Page 331 - Sensors and Control Systems in Manufacturing

288
S i x
Cha p te r

• The absolute algorithm. Compares color on the basis of abso-
lute voltages.
• The relative algorithm. Compares color on the basis of relative
percentages of each RGB component voltage.
The choice of algorithm depends on sensing distance variation
and the type of color distinction one needs. If the outputs vary exces-
sively with distance when the absolute algorithm is used, a relative
(ratio) algorithm must be considered. While a relative algorithm
does not retain the lightness information, it greatly reduces unwanted
distance-related variations. The relative algorithm shows changes in
chromaticity (hue and chroma) that exist in most color differences. If
the sensing distances can be held constant, the absolute algorithm
works well at detecting subtle changes in density (shades) of a single
color.

6.4 Optimum Detectors in Light Sensors
Deciding on the type of detectors to use for light sensing applications
can be overwhelming, given the kinds available, which include pho-
todiodes, phototransistors, photodarlingtons, photomultiplier tubes,
photo-resistors, integrated circuits, various hybrids, and even ther-
mopiles. A vital insight on selecting the appropriate approach for
ultraviolet, visible, and near-infrared light sensing applications is
imperative.
The light source spectral characteristics require optical power mat-
ing of various electronic packages with the following constraints:

• Image size
• Signal-to-noise ratio
• Frequency bandwidth
• Cost

Most corporations do not have resident experts in the fields of
modern optics—especially in the area of light detection. As a result,
when a new product is being developed, the light sensing design
project is usually assigned to a mechanical or electrical engineer. To
meet project schedules (for these optics for non-experts), individuals
must acquire the necessary knowledge on the various light sensing
methods available. A simple guideline briefly describing a variety of
light sensing technologies and options is presented next.

6.4.1 Available Light Sensing Options
Light sensing applications vary widely from specialized scientific
instrumentation that needs to detect individual light particles