Banner S18 Series - Manual (Page 3)

reflective (matte) background, or angle either the sensor or the background (in any plane) so the background does not
reflect light back to the sensor (see Figure 4 on page 3). Position the background as far beyond the cutoff distance as
possible.
An object beyond the cutoff distance, either stationary (and when positioned as shown in Figure 5 on page 3), or
moving past the face of the sensor in a direction perpendicular to the sensing axis, can cause unwanted triggering of the
sensor if more light is reflected to the near detector than to the far detector. The problem is easily remedied by rotating
the sensor 90° ( Figure 6 on page 3). The object then reflects the R1 and R2 fields equally, resulting in no false
triggering. A better solution, if possible, may be to reposition the object or the sensor.
Color Sensitivity
The effects of object reflectivity on cutoff distance, though small, may be important for some applications. It is expected
that at any given cutoff setting, the actual cutoff distance for lower reflectance targets will be slightly shorter than for
higher reflectance targets (see Performance Curves on page 4). This behavior is known as color sensitivity.
For example, an excess gain of 1 for an object that reflects 1/10 as much light as the 90% white card is represented by
the horizontal graph line at excess gain = 10. An object of this reflectivity results in a far limit cutoff of approximately 20
mm (0.8 inches), for the 25 mm (1 inch) cutoff model for example; thus 20 mm represents the cutoff for this sensor and
target.
These excess gain curves were generated using a white test card of 90% reflectance. Objects with reflectivity of less than
90% reflect less light back to the sensor, and thus require proportionately more excess gain in order to be sensed with the
same reliability as more reflective objects. When sensing an object of very low reflectivity, it may be especially important
to sense it at or near the distance of maximum excess gain.
R1 = Near Detector
R2 = Far Detector
E = Emitter
S18FF
Cutoff
Distance
Reflective
Background
Fixed
Sensing
Field
E
R2
R1
Strong
direct
reflection
to R1
Core of
emitted
beam
Figure 3. Reflective Background - Problem
R1
R2
E
R1 = Near Detector
R2 = Far Detector
E = Emitter
S18FF
Core of
Emitted
Beam
Cutoff
Distance
Reflective
Background
Fixed Sensing
Field
Strong
Direct
Reflection
Away From
Sensor
Figure 4. Reflective Background - Solution
Cutoff
Distance
R1 = Near Detector
R2 = Far Detector
E = Emitter
S18FF
E
R2
R1
Fixed
Sensing
Field
Reflective
Background
or Moving Object
Figure 5. Object Beyond Cutoff - Problem
E = Emitter
R1 = Near Detector
R2 = Far Detector
S18FF
E, R1, R2
Fixed
Sensing
Field
Cutoff
Distance
Reflective
Background
or Moving Object
Figure 6. Object Beyond Cutoff - Solution
A reflective background object in this position or moving across
the sensor face in this axis and direction may cause false sensor
response.
A reflective background object in this position or moving across
the sensor face in this axis will be ignored.
S18 Sensors (DC-Voltage Series)
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