Delta CS64A - Manual (Page 13)

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6. Threshold Setting
To differentiate between colors, set up thresholds on the Red/NIR, Green/NIR, and
Blue/NIR ratios.
If a sample is different from every other sample in one of the ratios, a set of thresholds
(upper and lower) on that ratio can sort that color from all of the others.
Since the Red and NIR colors are the strongest, it is often advantageous to use the
Red/NIR to sort as many of the samples as possible, setting secondary thresholds on the
Blue/NIR and Green/NIR ratios as needed to sort the remaining samples. You may not
need to use all of the ratios, often one or two will be enough for a given set of samples.
The larger the ratio difference between samples, the better the system will work. The
minimum difference required for reliable operation depends on the variability of the
colors, the presentation, and the ambient conditions. The presentation of the samples
includes variations in the distance from the CS64A to the samples, the angle of the
samples, etc. Ambient conditions include extraneous light, temperature, etc.
Generally a ratio difference of 10% or more will provide good result. If there is a ratio
difference in more than one ratio, the differences can be combined before checking
against thresholds.
As explained in the previous section, the amplitudes typically cannot be used directly
since they vary with distance, dirt, etc.
Sometimes, however, it is necessary to use thresholds on the NIR amplitude to
differentiate between shades of the same color. If the NIR amplitude is used:
1. The sensor to target distance must be constant,
2. The sensor window must be kept clean and/or
3. There must be sufficient variation in the NIR amplitude.
For very clean and controlled conditions, a 10% difference in the NIR amplitude may be
adequate, while more difference will allow for more distance variation and more dirt
buildup on the sensor window.
Of course, in your program, you will want to make the thresholds variables so they can
be adjusted if required.
7. Characteristics
7.1 Temperature
The feedback circuitry automatically compensates for the normal LED intensity variation
with temperature yielding a typical stability of 0.1%/C for the Red/NIR, Blue/NIR, and
Green/NIR ratios (0.3%/C maximum). In the most critical applications, the CS64A
should be maintained at a constant temperature since the LED color varies slightly with
temperature.