Light Intensity Measurement Quantities

Phuc Gia® is an organization that provides information about light measurement quantities (photometric quantities) to individuals, organizations, and enterprises both domestically and internationally. This information is directly relevant to businesses. Therefore, please refer to the article below as Phuc Gia® will clarify these issues.

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Luminous Intensity

Luminous Intensity is a fundamental optical quantity used in measuring light source parameters; it is one of the seven basic units of the International System of Units (SI): m (meter), kg (kilogram), s (second), A (Ampere), K (Kelvin), mol (mole), and cd (candela). The concept of luminous intensity expresses the density of energy emitted from a light source in a specific direction, or it can be defined as the luminous flux in a given direction emitted per unit solid angle (1cd = 1 lumen/steradian). Since October 1979, the CIE (International Commission on Illumination) has introduced a new definition for the candela: “The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of wavelength = 555nm and that has a radiant intensity in that direction of 1/683 w/steradian.”

The unit of luminous intensity is the candela (cd), with the word candela meaning “candle” in Latin. A typical candle emits light with a luminous intensity of approximately one candela. If emission in certain directions is blocked by an opaque barrier, the light source still maintains an intensity of about one candela in the unblocked directions.

To represent the distribution of luminous intensity in space, a polar coordinate system is often used, where the origin is the light source and the endpoints are the luminous intensity vectors. In practice, this diagram is represented in a plane or semi-plane by drawing the curve resulting from the intersection of this surface with certain defined meridian planes. For light sources with circular symmetry (rotationally symmetric), only one cut by a meridian plane is sufficient. Luminous intensity measurements must be performed in specialized measuring chambers using a device called a goniophotometer.

Luminous Flux

Luminous Flux is the photometric quantity that indicates the radiant power of the light beam emitted from a light source, or, alternatively, luminous flux is the flow of light emitted from a light source in all directions over one second. The unit of measurement for luminous flux is the lumen (lm). To measure the luminous flux of a common artificial light source, a specialized measuring device called a Photometer or Integrating Sphere is typically used.

Luminance

To characterize the ability of a light source or a reflective surface to emit light that causes the sensation of glare to the eye, the definition of Luminance is introduced. Luminance is a quantity that determines the luminous intensity emitted per unit area of a surface in a specific direction. It estimates the light that the human eye can perceive and depends on the direction of observation. Luminance plays a fundamental role in lighting engineering, serving as the basis for concepts related to perception and visual comfort. The unit of measurement for luminance is candela per square meter (cd/m²).

Illuminance

Illuminance (E) is the quantity characterizing an illuminated surface, representing the density of luminous flux over a surface with area (S). The unit of illuminance is Lux (Lx), where one lux is the luminous flux density of a light source of 1 lumen over an area of 1 m² (1 lux = 1 lm/m²). When the illuminated surface is non-uniform, the illuminance is calculated as the arithmetic mean of the illuminance at various points.
The concept of illuminance, besides the light source, is also related to the position of the illuminated surface. When considering the light source as a point (O) with luminous intensity I radiating onto an elemental surface (dS) at a distance (R) from (O), the illuminance on the elemental surface (dS) will change with the relative inclination of the surface (the angle between the normal to (dS) and the direction (R)) and is inversely proportional to the square of the distance (R).

Correlated Color Temperature (CCT)

The correlated color temperature of a light source, expressed on the Kelvin (K) scale, is the expression of the color of the light it emits. Imagine an iron bar: when cool, it is black; when heated steadily, it glows orange; continued heating turns it yellow; and continued heating until it becomes “white hot.” At any point during this heating process, we can measure the temperature of the steel bar in degrees Kelvin (0ºC corresponds to 273.15 K) and assign that value to the color produced.

For incandescent lamps, the color temperature is precisely the physical temperature of the filament itself. For fluorescent lamps, discharge lamps (generally, all non-incandescent lights), the color temperature is only representative, determined by comparing it to the equivalent temperature of a heated black body radiator. When speaking of a lamp’s color temperature, one immediately gets the feeling of whether the light source is “warm,” “neutral,” or “cool.” Generally, the lower the temperature, the warmer the source, and conversely, the higher the temperature, the cooler the source.

Color Rendering Index (CRI or Ra)

The Color Rendering Index (also referred to as: Color Rendition, Color Rendering Quality, or Color Rendering Index) is a quantity that indicates the ability of an artificial light source compared to an ideal or natural light source when assessing the color fidelity of an illuminated object. Light sources with high CRI are desirable in color-critical applications, such as vanity desks, fashion shops, infant care, and art restoration, where the highest possible CRI should be used.

The Color Rendering Index is symbolized as CRI (or Ra). The highest CRI value is 100. CRI = 100 is the color rendering index of a standardized light source whose light is identical to daylight. The CRI of other light sources will be less than 100, for example, a Halogen incandescent bulb has CRI ~ 100, a fluorescent lamp has CRI ~ 50, an LED lamp has CRI > 70, and the CRI of a low-pressure sodium lamp is a negative value.

The concept of the CRI was first introduced in 1964. Essentially, the index is measured as a reference result based on the appearance of eight color patches when illuminated by a test source, compared to the same color patches displayed under a standard light source (CRI = 100). However, this evaluation method does not fully account for the saturation of the light source. This means that if one or two colors render poorly while all other colors render very well, the CRI index is not penalized heavily and still achieves a high value. Therefore, in 2005, the CIE (International Commission on Illumination) introduced a new concept to address the inadequacy of CRI, and this new index is called the Color Quality Scale (CQS).

The Color Quality Scale (CQS) is a quantitative measure of a light source’s ability to accurately reproduce the colors of objects illuminated by that source. Similar to CRI, the highest value of CQS is 100; however, CQS does not have negative values, so the CQS scale ranges from 0 to 100. CQS is designed such that its score is reduced if one or two colors appear poorly, even if all other colors are rendered well, and it assigns a score of 0 to light sources that are virtually monochromatic (such as low-pressure sodium lamps).

Luminous Efficacy

Luminous Efficacy (or often simply called luminous efficiency) fully expresses the ability of a light source to convert consumed energy into luminous energy. Luminous efficacy is the ratio between the luminous flux emitted by the light source and the electrical power consumed by the light source, meaning how many lumens are produced per watt of electricity. The unit of measurement for luminous efficacy is lm/W.

In practical application, the light source (lamp) rarely operates independently; it must be accompanied by the entire luminaire system. Most light sources require the installation of additional electrical components to operate. For a luminaire designed to meet specific lighting requirements, besides the losses from these associated electrical components, there are also luminous flux losses due to the optical chamber structure, heat dissipation issues, etc., of the luminaire itself. Therefore, it is necessary to clearly distinguish between the luminous efficacy of the light source and the luminous efficacy of the entire luminaire.

Typically, light source manufacturers only publish the luminous efficacy of the light source measured under standard conditions, without accounting for the losses from associated electrical components. Luminaire manufacturers, depending on the type of luminaire, may publish the luminous efficacy of the light source, the luminous efficacy of the entire luminaire, or both simultaneously.

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The reasons why all enterprises choose Phuc Gia® as the leading unit in consulting for Food Safety Conformity Declaration and Food Safety Regulatory Compliance Declaration, Energy Labeling, Conformity Certification, Cosmetic Declaration, and Logistics Services:

Phuc Gia® has over 10 years of experience in the industry, and is the first unit with sufficient capability to provide Customs services to over 500 large and small organizations In & Out of Vietnam, including: Sharp (2012); Samsung (2012); Hitachi (2013); Electrolux (2013); Panasonic (2013); LG (2013); Sony (2013); Siemens (2013); Mitsubishi (2013); GE (2013); Haier (2014); Toshiba (2014); Carrier (2014); Philips (2014); HappyCook (2015); General (2015); TCL (2015); Alaska (2015); Casper (2015); Gree (2016); Hải Hà (2016); VinMart (2017)…

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• This is a concern shared by many customers before choosing Phuc Gia® as their Customs Services provider.
• Over the past 10 years, Phuc Gia® has served over 500 large and small Domestic and International Enterprises. More than 90% of the enterprises that have used Phuc Gia®’s Customs Services agree that the quality of the Service is commensurate with the money they invest.
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