Stranica 21 [21]
CHARISM A I 7
a
— Spectral density of the radiation source and non-linear camera response
Two crucial factors in achieving reproducible and comparable brightness distribution or
luminance in reflected images are the spectral density of the radiation source used to
illuminate the object and the linearity of the camera used to record the image.
The spectral density of a radiation source describes how the intensity of the radiation emitted
varies with wavelength. It is an intrinsic property of any source, described by its spectral
power distribution (SPD).** An ideal illuminant (e.g. the CIE standard illuminant E, Figure 1-9)
would have an equal distribution of intensity (an equal energy spectrum) across all the
wavelengths produced.
200 r
150F 1
z
Relative power
wv
o
ka
400 450 500 550 600 650 700
Wavelength A (nm)
Figure 1-9. Spectral power distribution of the CIE standard illuminant E . "
The intensity of the reflected wavelengths would then only be dependent on the properties of
the surface. In reality however, the intensity of most radiation sources is not homogeneous
across the wavelengths produced (the CIE standard illuminant E is a hypothetical radiation
source). Figure 1-10 shows the difference in luminance observed for visible-reflected images
as a result of capturing these with illuminants which have very different spectral power
distributions: Figure 1-10(a) was captured with tungsten lamps (as represented by the CIE
Standard illuminant A, shown), whilst Figure 1-10(b) was captured with flashes (as
represented by the CIE standard illuminants D50 or D55). As may be observed the very
different distribution of intensity across the wavelengths produced by these radiation sources,
has direct repercussions on the luminance of reflected images, and for visible-reflected
images, also seriously impacts the colour-rendering of images." The warmer wavelengths
prominent in incandescent sources (Such as tungsten lamps) give rise to images with a
redder colour cast, whereas the more even distribution of wavelengths in the source
representative of daylight (such as D55 flashes) yield more balanced colour rendering of the
image.
Version No. 1.0 13 Date : 14/10/2013