LUX? LUMEN? WATT? CANDELA?
The following is an overview of common vocabulary used when describing and discussing light. An attempt has been made to keep the terms simple and easy to understand. For detailed technical information, see the corresponding technical pages.
First, let us take a look at how we measure light.
Previously the indications on a light bulb box, 25, 40, 60, 75 or 100 watts, provided a general idea of how bright a light would be in your home living room. Today, many more factors need to be taken into account in order to assess the quality and effectiveness of a light. It is no longer enough to talk about watts.
In principle, start by differentiating between the physical variables (such as wattage) and photometric qualities (for example, lumens) which measure the effects or perceived brightness of light to the human eye. Keep in mind that eye sensitivity is personal and varies greatly depending on an individual’s age, eyes and time of day, therefore extremely relative.
Measuring Physical Qualities
In physics, a watt is a unit of power used to quantify the rate of energy transfer over a period of time. A watt can serve to measure a water heater, the output of mechanical work or the electrical power converted into light.
Radiant intensity, radiant power, radiance and irradiance are other physical qualities of light that can be measured with watts.
In our modern times, light measurement in watts or other specifications such as lumen/luminous flux, lux/illuminance are limited, for example in the case of LED, because do not describe enough about the actual light output.
Measuring Light Quantity
Luminous flux/lumen is the amount of light emitted from a light source (bulb). This is measured in lumens (lm) which is the total light output of the bulb, regardless of the direction the light radiates, and corresponds to the ‘performance’ of a light bulb in watts.
Illuminance is the quantity of light reaching a given surface. It is measured in lux (lx) per square meter and corresponds to ‘intensity’ of light.
One lux means 1 lumen per m2, which is approximately the amount of light created by a candle at a distance of 1 meter gives off.
The light that illuminates a surface/person is being measured, not the light being emitted from the bulb. Illuminance is related to distance. The ratio of luminous flux and illuminance depends on the ratio of distance to illuminated area. Simply put, illuminance decreases when distance increases; the higher a light, the less a space/room is lit.
Certain bulbs, such as reflectors or LEDs, do not emit light uniformly in all directions. The amount and intensity of light given off will depend on the angle the lamp is being looked at. The candela (cd) measures the luminous intensity being emitted in a certain direction which corresponds to the ‘intensity’ of light.
Luminance is a measure of luminous intensity of light travelling in one direction on a projected area and is measured in candelas per m2. It describes the quality and brightness of light (illuminated or self-luminous) the eye perceives on a given surface. Take a computer screen/display for example. When describing normal room lighting, referring to https://en.wikipedia.org/wiki/Luminous_intensity is more practical.
Terms such as ‘lux’ and ‘lumen’ have been in use since the Middle Ages. Early church fathers called the natural light God created on the first day of creation ‘lux’. Lux was transformed into ‘lumen’, considered a holy consecrated light, and when this light passed through a physical body, it was called ‘illumination’, a light that elevated the mind and renewed the spirit.
Measuring Light Efficacy
The light output is a measure of well a light source produces visible light. It is the ratio of luminous flux emitted by a light source to electrical power and is measured in lumens/watt (lm/W). The larger the value, the greater the luminous flux of a lamp that can be used by the eye. The light output plays a role in the energy-related evaluation of a light source, its efficiency.
Light Emission and Direction
To a great extent, the direction in which a light source emits its light determines its effects. In the age of the incandescent lamp, this was only influenced by where the lamp shone and light emanated in a room depending on where it was placed.
In essence, every light source shines in a particular direction, even the sun shines from the sky to the earth. A cloud covered sky acts as a light filter, which diffuses the bright sunlight and provides a general sense of brightness without particular direction.
For photographers and in the theater, the spatial effects achieved by the direction of light is of fundamental importance, which is why specialists turn to spotlights and flashes.
Since the introduction of LEDs, however, lights now differ in terms of their radiation effects. The smaller the light source and the stronger the luminance, the greater the influence on the radiation spectrum. A small LED chip produces a visibly different light effect than the diffused glow of an incandescent or fluorescent bulb.
Therefore, there are now two different EU labeling regulations: No 1194/2012/EC for directed/focused light (mainly LED and other reflectors) and No 244/2009 / EC for non-directional/unbundled light (all other household lamps and LED with diffuser or optics). Since in reality these two areas can not clearly be separated, the following legal threshold was established stating: a ‘lamp with concentrated light’ emits at least 80% of its luminous flux at a certain angle.1
Focused or directional light illuminates clearly defined areas, it spreads little to no stray light and produces hard edges, high contrasts and delimited shadows.
Small LED chips in lamps produce an almost punctiform/dotted circular light, which spreads very unevenly over a larger area. It then forms, for example, a bright spot of light in the center of the light in whichever direction it is shining. For household use, such lamps are sold as "spotlights.”
Several LED chips are usually installed together within one lamp and can be changed with different optics to vary their lighting and radiation.
As a rule, however, there remains a relatively sharply delineated light space with little stray light. As a result, for example, LED street and highway lights have unlit dark spaces between them, which generates feelings of insecurity in many people.
Reflectors (with or without parabolic mirror collectors), opaque lamp shades or flaps can also bundle incandescent lamps, headlamps and other wider scattered light sources. These lights are used for example in photography or on stage to highlight certain areas.
The light of an incandescent, halogen or fluorescent lamp radiates evenly, not only in one direction.
The result is a diffused, scattered, soft light with low contrasts, which creates bright shadows with blurred edges which can shine far into the room (with lots of stray light). Bright, translucent lampshades or filter discs made of frosted glass or similar reinforce this effect.
When a light source does not emit its light directly, but instead shines on a surface that is as bright as possible, such as a bright room wall, or in a light-opaque lampshade, the light bounces off this area into the room. The result is a particularly wide-spread, soft, indirect light, which opens up many possibilities for creating atmosphere in interior design.
Indirect light is also popular in portrait photography because it softens blemishes and contrasts.
1 The German Federal Environment Department clearly illustrated the differences on its pages: www.umweltbundesamt.de/themen/klima-energie/energiesparen/licht/haeufige-fragen-thema-licht
Light and Colour
Another key feature of light is its colour. Isaac Newton first discovered the spectral colors when he refracted white light with a prism so light’s component colours could be seen and the rainbow understood. Despite Goethe’s theory of colour being controversial among physicists, it has been scientifically confirmed that the spectrum of colours occurs where these coloured edges overlap and are explained by the interaction of lightness and darkness.2
In the production process of commercial LEDs (see: lamps and lights/LEDs) very often a high blue content present.
Too much blue in light not only keeps us awake but can also be harmful: exposure to blue light before sleeping disturbs the sleep rhythm, excess blue light damages the retina of the eye (photoretinitis, causes macular degeneration).
In addition, colour temperature and the colour rendering index play a role in assessing the color of an artificial light source.
The light spectrum is defined as the visible part of all electromagnetic waves. For historical reasons, these waves are subdivided different areas based on wavelength; from very short, high-energy altitude and gamma rays to X-rays and UV rays(ultraviolet) (invisible but effective). The waves change from purple to blue, yellow, green, orange and red to the longer invisible infrared (IR Radiation), until they come into the range of micro and radio waves and the various alternating currents.
Daylight offers the full spectrum of color and wave nuances, it also changes during the day and depends on weather and seasons. Artificial light can only reproduce part of the light spectrum. Incandescent and halogen bulbs have a relatively balanced spectrum with a high proportion of red, which gives the light an effect most people describe as pleasant or comfortable. LEDs, even white ones, always have a pronounced blue component, which increases with usage. The quality of the spectrum of LED lamps can vary greatly due to type and production. Fluorescent lamps and compact fluorescent lamps (energy-saving lamps) always have an unbalanced spectrum with gaps between the individual colors. (Line or band spectrum, see Lamps and Lights.
Wavelength alone (see Light Spectrum) only represents colour quality in monochromatic light such as LED. The kelvin (K) is used to describe colour temperature in terms of the impression or ‘mood’ created by the color of a lamp (cool colours, warm colours).
Temperature serves as a point of reference, which requires black glowing titanium for a particular colour to shine. The glow increases from red to yellow and white to blue. Therefore - unlike one might think - blue light has a high color temperature, but red light a low color temperature.
LED lamps are also classified by their colour temperature. ‘Warm white’ - generally perceived as a ‘cozy mood lighting’, is usually 2,800 K, or below 3,300 K. A ‘neutral’ colour temperature is between 3,300 and 5,000 K. Colour temperatures from 5,000 K are called ‘daylight white’. Natural daylight averages between 6,500 and 15,000 K and no artificial light reaches such a high level.
Even though the colour temperatures are measurable, it is difficult to classify their mood value. For example, the approximate value of artificial ‘daylight white, is experienced by many as unpleasantly bright for indoor lighting.
Colour rendering is the ability of a given light source to reveal colours faithfully in comparison to a natural light source. Light sources with different spectral ranges of radiation can change how colours appear.
Human skin colour in particular, with its varied colour composition, can be different in artificial light. Even clothing or food will look very different in store light compared to a natural light source.
In the general within the colour rendering index (CRI), eight reference colours are measured, which serve as ‘references’, to evaluate colour reproduction. Daylight and incandescent lamps always have an excellent colour rendering ability due to their continuous spectrum and are rated with the CRI value 100. Fluorescent and other discharge lamps tend to have poor colour rendering values (CRI 70-90). As for LEDs, the colour rendering depends on the quality of each lamp (CRI 75-95).
1 In: Arthur Zajonc: Catching the Light: The Entwined History of Light and Mind. Oxford 1993
2 Matthias Rang: Phänomenologie komplementärer Spektren. Phänomenologie in der Naturwissenschaft, Bd. 9, Berlin 2015; and Matthias Rang, Oliver Passon, Johannes Grebe-Ellis: Optische Komplementarität. Experimente zur Symmetrie spektraler Phänomene. Physik Journal 16-2017 Nr. 3, S.43-49