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6 / 192018-08-16 | Document No.: AN097
Wavelength
The SFH 4780S features a typical centroid wavelength of 810 nm (see also
Figure 4), to match best the requirements for iris illumination for all eye colors.
Figure 4: Definition of wavelength parameters for the SFH 4780S. λ
peak
≠ λ
centroid
(due
to the asymmetric spectrum)
Further wavelength specifications are:
• Peak wavelength (λ
peak
) is the peak wavelength of the spectral density
curve (in most applications it is of little significance). This is typically 820 nm
for the SFH 4780S.
• Full-width at half-maximum (FWHM, Δλ): sometimes also called spectral
bandwidth. It is the wavelength distance between the spectral points where
the spectral density S(λ) is 50 % of the peek value. This is typically around
30 nm for the SFH 4780S (see Figure 6 for more details).
• Center wavelength (λ
0.5 m
) is the wavelength halfway between the two
spectral density of 50 % of the peak value. This typically around 815 nm.
• Centroid wavelength (λ
centroid
) is the mean wavelength (see equation 1). It
divides the spectrum in two equal parts. It is the most important definition
for non-visual systems (like iris recognition) and relevant for this kind of
application.
For a symmetrical spectrum λ
peak
, λ
0.5 m
and λ
centroid
are identical. However, the
high efficient SFH 4780S features a slightly asymmetrical spectrum.
The SFH 4780S has very tight wavelength specifications and no secondary peak,
e.g. λ
centroid
is typically within ± 13 nm. Additionally, the device features a low
Wavelength [nm]
Rel. spectral emission
1.0
0.8
0.6
0.4
0.2
0.0
740 760 780 800 820 840 860 880
O
peak
FWHM
O
centroid

centroid
 S d
1
2

S d
1
2

--------------------------------------
= 1