Problem solving for Banner's QCM series of color sensors

1. Discrete photodiode with color filter

The traditional way to sense color is to combine three or four photodiodes on a single chip, with red, green, and blue color filters placed on the surface of the photodiodes (usually two blue color filters are combined together to compensate for the low sensitivity of the silicon wafer to blue light). Independent transimpedance amplifiers feed the output of each photodiode into an A/D converter with a typical resolution of 8 to 12 bits. The output of the A/D converter is then fed into a microcontroller or other type of digital processor.

The main advantage of this approach is flexibility, as the gain and bandwidth of the amplifier, as well as the speed and resolution of the A/D converter, can be adapted to the requirements of the specific application, allowing the design to be adjusted to achieve a performance and cost trade-off. This flexibility comes at the cost of increased design complexity and a demanding board layout for analog circuits. The main applications of this solution include high-speed process inspection in industrial control where short response times are required, or applications where variable lighting conditions require arbitrary adjustment of gain and speed.

2. Integrated optical-voltage converter

Another approach is to combine a photodiode, color filter, and transimpedance amplifier for a single color spectrum band on a single chip. As with the discrete implementation, the outputs of the three components are fed into an external three-channel A/D converter, which is then digitized. Texas Advanced Optoelectronic Solutions (TAOS) offers TSLR257, TSLG257, and TSLB257 (see Figure 1) as examples of these components.

This approach requires fewer components than discrete photodiodes, and because the noise-sensitive analog circuitry sits on top of the chip, it compresses the board footprint, reduces installation costs, and simplifies design and board layout. The disadvantage is that the gain and sensitivity of the sensor cannot be changed dynamically. Examples of applications for this approach include systems with well-defined lighting conditions, spatial constraints, sensitivity requirements, or systems with high requirements for time-to-market or design cycles.

3. Integrated optical-frequency converter

The third method is to convert the light intensity directly into a pulse train whose frequency is proportional to the intensity of the red, green, and blue light components of each red, green, and blue channel, respectively. A direct interface to a microprocessor or another digital processor eliminates the need for an additional A/D converter. TAOS' TCS230 is an example of such a device. It divides the red, green, and blue sensor-filter combination (and an additional "clean" sensor with no filter) into a grid, thus spreading the elements across the sensing area, eliminating the need for a light diffuser. Connecting each color of photodiode in parallel will eventually balance out any uneven illuminance.

Problem solving for Banner's QCM series of color sensors

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