Common Units and Symbols for Photonics

When you were in grade school you probably learned the "metric system" and found it much too complicated with all those strange prefixes to be memorized. Most likely, it was never explained that when you actually use the metric system (now called the SI, or International System) you don't use the whole list of prefixes (just as you don't use all the possible "English system" units either).

We only need to introduce a few measurement units:

These three unis, plus a couple more (e.g., for temperature and electical charge) are sufficient to defind any quantity of interest in physics. For example, speed is expressed in meters/second (length and time) and density is in kilograms /cubic meter (mass and length).

Your science teacher probably told you how the metric units are related to English units, however, when you use metric units exclusively you will begin to think in metric units and you won't need to do the conversions back to English units.

For our purposes, the prefixes below are the ones we will use most often. Note that all but centi are powers of 10 that are multiples of 3. (Centi is usually used with meter to express lengths around the size of your computer keyboard. If "inch" would be the appropriate unit, then cm is usually appropriate.)

PREFIX

SYMBOL

FACTOR (decimal)

FACTOR (scientific)

nano

n

0.000 000 001

10-9

micro

µ (Greek letter "mu"

0.000 001

10-6

milli

m

0.001

10-3

centi

c

0.01

10-2

kilo

k

1 000

103

Mega

M

1 000 000

106

Giga

G

1 000 000 000

109

Here are some examples: