Spectral Measurement Applications

Jeti GmbH, located in Jena, Germany, develops and produces spectroscopic equipment. Given the numerous benefits offered by USB devices, Jeti had decided to implement USB interfaces on their spectral measurement devices - specbos 1000 (spectrometer) and specbos 1100 (spectroradiometer). Using a Mitsubishi microprocessor, the Philips PDUSBD12 USB interface for communication over USB, and data acquisition software development tools from National Instruments - Lab Windows CVI, Jeti set out to create a USB spectrometer*(specbos 1000) and a USB spectroradiometer**(specbos 1100).

Jeti's spectrometers, specbos 1000, 2000, 3000 are used by vendors of various types of "illumination" devices such as television sets, car-displays, LCD displays, LED displays, traffic lights, lamps, that have to comply with certain standards, for example color, brightness and spectral dissipation. Jeti's spectroradiometer, specbos 1100, measures diffuse light sources, screens and other such devices.

Prior to implementing the USB standard, Jeti's spectrometers were based on the RS232 serial port communications standard. The motivation for implementing a USB interface were achieving true plug and play capabilities, fast and secure data transmission, and simple cabling. Having taken on the task of implementing USB, Jeti's engineers were unaware of the daunting task that lay ahead of them in the form of developing their applications and USB hardware access.

A General Description of the Products
Please refer to the block diagram below. The specbos 1000 (spectrometer) and specbos 1100 (spectroradiometer) operate as follows: Light from an application specific measurement head is guided through fiber to the spectrograph. The spectrograph is made up of the fiber-input, the grating and the photodiode-array. The grating diffracts the light into the different parts of the spectrum. The diffracted light is then imaged on to the photodiode-array. The photodiode-array's video signal shows the spectral distribution of the light in pixel format. This analog information is converted into digital values with a 16-bit analog to digital converter (ADC). The values from this converter are read by the micro-controller and calculations are performed (for example, pixel-wavelength relationships), so the spectra can be transferred to an application program that resides on a PC. In this application, further calculations can be done to determine color coordinates, radiation data of light sources (luminance, illuminance), width of film and many other properties, depending on the application.

The Development Process
Jeti developed their hardware for the specbos 1000 and specbos 1100 devices, using the Philips PDUSBD12 as the USB interface circuit for the devices. They then developed the firmware for their hardware and began to search for tools to develop the PC application software and USB access.

Jeti began development using Microsoft's DDK to develop the USB driver. However, when the project progressed slowly proving that interfacing to a USB-device was much more complicated than interfacing an RS232-driven measurement device, the chief engineer recommended commercial tools as an option. After doing market research, Jeti discovered WinDriver. WinDriver provided access to the USB-communication interface and enabled achievement of high-speed data transmission between the measurement device and the PC with the special application software.

Using the fully featured evaluation version of WinDriver, Jeti jump-started their development process, achieving an effective transmission speed of approximately 4 Mbit/sec, 40 times faster than the transmission speed achieved with the RS232 devices of 115,2 Kbit/sec. Typically, data transmission of 2 to 4 Kbytes is required. The minimum measurement time is approximately 1 ms, and bulk transfer is used for a secure transmission. (False data from transmission faults that may occur in isochronous mode would be a serious error.)

The application software is based on Lab Windows CVI data acquisition software development tools from National Instruments. Lab Windows CVI is a development tool for the programming of data acquisition software for measurement systems. The WinDriver functions (from Jungo's windrvr.sys) for USB data communication between the PC and specbos device are called from Jeti's software, which was developed by using Lab Windows CVI. This software provides a virtual measurement instrument on the PC with buttons for measurement functions, different diagrams or numeric displays for displaying the results of the measurements - for example a spectrum of the measured light source, color coordinates, integral intensity, luminance, illuminance etc. The software is able to interface to spreadsheets (e.g. Excel) for documentation or to store the results on hard disk.