Machine-vision applications often rely on color to determine a go/no-go condition and medical, chemical, and other equipment may require quantitative color measurements. A small color sensor device--the OPB780--from Optek Technology (www.optekinc.com) might do the trick when a full-blown color camera breaks your design budget.

Developers can use the Color Sensor Evaluation Kit (OPB780KIT) to test a sensor and see how it might handle their applications. The kit includes a mounted sensor, a USB-based controller board, a USB cable, and a USB memory stick with USB drivers and an application program.  After getting past a simple USB comm-port problem, the kit worked well and I recommend it to anyone who must measure color and wants to experiment with an inexpensive way to solve the problem. The kit sells for $US 125.00.

This photo of the underside of the test fixture shows the cavity that houses the sensor integrated circuit and the white LED.

The OPB780 reflective color sensor converts light intensity to a square-wave frequency signal for each of three colors, red, green, and blue, and for white light. Think of it as a light-intensity-to-frequency converter.  The sensor uses groups of 16 photodetectors placed behind a filter for each color and a group of 16 photodetectors with no filter (clear) to detect white light. You can use a white LED to illuminate the surface of the material under test. The kit comes with a white LED mounted on a test fixture, but you would have to include your own white LED in a design.

Two digital inputs to the sensor select one color--red, green, blue, or clear--at a time.  After selecting a color setting, a microcontroller or other device measures the frequency output for the associated color.  Thus, if you need to measure only green and blue color concentrations, you change the logic state on the sensor's S2 input and measure green (S2 = high) or blue (S2 = low).  Input S3 is set to a logic high. The frequency output will drive a standard TTL or CMOS input over a short distance.  The sensor in the kit uses a 9.5 inch ( 24 cm) cable between the sensor and the USB interface board.

The kit comes with a quick-start guide that describes how to load software from the USB stick.  Find a complete 19-page kit data sheet at:  http://www.optekinc.com/datasheets/opb780kit.pdf.  This data sheet includes a complete schematic diagram and code listings for the Zilog Z8 MCU on the small controller board.

The USB drivers and the application program loaded successfully, but the application could not find the USB virtual serial port set on my lab PC: COM17.  An email to Optek led to a call from Trevor Schelp, a senior product engineer at Optek.  He explained that because the application software expects to "see" a serial port from COM1 to COM16, it wouldn't respond to my USB COM17 port.  No problem, said Schelp. Instead of using Optek's application, start Hyper Terminal, set the serial port to COM17, 9600 bits/sec, 8 bits, no parity, and 1 stop bit. Press the A key followed by Enter, and the eval board communicates with the PC and displays red, green, blue, and clear values on the screen.  That did the trick.

The sensor measures the red area on a Netflix free-trial card.

Commands let users measure individual color intensities, all color intensities, set the sensor integration time and enable/disable the white LED in the test fixture.  I tested white copier paper, a blue-green ESD mat, a red Netflix "free offer" card, a yellow book cover, and a sheet of blue paper.  The unfiltered white sensors overflowed on the white paper even when I cut the integration time from 200 to 1 msec. But in a real application you also could control the intensity of the light source.

The Hyper terminal display screen shows the commands you can use and the results from several color measurements.

I welcome your comments about experiences measuring color in an application. 
--Jon Titus