This new lowcost radiant flux meter system gives direct radiometric measurements in the infrared visible and ultraviolet regions of the spectrum It zeros and calibrates itself too

By Charles L. Hicks and Michael R. Mellon

Optical measurement is one of the oldest and most fundamental areas of science, yet it's still one of the least mature. Optical energy is generally considered to be the portion of the electromagnetic spectrum between microwave and x-ray frequencies.* Measurements in this region are a separate area of science because optical sources, detectors, and techniques differ markedly from those used in the microwave or x-ray regions of the spectrum. Within the optical region lie the visible wavelengths, those to which the human eye is sensitive.

The need for reliable optical measurements is greater today than ever before, and is felt in such diverse areas as laser development and applications, communication systems, air pollution studies, medicine and astronomy.

Some optical measurements can be made with a high degree of accuracy. Velocity and wavelength are two that can. On the other hand, techniques for measuring the absolute intensity of optical radiation have lagged behind corresponding methods at lower frequencies. This is partly due to the importance of the lower frequencies in radar, missile guidance, and communications, and the consequent emphasis placed on research in these areas during the last three decades. But it's also partly because of the fundamental nature of optical radiation. Everything radiates optical energy: the earth radiates to space, the human body radiates to the environment, every object in a detector's field of view radiates, and the detector itself radiates optical energy to its environment. What's more, major complications are introduced by the geometry of the optical system: factors such as the detector's field of view and the source's size, shape, and distance from the detector must be taken into account. It's easy to see that experimental technique is a critical

factor in optical measurements.

An optical researcher who builds his own power measurement system must solve numerous problems. He must first decide whether he wants the readout to be in photometric or radiometric units (see box, page 15). Next a detector must be chosen, its job being to convert optical power to a measurable voltage or current. There are quantum detectors such as photomultipliers, photodiodes, and phototransistors, and there are thermal detectors such as thermistors, thermopiles, and pyro-electric detectors. Quantum detectors are generally quite fast and have high sensitivity, but their sensitivity depends on the wavelength of the optical radiation. Thermal detectors in general are relatively slow and less sensitive but have the virtue of uniform sensitivity over wide portions of the spectrum (millions of gigahertz in some cases!).

Once the detector is chosen a voltmeter or ammeter to measure its output must be selected. It must be compatible with the detector or a special interface must be built. Finally, the researcher must figure out how to keep his system calibrated and what the worst-case error is likely to be, and then document what he has done so others can use the system.

A Better Way

It's now possible to avoid these selection and interfacing problems, yet have an optical power measurement system that satisfies most requirements for speed, sensitivity, and flat spectral response, automatically zeros and calibrates itself, and is accurate within ±5%. A unique new thin-film thermopile detector, used in the HP Model 8334A Detector, and a precision nanovolt-

Flux Meter Schematic

Fig. 1. Easy-to-use, low-cost HP 8330A/8334A Radiant Flux Meter System measures optical power in the ultraviolet, visible, and infrared regions of the electromagnetic spectrum. It reads directly in absolute radiometric units without spectral calibration curves. Maximum absolute uncertainty is less than ±5% of full scale.

Fig. 1. Easy-to-use, low-cost HP 8330A/8334A Radiant Flux Meter System measures optical power in the ultraviolet, visible, and infrared regions of the electromagnetic spectrum. It reads directly in absolute radiometric units without spectral calibration curves. Maximum absolute uncertainty is less than ±5% of full scale.

meter, the HP 8330A Radiant Flux Meter, give unambiguous readings of irradiance (power per unit area) from the vacuum ultraviolet region of the spectrum to the infrared.

The system (Fig. 1) has ten overlapping irradiance ranges, from 3 ^W/cm2 to 100 mW/cm2 full scale, suitable for measuring power output from a wide variety of sources such as lasers, gas-discharge devices, incandescent lamps, cathode-ray tubes, light-emitting diodes, infrared sources, and blackbody radiators. No charts or calibration curves are needed because the standard system's spectral response is uniform within ±3% over a wavelength range of 0.3 urn to 3.0 /im.* This range can be extended to less than 0.2 ^m and more than 15 ^m using different optical window materials in the detector. With the automatic zero feature, readings can be compensated for background radiation up to ±100 /tW/cm2. This allows operation under normal laboratory light conditions. The automatic calibration feature gives the user confidence in his measurements; it assures that overall system accuracy, including meter and detector, is within ±5%.

Fast, Broadband Thin-Film Thermopile

The key to the performance of the new optical power measurement system is the HP-developed thin-film thermopile detector. It converts optical power—ultra* 1 Am = 10-' meter = 1 micron = 10,000 A

violet or infrared or anything in between—to a dc voltage directly proportional to the power. The thin-film construction minimizes the detector's thermal and inertial mass, thereby giving it fast response and high immunity to mechanical shock. It also allows small geometry, so small that 64 individual thermocouples fit in an area only 0.43 centimeter square. Fig. 2 is a photograph of the detector.

Thermopile construction begins with a sheet of aluminum foil approximately 0.004 inch thick. It's first anodized on one side and then chemically etched on the other to produce an 8 X 8 array of 64 square windows— areas where the aluminum has been removed to leave only a thin (750A) transparent layer of aluminum oxide. Next, antimony and bismuth, the thermocouple materials, are deposited on the anodized side in overlapping patterns (see Fig. 3). The patterns are such that one antimony-bismuth junction is over a window area, the next is over the thicker aluminum-foil substrate, or nonwindow area, the next is again over a window, and so on. The junctions over the solid non-window areas are the 'cold' or reference junctions. The aluminum-foil substrate is thick compared to the oxide, antimony, and bismuth layers, so the substrate acts as a heat sink and tends to hold the junctions over it at a uniform temperature near ambient. The junctions over the thin window areas are the 'hot' junctions. To make them hot, a black optical absorber is deposited over the window areas.

8330a Radiant Flux Meter
Fig. 2. Unique, HP-designed thin-film thermopile detector has fast five-millisecond response, low drift, and high immunity to mechanical shock.

Reference (Cold) Junction

/ Measuring L(Hot) Junction

Optical Absorber (Gold-Black)

Reference (Cold) Junction

/ Measuring L(Hot) Junction

Optical Absorber (Gold-Black)

I- Aluminum Substrate Heat Sink

Reference' (Cold) Junction

Fig. 3. Thin-film thermopile consists of 64 antimony-bismuth thermocouples connected in series.

I- Aluminum Substrate Heat Sink

Reference' (Cold) Junction

Fig. 3. Thin-film thermopile consists of 64 antimony-bismuth thermocouples connected in series.

Non-window areas remain reflective. Each hot-cold junction pair constitutes a thermocouple, so the complete thermopile has 64 thermocouples. All are connected in series.

When the thermopile is exposed to optical radiation the black junctions absorb energy and their temperature increases while the shiny, heat-sunk junctions reflect energy and remain near ambient temperature. This temperature difference results in a thermoelectric voltage. For antimony-bismuth couples, this voltage is about 100 //,V/°C. Since the thermopile has 64 couples connected in series it has an overall sensitivity close to 6.4 mV/°C.

The black optical absorber is gold-black,* chosen primarily because of its high ratio of absorbence to mass, and because its absorbence is constant from the vacuum ultraviolet to the far infrared. Thanks to its

* Gold-black is pure gold evaporated so that it forms an extremely rough surface, so rough that it appears black because it absorbs nearly all incoming optical radiation.

low mass, the detector responds in less than five milliseconds to a change in optical power, much faster than other thermal detectors, which often take several seconds to respond. And because of the constant absorbence of gold-black, the detector's response is limited primarily by the optical window placed in front of it. The window isn't necessary, but it reduces noise caused by air turbulence and prolongs the life of the detector by keeping out dust and chemically corrosive atmospheres. Fig. 4 shows the detector's response to different wavelengths with several types of windows.

At very long wavelengths in the infrared (greater than 40 /<m) the thermopile's absorbing efficiency drops because some of the radiation is reflected. To minimize this effect the thermopile is mounted at the focus of a gold-plated hemispherical dome which re-reflects to the thermopile much of the energy that's reflected from it. The thermopile is sealed in the dome and the entire assembly is placed in contact with a massive aluminum block for temperature stabilization. To minimize the effects of handling and rapid ambient-temperature fluctuations, the assembly is isolated from the impact-resistant plastic case.

A field-stop aperture in the detector assembly restricts the detector's field of view to a solid angle of 0.1 steradian.** This makes it easy to convert the system's irradiance readings to radiance units (W/cm2/sr); you just multiply by 0.1. Under appropriate conditions, readings can also be converted to radiant flux, which has units of watts. Irradiance, radiance, and radiant flux are radiometric units typically used for measuring optical power from point sources, wide-area sources, and beams, respectively (see page 15).

On the front of the detector housing is a removable bezel with 30 mm camera threads for mounting adapters to hold lenses, filters, or shutters. Behind the bezel is a %-in diameter cavity which holds filters at the same temperature as the thermopile to minimize self-emission in the infrared region.

A Sensitive Nanovoltmeter

The successful application of this sensitive detector required that the input amplifier of the 8330A Radiant Flux Meter be an ultra-stable dc amplifier capable of making reliable measurements in the nanovolt range, f

The amplifier is a synchronous design (see Fig. 5) which uses a precision electromechanical chopper to convert the dc output voltage from the detector to a

** 0.1 steradian is equivalent to 10.5° linear half-angle from the optical axis, t 1 nanovolt = 10-' volt = 0.000000001 volt.

With Infrasil* Fused Quartz Window (standard)

Visible

Response of Windowless HP 8334A Detector

With Thallium Bromoiodide (KRS-5)t Window

"Amersil, Inc. tHarshaw Chemical Co.

Microwaves-

Response of Human Eye

1.0 10 100 Wavelength (um)

1000

3,000,000 300,000 30,000 3,000 Frequency (GHz)

107-Hz ac voltage with an amplitude proportional to the dc voltage. Low-level ac voltages are much easier to amplify than dc voltages, since dc drift can be eliminated in ac amplifiers.

After the dc voltage is converted to ac it's amplified by narrow-band amplifiers which reject noise and power-line-related interference. Once amplified, the voltage is converted back to dc and displayed on the meter. There's an optimum ratio between the amounts of ac and dc amplification in such a system. If there's too much ac amplification the system will tend to saturate on line-related interference. If there's too much dc amplification, drift increases. In the final design temperature-induced drift referred to the input is typically less than one nano-volt per °C and line-related interference isn't a problem. Dynamic range is approximately 100 dB.

Noise was a primary consideration in the design of the amplifier system, since it's the overall noise level that determines the smallest amount of optical power that can be measured. For this reason the first amplifier stage (after the chopper and an input transformer) is a special low-noise junction-FET amplifier. Referred to the primary of the input transformer, that is, to the point where the detector output enters the meter, the noise attributable to the entire 8330A meter is less than the thermal noise in the detector. Thus the system's sensitivity is limited more by the thermal-noise characteristics of the detector than by the amplifier.

Automatic Zeroing

It's convenient to be able to use an optical power

Fig. 4. Flat response of detector allows accurate broadband measurements. Spectral range varies with transmission characteristics of window mounted in front of thermopile.

meter in normal laboratory light as well as in dark rooms. It should therefore have a zero-suppression capability so the user can compensate for background or ambient radiation and measure only the source rather than the source plus the background. Instead of the usual manually operated zero control, the 8330A has an automatic pushbutton meter-zeroing system. Pressing a single switch zeros the system with electronic speed and accuracy, and the user doesn't have to take his eyes off his experiment.

The zeroing circuit (see Fig. 5) consists essentially of a comparator amplifier and a long-term analog memory connected in a negative-feedback loop. When the front-panel MODE switch is moved to ZERO, the voltage across the meter is electronically compared to a zero-voltage reference. If there's an offset the difference voltage is amplified and channeled through a MOSFET source follower back to the input, where it nulls out the offset. The circuit will reduce any offset up to ±300 times the lowest meter range to less than 2% of the lowest range in less than two seconds.

After zeroing, the user returns the MODE switch to its OPERATE position. This causes the output of the comparator amplifier to be disconnected from the analog memory, which is a polystyrene capacitor connected to the gate terminal of the MOSFET. The voltage that was required to zero the instrument then remains on the capacitor because the only discharge paths are via surface leakage and through the insulated gate of the MOSFET, both very high impedances. The discharge time constant

Multitudinous Applications

Versatility and performance make the HP 8330A/8334A

System useful in diverse ways in optics, process control, analytical science, and other fields. Here are some typical uses.

Electro-optical Measurements

■ radiant power from optical sources such as lasers,* monochromators, gas-discharge devices, incandescent lamps, CRT's, LED's, infrared sources, blackbody radiators, ultraviolet sources

■ analysis of optical input/output and memory devices for computers

■ polarization studies (with polarizing filter)

■ direct comparison of emissions at different wavelengths from continuous or discrete sources

■ precision calibration of other optical detectors over broad spectral regions

■ photographic and holographic exposure levels

■ transmission and reflection characteristics of filters, lenses, mirrors, optical coatings, thin-films, liquids, and gases

■ wideband output leveling of sources and monochromators

■ determination of spectral outputs of sources (with tunable optical filter or monochromator)

■ infrared research, development, and production

■ educational demonstrations.

Process Control and Analytical Science

■ Remote, non-contacting temperature measurement of physical objects using infrared radiation (useful for moving, liquid, or semi-plastic objects and for inaccessible, radioactive, or corrosive environments)

watts/cm2 = e <r(T4 — T04), where e = emissivity of surface of emitter a = Stefan-Boltzmann constant = 5.67 x 10"10 watts/cm2/K4 T = unknown temperature in K of emitter To = temperature in K of detector (normally ambient temperature)

■ infrared mapping of temperature

■ rapid detection of small temperature differentials

■ ambient illumination measurements

■ ambient sunlight level measurements for agricultural and photochemical air pollution studies

■ photobiological studies of plant growth and photosynthesis

■ wideband optical detection system for optical spectroscopy, useful for ultraviolet, visible, and infrared spectroscopy at discrete wavelengths with narrowband filters

■ color control and analysis (tri-stimulus)

control of ultraviolet-activated chemical processes such as photoetching of printed circuits, microcircuits and chemical milling processes monitoring laser power in laser micro-machining applications.

* Laser beams should be attenuated and/or diffused for best results.

of the analog memory is extremely long—typically several weeks. To preserve this long time constant the memory circuit is thoroughly cleaned and encapsulated in a silicone compound to keep out moisture.

Automatic Calibration

Among the significant contributions of the 8330A/ 8334A system is its built-in self-calibrating feature. Instead of requiring the user to make screwdriver adjustments while measuring an external optical standard, the new system has a completely self-contained precision electronic calibrator. It's made possible by the fact that an ac voltage superimposed across the output terminals of the thermopile detector will dissipate power in the thermopile and cause a temperature rise, and this in turn will cause a thermoelectric voltage to be generated. Thus it's possible to substitute lower-frequency ac power for optical power when calibrating the system. In the 8330A a precision ac calibration voltage is derived from an electronic oscillator which operates at a frequency of 10 kHz.

Calibration is done automatically by a feedback technique similar to that used for automatic zeroing. The same front-panel MODE switch that's used for zeroing the instrument also has a CALIBRATE position. When the switch is placed in this position several things happen. First, regardless of the range setting, the instrument internally switches itself to the 3 mW/cm2 range. At the same time, the precision internal electronic calibrator is connected across the thermopile detector, causing a minute temperature rise and a corresponding thermoelectric output voltage. The meter voltage is then electronically compared to 1.000 volt (full scale). If the gain of the system is properly adjusted the calibrator power will cause a detector output sufficient to give a full-scale reading on the 8330A meter. If it doesn't, the output comparator amplifier will sense a difference or error voltage. The comparator will amplify the difference and feed a correcting voltage through an analog memory circuit to the gate of a junction FET, which is used as a variable resistor in the feedback loop of one of the ac amplifiers. If the detector sensitivity is too high the correcting voltage will reduce the gain of the system to compensate for the high sensitivity. If the sensitivity is too low it will raise the gain.

This process is entirely controlled by the single frontpanel switch and takes about one second. The MODE switch is then returned to the OPERATE position. This disengages the calibrator, returns the instrument to the range indicated on the front panel and disconnects the

Optical / Radiation

Optical / Radiation

Optical Radiation Sensor Circuit

Comparator Servo Amplifier

Comparator Servo Amplifier

Fig. 5. Sensitive, low-noise input amplifier doesn't degrade detector characteristics. Automatic zero system suppresses background radiation. Automatic calibration system eliminates time-consuming calibration against external standards and increases user confidence.

comparator's output from the memory circuit. Again, however, the voltage that was required to calibrate the

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  • Tobold
    Who calibrates uv meters?
    10 years ago