LED Technical
2019-12-04
Color Temperature and Color Rendering (Kelvin and CRI)
There are two standard measurements for the color characteristics of light: "color rendering index" (CRI), a term used to describe the extent to which an artificial light source is able to render the true color of objects as seen by natural outdoor sunlight which has a CRI of 100, and "color temperature", which expresses the color appearance of the light itself.
Color Rendering Index (CRI): Incandescent is used as the base reference of 100 CRI. Compact fluorescent lamps are graded at 82-86 CRI, which is considered high quality color rendering. CRI is a more important consideration for retail lighting design than it is for office lighting.
Any CRI rating of 80 or above is considered high and indicates that the source has good color properties. Incandescent lamps and daylight have a CRI of 100, the highest possible CRI. The higher the CRI of the light source, the "truer" it renders color.
Color Temperature (Kelvin): Refers to the way color groups are perceived – the psychological impact of lighting. Color temperature is how cool or warm the light source appears.
The color temperature of a light source is a numerical measurement of its color appearance. This temperature is based on the principle that any object will emit light if it is heated to a high enough temperature and that the color of that light will shift in a predictable manner as the temperature is increased. This system is based on the color changes of a black metal as it is heated from a cold black to a white hot state. As the temperature increases, the color would shift gradually from red to orange to yellow to white and finally to a blue white. Color temperature is measured in degrees Kelvin (K). Colors and light sources from the red/orange/yellow side of the spectrum are described as warm (incandescents) and those toward the blue end are referred to as cool (natural daylight).
The sun, for example, rises at approximately 1800 Kelvin and changes from red to orange to yellow and to white as it rises to over 5000 Kelvin at high noon. It then goes back down the scale as it sets.
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Color Temperature - Common Applications |
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Kelvin |
Associated Effects & Moods |
Appropriate Applications |
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2700° |
Friendly, personal, intimate |
Homes, libraries, restaurants |
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3000° |
Soft, warm pleasing light |
Homes, hotel rooms and lobbies, restaurants, retail stores |
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3500° |
Friendly, inviting, non-threatening |
Executive offices, public reception areas, supermarkets |
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4100° |
Neat, clean, efficient |
Office, classrooms, mass merchandisers, showrooms |
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5000° |
Bright, alert |
Graphic industry, hospitals |
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6500° |
Bright, cool |
Jewelry stores, beauty salons, galleries, museums, printing |
Lighting and Electrical Glossary
Ampere
A unit expressing the rate of flow of electric current.
(Design) Amperes
The approximate current which the lamp will draw at design volts
ANSI (American National Standards Institute)
The organization that develops voluntary guidelines and produce performance standards for the electrical and other industries.
Average Rated Life
An average rating, in hours, indicating when 50% of a large group of lamps have failed, when operated at nominal lamp voltage and current; manufacturers use 3 hours per start for fluorescent lamps and 10 hours per start for HID lamps when performing lamp life testing procedures; every lamp type has a unique mortality curve that depicts its average rated life. For PHOTO-OPTIC lamps average rated life refers to the operating period after which on statistical average, 50% of the lamps will perform within their specified values.
Ballast
A device used with an electric-discharge lamp to obtain the necessary circuit conditions (voltage, current and waveform) for starting and operating; all fluorescent and HID light sources require a ballast for proper operation. Ballasts have two primary functions: 1) start the lamp and 2) control operation of the lamp once it has started. High frequency electronic ballasts operate lamps more efficiently (30 - 40% at equivalent light output) and eliminate the hum and visible flicker normally associated with standard magnetic ballasts. Electronic ballasts also typically have better power quality than magnetic ballasts (higher power factor and lower THD).
Ballast Efficacy Factor (BEF)
Relative light output (ballast factor) divided by input power (watts). Used to measure the level of efficiency of similar ballast models. For example, the OSRAM SYLVANIA QT2X32IS which has a ballast factor of 0.90 and input watts of 59 (BEF=1.53), is more efficient than competitors' electronic ballasts with ballast factor of 0.875 and input watts of 62 (BEF=1.41).
Ballast factor (BF)
Relative light output as compared to a reference ballast (i.e. BF of 0.90 would yield 90% of a lamp's rated lumens. The measured ability of a particular ballast to produce light from the lamp(s) it powers; ballast factor is derived by dividing the lumen output of a particular lamp/ballast combination by the lumen output of the same lamp(s) on a reference ballast.
Ballast life
Bulborama ballasts are designed to have a life expectancy of 60,000 hours. To maximize life, ambient temperature should be kept as low as possible. It is also important to maintain effective dissipation of heat using the lighting fixture as a heatsink for the ballast enclosure.
Ballast losses
Power consumed by a ballast that dissipates as heat instead of being converted into light. Electronic ballasts operate more efficiently than magnetic or hybrid ballasts. A typical ballast loss for a standard two lamp magnetic ballast is 20 watts, which an electronic equivalent would only be 7 watts.
Ballast types
There are three types of lighting ballasts: 1) Magnetic: an inefficient device that uses a core and coil assembly transformer to perform the minimum functions required to start and operate the lamp; 2) Hybrid or "low frequency electronic": essentially a magnetic ballast with a few electronic components that switch off voltage to the lamp coil once the lamp has started. A minimal increase in efficiency is obtained via more expensive magnetic core material and the absence of power to the lamp coils during operation; 3) High frequency electronic: a ballast that operates lamps at frequencies above 20,000 Hz. Maximum efficiency is obtained through the use of electronic circuitry and optimum lamp operating characteristics.
Base
The lamp base mechanically holds the lamp in place in the application. The lamp base directly or indirectly (via a cable or lead-in wires) conducts electricity from the circuit to the lamp and can be designed to dissipate heat. Lamp bases should be operated within specified temperature range.
Beam angle
Also called the beam spread; the angular dimension of the cone of light from reflectorized lamps encompassing the central part of the beam out ot the angle where the intensity is 50 percent of maximum.
Bulb
Hard, soft or quartz glass enclosure, which can contain a vacuum, elemental inert gas or metal and a means of light generation (filament or electrodes).
Candela (cd)
The unit of measure indicating the luminous intensity (candlepower) of a light source in a specific direction; any given light source will have many different intensities, depending upon the direction considered.
Candlepower distribution
A curve that represents the variation in luminous intensity (expressed in candelas) in a plane through the light center of a lamp or luminaire; each lamp or lamp/luminaire combination has a unique set of candlepower distributions that indicate how light will be spread.
Center Beam Candlepower (CBCP)
The intensity of light produced at the center of a reflector lamp, expressed in candelas.
CFL
Compact Fluorescent Lamp or Compact Fluorescent Light Bulb.
Color rendering index (CRI)
The Color Rendering Index (CRI) measures the effect a light source has on the perceived color of objects and surfaces. High CRI lights makes virtually all colors look natural and vibrant. Low CRI causes some colors to appear washed out or even to take on a completely different hue.
Color temperature (CT)
Color temperature, which is measured in Kelvin, indicates whether a lamp has a warm, midrange or cool color appearance. "Warm" light sources have a low color temperature (2000-3000K) and feature more light in the red/orange/yellow range. Light with a higher color temperature (>4000K) features more blue light and is referred to as "cool."
Compact Fluorescent Lamps (CFL)
Compact fluorescent lamps employ small diameter tubes that are bent so they begin and end in a ceramic base. This allows them to be produced in a wide variety of configurations, greatly extending the applications for fluorescent lighting and offering innovative energy efficient lighting solutions.
Correlated Color Temperature (CCT)
A specification of the color appearance of a lamp, relating its color to that of a reference source heated to a particular temperature, measured in degrees Kelvin (K); CCT generally measures the "warmth" of "coolness" of light source appearance.
Current
A measure of the flow of electricity, expressed in amperes (A).
Décor
Decorative lamps, such as candelabra or post lights, in a variety of shapes and bases.
Description
See ordering abbreviation.
Double-ended
Lamps that have two bases opposite one another for series electrical connection, mechanical mounting and heat dissipation.
Efficacy
The rate at which a lamp is able to convert power (watts) into light (lumens). A watt of electricity is the amount of power in and a lumen or light is the amount of power out. Represented in lumens per watt and found by dividing the light output in lumens by the electrical power input (to the lamp). Also see LPW performance.
Electronic ballasts
The electric arc in any fluorescent system is generated by a ballast. The ballast starts the lamp, then limits its operating current and provides power factor correction. Modern electronic ballasts perform these functions with great efficiency and provide other control functions as well.
EMI/RFI
Ballasts contain circuits that limit electrical noise conducted onto the power line or radiated through the air, otherwise referred to as EMI/RFI. Bulborama ballasts comply with FCC 47 CFR Part 18, Non-Consumer limits for commercial applications.
End Foot Candles (EFC)
A measure of that portion of the total light output of a T-2 lamp that passes through a .250" orifice placed at the end of the lamp.
Energy
A measure of work done by an electrical system over a given period of time, often expressed in kilowatt-hours (kWh).
Filament
A tungsten wire purposely positioned inside a lamp bulb, that when heated electrically generates radiation in the visible, infrared and ultraviolet ranges. Tungsten material replaced carbon almost universally, as it has great tensible strength, and is very durable. However, the basic reason for its selection as the best filament material is the fact that it can be burned very near its melting point without evaporating rapidly. Lamp filaments are offered in a variety of designs optimized for specific applications.
Floodlight
A reflectorized lamp whose emitted beam pattern is enlarging. Also a luminaire consisting of lamp and reflector at fixed distance providing a wide field of illumination.
Fluorescent lamp
High efficiency lamp that uses an electric discharge through low-pressure mercury vapor to produce ultra-violet (UV) energy. The UV excites phosphor materials applied as a thin layer on the inside of a glass tube that makes up the structure of the lamp. The phosphors transform the UV to visible light.
Footcandle (fc)
A unit of illuminance equal to 1 lumen per square foot.
Frequency
The number of times per second that an alternating current system reverses from positive to negative and back to positive, expressed in cycles per second or hertz (Hz).
Fusing
All Bulborama ballasts contain inherent electrical protection. Although there is no need to externally fuse the ballast, should code or regulation require one, 3 amp slow blow fuses are recommended.
Glare
Excessive brightness that may be caused by either direct of indirect viewing of a light source.
Glow to arc transition
In order to achieve full rated lamp life, a ballast should start a lamp so that the time from when the lamp begins to glow to the time the lamp arc strikes should be as short as possible. Bulborama instant start ballasts typically accomplish this task within 50 msec.
Grounding
The ballast case and fixture must always be grounded. The grounding helps assure safety, proper lamp starting, and acceptable EMI/RFI performance.
Halogen lamps
(Tungsten-halogen lamp) high pressure lamps containing halogen gases which allow the filaments to operate at higher temperatures and higher efficacies. Halogen lamps use a filament, but since it is sealed in a pressurized capsule containing halogen gas, the lamp provides brighter, whiter light with better color characteristics, longer service life and improved energy efficiency.
Harmonic
An electrical frequency that is an integer multiple of the fundamental frequency; for example, if 60 Hz is the fundamental frequency, then 120 Hz is the second harmonic and 180 Hz is the third harmonic; some electronic devices, such as ballasts or power supplies, can cause harmonic distortion, directly affecting power quality.
High-intensity discharge (HID) lamps
In HID lamps, an arc passing between two cathodes in a pressurized tube cause various metallic additives to vaporize and release large amounts of light. All HID lamps offer outstanding energy efficiency and service life. Metal halide lamps also offer good to excellent color rendering index (CRI).
Hot ignition
The restarting of a previously operating lamp shortly after turn-off. Hot ignition is a high performance feature in many OSRAM discharge lamp types.
Illuminance
Light arriving at a surface, expressed in lumens per unit area; 1 lumen per square foot equals 1 footcandle, while 1 lumen per square meter equals 1 Lux.
Incandescent lamps
A light source that generates light utilizing a think filament wire (usually tungsten) heated to white heat by an electric current passing through it. Regular incandescent lamps produce light by passing an electric current through a filament in a vacuum or gas-filled bulb. They provide low initial cost, good color rendition and excellent optical control.
Instant start (IS) vs. rapid start (RS)
Instant start (high voltage is applied across the lamp with no preheating of the cathode) is the most energy efficient starting method for fluorescent lamp ballasting. IS ballasts use 1.5 to 2 watts less per lamp than rapid start ballast (low voltage is applied to the cathodes prior to lamp ignition and is maintained throughout operation). Other IS ballast benefits typically include parallel lamp circuitry, longer remote wiring distance, easier installation due to less complicated wiring, and capability to start lamps at 0 degrees (versus 50 degrees F for rapid start).
K-factor
A measurement that quantifies the effect of non-linear equipment, such as lighting ballasts, on an electrical system. Lighting systems should be designed so that the transformer rating is sufficient for the ballast used (typically K-factor <4). All Bulborama ballasts meet this specification.
Lamp
Manufactured light source; synonymous with light bulb; the three broad categories of electric lamps are incandescent, fluorescent and high-intensity discharge.
Lamp Current Crest Factor (LCCF)
The ratio of peak lamp current to the RMS (average) lamp current. Lamp manufacturers require a LCCF of less than 1.70 in order to achieve full lamp life. Values less than 1.70 do not achieve higher than rated lamp life.
Lamp Disposal
When disposing of spent lamps, always consult federal, state, local and/or provincial hazardous waste disposal rules and regulations to ensure proper disposal.
Lamp flicker
High frequency electronic ballasts provide a minimal level of lamp flicker. Lamp flicker from magnetic ballasts can cause eye fatigue for some people.
Light
Radiant energy that is capable of producing a visual sensation.
Light bulb
see Bulb
Light Center Length (LCL)
The distance from a specified reference point on a lamp base to its light center, typically expressed in inches.
Linear fluorescent lamps
In a fluorescent lamp, an electric arc passing between cathodes in a tube excites mercury vapor and other gases and produces UV radiant energy. A phosphor coating on the tube then converts this energy to visible light. Fluorescent lamps are very energy efficient and provide a wide range of color responses.
LPW performance
Lumens Per Watt. The number of lumens produced by a lightsource for each watt of electrical power supplied to the light source. Also see Efficacy.
Lumen depreciation
The decrease in lumen output of a light source over time; every lamp type has a unique lumen depreciation curve (sometimes called a lumen maintenance curve) depicting the pattern of decreasing light output.
Lumens
A unit of luminous flux; overall light output; quantity of light, expressed in lumens. For example, a dinner candle provides about 12 lumens and a 60-watt soft white incandescent lamps provides about 840 lumens.
Luminaire
A light fixture; the complete lighting unit, including lamp, reflector, ballast, socket, wiring, diffuser and housing.
Luminance (L)
Light reflected in a particular direction; the photometric quantity most closely associated with brightness perception, measured in units of luminous intensity (candelas) per unit area (square feet or square meters).
Lux (lx)
A unit of luminance equal to 1 lumen per square meter.
Maximum Overall Length (MOL)
The total length of a lamp, from top of bulb to bottom of base, typically expressed in inches.
Mean Spherical Candela (MSCD)
The average value of the luminous intensity of a light source in all directions. To convert MSCD to Lumens, multiply by 4?(12.57).
MTBF - Mean Time Between Failures
A calculation of ballast life based on thermal conditions, component values, and circuit characteristics used to develop relative predictions of ballast life.
Medium pin
Referring to the lamp base pin diameters. Often referencing fluorescent lamps (T-8F and T-12F).
Nominal watts
Represents the rated wattage consumption period. Represents the energy used to produce light. Watts= Volts x Amperes. Also see Watt.
Operating Position
All Bulborama lamps must be operated within the specified operational positions (base, filament, electrodes).
Ordering abbreviation code
Provides a shorthand description of the lamp, using a unique code which can be used when ordering a lamp if you do not know the item number. An example would be: 50PAR20/FL 130V, which translates to a 50-watt PAR20 flood 130 volt halogen par lamp.
Parallel vs. series
Ballasts with parallel lamp circuitry have the benefit of companion lamps remaining lit, even if one of the lamps operated by the ballast should fail. Systems with series lamp wiring (magnetic ballasts and many competitors' electronic types) result in all lamps operated on the ballast going out if one should fail.
PAR lamps
Usually halogen lamps, means parabolic reflector lamps. A lamp fixed within a parabolic reflector, a lamp system that can use incandescent, halogen and HMI lamp types. Numeric portion of PAR description indicates actual parabolic reflector diameter size in 1/8th inch units (example: PAR 64 is a lamp whose diameter is 64/8th inch or 8 inches). Example: PAR 36, 38, 46, 56 and 64 types.
Photo-Optic lamps
Photo-Optic lamps employ a variety of technologies to meet the very precise levels of performance required by the entertainment industry, science, medicine and other high-tech fields.
Power
The rate at which energy is taken from an electrical system or dissipated by a load, expressed in watts; power that is generated by a utility is typically expressed in volt-amperes.
Power factor
A measure of the effectiveness with which an electrical device converts volt-amperes to watts; devices with power factors (0.90) are "high power factor" devices.
Reflectance (icon)
The percentage of light reflected back from a surface, the difference having been absorbed or transmitted by the surface. See Reflection.
Reflection
If a light ray strikes a mirror- life surface, it is reflected. The angle of incidence equals the angle of reflection; this is called specular reflection. When a ray strikes a mat surface, light is reflected uniformly in all directions. This is called diffuse reflection. With the reflectors the rays of light are pencilled in the proper direction. See Reflectance.
Reflector
An optical device to reflect light. PHOTO-OPTIC reflector lamps utilize ellipsoidal (converging light rays) or parabolic (collimating light rays) reflectors. Dichroic coated reflectors are designed to reflect visible light and pass through unwanted infrared wavelengths.
Resistance (R)
A measure of resistance to flow of current, expressed in ohms.
Safety
Ballasts should be installed and operated in compliance with the National Electric Code (NEC), Underwriters Laboratories Inc. (UL) requirements, and all applicable codes and regulations. As it is possible to come in contact with potentially hazardous voltages, only qualified personnel should perform ballast installation. All installation, inspection, and maintenance of lighting fixtures should be done with the power to the fixture turned off.
Single pin
Single pins have a mini can or D.C. bay base whereas bi- pin lamps have a bi- pin base.
Single-ended
Lamps having a single lamp base or point of electrical connection.
Spectral Power Distribution (SPD)
A curve illustrating the distribution of power produced by the lamp, at each wavelength across the spectrum.
Spotlight
A luminaire using halogen/incandescent or a high intensity discharge (HID) lamp that produces a narrow beam angle designed to illuminate a specifically defined area.
Total harmonic distortion (THD)
Excessive THD (defined by ANSI as greater than 32%) may cause adverse effects to the electrical system. THD levels below 20% provide optimal system compatibility, but levels below 10% may not add any practical benefit. 10% THD types may also introduce excessive in-rush current unless circuitry is added that limits in-rush levels.
TCLP Test
(Toxicity Characteristic Leaching Procedure), Federal EPA regulations (RCRA of 1990) have define a TCLP test to determine whether wastes are to be treated as hazardous or non-hazardous.
Tungsten Halogen Cycle
Halogen light sources utilizing the halogen regenerative cycle to prevent blackening of the lamp envelope during life.
Voltage (E)
A measure of electrical potential, expressed in volts (V).
Watt
A unit of electrical power. Lamps are rated in watts to indicate power consumption. Also see Nominal watts.
Wavelength (icon)
Distance between two successive points of a periodic wave; the wavelengths of light are typically expressed in nanometers (nm), or billionths of a meter.
Working Distance
As a function of an elliptical reflector, light is collected and converged into a specific area a certain distance in front of the lamp. Lamp alignment can be provided for specific illumination and color qualities at the designated area.
Lumens,Illuminance,Foot-candles
In defining how bright something is, we have two things to consider.
1. How bright it is at the source- How Bright is that light?
2. How much light is falling on something a certain distance away from the light.
Some Definitions.
We're in America , so we are going to talk about units of measurement that concern distance in feet and inches. So, we will use some terms that folks in Europe don't use. We're going to talk about "foot-candles".
This one's simple. Get a birthday cake candle. Get a ruler. Stick the candle on one end of the ruler. Light the candle. Turn out the lights. Sing Happy Birthday to Doc. It was his 47th on the 23rd. OK, quiet down. Enough of that nonsense. One foot-candle of light is the amount of light that birthday cake candle generates one foot away.
That's a neat unit of measurement. Why? Say you have a lamp. You are told it produces 100 foot candles of light. That means at one foot from the lamp, you will receive 100 foot candles of light.
But here's where it gets tricky. The further away you move the light from what you want to illuminate, the less bright the light seems! If you measure it at the light, it's just as bright. But when you measure at the object you want illuminated, there is less light! A Physics teacher is going to tell you that light measured on an object is INVERSELY PROPORTIONAL to the distance the object is from the light source. That's a very scientific and math rich way of saying, the closer you are to the light bulb, the brighter that bulb is. Or, think of it this way. You can't change how much light comes out of your light bulb. So, to make more light on an object, you have to either move the light closer, or add more lights.
LUMENS.
A LUMEN is a unit of measurement of light. It measures light much the same way. Remember, a foot-candle is how bright the light is one foot away from the source. A lumen is a way of measuring how much light gets to what you want to light! A LUMEN is equal to one foot-candle falling on one square foot of area.
So, if we take your candle and ruler, lets place a book at the opposite end from the candle. We'd have a bit of a light up if we put the book right next to the candle, you know. If that book happens to be one foot by one foot, it's one square foot. Ok, got the math done there. Now, all the light falling on that book, one foot away from your candle equals both…….1 foot candle AND one LUMEN!
Ah, we've confused you. Let's split off from this and talk about the difference between RADIANCE and ILLUMINANCE.
RADIANCE is another way of saying how much energy is released from that light source. Again, you measure it at the source. Unless you're talking about measuring the radiance of something intensely hot, like the Sun. Then you might want to measure it at night, when it's off.
ILLUMINANCE is what results from the use of light. You turn your flashlight on in a dark room, and you light something up. That's ILLUMINANCE. Turning on a light in a dark room to make the burglar visible gives you ILLUMINANCE. It also gives you another problem when you note the burglar is pointing your duck gun at your bellybutton.
Illuminance is the intensity or degree to which something is illuminated and is therefore not the amount of light produced by the light source. This is measured in foot-candles again! And when people talk about LUX, it's illuminance measured in metric units rather than English units of measure. To reinforce that, LUX is the measurement of actual light available at a given distance. A lux equals one lumen incident per square meter of illuminated surface area. They're measuring the same thing, just using different measurement units.
Pretend you're an old photographer, like O. Winston Link, or Ansel Adams. These two gods of black and white photography (and a print made by either can fetch quite a hefty sum of money these days) used a device called a light meter to help them judge their exposure. (There is another way of judging exposure-that's when someone whispers in our ear at a cocktail party, "You silly twit, your fly's come undone!").
These light meters were nifty devices. You could use it to show how much light was falling on an object, light from the sun, and reflected light energy from every thing else. Or you could use it to show how much light energy was reflected off the object itself.
All this brings back two points. Well, three.
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The first point is if we measure the output of a light at the source that gives us one thing.
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The second point is that we use an entirely different unit of measure if we are measuring the results of that light's output.
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The third point is the instructor is right off his trolley, isn't he?
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Now back to the book at the end of the ruler.
We've measured two different things. We have a unit of measure for how much light is produced. We Yankees express that as a foot-candle. Being lazy, we use it all over the place.
Candlepower
Candlepower is a way of measuring how much light is produced by a light bulb, LED or by striking an arc in a Carbon-Arc spotlight. Is it a measure of how much light falls upon an object some distance away? No. That's illuminance. Is it a measure of how well we see an object that is illuminated by that light source? No. That's something all together different, and we are not going there!
Nowadays we use the term CANDELA instead of candlepower. Candlepower, or CANDELA is a measure of how much light the bulb produces, measured at the bulb, rather than how much falls upon the thing you want to light up. Further confusing the matter is beam focus. That's how much candlepower can be focused using a reflector/lens assembly. Obviously, if you project all your light bulbs intensity at a given spot, or towards something, it will be more intense, and the illuminance will be higher.
And here comes the confuser! A candlepower as a unit of measure is not the same as a foot-candle. A candlepower is a measurement of the light at the source, not at the object you light up.
And a candela is the metric equivalent of the light output of that one candle, based on metric calculations. And since using a candle is rather imprecise, the definition was amended to replace a light source using carbon filaments with a very specific light source, see the following:
The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
The above from the National Institute of Standards Reference on Constants, Units, and Uncertainty.
Candlepower is a measure of light taken at the source-not at the target. Foot-candles tell us how much of that light is directed at an object we want to illuminate.
Now, lets convert the lumens, a metric unit of light measurement, to candlepower.
We understand a candle radiates light equally in all directions, its output, in this consideration is not focused by any mechanical means (lenses or reflectors). Pretend for a moment that a transparent sphere one meter in radius surrounds your candle. We know that there are 12.57 square meters of surface area in such a sphere. Remember your Solid Geometry classes?
That one candle (1 Candlepower/Candela) is illuminating equally the entire surface of that sphere. The amount of light energy then reflected from that surface is defined thusly:
The amount of energy emanating from one square meter of surface is one lumen. And if we decrease the size of the sphere to one foot radius, we increase the reflected energy 12.57 times of that which fell on the square meter area.
LUX is an abbreviation for Lumens per square meter.
Foot-candles equal the amount of Lumens per square feet of area.
So, that one candlepower equivalent equals 12.57 lumens.
And for you figuring out LED equivalents, first you must know how many lumens your LED's each produce. Then divide that value by 12.57 and you have candlepower of the LED. You don't have foot-candles, remember foot-candles are illuminance. And we are measuring radiance.
In Summation:
Candlepower is a rating of light output at the source, using English measurements.
Foot-candles are a measurement of light at an illuminated object.
Lumens are a metric equivalent to foot-candles in that they are measured at an object you want to illuminate.
Divide the number of lumens you have produced, or are capable of producing, by 12.57 and you get the candlepower equivalent of that light source.
We've now converted a measurement taken some distance from the illuminated object, converted it from a metric standard to an English unit of measure, and further converted it from a measure of illumination to a measure of radiation!
Light Conversion Link
Conversion Table
|
To convert from |
into |
multiply by ... |
|
Lumens |
Candela steradian |
1.0 |
|
Lumens |
Candle power (spherical) |
0.07958 |
|
Lumens |
Watts |
0.0015 |
|
Lumens per square centimeter |
Lamberts |
1 |
|
Lumens per square centimeter |
Lux |
10000 |
|
Lumens per square centimeter |
Phots |
1 |
|
Lumens per square foot |
Foot candles |
1 |
|
Lumens per square foot |
Foot lamberts |
1 |
|
Lumens per square foot |
Lumens /Square meter |
10.76391 |
|
Lumens per square foot |
Lux |
10.76396 |
|
Lumens per square meter |
Foot candles |
0.0929 |
|
Lumens per square meter |
Lumens/Square foot |
0.0929 |
|
Lumens per square meter |
Phots |
0.0001 |
|
Lumens per square meter |
Lux |
1 |
|
Lux |
Foot Candles |
0.0929 |
|
Lux |
Lumens/square meter |
1 |
|
Lux |
Phots |
0.0001 |
LED Characteristics
Most LEDs have their characteristics specified at a current of 20 mA. If you want really good reliability and you are not certain you don't have worse-than-average heat conductivity in your mounting, heat buildup in wherever you mount them, voltage/current variations, etc. then design for 15 milliamps.
How to make 15 milliamps flow through the LED:
First you need to know the LED voltage drop. It is safe enough to assume 1.7 volts for non-high-brightness red, 1.9 volts for high-brightness, high-efficiency and low-current red, and 2 volts for orange and yellow, and 2.1 volts for green. Assume 3.4 volts for bright white, bright non-yellowish green, and most blue types. Assume 4.6 volts for 430 nM bright blue types such as Everbright and Radio Shack. Design for 12 milliamps for the 3.4 volt types
and 10 milliamps for the 430 NM blue.
Chromatic Diagram
You can design for higher current if you are adventurous or you know you will have a good lack of heat buildup. In such a case, design for 25 ma for the types with voltage near 2 volts, 18 ma for the 3.4 volt types, and 15 ma for the 430 NM blue.
Meet or exceed the maximum rated current of the LED only under favorable conditions of lack of heat buildup. Some LED current ratings assume some really favorable test conditions - such as being surrounded by air no warmer than 25 degrees Celsius and some decent thermal conduction from where the leads are mounted. Running the LED at specified laboratory conditions used for maximum current rating will make it lose half its light output after rated life expectancy (20,000 to 100,000 hours) - optimistically! You can use somewhat higher currents if you heat-sink the leads and/or can tolerate much shorter life expectancy.
Next, know your supply voltage. It should be well above the LED voltage for reliable, stable LED operation. Use at least 3 volts for the lower voltage types, 4.5 volts for the 3.4 volt types, and 6 volts for the 430 NM blue.
The voltage in most cars is 14 volts while the alternator is successfully charging the battery. A well-charged 12 volt lead-acid battery is 12.6 volts with a light load discharging it. Many "wall wart" DC power supplies provide much higher voltage than specified if the load is light, so you need to measure them under a light load that draws maybe 10-20 milliamps.
Next step is to subtract the LED voltage from the supply voltage. This gives you the voltage that must be dropped by the dropping resistor. Example: 3.4 volt LED with a 6 volt supply voltage. Subtracting these gives 2.6 volts to be dropped by the dropping resistor.
The next step is to divide the dropped voltage by the LED current to get the value of the dropping resistor. If you divide volts by amps, you get the resistor value in ohms. If you divide volts by milliamps, you get the resistor value in kilo-ohms or k.
Example: 6 volt supply, 3.4 volt LED, 12 milliamps. Divide 2.6 by .012. This gives 217 ohms. The nearest standard resistor value is 220 ohms.
If you want to operate the 3.4 volt LED from a 6 volt power supply at the LED's "typical" current of 20 ma, then 2.6 divided by .02 yields a resistor value of 130 ohms. The next higher popular standard value is 150 ohms.
If you want to run a typical 3.4 volt LED from a 6 volt supply at its maximum rated current of 30 ma, then divide 2.6 by .03. This indicates 87 ohms. The next higher popular standard resistor value is 100 ohms. Please beware that I consider the 30 ma rating for 3.4-3.5 volt LEDs to be optimistic.
One more thing to do is to check the resistor wattage. Multiply the dropped voltage by the LED current to get the wattage being dissipated in the resistor. Example: 2.6 volts times .03 amp (30 milliamps) is .078 watt. For good reliability, I recommend not exceeding 60 percent of the wattage rating of the resistor. A 1/4 watt resistor can easily handle .078 watt. In case you need a more powerful resistor, there are 1/2 watt resistors widely available in the popular values.
You can put LEDs in series with only one resistor for the whole series string. Add up the voltages of all the LEDs in the series string. This should not exceed 80 percent of the supply voltage if you want good stability and predictable current consumption. The dropped voltage will then be the supply voltage minus the total voltage of the LEDs in the series string.
Do not put LEDs in parallel with each other. Although this usually works, it is not reliable. LEDs become more conductive as they warm up, which may lead to unstable current distribution through paralleled LEDs. LEDs in parallel need their own individual dropping resistors. Series strings can be paralleled if each string has its own dropping resistor.
SUGGESTED LIGHT LEVELS: (footcandles)
Offices, Classrooms 30-100
Industrial High-bay 30-50
Manufacturing (on task) 30-150
Hallways, Corridors 5-10
Merchandising 30-150
Roadway 0.3-1
Baseball, home plate (PRO) 350
Indoor Parking 2-5
Outdoor Parking 0.5-2.0
Illumination is measured in footcandles. One footcandle is the illumination one foot from a standard candle; 30 foot-candles (abbreviated fc) is thirty times higher.
A typical light level for an indoor space is 30-50 fc.
The human eye is very adaptable: full sunlight is about 8000 fc at noon; full moonlight is only 0.01 fc (one hundredth of a foot-candle!). If you reduce the light level people may adapt but there may be a cost.
Lower light levels can lead to poor morale, reduced productivity, increased errors and so on.
There has been speculation that the human errors that caused nuclear problems at Three-Mile Island and Chernobyl could have been avoided if the light levels in the control room had been higher: workers would have been more alert, even at 4:00 a.m. Talk about the cost of poor lighting!
There are intelligent ways of reducing energy costs (retrofit with newer technologies) and there are "not-so-intelligent" ways of reducing energy costs (simply remove one lamp from a 3-lamp fixture).
What Does an "IP" Rating Mean?
An "IP Code" which can usually be found in a brochure or user's manual. The "IP Code" stands for "International Protection Rating" or "Ingress Protection Rating" which classifies the level of protection that electrical appliance provide against the intrusion of solid objects or dust, accidental contact, and water. To be clear it has NOTHING to do with how well a particular device can withstand a drop or shock.
The code is expressed as IPXX with "X" being numbers. For example, has an "IP" Rating of 55.
The first numerical digit indicates the level of protection against solid particles such as dust, dirt or other matter. The second numerical digit indicates the level of protection from harmful ingress of water. The higher the values of each number the higher the resistance to these contaminants. With lower values are more likely intended for use in offices, schools, churches and similar "carpeted areas." With higher numerical "IP" ratings are designed for use anywhere, but especially in environments where dust and moisture could be prevalent. You can use an "IP" number to determine if it's built for the environment you intend to use it or store it in. If you find that it has a letter "X" in its "IP" Rating, it means a value has not been assigned for that type of ingress. For example, you may find a product with an "IP" Rating of "X4."
Here's the scale you can refer to:
First Number Effective Against
0
No protection against contact and ingress of objects
1
Any large surface of the body, such as the back of a hand,
but no protection against deliberate contact with a body part
2
Fingers or similar objects
3
Tools, thick wires, etc.
4
Most wires, screws, etc.
5
Dust protected--Ingress of dust is not entirely prevented,
but it must not enter in sufficient quantity to interfere with
the satisfactory operation of the equipment
6 Dust tight--No ingress of dust; complete protection
The second numerical digit indicates the level of protection from harmful ingress of water: Second Number Effective Against
0 Not protected
1 Dripping water
2 Dripping water when tilted up to 15 degrees
3 Spraying water
4 Splashing water
5 Water jets
6 Powerful water jets
7 Immersion up to 1 meter
8 Immersion beyond 1 meter
