C. Macfarlane et al. Agricultural and Forest Meteorology 100 2000 155–168 157
morphology and stand structure E. nitens Deane and Maiden Maiden and E. grandis W. Hill ex Maiden;
Dye, 1993; Battaglia et al., 1998. We also compared our photographic technique with destructive sampling
allometry in plantations of E. globulus with different canopy structures. Our objectives were:
1. To investigate the effect of photographic exposure and image processing on estimates of L in plan-
tations of E. globulus. 2. To examine the effect of stand structure and sam-
pling position on L estimated from hemispherical photography.
2. Theory
Chen et al. 1991 suggested that photographs should be overexposed by 1–2 stops relative to the
brightness of the sky outside the canopy to obtain accurate estimates of L
e
from hemispherical photog- raphy. Exposure is the amount of light acting on the
emulsion of the film or paper and is determined by the lens aperture f number and shutter speed Grimm
and Grimm, 1997. Built-in light camera meters mea- sure the brightness or luminance of the subject being
photographed and the camera calculates ‘automatic’ exposure settings assuming the light comes from a
mid-gray surface 18 visible reflectivity; Unwin, 1980. The degree of overexposure or underexpo-
sure of a photo image can be expressed simply by the relative exposure value EV
R
where EV
R
= 0 is
‘automatic’ exposure, EV
R
= 1 is one stop of over-
exposure and EV
R
= − 1 is 1 stop of underexposure
Unwin, 1980. A change in EV
R
of 1 stop represents a halving or doubling of the amount of light reaching
the film. Therefore, to make an unobscured overcast sky 18 visible reflectivity appear completely white
100 visible reflectivity should require 2.5 stops of overexposure EV
R
= 2.5. Overexposing the image
also increases the uniformity of the sky brightness Wagner, 1998.
However, digital grayscale images are typically converted to black and white prior to analysis using
a threshold algorithm which classifies pixels as black or white based on their brightness. In this process, not
only completely ‘white’ pixels are classified as sky but any pixel with a brightness value above a ’threshold’
value. If a constant threshold value of 50 brightness is used, then only 1.5 stops of overexposure should
be required to make an unobscured overcast sky appear completely white 50 visible reflectivity.
This agrees well with the 1–2 stops of over-exposure suggested by Chen et al. 1991.
Assuming that foliage is completely black, the ‘cor- rect’ EV
R
metered below the canopy should decrease below 1.5 as the proportion of light penetrating be-
low the canopy decreases below 100 and could be derived from Eq. 1, where I
D
is the fraction of light transmitted beneath the canopy. For example, beneath
a canopy through which 18 of the light above the canopy penetrated, EV
R
= − 1 should be required.
‘Automatic’ exposure would be correct for a canopy through which 36 of the light was transmitted.
EV
R
= log
2
I
D
0.36 1
In this study, the diffuse non-interceptance of light τ ; Welles and Norman, 1991, calculated using the soft-
ware from the PCA Licor, 1991;see Section 3, was used as an estimate of I
D
to predict EV
R
from Eq. 1.
3. Materials and methods
