider in a light for fluorescence diving and imaging. We closed by saying that the right question to ask is not ‘How bright is this light (in lumens)?’ but ‘How bright a fluorescence does this light produce in the things it shines on?’ So let’s talk about what contributes to fluorescence brightness.
Fluorescence is a process in which electromagnetic radiation is absorbed at one wavelength and then re-emitted at a different, longer wavelength. In our case the electromagnetic radiation is light, so fluorescence in effect converts light of one wavelength (color) to another.
Here’s what is going on when you point your light source as some subject to make it fluoresce:
The ‘but not all’ is the key. Fluorescence is a percentage game, and it is a game that is somewhat stacked against you. Each step is necessary, and at each step only a fraction of the energy that meets the criterion will go on to the next step. So we want to do whatever we can to tip the balance in our favor. We’ll look at these steps in reverse.
Step #3 – This is out of our control, so we won’t worry about it too much. It is entirely dependent on the internal properties of the subject. Some molecules are just more efficient than others at converting absorbed light to fluorescence. Many organisms don’t fluoresce at all. This is why, for the same excitation, you see large differences in fluorescence brightness.
Step #2 – there are two things involved in whether incident light will be absorbed by the molecule that produces fluorescence.
Step #1 – What is important is not how much energy comes out of the light, but how much reaches the subject. This is where the light’s beam pattern is important. The broader the beam, the less energy is delivered to any given area and the dimmer the fluorescence. There is a direct relationship between light energy in and fluorescence out, so you want to get as much energy as possible on the fluorescing subject.
So the right measurement to specify for a light for exciting fluorescence is not how much energy it emits in all directions (the normal way of claiming a lumen rating for a white light). What is important is the energy delivered per unit area, usually stated in units of something like microwatts per square centimeter with the light source at some specified distance. The more energy delivered per unit area, the brighter the fluorescence for any given subject. It is important to state the distance at which the rating measurement was made. The beam spreads out with distance, so the further away your subject is from the light, the lower the energy per unit area.