How is brightness determined

One lux is 1 lumen metre You are perhaps most likely to come across it in specifications for the sensitivity of electronic imaging devices such as video cameras.

Highly sensitive sensors need only a few lux to operate, gathering in only a few photons. A less sensitive sensor can compensate by increasing its area, as the pupil of your eye does when the light grows dim. There is one more step of interest here.

Just as power is of most interest to humans in practical situations remember the unit horsepower , to show how many horses a new-fangled steam engine could replace , so lighting is of most interest when we put it to work. The candlepower performed the same function as the horsepower for many years as the unit of light intensity. Take one standard candle — of fixed size and constituents — and compare its output with the source that needs calibrating.

The luminous intensity of a source depends on the power emitted in a particular direction, as we're usually interested in lighting something, not just lighting. The modern unit is the candela. If a source is of 1 candela, uniformly in all directions, then it emits a luminous power of 4PI lumens.

Intensity is energy delivered per unit time per unit area. The intensity of light is a measurement of photon irradiance, which is the number of photons delivered per square meter per second.

You can measure intensity with a photoelement, such as a solar cell or a photomultiplier, which converts light to electric current. As the electric current varies strictly with the intensity of the light, an objective measurement is possible.

The more energetic the photon the shorter its wavelength , the fewer will be the number of photons required for a given intensity. Brightness is a "subjective" quality of light. It depends on the perception of whoever is viewing the light. It can't be objectively measured, but it can be scaled, so that the same viewer or viewers with similar perceptions can agree that certain light is more or less bright.

In astronomy, for example, stars may be graded according to their apparent magnitude, which is their brightness in comparison to a very bright benchmark star. Brightness may also be called luminous flux. Photon is a term used to describe the particle attribute of light. A photon may be considered the smallest packet of energy into which light can be separated. You could deliver greater intensity by emitting light of a shorter wavelength, or by increasing the surface area emitting the light greater surface area means more electrons emitting more photons.

Photons are considered to be "massless particles", but since a photon has energy, it must have mass. This conundrum is solved by saying that photons have relativistic mass when they are traveling, but possess zero rest mass because they are never at rest. The wavelength of a photon determines its energy, and its energy determines its wavelength. The shorter the wavelength, the shorter the cycle of the wave, the greater its frequency, and the more periods of the wavelength can be crammed into a unit time.

In buildings, lux can be measured with a device called lux meter which will give an accurate reading of the illuminance. In addition to light itself, there are other factors to consider when measuring the amount of light emitted. Most lamps are encased inside a light fitting consisting of elements such as diffusers and reflectors. These elements, along with things including the shape and number of lamps, will have an effect on the brightness and distribution of light from the luminaire.

Luminaire efficacy factors these elements in to give a more accurate measurement of the lumens given off by the entire luminaire, in a fully assembled state. This ratio is basically the number of lumens a light gives off for each watt of power it consumes, referred to technically as luminous efficacy.

The candela cd is the standard unit of luminous intensity or brightness of a light in a specific direction. Because this is a biological process, there is a wide range of human diversity, and so color and light will be perceived differently by each person. You may recall the social phenomenon that divided the internet as to whether t he dress was white and gold or blue and black. The dress was perceived in different colors.

There is a science behind why no one agreed on the color. An explanation for the difference in color perspective was explained by Bevil Conway, a neuroscientist who studies color and vision at Wellesley College. In addition, as you can see in the image above, the cone peaks at nanometers.

This would indicate that under normal lighting conditions such as daylight, the eye would be the most sensitive to the yellowish-green color.

The light at this wavelength produces the highest brightness when compared to light at the other wavelengths. From the sun to automotive headlights to an LED single color indicator, light sources are all around us. Different types of lights affect the way our eyes view colors. In daylight, we tend to see blue and green most. In this section, we compare common items such as billboards, headlights and smartphones to their respective "brightness" and its units of measurement.

First, let's give you a baseline from the Sun and Moon in terms of brightness. The sun is the Earth's main source of light. The a pparent magnitude is a measure of the star's flux brightness. The Sun's apparent magnitude is The moon's surface reflects the light of the Sun.

Its apparent magnitude is To fully understand the concept of how the moon is 14x fainter than the Sun, but only half its actual apparent magnitude, we need to appreciate logarithmic scale. Logarithmic scale is a "scale of measurement that uses the logarithm of a physical quantity instead of the quantity itself.

On a logarithmic scale, each tick mark on the scale is the previous tick mark multiplied by some number. WinMate, a global leader in developing advanced mobile technologies, compared different lighting conditions and its source. You can view the chart here. It would make sense if the brighter the display, the better it can compete with the Sun. At night, the sign is competing instead with the Moon, and so it appears much brighter even when operating at the same intensity.

With evolving technology, many states in the U. Code of Industry Principles agrees fully stating, " We are committed to ensuring that the ambient light conditions associated with standard-size digital billboards are monitored by a light-sensing device at all times. Brightness regulations are an important aspect when implementing electronic signs. Lumens are the standard measurement of light output in automotive headlights.

A car headlight is around lumens with a high-beam headlight at lumens. For headlights, it depends on the application and the desired effect. The article goes on to say, "Improved LED performance, lower power consumption, and flexible design were the first enablers.