DOCSLIB.ORG

Practical Guide to the International System of Units (SI)

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Practical Guide to the International System of Units (SI) Practical Guide to the International System of Units (SI) Table 1 Prefixes Table 2 SI Units Symbol Prefix Factor Symbol Unit Name Quantity (general symbol ) Definition 24 Y yotta 10 General Purpose 21 Z zetta 10 m meter length ( l) BASE UNIT 18 E exa 10 kg kilogram mass ( m) BASE UNIT 15 P peta 10 s second time ( t) BASE UNIT 12 T tera 10 K kelvin temperature ( T) BASE UNIT G giga 10 9 m² square meter area ( A) m² M mega 10 6 m³ cubic meter volume ( V) m³ k kilo 10 3 N newton force ( F) kg m/s² h hecto 10 2 J joule energy ( E) Nm * da deka 10 1 W watt power ( P) J/s 10 0 = 1 Pa pascal pressure ( p) N/m² d deci 10 –1 Hz hertz frequency 1/s * c centi 10 –2 Electromagnetic C coulomb charge ( Q) As ≈ 6.24 × 10 18 e m milli 10 –3 A ampere current (I) BASE UNIT = C/s µ micro 10 –6 V volt voltage ( V) J/C = W/A n nano 10 –9 Ω ohm resistance ( R) V/A –12 p pico 10 S siemens conductance ( G) 1/Ω = A/V –15 f femto 10 F farad capacitance ( C) C/V –18 a atto 10 Wb weber magnetic flux Vs –21 2 z zepto 10 T tesla magnetic flux density Wb/m –24 y yocto 10 H henry inductance ( L) Wb/A * The four shaded prefixes (hecto, Dimensionless deka, deci, and centi) are irregular amount of substance (n) BASE UNIT mol mole 23 in that they are not powers of 1000. (number of particles) = g/Da = g/u ≈ 6.02 × 10 They are generally limited to cm, rad radian plane angle 1/2 π of a circle m², and m³. See sections 4, 5, 7, 8. sr steradian solid angle 1/4 π of a sphere Powers of two—the binary numbers Visible Light used in computers—have their own corresponding system of prefixes, lm lumen Luminous flux cd sr = 1/683 W @ 540 THz modeled on SI although they’re not SI: cd candela luminous intensity ( ) BASE UNIT = lm/sr 60 I Ei exbi = 2 ≈ 1.15 E 2 Pi pebi = 2 50 ≈ 1.13 P lx lux illuminance ( E) lm/m Ti tebi = 2 40 ≈ 1.09 T Gi gibi = 2 30 ≈ 1.07 G Radiology & Biochemistry 20 Mi mebi = 2 ≈ 1.05 M Bq becquerel radioactivity 1/s (decays per second ) Ki kibi = 2 10 ≈ 1.02 k Gy gray absorbed dose ( D) J/kg ( of body mass ) Sv sievert dose equivalent ( H) Gy Q (Q = quality factor ) © 2006 by Dennis Brownridge Revised 2008-01-28 13:25 kat katal catalytic activity mol/s 2 Understanding measurement and SI Rules. For clarity and simplicity, there are international 1. Advantages and history rules for expressing measurements, summarized in Table 7 The International System of Units, abbreviated SI , is the (pages 12 & 13) with correct examples and common mis- simplified modern version of the various metric systems. takes. Study them carefully. You will encounter errors, Most metric units are not part of SI, so don’t call it “the even in textbooks. Follow this guide, not the textbooks. metric system.” SI has many advantages: • No conversions. Only one unit for each quantity. 2. Base & derived units (Table 2) • No numbers to memorize. Derived units are defined A quantity is a physical measurement, such as length, without numerical factors. time, volume, area, energy, temperature, and mass. Hun- • No fractions. Decimals only. dreds of different quantities are used in science, technol- • No long rows of zeroes . Prefixes replace them. ogy, and everyday life. Quantities are represented with • Complete. SI can measure any physical quantity. italic letter symbols. A specific quantity consists of a num- • Coherent. Units follow natural laws and can be ma- ber times a unit. A unit is a standard of measurement. Units nipulated algebraically. are represented with normal (upright) letter symbols. For • Only 30 units. Compared to hundreds of non-SI units. simplicity, we usually write units with symbols rather than • Clear symbols. Unique, unambiguous letter symbols. spelling them out. For example, 5 m is a quantity (length) • World standard. Virtually all measurements are based consisting of the number 5 times the unit meter (m). Write on SI, even if expressed in other units. symbols exactly as they appear in the tables. They are sym- bols, not abbreviations, and are case sensitive. Always Non-SI units. Any unit not on Table 2 (page 1) is a non-SI leave a space between the number and unit. (See Rules 2 unit. Hundreds of them are in use, especially in the USA. through 11, pages 12 & 13.) They include a chaotic hodgepodge of old metric, Babylo- nian, and European units, plus units invented for various SI is built on seven fundamental standards called base industries and scientific fields. None of them are really nec- units , included in Table 2 (page 1) and defined in Table 9 essary, since all physical quantities can be measured in SI. (page 14). All other SI units are derived by multiplying, dividing, or powering these base units in various combina- History. SI was introduced in 1960. However, its roots go tions. No numerical factors are involved (except for the back a century earlier when British scientists developed the sievert). SI is a coherent system, which means that derived first coherent measurement system, based on physical laws units are defined by the same laws or equations as the and metric base units. SI is governed by the General Con- quantities they measure. For example, the quantity called ference of Weights and Measures (CGPM), an international speed ( v) is defined as distance per time . Per means treaty organization founded in 1875. The United States is a “divided by.” Distance is a kind of length. In symbols, charter member. The names of most SI units (those with capitalized symbols) honor the scientists who discovered v = d/t (definition of speed) the laws for the corresponding quantity. It is truly an inter- national system. Only two of the 30 SI units (meter and The SI unit of speed is therefore meter per second —the kilogram) were part of the original metric system intro- unit of length divided by the unit of time. In symbols, you duced in France in the 1790s. can show division three different ways: a slash, horizontal bar, or negative exponent. For example, you may write “8 SI in the United States. Americans were intimately in- meters per second” as 8 m volved in developing SI. For example, the energy and elec- 8 m/s 8 m s–1 s trical units (joule, watt, volt, ampere, etc.) were established at conferences in Chicago in the 1880s. But paradoxically, The slash and negative-exponent styles keep everything the United States is virtually the only nation where anti- neatly on one line, while the horizontal-bar style is useful quated non-SI units still predominate in everyday life. The in calculations, for ease in canceling. U.S. Constitution (Article I, section 8) gives Congress the job of establishing a measuring system. After detailed stud- To multiply unit symbols, use a middle dot ( ), Unicode ies, Congress legalized metric units in 1866. Since 1893, all character 00B7. Don’t pronounce or spell it out. Example: American measuring units have been based on SI stan- dards. In 1975 and 1988, after still more studies, Congress 15 N m ( pronounced “15 newton meters”) adopted SI as the preferred U.S. system, but failed to pass effective laws enforcing it. Nevertheless, a growing frac- See Rules 13 and 14, page 13. tion of the American economy is now largely SI, including automobiles, heavy equipment, farm machinery, medicine, science, electronics, computers, cameras, bicycles, the mili- tary, and several sports. 3 and down) have lowercase symbols and mostly end in “o.” 3. Characters not on the keyboard The symbol is the first letter of the name, except for µ SI requires four characters not shown on a computer key- (micro) and da (deka). The letter µ is the Greek “m,” called board. In any application, you can insert them by holding mu, which is Unicode character 00B5. Be sure to write let- down the Alt key while you type a four-digit code on the ters carefully. There is a big difference between M (mega = number pad (fourth column of Table 3 below). Num Lock 10 6) and m (milli = 10 –3), and an even bigger difference be- must be on. However, in Microsoft Word™ it’s much eas- tween P and p, Z and z, and Y and y. ier to use shortcut keys, which are easily to set up. Click Insert, Symbol, enter the Unicode character number, click Regular prefixes. Choose a prefix that minimizes place- Shortcut Key, press the desired shortcut keys, Assign, holding, non-significant zeroes (Rule 12, page 13). Most of Close. Thereafter, you can insert the symbol by just press- the prefixes are multiples of a thousand, or 10 3 (10 3, 10 6, ing the shortcut key. The key assignments in the right hand 10 9, 10 12 , etc.) These regular prefixes, shown without shad- column of Table 3 are recommended, so you won’t disable ing on Table 1, may be used with any SI unit. Think of any of Word’s default shortcuts. For combinations you use them as a ladder. Going up the ladder multiplies a unit in frequently, the AutoCorrect feature is even easier (Tools, steps of 1000 or 10 3. Include 10 0 (=1) as a step, although it AutoCorrect Options...).
Load more

Recommended publications
  • A Learner's Guide to SI Units and Their Conversion
    A Learner's Guide to SI Units and their Conversion October 2004 Workbook for students Learner's Guide to SI Units and their Conversion their and Units SI to Guide Learner's Edexcel A London Qualifications is one of the leading examining and awarding bodies in the UK and throughout the world. It incorporates all the qualifications previously awarded under the Edexcel and BTEC brand. We provide a wide range of qualifications including general (academic), vocational, occupational and specific programmes for employers. Through a network of UK and overseas offices, our centres receive the support they need to help them deliver their education and training programmes to learners. For further information please call Customer Services on 0870 240 9800, or visit our website at www.edexcel.org.uk Authorised by Jim Dobson Prepared by Sarah Harrison All the material in this publication is copyright © London Qualifications Limited 2004 CONTENTS Introduction 1 What are units? 2 Operations with units 5 Submultiple and multiple units 9 Conversion of units 11 Conversion examples and exercises 13 Length 13 Area 14 Volume 15 Mass 16 Time 17 Temperature 18 Density 19 Force 20 Stress and pressure 21 Answers to exercises 23 Introduction One of the important areas where Science and Technology students need support is in the conversion of units. This booklet is designed to be useful for students in all Science, Technology and Engineering subjects. This booklet has been produced to: S introduce students to SI base and derived units and S help students with the conversion of multiple and sub-multiple units to SI base and derived units.
    [Show full text]
  • Illumination and Distance
    PHYS 1400: Physical Science Laboratory Manual ILLUMINATION AND DISTANCE INTRODUCTION How bright is that light? You know, from experience, that a 100W light bulb is brighter than a 60W bulb. The wattage measures the energy used by the bulb, which depends on the bulb, not on where the person observing it is located. But you also know that how bright the light looks does depend on how far away it is. That 100W bulb is still emitting the same amount of energy every second, but if you are farther away from it, the energy is spread out over a greater area. You receive less energy, and perceive the light as less bright. But because the light energy is spread out over an area, it’s not a linear relationship. When you double the distance, the energy is spread out over four times as much area. If you triple the distance, the area is nine Twice the distance, ¼ as bright. Triple the distance? 11% as bright. times as great, meaning that you receive only 1/9 (or 11%) as much energy from the light source. To quantify the amount of light, we will use units called lux. The idea is simple: energy emitted per second (Watts), spread out over an area (square meters). However, a lux is not a W/m2! A lux is a lumen per m2. So, what is a lumen? Technically, it’s one candela emitted uniformly across a solid angle of 1 steradian. That’s not helping, is it? Examine the figure above. The source emits light (energy) in all directions simultaneously.
    [Show full text]
  • 2.1 Definition of the SI
    CCPR/16-53 Modifications to the Draft of the ninth SI Brochure dated 16 September 2016 recommended by the CCPR to the CCU via the CCPR president Takashi Usuda, Wednesday 14 December 2016. The text in black is a selection of paragraphs from the brochure with the section title for indication. The sentences to be modified appear in red. 2.1 Definition of the SI Like for any value of a quantity, the value of a fundamental constant can be expressed as the product of a number and a unit as Q = {Q} [Q]. The definitions below specify the exact numerical value of each constant when its value is expressed in the corresponding SI unit. By fixing the exact numerical value the unit becomes defined, since the product of the numerical value {Q} and the unit [Q] has to equal the value Q of the constant, which is postulated to be invariant. The seven constants are chosen in such a way that any unit of the SI can be written either through a defining constant itself or through products or ratios of defining constants. The International System of Units, the SI, is the system of units in which the unperturbed ground state hyperfine splitting frequency of the caesium 133 atom Cs is 9 192 631 770 Hz, the speed of light in vacuum c is 299 792 458 m/s, the Planck constant h is 6.626 070 040 ×1034 J s, the elementary charge e is 1.602 176 620 8 ×1019 C, the Boltzmann constant k is 1.380 648 52 ×1023 J/K, 23 -1 the Avogadro constant NA is 6.022 140 857 ×10 mol , 12 the luminous efficacy of monochromatic radiation of frequency 540 ×10 hertz Kcd is 683 lm/W.
    [Show full text]
  • Guide for the Use of the International System of Units (SI)
    Guide for the Use of the International System of Units (SI) m kg s cd SI mol K A NIST Special Publication 811 2008 Edition Ambler Thompson and Barry N. Taylor NIST Special Publication 811 2008 Edition Guide for the Use of the International System of Units (SI) Ambler Thompson Technology Services and Barry N. Taylor Physics Laboratory National Institute of Standards and Technology Gaithersburg, MD 20899 (Supersedes NIST Special Publication 811, 1995 Edition, April 1995) March 2008 U.S. Department of Commerce Carlos M. Gutierrez, Secretary National Institute of Standards and Technology James M. Turner, Acting Director National Institute of Standards and Technology Special Publication 811, 2008 Edition (Supersedes NIST Special Publication 811, April 1995 Edition) Natl. Inst. Stand. Technol. Spec. Publ. 811, 2008 Ed., 85 pages (March 2008; 2nd printing November 2008) CODEN: NSPUE3 Note on 2nd printing: This 2nd printing dated November 2008 of NIST SP811 corrects a number of minor typographical errors present in the 1st printing dated March 2008. Guide for the Use of the International System of Units (SI) Preface The International System of Units, universally abbreviated SI (from the French Le Système International d’Unités), is the modern metric system of measurement. Long the dominant measurement system used in science, the SI is becoming the dominant measurement system used in international commerce. The Omnibus Trade and Competitiveness Act of August 1988 [Public Law (PL) 100-418] changed the name of the National Bureau of Standards (NBS) to the National Institute of Standards and Technology (NIST) and gave to NIST the added task of helping U.S.
    [Show full text]
  • Recommended Light Levels
    Recommended Light Levels Recommended Light Levels (Illuminance) for Outdoor and Indoor Venues This is an instructor resource with information to be provided to students as the instructor sees fit. Light Level or Illuminance, is the amount of light measured in a plane surface (or the total luminous flux incident on a surface, per unit area). The work plane is where the most important tasks in the room or space are performed. Measuring Units of Light Level - Illuminance Illuminance is measured in foot candles (ftcd, fc, fcd) or lux (in the metric SI system). A foot candle is actually one lumen of light density per square foot; one lux is one lumen per square meter. • 1 lux = 1 lumen / sq meter = 0.0001 phot = 0.0929 foot candle (ftcd, fcd) • 1 phot = 1 lumen / sq centimeter = 10000 lumens / sq meter = 10000 lux • 1 foot candle (ftcd, fcd) = 1 lumen / sq ft = 10.752 lux Common Light Levels Outdoors from Natural Sources Common light levels outdoor at day and night can be found in the table below: Illumination Condition (ftcd) (lux) Sunlight 10,000 107,527 Full Daylight 1,000 10,752 Overcast Day 100 1,075 Very Dark Day 10 107 Twilight 1 10.8 Deep Twilight .1 1.08 Full Moon .01 .108 Quarter Moon .001 .0108 Starlight .0001 .0011 Overcast Night .00001 .0001 Common Light Levels Outdoors from Manufactured Sources The nomenclature for most of the types of areas listed in the table below can be found in the City of Los Angeles, Department of Public Works, Bureau of Street Lighting’s “DESIGN STANDARDS AND GUIDELINES” at the URL address under References at the end of this document.
    [Show full text]
  • 2019 Redefinition of SI Base Units
    2019 redefinition of SI base units A redefinition of SI base units is scheduled to come into force on 20 May 2019.[1][2] The kilogram, ampere, kelvin, and mole will then be defined by setting exact numerical values for the Planck constant (h), the elementary electric charge (e), the Boltzmann constant (k), and the Avogadro constant (NA), respectively. The metre and candela are already defined by physical constants, subject to correction to their present definitions. The new definitions aim to improve the SI without changing the size of any units, thus ensuring continuity with existing measurements.[3][4] In November 2018, the 26th General Conference on Weights and Measures (CGPM) unanimously approved these changes,[5][6] which the International Committee for Weights and Measures (CIPM) had proposed earlier that year.[7]:23 The previous major change of the metric system was in 1960 when the International System of Units (SI) was formally published. The SI is a coherent system structured around seven base units whose definitions are unconstrained by that of any other unit and another twenty-two named units derived from these base units. The metre was redefined in terms of the wavelength of a spectral line of a The SI system after the 2019 redefinition: krypton-86 radiation,[Note 1] making it derivable from universal natural Dependence of base unit definitions onphysical constants with fixed numerical values and on other phenomena, but the kilogram remained defined in terms of a physical prototype, base units. leaving it the only artefact upon which the SI unit definitions depend. The metric system was originally conceived as a system of measurement that was derivable from unchanging phenomena,[8] but practical limitations necessitated the use of artefacts (the prototype metre and prototype kilogram) when the metric system was first introduced in France in 1799.
    [Show full text]
  • Relationships of the SI Derived Units with Special Names and Symbols and the SI Base Units
    Relationships of the SI derived units with special names and symbols and the SI base units Derived units SI BASE UNITS without special SI DERIVED UNITS WITH SPECIAL NAMES AND SYMBOLS names Solid lines indicate multiplication, broken lines indicate division kilogram kg newton (kg·m/s2) pascal (N/m2) gray (J/kg) sievert (J/kg) 3 N Pa Gy Sv MASS m FORCE PRESSURE, ABSORBED DOSE VOLUME STRESS DOSE EQUIVALENT meter m 2 m joule (N·m) watt (J/s) becquerel (1/s) hertz (1/s) LENGTH J W Bq Hz AREA ENERGY, WORK, POWER, ACTIVITY FREQUENCY second QUANTITY OF HEAT HEAT FLOW RATE (OF A RADIONUCLIDE) s m/s TIME VELOCITY katal (mol/s) weber (V·s) henry (Wb/A) tesla (Wb/m2) kat Wb H T 2 mole m/s CATALYTIC MAGNETIC INDUCTANCE MAGNETIC mol ACTIVITY FLUX FLUX DENSITY ACCELERATION AMOUNT OF SUBSTANCE coulomb (A·s) volt (W/A) C V ampere A ELECTRIC POTENTIAL, CHARGE ELECTROMOTIVE ELECTRIC CURRENT FORCE degree (K) farad (C/V) ohm (V/A) siemens (1/W) kelvin Celsius °C F W S K CELSIUS CAPACITANCE RESISTANCE CONDUCTANCE THERMODYNAMIC TEMPERATURE TEMPERATURE t/°C = T /K – 273.15 candela 2 steradian radian cd lux (lm/m ) lumen (cd·sr) 2 2 (m/m = 1) lx lm sr (m /m = 1) rad LUMINOUS INTENSITY ILLUMINANCE LUMINOUS SOLID ANGLE PLANE ANGLE FLUX The diagram above shows graphically how the 22 SI derived units with special names and symbols are related to the seven SI base units. In the first column, the symbols of the SI base units are shown in rectangles, with the name of the unit shown toward the upper left of the rectangle and the name of the associated base quantity shown in italic type below the rectangle.
    [Show full text]
  • International System of Units (SI)
    International System of Units (SI) National Institute of Standards and Technology (www.nist.gov) Multiplication Prefix Symbol factor 1018 exa E 1015 peta P 1012 tera T 109 giga G 106 mega M 103 kilo k 102 * hecto h 101 * deka da 10-1 * deci d 10-2 * centi c 10-3 milli m 10-6 micro µ 10-9 nano n 10-12 pico p 10-15 femto f 10-18 atto a * Prefixes of 100, 10, 0.1, and 0.01 are not formal SI units From: Downs, R.J. 1988. HortScience 23:881-812. International System of Units (SI) Seven basic SI units Physical quantity Unit Symbol Length meter m Mass kilogram kg Time second s Electrical current ampere A Thermodynamic temperature kelvin K Amount of substance mole mol Luminous intensity candela cd Supplementary SI units Physical quantity Unit Symbol Plane angle radian rad Solid angle steradian sr From: Downs, R.J. 1988. HortScience 23:881-812. International System of Units (SI) Derived SI units with special names Physical quantity Unit Symbol Derivation Absorbed dose gray Gy J kg-1 Capacitance farad F A s V-1 Conductance siemens S A V-1 Disintegration rate becquerel Bq l s-1 Electrical charge coulomb C A s Electrical potential volt V W A-1 Energy joule J N m Force newton N kg m s-2 Illumination lux lx lm m-2 Inductance henry H V s A-1 Luminous flux lumen lm cd sr Magnetic flux weber Wb V s Magnetic flux density tesla T Wb m-2 Pressure pascal Pa N m-2 Power watt W J s-1 Resistance ohm Ω V A-1 Volume liter L dm3 From: Downs, R.J.
    [Show full text]
  • Worksheet 2 the Metric System and SI Units
    Unit Conversions Worksheet 2 The Metric System and SI Units 1 © MathTutorDVD.com Here is a helpful list of all the prefixes and their exponents and abbreviations: 109 giga G one billion 106 mega M one million 103 kilo k one thousand 102 hecto h one hundred 101 deca da ten 10-1 deci d one tenth 10-2 centi c one hundredth 10-3 milli m one thousandth 10-6 micro µ one millionth 10-9 nano n one billionth 10-12 pico p one trillionth 2 © MathTutorDVD.com 1. Give the abbreviation for each of the SI base units below. a. kilogram b. Kelvin c. meter d. second 2. State what quantities the following SI units are used to measure. a. kg b. m c. s d. K 3. Give the power of 10 that each of the following prefixes represents. a. kilo b. micro c. centi d. deca e. mega 3 © MathTutorDVD.com 4. Write the following numbers in their base units. a. 5 kilometers b. 12 centimeters c. 2 micrograms d. 4.1 megagrams 5. Write the abbreviations for the following measurements. a. 67 micrograms b. 831 kilometers c. 1.2 meters d. 791 megagrams 4 © MathTutorDVD.com 6. Express the following numbers with an appropriate SI prefix (e.g., 5,000 g = 5 kg). a. 7800 m b. 5.0 x 10-6 g c. 7.8 x 106 m d. 1.6 x 10-3 g 7. Convert the following to scientific notation using the base SI unit. a. 4.51 microseconds b. 6700 grams c.
    [Show full text]
  • The International System of Units (SI)
    NAT'L INST. OF STAND & TECH NIST National Institute of Standards and Technology Technology Administration, U.S. Department of Commerce NIST Special Publication 330 2001 Edition The International System of Units (SI) 4. Barry N. Taylor, Editor r A o o L57 330 2oOI rhe National Institute of Standards and Technology was established in 1988 by Congress to "assist industry in the development of technology . needed to improve product quality, to modernize manufacturing processes, to ensure product reliability . and to facilitate rapid commercialization ... of products based on new scientific discoveries." NIST, originally founded as the National Bureau of Standards in 1901, works to strengthen U.S. industry's competitiveness; advance science and engineering; and improve public health, safety, and the environment. One of the agency's basic functions is to develop, maintain, and retain custody of the national standards of measurement, and provide the means and methods for comparing standards used in science, engineering, manufacturing, commerce, industry, and education with the standards adopted or recognized by the Federal Government. As an agency of the U.S. Commerce Department's Technology Administration, NIST conducts basic and applied research in the physical sciences and engineering, and develops measurement techniques, test methods, standards, and related services. The Institute does generic and precompetitive work on new and advanced technologies. NIST's research facilities are located at Gaithersburg, MD 20899, and at Boulder, CO 80303.
    [Show full text]
  • Commission Services Working Document
    COMMISSION OF THE EUROPEAN COMMUNITIES Brussels, 22 December 2006 COMMISSION STAFF WORKING DOCUMENT on units of measurement (Directive 80/181/EEC) EN EN TABLE OF CONTENTS 1. Introduction.................................................................................................................. 3 2. Extension of the international SI-system ..................................................................... 3 3. Multiple indication of SI units ..................................................................................... 4 4. So-called Arbitrary units (U) ....................................................................................... 4 5. Ratios (not expressed in units) ..................................................................................... 5 6. Non-SI Units required by International Treaties in transport ...................................... 5 7. Non-SI Units used in Traditional industries and in new markets ................................ 5 8. Date of Supplementary indications in non-SI units ..................................................... 6 9. Scope of the directive................................................................................................... 7 10. Stakeholder Consultation ............................................................................................. 8 EN 2 EN 1. INTRODUCTION The use of units of measurement is prescribed in the European Union by means of COUNCIL DIRECTIVE 80/181/EEC of 20 December 1979 on the approximation of the laws of the Member States
    [Show full text]
  • Problems with Foot-Candles, Lux and Lumens
    TECHNICALLY SPEAKING Problems with Foot-candles, Lux By Erik Runkle and Bruce Bugbee and Lumens here are three ways to describe light inten- best way to predict photosynthesis. The number of pho- sity, but only one of them is appropriate tons in the photosynthetic waveband (400 to 700 nm) for photosynthesis. Increasing use of light- is typically reported as micromoles per square meter and emitting diodes (LEDs) means that the second, or µmol·m-2·s-1. This system considers all colors appropriate unit for light measurement is of light equally and is the most appropriate way to mea- moreT important than ever. Although light quality and sure light intensity for photosynthesis and plant growth. timing are powerful tools for altering plant shape, light Measuring moles of photons is not biased toward human intensity provides the horsepower to drive photosynthesis. vision; in fact, an intensity of deep blue and deep red light Photometric: Foot-candles and lux. These units that appears dim to us can be bright light for plants. of measurement are based on the perceived brightness to Measuring light intensity in foot-candles or lux the human eye. Our eyes perceive green and yellow light is especially unsuitable when using LEDs for plant much better than blue or red light, so this measurement growth. Blue and red LEDs are most often used for system is completely biased toward people and is not plant lighting because: appropriate for plants. Since most lighting applications 1. They are the most energy-efficient colors of LEDs are for people, this system is used by lighting professionals 2.
    [Show full text]