Materials that absorb sunlight well include dark surfaces, water and metal. The sun's light energy arrives as a mixture of visible light, ultraviolet and infrared; some materials absorb all these wavelengths well, while others are better suited to a certain restricted type of light. Most materials convert absorbed sunlight into heat energy; living things, however, turn the sun's rays into chemical energy and the building blocks of life.
Water: Global Heat Reservoir
All water absorbs a lot of energy from the sun, with the amount absorbed directly dependent on how large the body of water is. For example, an ocean will absorb more energy than a lake. Water releases the energy back into the surrounding air very slowly. This is why when you go to the beach, temperatures are generally a few degrees cooler than they are further inland, as the water takes in much of the heat.
Spirulina: Efficient Algae
Spirulina, an algae found in open sources of warm, fresh water, absorbs energy from the sun. Called the "sun food," spirulina contains phycocyanin which helps the plant absorb the entire light spectrum, enabling it to absorb more sun energy than other plants. It is often used as a dietary supplement as it stimulates red blood cell production and can also protect cells against aging.
The Concrete Jungle
Concrete absorbs solar energy well, which is why sidewalks tend to get so hot under direct sunlight. Partly for this reason, concrete is not a popular building material for homes or office spaces. Painting concrete can make a slight change in solar energy absorption. For example, white paint will deflect more light while black paint will absorb more. However, the difference is minimal, particularly if the concrete is in direct sunlight.
Darker Means Hotter
Darker colors tend to absorb more energy from the sun than objects with lighter colors. Someone wearing a white T-shirt in the summer will find that he is cooler than someone wearing a black or dark-colored shirt. This is true of all materials which have dark colors. Other dark surfaces include blacktops, paved roads or rooftops.
Warming up to Metal
Most metals absorb solar energy well, as anyone who has touched a car sitting outside in the sun for some time should know, although you may notice that a white car is a tad cooler to the touch than a black one thanks to the lighter color. Building features made of copper, stainless steel or other metals retain the sun's energy.
How Do I Measure Lux Lighting?
The electromagnetic spectrum spans radio waves, with wavelengths of a millimeter or more, to gamma rays, with wavelengths less than a trillionth of a meter. Ultraviolet light has wavelengths from about 1 to 400 nanometers (nm), where a nanometer is 10-9 meters. Visible light, or more simply light, is electromagnetic radiation in the range of approximately 400 to 750 nm, while infrared light has wavelengths from about 750 to 2500 nm.
Radiant and Luminous Flux of a Light Source
Like all electromagnetic radiation, light is a form of energy. Radiant flux, or power, is the total energy per second emitted from a source. Radiant flux is expressed in watts (W) and results from the output at all wavelengths.
Unlike radiant flux, the luminous flux of a source is the output power in only visible wavelengths, which excludes ultraviolet and infrared radiation. In SI units, luminous flux is expressed as lumens (lm) and is determined by weighting the output power to account for the human eye’s sensitivity to various wavelengths.
For example, a 100-W incandescent bulb consumes 100 watts of electrical power to produce only 10 watts of visible light. The remaining power becomes 90 watts of infrared light and waste heat, which are visually useless. The radiant flux includes infrared light and would be close to 100 watts. The luminous flux, however, would be only the 10 watts of power in visible wavelengths, weighted to reflect our eyes' response, to yield about 1,600 lumens.
Relationship Between Luminous Flux and Lux
Luminous flux measures how we perceive the brightness of a light source. Many situations, however, require knowing the amount of light shining on an object. To have a safe warehouse space with good visibility, a property manager must determine floor-level illumination with a light meter. Illuminance is the amount of luminous flux per unit area and is expressed as lux (lx) in SI units. One lux is one lumen per square meter and therefore measures the human perception of the brightness of light at a given point.
How Light Diminishes With Distance
For a source that shines equally in radial directions, the amount of light on a unit area decreases by the inverse-square law. If a certain light bulb shines on a spot 1 meter away, the illuminance there might be 900 lux. A spot 2 meters away would be at double the distance and therefore would have only 1/22, or one-quarter the illuminance, which would be 225 lux. A spot 3 meters away would be at triple the distance and would have only 1/33, or one-ninth the amount of light, only 100 lux.
If the light originates from a source that is not a point, such as from a tubular fluorescent bulb, or if the light is manipulated by lenses, mirrors or reflectors, then the inverse-square law may not apply.
Lux Measurement Procedure
The human eye has maximum sensitivity to light in the band of green-yellow wavelengths, from about 520 to 580 nm. This sensitivity falls to less than 15 percent of the maximum for red at one end of the spectrum and 10 percent for blue at the other end. Most lux meters use a single silicon sensor with optical filters to imitate this sensitivity profile to various degrees.
Lux meter measurements are generally accurate with incandescent lights because the spectral output is very similar for all incandescent sources. However, the many types of LEDs and fluorescent lights (high-intensity discharge, metal halide, high-pressure sodium and cool white office lights), each have different spectral profiles. High-quality lux meters closely reproduce the eye’s visual response and can accurately measure these other light sources, too.
Lux meters that poorly follow the eye’s sensitivity curve require a color correction factor (CCF) that adjusts the lux meter measurement for LEDs or fluorescent lights. Therefore, you need to know the type of light you are measuring and use the appropriate CCF.
The lux measurement procedure simply requires positioning a meter’s sensor on the surface or location where you wish to measure the incident light. The sensor should face the light source at a right angle. If the sensor is not perpendicular to the light, the measurement will be incorrect, though some lux meters have a cosine correction to account for the angle. Meters that require a color correction factor may have a means of inputting the CCF to adjust the result for LEDs or fluorescent lights; otherwise, you will have to manually multiply the measured lux by the CCF.
Some Facts About Visible Light Waves
Visible light, which travels at a dizzying 186,282 miles per second through space, is just one part of light's broad spectrum, which encompasses all electromagnetic radiation. We can detect visible light because of cone-shaped cells in our eyes that are sensitive to the wavelengths of some forms of light. Other forms of light are invisible to humans because their wavelengths are either too small or too large to be detected by our eyes.
The Hidden Nature of White Light
What we call white light is not a single color at all but the full spectrum of visible light all combined. For most of human history, the nature of white light was completely unknown. It wasn't until the 1660s that Sir Isaac Newton discovered the truth behind white light using prisms – triangular bars of glass – to break the light into all of its different colors and then reassemble them again.
When white light goes through a prism, its component colors are separated, revealing red, orange, yellow, green, blue, indigo and violet. This is the same effect you see when light passes through water droplets, creating a rainbow in the sky. When those separated colors shine through a second prism, they are brought back together to form a single beam of white light.
The Light Spectrum
White light and all the colors of the rainbow represent a small part of the electromagnetic spectrum, but they are the only forms of light we can see because of their wavelengths. Humans can only detect wavelengths between 380 and 700 nanometers. Violet has the shortest wavelength we can see, while red has the largest.
While we don't normally call other forms of electromagnetic radiation light, there is little difference between them. Infrared light is just outside our vision with a wavelength bigger than red light. Only with instruments like night-vision goggles can we detect the infrared light generated by our skin and other heat-emitting objects. On the other side of the visible spectrum, smaller than violet light waves are ultraviolet light, X-rays and gamma rays.
Light Color and Energy
Light color is usually determined by the energy being produced by the source that emits it. The hotter an object is, the more energy it radiates, resulting in light with shorter wavelengths. Cooler objects create light with longer wavelengths. For example, if you fire up a blowtorch, you will find its flame is red at first, but as you turn it up, the color becomes blue.
Similarly, stars emit different colors of light because of their temperatures. The surface of the sun has a temperature around 5,500 degrees Celsius, causing it to emit a yellowish light. A star with a cooler temperature of 3,000 C, like Betelgeuse, emits red light. Hotter stars like Rigel, with a surface temperature of 12,000 C, emit blue light.
The Dual Nature of Light
Experiments with light in the early 20th century revealed that light had two natures. Most experiments showed that light behaved as a wave. For example, when you shine light through a very narrow slit, it expands as a wave does. In another experiment, however, called the photoelectric effect, when you shine violet light on sodium metal, the metal ejects electrons, suggesting that light is made of particles called photons.
In fact, light behaves as both a particle and a wave and appears to change its nature based on which experiment you conduct. In the now-famous two-slit experiment, when light encounters two slits in a single barrier, it behaves as a particle when you are looking for particles but also behaves as a wave if you are looking for waves.
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