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  • Letters To The Editor

Photonics Spectra
Feb 2012
Changes in Optical Radiation Instrumentation

I have been involved in the light or optical radiation measurement industry for more than four decades. In the early years, a major strength was that we continually provided new, often unique, reliable instruments along with the assurance of proper stable calibration, based on internal hardware. We also supplied calibrated incandescent sources, with and without special filters, allowing the user to maintain calibration accuracy using an external standard.

The calibration of a photometer, radiometer or colorimeter is fundamentally the comparison of the output of the instrument system to the characteristics of a known standard, be it a source or similar instrument that exhibits a certified characteristic, and then adjusting the instrument to read the known parameter(s) correctly.

For example, if a calibrated source with an output of 1000 lx is measured by an illuminance meter, the meter should accurately read 1000 lx. If the instrument doesn’t read the correct value, it must be adjusted or calibrated. A variety of methods can accomplish this.

The values of internal variable resistors, or values stored in firmware or software, can be changed, or perhaps some other type of mechanical adjustment can be made. Although this is an obvious oversimplification of the calibration process – and does not consider all of the precision issues such as amplifier zero, dark current offsets, linearity errors, dynamic range tracking and major geometry variables – the concept applies to all such instruments and systems. As for these inherent precision errors, instrument manufacturers have always tried to limit or reduce them and, with varying success, have done a good job.

However, readings that are accurate under one set of conditions are not guaranteed to be accurate when conditions change. Many of the above-listed error sources, unless fully accounted for, can lead to highly inaccurate measurements. Users must be able to verify the precision of their instrument or system either on their own or by consulting a qualified and competent third party.

So much depends upon the accuracy a manufacturer has built into an instrument; I see no reason why this type of calibration or verification information should not be made available to the consumer/end user, along with a disclaimer cautioning against possible problems.

In the past, most manufacturers routinely included service instructions in operator manuals. But today’s consumer will be hard-pressed to find such factory support documentation or software that allows calibration or verification by anyone other than the manufacturer, despite increasing instrument and configuration complexity. Not allowing the customer (or a technically qualified third party) access to calibration resources has great potential drawbacks for the end user.

It limits the customer to only one source for service or calibration, which in turn dictates the cost, turnaround and calibration cycle time. This greatly limits flexibility for the customer, especially if the manufacturer encounters difficulties, if the user wishes to calibrate or verify calibration against internal standards, or if the customer wants to employ another facility.

Furthermore, fundamental calibration methods are not always adequate for special applications – for example, ultraviolet radiation measurements.

Imagine the problems that might exist if companies like Hewlett-Packard, Tektronix, Keithley or Fluke required that their test instruments be returned only to them for service and calibration. Or what if an automotive manufacturer did the same? What if car dealers didn’t have the necessary documentation to allow themselves and others to reliably service their vehicles?

These consumer companies were insightful enough to provide resources that allow a choice as to where we might have our instruments repaired or calibrated, and that allow free-market competition to perform the best possible repairs or calibrations at reasonable costs and delivery dates. Of course, the contrast in volume between test instruments and vehicles, along with other differences, is so significant that there is no comparison. However, if you’re a customer with a production line down for an extended period of time because you can’t quickly get your instruments returned, that logic will most assuredly not soothe upper management, your customers, or, in some cases, your investors.

If you are considering the purchase of a new instrument or system for optical radiation measurements, give careful attention to these issues. It may also be prudent to contact the manufacturer and explore whether arrangements can be made to provide service and calibration information.

Bob Ruff
President, Opto-Cal
Lakeside, Calif.


Two photo captions in the January 2012 Photonics Spectra article “Surprising Surges, Cautious Expectations, Positive Outlooks” were swapped. The photo on p. 64, courtesy of Jenoptik, depicts diode lasers at the electroplating stage; the photo on p. 66 features Sydor Optics products and was taken by Laura S. Marshall.

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