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Lighting Glossary

Your ultimate Guide to Lighting Vocabulary.

Accomodation

Accomodation is the process that enables the eye to focus on different objects. As an object approaches, the lens becomes more curved, and it flattens out for distant objects. Prescription glasses help address the eye’s limitations in adjusting the lens shape for optimal clarity.

Adaptation

Adaptation relates to the pupil's size and the retina's sensitivity. In low light, the pupil opens wider, and it becomes smaller as light levels rise. Changes also occur in the photochemical substances in the retina. It generally takes more time to adjust from bright to dark environments, such as when entering a dark theatre during the day, than to transition from dark to light.

Beam Angle

The beam angle is defined as the angle at which the luminous intensity of a luminaire drops to half of its maximum value.

Colour Rendering Index
The Color Rendering Index (CRI) measures how accurately a lamp displays colours compared to a reference source with a CRI of 100.
In simple terms, it reflects a light source's ability to reveal the true colours of objects. A higher CRI means that colours will appear more "natural," similar to how they look in daylight. Conversely, a light source with a low CRI can distort colours. When we refer to "natural," we mean colours as seen in sunlight.
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Colour Temperature
Picture a piece of metal heated to a high temperature; it begins to glow. When the temperature reaches a certain point, it emits light through incandescence.
As the temperature rises, the light shifts to a whiter hue. This concept is used to define the colour of a light source, known as its Color Temperature. Light sources up to 3000 K offer a warm feel, those around 4000 K provide a neutral tone, and those at 5000 K and above present a cooler appearance.
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Illuminance

Illuminance measures the amount of luminous flux from a light source that strikes a specific surface. It is calculated as the ratio of luminous flux to the area of that surface. In the SI system, the unit of illuminance is Lux, while in the FPS system, it is measured in foot-candles, with one foot-candle equal to 10.76 Lux. The unit is represented as Lux (Lm), and its symbol is E.

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Inverse Square Law

As a surface illuminated by a light source moves farther away, it appears dimmer. In fact, it diminishes in brightness much more quickly than the distance increases. This phenomenon is described by the inverse square law, which relates illuminance (E) to intensity (I) with the formula: E = I/d², where d is the distance from the light source. For instance, if the illuminance on a surface is 40 Lux at a distance of 0.5 meters from the light source, it decreases to 10 Lux when the distance increases to 1 meter, as illustrated in the figure above.

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Luminance

Luminance refers to the luminous intensity of a surface in a specific direction per unit area as observed from that direction, essentially describing the brightness of the surface. Unit: Candela/sq.m Symbol: L

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Luminous Flux

Luminous flux is the total amount of light emitted by a light source. It represents the power of the light provided by the source and quantifies the energy released over time in the form of visible radiation. Unit: Lumen Symbol: (Lm)

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Luminous Efficacy

Luminous efficacy is the ratio of the total luminous flux emitted to the total input power (wattage) of the lamp. It can be calculated using the formula: Luminous efficacy = Luminous flux/Wattage.

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Luminous Intensity

Luminous intensity describes the amount of light emitted from a light source in a specific direction. Unit: Candela (Cd) Symbol: I

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Photometry

To standardize light sources and luminaires and assess the adequacy of lighting for different tasks, a scientific measurement system known as photometry has been established. Photometry is a crucial aspect of illumination engineering, covering everything from highly precise measurements performed in laboratories by standard institutes for calibrating reference sources to routine assessments in research, quality control, and field evaluations. While some field measurements can be very accurate, others often serve to provide a general overview of current lighting conditions.

Refraction
Reflection of light is one of the most well-known properties of light, as it allows us to see the objects around us.
Light from a source, such as a sun or a lamp, travels in a straight line until it encounters an object, at which point it can be absorbed, transmitted, or reflected. Surfaces that are silver or white reflect the most light. A smooth, flat silver surface behaves like a mirror, producing a perfect reflection of the surroundings.
Refraction
Light transmitted through a medium typically deviates from its original straight path.
This phenomenon is commonly observed with transparent objects like glasses and lenses, where items viewed through them may appear larger, smaller, or distorted. For instance, when a stick is partially submerged in water, it looks bent at the surface.