2. MEASUREMENTS 
  
The concept of "spectral signature" is still being discussed from 
the terminological and definition points of view (c.f. Sievers, 
1977). A detailed discussion on the physical phenomena involved 
in the concept was recently given by Boehnel (1977). Fisher 
(1977) has discussed the implementation of measuring the incident 
and reflected radiation and Kriebel (1977) the definition of the 
spectral reflectance factor with and without the reflection standard. 
2.1 Sample plots 
The sample plots selected for telespectroradiometer measurements 
were located in pure, even-aged ScotS pine (Pinus silvestris), Norway 
spruce (Picea abies) and White birch (Betula verrucosa) stands. 
Geographically, the measurements were concentrated around three cities 
in South-Finland:  Kouvola, Jämsänkoski and Vilppula, each 100-150 
km apart from each other. The sample stands were divided into three 
size classes:  seedling, thinning and mature stands. Among them, 
four different site type categories were represented. The age, timber 
volume, mean height and diameter (DBH) were measured on the ground for 
every stand. The rectangular sample plots were chosen on flat ter- 
rain. Although a number of plots were measured in July 1976, only 
39 plots measured in August were accepted for further analysis in 
this study. The reasons for rejecting the early measurements were 
hazy or windy weather and minor problems with the platform. 
2.2 Equipment used 
The equipment employed consisted of a telespectroradiometer system, 
a TV-camera with monitor, a motor driven camera, and a helicopter 
as platform vehicle. The telespectroradiometer system comprises op- 
tical unit, radiometer unit -and plotting unit. The spectral range 
of the monochromator in the optical system is from 400 to 1100 nm, 
and the spectral response is adequate from 380 to 1050 nm. The sys- 
tem which is illustrated in Fig. 1 was installed on board a light 
weight helicopter. The input optics emerging through the removed 
door were pointed at the ground or sky when recording the reflected 
radiation or incident radiation, respectively. The pilot could