What is Cut Off Angle (of a Luminaire)

Horace He

Last Updated: December 26, 2023

What is Cut Off Angle (of a Luminaire)

The cut off angle of a luminaire is the angle at which the light emitted from a luminaire is directed and controlled. It is measured from the lowest point upwards, between the vertical axis and the first row where the bare light source (lamp) is not visible. The cut off angle helps determine the level of glare and visual comfort provided by the luminaire.

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The Illuminating Engineering Society (IES) defines a cutoff fixture as having specific intensity thresholds at or above certain angles. For example, a full cutoff angle luminaire completely blocks the light from being emitted above the horizontal plane, minimizing light pollution and reducing glare. On the other hand, a semi cutoff angle luminaire allows a small amount of light to be emitted above the horizontal plane, striking a balance between controlling light spillage and providing some upward light distribution. Lastly, a noncutoff angle luminaire has no restrictions on the light distribution, emitting light in all directions.

The specific cutoff angle and its variations can vary depending on the application and lighting standards. Different lighting fixtures may have different cutoff angles based on their design and intended use.

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Frequently Asked Questions

What Is the Definition of a Cut Off Luminaire

A cut off luminaire, also known as a cutoff fixture, is a type of lighting fixture that has a specific light distribution. In this type of luminaire, the candlepower does not exceed 2.5% of the lamps’ rated initial lumen output at a 90-degree angle above nadir. Additionally, it does not exceed 10% of the lamps’ rated initial lumen output at a vertical angle of 80 degrees above nadir.

What Does Full Cut Off Mean

Full cut-off refers to the condition where the light emitted from the fixture does not extend beyond its bottom.

What Is the Difference Between Semi-Cutoff and Full Cutoff

Traditional wall packs distribute light in all directions, whereas semi-cutoff wall packs direct the light downwards and outwards. On the other hand, full cutoff wall packs restrict the distribution of light to below the luminaire. Full cutoff wall packs are designed to comply with dark sky regulations and effectively minimize light trespass and light pollution.

What Is Cut Off Angle

Cut-off angle refers to the angle at which the bare source of light is no longer visible, measured from the lowest point between a vertical line from the center of the lamp extended to the ground and the first line of sight.

A fully shielded fixture is designed in such a way that the light source is not visible to people or wildlife in the vicinity. By using fully shielded lighting, you can effectively prevent light from trespassing onto your neighbors’ property, as well as minimize glare and skyglow.

What Is a Fully Shielded Light Fixture

“Fully shielded” refers to light fixtures that have been designed in a way that ensures the emitted light rays, whether directly from the lamp or indirectly from the fixture, are directed downwards below a horizontal plane. This shielding is achieved by positioning the lowest point on the fixture where light is emitted below this plane.