To focus an instrument that will be used in the field or out-of-doors, first set the instrument up in subdued lighting in a laboratory. There you will be able to see the light being emitted by the Light Emitting Diode (LED) and determine the distance to spot size ratio. Then, when you are in the field, you can determine the area being measured by knowing that the ratio of the distance to spot size will remain constant as it was in the lab.