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N. Blanik
11.3 Illumination Concepts
Since PPGI measures the amount of light reflected from skin surface, sufficient
proper source of adequate illumination is of utmost importance for this method.
Three different light sources can be used:
• daylight,
• environmental light or,
• artificial light.
Daylight, or rather sunlight, offers strong illumination in the visible and infrared
spectrum. The drawbacks of this source of illumination are that it cannot be manipulated by the experimenter. Moreover, fluctuations in lighting conditions due to cloudiness or progression of daytime, would manifest in disturbances in illumination intensities. Since a majority of PPGI measurements require a steady and constant level of
brightness, either the changing intensity must be logged separately for computational
compensation, or daylight must be cut out totally.
Using indoor environmental light—in most cases ceiling lights—ensures constant
illumination. The wavelength of the illuminant would determine coverage of the
visible spectrum and small portions of the near-infrared spectrum. Old-fashioned
incandescent light would cover larger portions of the infrared spectrum. Indoor illumination is customarily powered by 230 V alternating voltage, respectively, 50 Hz or
60 Hz noise could couple with the recordings depending on the camera’s sampling
frequency. The disturbing effects of noise, however, may be overcome with the help
of a matching notch filter.
Despite the fact that environmental illumination possesses the attribute of homogeneous distribution, reflections, and shadowing by objects outside, the visible field
of the camera may influence lighting condition of the monitored object. Figure 11.8
demonstrates how a PPGI signal was disturbed by changing lighting conditions
caused by a person walking by in close proximity to the measuring field [10].
To avoid such illumination artifacts, it is recommended that artificial illumination
be used as the source of light, because it homogeneously distributed specifically
on the subject being measured. If possible, the surroundings should be darkened.
LED illumination panels, in particular, have proven to be appropriate for this task.
In Fig. 11.9, two different types of such panels are shown. The fundamental benefits
of LED use include:
• widely used and available at comparatively low cost,
• large number of narrow-band wavelengths or wide-band spectra are available,
• energy efficient power-LEDs,
• option of selecting narrow or wide beam characteristic,
• easy installation and combination with existing measurement setup,
• short switching times; possible synchronization with camera frame grabbing,
• small drift in brightness.
N. Blanik
11.3 Illumination Concepts
Since PPGI measures the amount of light reflected from skin surface, sufficient
proper source of adequate illumination is of utmost importance for this method.
Three different light sources can be used:
• daylight,
• environmental light or,
• artificial light.
Daylight, or rather sunlight, offers strong illumination in the visible and infrared
spectrum. The drawbacks of this source of illumination are that it cannot be manipulated by the experimenter. Moreover, fluctuations in lighting conditions due to cloudiness or progression of daytime, would manifest in disturbances in illumination intensities. Since a majority of PPGI measurements require a steady and constant level of
brightness, either the changing intensity must be logged separately for computational
compensation, or daylight must be cut out totally.
Using indoor environmental light—in most cases ceiling lights—ensures constant
illumination. The wavelength of the illuminant would determine coverage of the
visible spectrum and small portions of the near-infrared spectrum. Old-fashioned
incandescent light would cover larger portions of the infrared spectrum. Indoor illumination is customarily powered by 230 V alternating voltage, respectively, 50 Hz or
60 Hz noise could couple with the recordings depending on the camera’s sampling
frequency. The disturbing effects of noise, however, may be overcome with the help
of a matching notch filter.
Despite the fact that environmental illumination possesses the attribute of homogeneous distribution, reflections, and shadowing by objects outside, the visible field
of the camera may influence lighting condition of the monitored object. Figure 11.8
demonstrates how a PPGI signal was disturbed by changing lighting conditions
caused by a person walking by in close proximity to the measuring field [10].
To avoid such illumination artifacts, it is recommended that artificial illumination
be used as the source of light, because it homogeneously distributed specifically
on the subject being measured. If possible, the surroundings should be darkened.
LED illumination panels, in particular, have proven to be appropriate for this task.
In Fig. 11.9, two different types of such panels are shown. The fundamental benefits
of LED use include:
• widely used and available at comparatively low cost,
• large number of narrow-band wavelengths or wide-band spectra are available,
• energy efficient power-LEDs,
• option of selecting narrow or wide beam characteristic,
• easy installation and combination with existing measurement setup,
• short switching times; possible synchronization with camera frame grabbing,
• small drift in brightness.