White light emitting diodes (LEDs) can be fabricated from blue LEDs with a yellow-emitting phosphor or substrate. These combined blue-yellow emissions fool the human eye into perceiving white light. This project will focus on white LEDs and their photometrics: luminous flux, luminous intensity, luminous efficacy, and illuminance.
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Electrostatics Notice
Users who mishandle LEDs can damage them through electrostatic discharge (ESD). ESD-damaged LEDs will exhibit abnormalities such as failure to light at low currents, low forward voltage, and high reverse leakage current. Anyone handling LEDs should wear a grounded anti-static wrist band or use similar anti-static countermeasures.
White LED Photometrics
Components Survival Guide: LEDs Part 1 covered color LEDs that emit only a narrow part of the visible spectrum. Effectively, LEDS are current-dependent light sources. White LEDs are no exception, but their wider distribution of colors makes certain metrics such as peak wavelength and dominant wavelength ineffective.
The human eye is well-adapted to sunlight (at sea level). Consequentially, mid-day sunlight is a common reference for “neutral” white light.
But an artificial light source may visibly favor one color or another…
- Warm white LEDs skewed toward yellow emissions
- Cool white LEDs skewed toward blue emissions
Fortunately, physicists created the idea of a color space or color gamut wherenumeric values align with different color combinations. As of the early 21st century, the Commission Internationale de l’éclairage (CIE) color spaces are the most relevant since they map these 3 color values to a 2-dimensional multi-colored chart. Thanks to the CIE 1931 diagram (shown in Figure 1), visible colors map to x y coordinates…
Correlated Color Temperature
Near the center of the CIE chart is an arc that approximates the white light of incandescent sources. This arc (the Planckian Locus) is marked with temperature graduations measured in degrees Kelvin (°K).
LED manufactures use the Correlated Color Temperature (CCT) scale to express where their white LED emissions fit along the Planckian Locus.
Table 1: Light Sources and their typical correlated color temperature
| LIGHT SOURCE | CORRELATED COLOR TEMPERATURE (°K) |
| Candlelight | 1,500 |
| Sunrise/sunset | 3,200 |
| 1 h before/after dusk/dusk | 3,400 |
| LED Luxeon White, 5 W | 5,500 |
| Sunny daylight around noon | 5,800 |
| Fluorescent lamp | 2,700-8,000 |
Two important notes…
- The CCT scale is a visual metaphor and does not express the actual temperature of an LED. Generally, LED junctions operate below 425 °K.
- Because terms warm white and cool white are rooted in human intuition and instinct, the CCT scale ironically contradicts it by associating cooler white with higher temperatures.
Color Rendering Index
Color Rendering Index (CRI) is another metric associated with white LEDs. Effectively, CRI answers the question: “compared to the sun, how accurate do colors look under this light source?” In other words, CRI expresses color fidelity.
The CRI scale ranges from 0 to 100. A CRI of 100 approximates mid-day sunlight. To an average adult, sources with CRI lower than 60 will appear tinted a particular color.
Table 2: Some light sources and their typical color rendering index (CRI)
| LIGHT SOURCE | COLOR RENDERING INDEX |
| Low-pressure sodium lamp | 0-18 |
| High-pressure sodium lamp | 25-82 |
| High-pressure mercury lamp | 16-58 |
| Warm White Fluorescent lamp | 55 |
| Cool white fluorescent lamp | 65 |
| LED Luxeon white 5 W | 70 |
| Deluxe warm white fluorescent lamp | 73 |
| Daylight fluorescent lamp | 79 |
| Metal halide lamp (4,200 K) | 85 |
| Deluxe cool white fluorescent lamp | 86 |
| Incandescent lamp (100 W) | 100 |
Low CRI is not necessarily bad. For example: low-pressure sodium lamps have high energy efficiency, yet low CRI. Together CRI and CCT are important factors in choosing a light source in locations were color rendering matters (example: textiles, hotels, galleries).
Luminous Flux (lumens)
Luminous flux (unit of measure: lumens, abbreviated: lm) rating describes the total light energy a light source emits per unit time (dQ/dt). This rating is weighted to the sensitivity of human vision and only accounts for visible light emissions.
In the 20th century, consumer-grade incandescent bulbs used wattage to express bulb brightness. In practice, this was acceptable only for informally comparing one incandescent product to another. But the lumen rating is more reliable since it describes the light leaving the source rather than the power entering.
Luminous Intensity (W sr-1)
Luminous intensity expresses the brightness of the LED beam in a general direction. The unit of measure: watts per steradian (W sr-1) expresses light power (watts) and divides it by a solid angle (steradians).
On the other hand, luminous flux expresses the total amount of light exiting the LED in any direction. Because of this, 2 light sources may have the same lumen rating, yet one may cast a brighter spot than the other (example: a spotlight versus a floodlight).
Luminous Efficacy (unit: lm W-1)
The luminous flux divided by the amount of power used gives the luminous efficacy of the LED (unit: η or lm W-1). This rating can help judge the efficiency of different lamps.
Illuminance (Illumination)
Illuminance (unit: 1 lux = l lm m-2) expresses the brightness of an illuminated surface positioned some distance away from the LED. This photometric is useful in expressing how bright a surface will appear under the light source.
Table 3: Some typical light sources and their Illuminances
| SCENE | ILLUMINANCE (lux) |
| Clear night under a full moon at zenth | 0.1 to 0.2 |
| Street lighting | 3 to 30 |
| Domestic lighting | 40 to 150 |
| Bright office | Up to 400 |
| Winter day (cloudy) | 3,000 to 4,000 |
| Summer day (clear) | 32,000 to 100,000 |
LED Disadvantages
LEDs have disadvantages related to heat and heat dissipation.
As forward current increases, an LED will grow warm under its own joule heating (albeit not nearly as much as an incandescent bulb). If the LED junction grows hot enough, it will reach a point where an increase in current causes a decrease in luminous flux. Effectively: the work-around is to either use aggressive heatsinking or simply add more LEDs. This is why LED assemblies often contain arrays of LEDs rather than a single giant LED.
LED application circuits and drive methods will be covered in the 3rd and final part: “Components Survival Guide: LEDs Part 3.”
References
| [1] | P. Flesch, “Light and Light Sources,” in Light and Light Sources – High Intensity Discharge Lamps, Berlin, Springer, 2006, pp. 1-50. |
| [2] | D. Tree, Artist, Red and black cars under low pressure sodium lamps.jpg. [Art]. Wikimedia Commons, 2013. |
| [3] | R. Hui, “Unifying PET theory with colorimetry,” in Photo-Electro-Thermal Theory for LED Systems, Basic Theory and Applications, Cambridge, UK, Cambridge University Press, 2017, pp. 61-63. |
| [4] | Haade, Artist, Solid Angle.png. [Art]. Wikimedia Commons, 2007. |
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