Introduction to Photometrics

The science of measuring light, photometry, specifically applies to light in a space. Photometrics gauges how humans perceive light — its coverage area, where light cuts off and the intensity of light in relation to distance from the light source.

In practical terms, photometrics shows whether a lighting plan meets the qualitative and quantitative lighting requirements for a project. Proper use of photometry can improve the user experience in a space and provide data results that can lead to more energy-efficient lighting, helping the property owner grasp where, how and what’s necessary to do to lower costs.

In short, photometrics is a design, logistics and construction tool. To understand essential photometrics principles, consider its six main means of measuring data.

Lumens: Lumen output, also known as luminous flux, brightness or light output, is a measure of the total quantity of visible emitted light by a light source per unit of time, weighted according to the human eye’s sensitivity to wavelengths of light, the study of which is known as luminous efficiency function. The reference point: a standard 100-watt incandescent light bulb produces about 1,200 – 1,500 lumens.

Candela: Luminous intensity, measured in candela, is the amount of light produced in a specific direction. Graphically, this data is gathered into polar formatted charts that pinpoint intensity of light at each angle away from a zero-degree lamp axis. The numeric information is also available in tabular form.

Lux: Lux, also known as illuminance, is the measurement of lumen output or luminous flux per unit area. In the study of photometrics, it is used as a measure of the intensity, as perceived by the human eye, of light that hits a surface. A flux of 1000 lumens, concentrated into an area of 1 square meter, lights up that square meter with an illuminance of 1000 lux. However, the same 1000 lumens, spread out over 10 square meters, produces a dimmer illuminance of only 100 lux.^[1]

Foot Candles: Illuminance, measured in foot candles, measures the quantity of light on a surface. Three factors determining illuminance include intensity of the luminaire in the direction of the surface, the distance from the luminaire to the surface and the angle of incidence of the arriving light.

Candelas/meter2: Candelas/meter2 measures the quantity of light that leaves a surface. This is what the eye perceives. This means gauges luminance, which reveals more about design quality and comfort than illuminance alone.

Cutoff: The cutoff angle, also known as the beam spread, of light is the angle between its vertical axis and the sight line, where brightness of a source (or its reflected image) is no longer visible. The cutoff angle is the deciding factor for the lighting designer to determine visual comfort in lighting. Deep cutoff optics provide low brightness luminaires, allowing the eye to see with more clarity. The shielding angle is complementary angle to the cutoff angle.

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