PRINCIPLES OF LIGHT

Basic Theory
Lighting Metrics: Quantity, Quality, Efficiency
     Quantity of Light
     Quality of Light
     Fixture Efficiency
Color Metrics
     Basic Principles
     Metrics
     Specifying Color

Basic Theory

Light is a form of radiant energy that travels in waves made up of vibrating electric and magnetic fields. These waves have both a frequency and a length, the values of which distinguish light from other forms of energy on the electromagnetic spectrum.

Visible light, as can be seen on the electromagnetic spectrum, represents a narrow band between ultraviolet light (UV) and infrared energy (heat). These light waves are capable of exciting the eye's retina, which results in a visual sensation called sight. Therefore, seeing requires a functioning eye and visible light.

Light can be produced by nature or by humans. "Artificial" light is typically produced by lighting systems that transform electrical energy into light. Nearly all lighting systems do so either by passing an electrical current through an element that heats until it glows, or through gases until they become excited and produce light energy.

Incandescent light sources are an example of the first method, called incandescence. Current is passed through a filament, which heats until it glows. Because this method is considered wasteful (most of the energy entering the lamp leaves it as heat instead of visible light, other light sources were pioneered that rely on the gaseous discharge method, including fluorescent, high-intensity discharge (HID) and low-pressure sodium light sources.

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Lighting Metrics: Quantity, Quality, Efficiency

Because some 85% of human impressions are visual, proper quantity and quality of light are essential to optimum performance. The mission of lighting management is to provide the optimum quantity and quality of light to its users at the lowest operating cost.

Lighting metrics are used to understand and predict how a lighting system will operate. They deal with quantity of light (light output and light levels), quality of light (brightness and color), and fixture efficiency (electrical efficiency and how much light leaves the fixture).

: Quantity of Light

Luminous Flux (Light Output). This is the quantity of light that leaves the lamp, measured in lumens (lm). Lamps are rated in both initial and mean lumens.

Initial lumens indicate how much light is produced once the lamp has stabilized; for fluorescent and high-intensity discharge (HID) lamps, this is typically 100 hours.

Mean lumens indicate the average light output over the lamp's rated life, which reflects the gradual deterioration of performance due to the rigors of continued operation; for fluorescent lamps, this is usually determined at 40% of rated life.

A number of factors affect a lamp's light output over time, including lamp lumen depreciation, the lamp's interaction with the ballast, supply voltage variations, dirt or dust on the lamp, and the ambient temperature in the fixture.

To avoid confusion, note that "lumen output" is a term also used to describe a fixture's light output, not just a lamp's. Even more factors can affect light output in this case, including the distribution characteristics of the fixture, fixture surface depreciation, and dirt and dust buildup.

Illuminance (Light Level). This is the amount of light measured on the workplane in the lighted space. The workplane an imaginary horizontal, tilted or vertical line where the most important tasks in the space are performed. Measured in lux, light levels are either calculated or, in existing spaces, measured with a light meter. One lux is one lumen per square meter. Lumens can be produced as either initial or maintained quantities.

Initial footcandles indicates a light level after new lamps are installed.

Maintained footcandles indicates a light level after light loss factors are considered over a period of time. Light loss factors include those affecting light output (see above) and also room surface reflectances, room size/proportions, dirt and dust buildup. While light output may describe either the output of a light source or fixture, maintained footcandles always takes into account the efficiency of the fixture in transmitting light to the workplane.

The human eye is a sophisticated piece of machinery; it is able to adjust to a wide range of light levels, including about 10,000 footcandles on a sunny day to about 0.01 footcandles under full moonlight. However, optimum ranges of light levels have been established for various tasks so that those tasks are performed most efficiently (reading a magazine, for example, would be difficult undermoonlight, while 10,000 foot candles would be excessive).

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: Quality of Light

Luminance (Photometric Brightness). The light that we actually see, brightness can be measured as the light leaving a lamp, or the light reflecting from an object's surface. If not controlled, brightness can produce levels of glare that either impair or prevent a desired task being performed. Glare can be described as direct or reflected glare, which can then result in discomfort or disability. Direct glare comes straight from the light source.

Reflected glare shows up on the task itself, such as a computer screen.

Discomfort glare does not prevent seeing makes it uncomfortable.

Disability glare prevents vision. A popular example is holding a glossy magazine at a certain angle; a veiling reflection results, impairing our reading of the page.Color. The color quality of a lamp is revealed as its color temperature rating and Color Rendering Index (CRI) rating.

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: Fixture Efficiency

There are two ways to look at a light fixture's (luminaire's) efficiency; one indicates how well the lighting system transforms electrical input into useful light output, and the other indicates how well the fixture itself transmits light from the lamp(s) to the workplane.

Electrical Efficiency. Lighting systems require electrical input to work. This input is measured in watts (W), a measure of required electric power. A lighting system's rated input wattage, therefore, is the amount of power required for it to work at any given instant of time.

Lamp manufacturers publish nominal wattage ratings for their lamps; when fluorescent and HID lamps are operated as a system with a ballast, however, a new rated wattage will result, published by the ballast manufacturer. Ballast manufacturers publish up to three input wattage ratings. The ANSI number is the result of a standardized ANSI test of that given ballast manufacturer's ballast operating a given compatible lamp type (often called the "bench test" because the lamps and ballasts are operated bare on a bench). The next one or two are the manufacturer's ratings for tests in actual open and/or enclosed fixtures.

One way to compare the electrical efficiency of lamp-ballast systems is to determine a common light output level, then compare the input wattage for various systems.A more popular way of achieving a comparison of the relative efficiencies of lighting systems is to use efficacy, expressed in lumens per watt (LPW or lm/W). To determine a system's efficacy, divide its lumen output by its rated input wattage.

Fixture Efficiency. The light fixture's physical characteristics will affect how much light will leave the fixture and how much will be directed at the task. Factors that affect the efficiency of the fixture include its shape, the reflectance of its materials, how many lamps are inside the fixture (and how close they are to each other), and whethershielding material such as a lens or louver is used to soften or scatter the light.

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Color Metrics

: Basic Principles

Light, like all forms of radiant energy, is represented on the electromagnetic spectrum. Traveling in waves, light is differentiated from other forms of radiant energy such as heat and X-rays by the frequency and length of its waveform. A narrow band on the spectrum is visible light, composed of different colors/wavelengths, from violet at 380 nanometers to red at 620-760 nanometers. An even balance of these light waves composes white visible light. To see this principle firsthand, look at a rainbow, which results from sunlight being refracted by droplets of moisture in the air, or simply shine a beam of white light through a glass prism to make a rainbow of colors appear on the other side.

Visible light cannot be seen, however. If we turned on a flashlight in a dark room, the beam of light we are seeing is actually light being reflected from a multitude of dust particles in the air. Therefore, we see objects only when light is reflected or emitted from them. And that is how we see color.

All objects are chemically oriented to absorb certain wavelengths of light and reflect others. The ones that are reflected are perceived by the human eye to be the color of the object. A red object being struck by visible white light will absorb all wavelengths except red, which is reflected, and so we see the object as red. A pure white object reflects all wavelengths and absorbs none. A pure black object absorbs all wavelengths and reflects none.

This is where a great amount of art comes into lighting because few lamp types produce pure white light. Some lamps produce light that is saturated in blue and green, others red and yellow. A red object struck by light that contains only blue and greenwavelengths would not appear red as if it were under sunlight. A low-pressure sodium lamp produces light saturated in yellow, which means that all objects struck by it will appear yellow, black or a shade of gray.

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: Metrics

To understand how a lamp's light will affect the color of objects in the space, three metrics are used, including spectral power distribution, color temperature and color rendering.

Spectral power distribution shows the visible light spectrum and the wavelength composition for the light from the lamp (see illustration). The spikes indicate that the light is stronger in revealing certain colors.

A spectral power distribution curve for a 400K lamp with a triphosphor (red, blue, green) coating to improve color rendering.

 

Color temperature, expressed on the Kelvin scale (K), is the color appearance of the lamp itself and the light it produces.

Imagine a block of steel that is steadily heated until it glows first orange, then yellow and so on until it becomes blue or bluish-white. At any time during the heating, we could measure the temperature of the metal in Kelvins (Celsius + 273) and assign that value to the color being produced, resulting in a "color temperature." Computer software performs this function for today's lamps, giving them a color temperature rating found in the manufacturers' literature.

For incandescent lamps, the color temperature is a "true" value; for fluorescent and high-intensity discharge (HID) lamps, the value is approximate and is therefore called correlated color temperature. In the industry, both terms - - color temperature and correlated color temperature - - are often used interchangeably. The color temperature of lamps makes them visually "warm," "neutral" or "cool" light sources.

Lamps with a lower color temperature (3500K or less) have a warm or red-yellow/orangish-white appearance. The light is saturated in red and orange wavelengths, bringing out warmer object colors such as red and orange more richly.

Color Temperature (Scale indicating light color of light source)/ unit: K (kelvin)

The color of a light emitting object (light source) is called its light source color. “Fluorescent lamps are classified, according to the color they emit, into cool white, daylight, etc., which are terms used to describe the light source color. For example, the light from incandescent lamps looks yellowish-orange, that from fluorescent lamps is whitish, and that from mercury lamps, bluish-white. Such light source colors are generally indicated by the color temperature (unit K: kelvin). Generally, the light that is radiated from an object of lower color temperature will look reddish. As the color temperature increases, the light becomes whiter, and as it further increases, the light looks bluish.

Lamps with a mid-range color temperature (3500K to 4000K) have a neutral or white appearance. The light is more balanced in its color wavelengths.
Lamps with a higher color temperature (4000K or higher) have a cool or bluish-white appearance. Summer sunlight has a very cool appearance at about 5500K. The light is saturated in green and blue wavelengths, bringing out cooler object colors such as green and blue more richly.

These three photos simulate the effects of color temperatures on objects. (Left) a warm light source is used, enhancing reds and oranges while dulling blues and greens; (Middle) a neutral source is used; (Right) a cool source is used enhancing blues and greens while dulling reds and oranges

Color rendering, expressed as a rating on the Color Rendering Index (CRI), from 0-100, describes how a light source makes the color of an object appear to human eyes and how well subtle variations in color shades are revealed. The higher the CRI rating, the better its color rendering ability.

According to the IESNA, color rendering is the "measure of the degree of color shift objects undergo when illuminated by the light source as compared with the color those same objects when illuminated by a reference source of comparable color temperature.“

Imagine two objects, one red, one blue, that are lighted by a cool light source with a low CRI. The red object appears muted while the blue object appears a rich blue. Now take out the lamp and put in a cool light source with a high CRI. The blue object still appears a rich blue, but the red object appears more like its true color.

Standard incandescent lamps enjoy a CRI rating of 100. Fluorescent lamps are in the 52-95 range, depending on the lamp. Advances in phosphor technology have enabled fluorescent and HID lamps to advance greatly in color rendering.

As stated in the IESNA definition, to compare any two given lamps, they must have the same color temperature for the comparison to have any meaning.

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: Specifying Color

When specifying color characteristics for a lamp, numerous psychological factors must be considered depending on the lighting goals for the space. Here are a few general tips.

Warm light sources are generally preferred for the home, restaurants and retail applications to create a sense of warmth and comfort, while neutral and cool sources are generally preferred for offices and similar applications to create a sense of alertness.

In addition, in retail applications, color is a critical design decision because buyers need to be able to choose products of the correct color, both to enhance the chance of its sale and to reduce the chance of it being returned once the buyer gets outside and sees it under sunlight. In this or any other application where the occupant needs to see the right color, good color quality is essential.

In other applications such as parking lots, color is not an important factor, so lowcolor-rendering lamps can be specified.

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