Basic Optical Principles
Metric System of Measurement

The basic unit of linear measurement in the metric system is the meter. It is often useful to compare units of measurement in the metric system with those of the English system of feet and inches which we use every day.

One Meter (m) is equal to:	39.37 inches
One inch is equal to:	24.4 millimeters (mm)

The two comparisons illustrated above should be memorized. They will help you in some of the practice exercises later in this section and can be useful when taking the ABO national certifying exam.

One Dollar is Equal To:	One Meter is Equal To:
10 dimes 100 cents 1000 mils (10 mils = 1 cent)	10 decimeters (dm) 100 centimeters (cm) 1000 millimters (mm)

The metric system of measurement is actually easier to use than the English system of inches and feet which we use every day. The metric system uses one basic unit of measurement, the meter. All other units are related to the meter by factors of ten. This can easily be compared to our monetary system of measurement which the above table illustrates.

If a basic unit of measurement in the metric system, such as the meter, is preceded by the prefix �Kilo� that unit is multiplied by one thousand. For example one kilometer is equal to 1000 meters, one kilogram equals 1000 grams and so on.

In our daily work in optics we are primarily concerned with millimeters which is one one thousandth of a meter. Facial measurements such as PD�s and bifocal segment heights are expressed in millimeters, as are lens and frame dimensions. However there is one other division of the meter which we need to consider in our field, and that is an incredibly small unit called the nanometer. There are one billion nanometers in a meter. On your PD ruler the smallest unit of measurement is a single millimeter. Within that single millimeter could fit one million nanometers. This unit of measurement becomes important when we cover the nature of light later in this session.