Calibrating the Sensor

The output is a square wave (50% duty cycle) with frequency (f_o) directly proportional to light intensity:

where

f_o is the output frequency
f_d is the output frequency for dark condition (when Ee = 0)
Re is the device responsivity for a given wavelength of light in kHz/(mW/cm²)
Ee is the incident irradiance in mW/cm².

f_d is an output frequency resulting from leakage currents. As shown in the equation above, this frequency represents a light-independent term in the total output frequency f_o. At very low light levels (dark colors), this dark frequency can be a significant portion of f_o. The dark frequency is also temperature dependent.

As f_o is directly proportional to frequency, it is possible to map between the frequency and RGB color value (0-255 for each of R, G and B) using linear interpolation.

Two points on the RGB line are well determined – pure Black (RGB 0, 0, 0) and pure White (255, 255, 255). The values returned by the sensor can be read using easily obtainable color swatches:

A black color card gives us the dark condition constant f_d. This is the origin (zero value) for the RGB straight line conversion.
A white color card gives us the extreme RGB point f_w, also known as white balance. Knowing f_d, this value can be used to scale all intermediate frequencies to a corresponding RGB value.

The proportional relationship is expressed by the standard straight line equation y = mx + b where

y is the reading obtained (in our case f_o)
x is the normalized RGB value
b is the value of y when x is 0 (in our case f_d)
m is the slope, or proportionality constant, of the line (in our case [f_w - f_d]/255.

The resulting equation is

or, rearranging to give us the desired RGB value

References

Yurish, Sergey Y 2005, ‘Intelligent Opto Sensors Interfacing Based on Universal Frequency-to-Digital Converter’, Sensors & Transducers Magazine (S&T e-Digest), Vol.56, Issue 6, June 2005, pp.326-334.

TAOS Inc 2003, ‘TCS230 Programmable Color Light-to-Frequency Converter’, Product Datasheet TAOS046 – February 2003.

Poynton, Charles 2005, ‘Sensing Color with the TAOS TCS230’, TAOS Inc., 2005-05-17.