# The Physical Properties of Light

There a number of terms used to describe and measure light. This video from Philips Lighting explains them well:

Luminous Flux (measured in lumens (lm))- flow of light energy through a given square meter of air. 1 lumen = 1 cd*sr (solid radian)

Luminous Intensity (measured in lumens per solid radian, or lumens per solid angle, lm/sr also known as candela (cd)) – amount of light energy at the surface. Intensity changes with direction from the source. Luminous intensity also changes with distance according to an inverse square law. The luminous intensity decreases with the square of the distance from the source. In other words, the intensity at 2 meters is 1/4 the intensity at 1 meter. At 3 meters, the intensity is 1/9 the intensity at 1 meter.

Illuminance – (measured in lumens per square meter (lm/m2  or lux(lx)) – density of light energy on a surface. Illuminance is dependent on distance and angle.

Typical illuminance levels:

• Incidental use: 200 lx
• General work/office use: 300 lx
• Task lighting: 5-700 lx
• Specialized work: 1000-1500 lx
• Midday sun: 32K – 100K lx

-Descottes, Hervé; Ramos, Cecilia E. (2013-07-02). Architectural Lighting: Designing With Light And Space (Architecture Briefs)

Exitance (aka emittance)- (measured in lumens per square meter (lm/m2) or lux(lx)) – total light exiting a surface. Think of the difference between the light coming from a black surface and a white surface of the same material. The white surface has higher exitance.

Luminance (measured in candelas per square meter, (cd/m2 ))- refers to the specific way light comes off a surface. Luminance is dependent on direction and specularity. Think of the difference between a mirror and a flat white surface, both of the same size. They have the same exitance, but the luminance from them is quite different.

Brightness is a subjective term referring to how we as humans perceive light. Though it’s often confused with luminance, our perception of brightness is dependent as much on our mood, energy, and other subjective factors as it is on luminance levels.

Luminous efficacy (measured in lumens per watt (lm/W)) – a measure of a light source’s ability to convert electrical energy into light. High luminous efficacy means less environmental impact for the same amount of light.

Hue describes light at different wavelengths, or what we see as colors. Humans can typically see wavelengths in a range from 380 to 78o nanometers, with our peak at about 550nm.

Lightness refers to a color’s subjective brightness (light blue versus dark blue), while saturation refers to the color’s purity or intensity against a counterpoint of gray.”

-Descottes, Hervé; Ramos, Cecilia E. (2013-07-02). Architectural Lighting: Designing With Light And Space (Architecture Briefs)

Color temperature (measured in degrees Kelvin (°K)) refers to the warmth or coolness of a hue. “[T]he Kelvin scale for color temperature of light sources ranges from reds (1,500k– 2,000k) to yellows (2,500k– 3,000k) to whites (3,000– 4,000k) to pale blues (4,000k– 6,500k) and then sky blues (6,500k and higher)— the higher the Kelvin temperature, the cooler the color hue.”

-Descottes, Hervé; Ramos, Cecilia E. (2013-07-02). Architectural Lighting: Designing With Light And Space (Architecture Briefs)

Color Rendering Index is a measure of how well a given light source renders the colors of an object. A source which renders all colors. Gas discharge lamps generally have low color rendering indices, because they emit a discontinous spectrum of light, while incandescent sources have a high CRI. The CRI of an LED depends on the phosphors used to make the source.

## Lamp? Bulb? Luminaire? Fixture?

Generally, the component that generates the light, what consumers might call a bulb is called a lamp. Lamps are mounted in fixtures or luminaires, which are the housings that hold the lamp and any diffuser or lenses as well as the controls for the lamp. Lenses channel and focus the light in a given direction, and diffusers spread the light out. Baffles or louvers are used to block the light so that it doesn’t shine directly on the eye. Beyond these basic terms, there are also some specific architectural or design terms, and stage lighting terms that are useful to know,

## Common Lighting Technologies

• Incandescent
• Tungsten-Halogen
• Gas Discharge lamps
• Low pressure – Mercury (common fluorescent), Sodium
• High Pressure – Mercury, Sodium, Xenon
• Dilectric Barrier Discharge
• Metal Halide
• Sulfur
• LED

## Comparison of Lighting Technologies

A comparison of the efficacy, power usage, color rendering, and lifetime of various lighting technologies. It’s worth noting that incandescent and halogen lighting are the sources that render color most faithfully across a broad spectrum.

This chart explains why lighting designers have been reluctant to switch from incandescent sources, despite the fact that they rank low on lumen efficacy.