10.5.1 Sound and light formulas
The measurement of sound and light is important as it relates to the sense of hearing and sight, as well as many industrial applications such as the use of sound waves for the detection of flaws in solids and in location and linear dis- tance measurement. Sound pressure waves can induce mechanical vibration and hence failure. Excessive sound levels produce noise pollution. Light and its measurement is used in many industrial applications for high-accuracy linear measurements, location of overheating (infrared), object location and position measurements, photo processing, scanning, readers (bar codes), and so forth.
Sound waves are pressure waves that travel through air, gas, solids, and liquids, but cannot travel through space or a vacuum unlike radio (electromagnetic) waves. Pressure waves can have frequencies up to about 50 kHz. Sound or sonic waves start at 16 Hz and go up to 20 kHz; above 30 kHz sound waves are ultrasonic. Sound waves travel through air at about 340 m/s (depends on temperature, pressure, and the like.). The amplitude or loudness of sound is measured in phons.
Sound pressure levels (SPL) are units often used in the measurement of sound levels and are defined as the difference in pressure between the maximum pres- sure at a point and the average pressure at that point. The units of pressure are normally expressed as follows:
1 dyn/cm2=1 ubar =1.45 ×10−5psi (10.8) where 1 N =105dyn.
Decibel (dB) is a logarithmic measure used to measure and compare ampli- tudes and power levels in electrical units, sound, light, and the like. The sen- sitivity of the ears and eyes are logarithmic. To compare different sound intensities the following applies:
Sound level ratio in dB = (10.9)
where I1 and I2 are the sound intensities at two different locations and are scalar units. A reference level (for I2) is 10−
16
W/cm2(average level of sound that can be detected by the human ear at 1 kHz) to measure sound levels.
When comparing different pressure levels the following is used:
Pressure level ratio in dB = (10.10)
where P1 and P2are the pressures at two different locations (note pressure is a measure of sound power, hence 20 log). For P2, 20 µN/m2 is accepted as the average pressure level of sound that can be detected by the human ear at 1 kHz and is therefore, the reference level for measuring sound pressures.
Typical figures for SPL are as follows: Threshold of pain 140 to 150 dB
Rocket engines 170 to 180 dB
Factory 80 to 100 dB
Light is ultra-high frequency electromagnetic wave that travels at 3 ×108m/s. Light amplitude is measured in foot-candles (fc) or lux (lx). The wavelength of visible light is from 4 to 7 ×10−7m. Longer wavelengths of electromag- netic waves are termed infrared and shorter wavelengths, ultraviolet. Light wavelengths are sometimes expressed in terms of angstroms (Å) where 1 Å = 1 ×10−10m. 20 10 1 2 log P P 10 10 1 2 log I I
Example 10.6 What is the wavelength of light in Å, if the wavelength in meters is 500 nm?
1 Å =10−10m
500 nm =500 ×10−9/10−10Å =5000 Å
Intensity is the brightness of light. The unit of measurement of light inten- sity in the English system is the foot-candle (fc), which is one lumen per square foot (lm/ft2). In the SI system the unit is the lux (lx) which is one lumen per square meter (lm/m2). The phot (ph) is also used and is defined as one lumen per square centimeter (lm/cm2). The lumen replaces the candela (cd) in the SI system. The dB is also used for the comparison of light intensity as follows:
Light level ratio in dB = (10.11)
where Φ1and Φ2 are the light intensity at two different points.
The change in intensity levels for both sound and light from a source is given by the following equation:
Change in levels = (10.12)
where d1and d2are the distances from the source to the points being considered.
Example 10.7 Two points are 65 and 84 ft from a light bulb. What is the difference in the light intensity at the two points?
Difference =
X-rays should be mentioned at this point as they are used in the process con- trol industry and are also electromagnetic waves. X-rays are used primarily as inspection tools; the rays can be sensed by some light-sensing cells and can be very hazardous if proper precautions are not taken.
10.5.2 Sound and light measuring devices
Microphones are transducers used to convert sound levels into electrical signals, i.e., electromagnetic, capacitance, ribbon, crystal, carbon, and piezoelectric microphones can be used. The electrical signals can then be analyzed in a spec- trum analyzer for the various frequencies contained in the sounds or just to measure amplitude.
Sound level meter is the term given to any of the variety of meters for meas- uring and analyzing sounds.
Photocells are used for the detection and conversion of light intensity into elec- trical signals. Photocells can be classified as photovoltaic, photoconductive, pho- toemissive, and semiconductor.
10 65 84 1 11 log10 dB = − . 10 log10 1 2 d d 10 log10 1 2 Φ Φ
Photovoltaic cells develop an emf in the presence of light. Copper oxide and selenium are examples of photovoltaic materials. A microammeter calibrated in lux (lm/m2) is connected across the cells and measures the current output.
Photoconductive devices change their resistance with light intensity. Such materials are selenium, zirconium oxide, aluminum oxide, and cadmium sulfide. Photoemissive materials, such as mixtures of rare earth elements (cesium oxide), liberate electrons in the presence of light.
Semiconductors are photosensitive and are commercially available as photodi- odes and phototransistors. Light generates hole-electron pairs, which causes leak- age in reversed biased diodes and base current in phototransistors. Commercial high-resolution optical sensors are available with the electronics integrated onto a single die to give temperature compensation and a linear voltage output with inci- dent light intensity are also commercially available. Such a device is the TSL 250. Also commercially available are infrared (IR) light-to-voltage converters (TSL 260) and light-to-frequency converters (TSL 230). Note, the TSL family is manufactured by Texas Instruments.
10.5.3 Light sources
Incandescent light is produced by electrically heating a resistive filament or the burning of certain combustible materials. A large portion of the energy emitted is in the infrared spectrum as well as the visible spectrum.
Atomic-type sources cover gas discharge devices such as neon and fluorescent lights.
Laser emissions are obtained by excitation of the atoms of certain elements. Semiconductor diodes (LED) are the most common commercially available light sources used in industry. When forward biased, the diodes emit light in the visible or IR region. Certain semiconductor diodes emit a narrow band of wave- length of visible rays; the color is determined by material and doping. A list of LEDs and their color is given in Table 10.1.
10.5.4 Sound and light application considerations
Selection of sensors for the measurement of sound and light intensity will depend on the application. In instrumentation a uniform sensitivity over a wide frequency range requires low inherent noise levels, consistent sensitivity
TABLE 10.1 LED Characteristics
Material Dopant Wavelength (nm) Color
GaAs Zn 900 IR GaP Zn 700 Red GaAsP N 632 Orange GaAsP N 589 Yellow GaP N 570 Green SiC — 490 Blue
with life, and a means of screening out unwanted noise and light from other sources.
In some applications, such as the sensing of an optical disc, it is only nec- essary to detect absence or presence of a signal, which enables the use of cheap and simple sensors. For light detection, the phototransistor is being used very widely, because of the ability with integrated circuits to put temperature correction and amplification in the same package for high-sensitivity. The device is cost effective and has good longevity.
Figure 10.7 shows the schematic symbols used for optoelectronic sensors and Table 10.2 gives a comparison of photosensor characteristics.
Figure 10.7 Schematic symbols for opto sensors.
TABLE 10.2 Summary of Opto Sensor Characteristics
Response Response
Type Device (µm) Time Advantages Disadvantages
Photo- Photo- 0.6–0.9 100 ms Small, Slow, hystersis, conductive resistor high-sensitivity, limited temperature
CdSlow cost
Photo- 0.6–0.9 10 ms Small, Slow,
resistor high-sensitivity, hystersis,
CdSe low cost limited temperature
Semi- Photo- 0.4–0.9 1 ns Very fast, Low-level
conductor diode good linearity, output
low noise
Phototransistor 0.25–1.1 1 µs Low frequency response, nonlinear Photo- Solar cell 0.35–0.75 20 µs Linear, Slow, low-level
Summary
A number of different types of sensors were discussed in this chapter. Sensors for measuring position, speed, and acceleration were introduced. The concepts of force, torque, and load measurements were discussed together with measur- ing devices. Also covered in this chapter were smoke and chemical sensors and an introduction to sound and light measurements and instruments.
The salient points covered are as follows:
1. The basic terms and standards used in linear and rotational measurements and the sensors used for the measurement of absolute and incremental posi- tion, velocity, and acceleration
2. Optical and magnetic sensors and their use as position measuring devices in linear as well as rotational applications and sensors used for distance meas- urement
3. Definitions of force, torque, couples, and load and the use of mechanical forces in weight measurements
4. Stress in materials, the use of strain gauges for its measurement, and instru- ments used for measurement
5. Smoke and chemical sensors were introduced with sensors and applications 6. An introduction to sound and light units and the units used in their meas-
urement with examples
7. Sound and optical sensing devices are given and the type of semiconductor devices used for light generation with their color spectrum
Problems
10.1 What force is necessary to accelerate a mass of 17 lb at 21 ft/s2?