Christian B. Luginbuhl
U.S. Naval Observatory Flagstaff Station
PO Box 1149
Flagstaff AZ 86002
cluginbuhl@nofs.navy.mil

Version 1.0
1 April 2003

What Does Fully Shielded Mean?

Definition of a Fully Shielded Light Fixture (cf. IDA Outdoor Lighting Code Handbook):

• A lighting fixture constructed in such a manner that all light emitted by the fixture, either directly from the lamp or a diffusing element, or indirectly by reflection or refraction from any part of the luminaire, is projected below the horizontal* as determined by photometric test or certified by the manufacturer. Any structural part of the light fixture providing this shielding must be permanently affixed.

In plain English - No light shines above the horizontal, from any part of the fixture.

* horizontal is a direction, and does not refer to any location on the fixture. Any light originating at any point of a light fixture (either from the lamp or after reflection or refraction from a reflector or lens) and directed even one degree above horizontal makes a fixture that is not fully shielded. The important character is the direction of the light ray, not the particular location on the fixture from which it originates. Some codes include phrases like "...light rays are projected below a horizontal plane running through the lowest point on the fixture where light is emitted" - these additional words are superfluous and often lead to confusion.

This term is intentionally less specific but simpler to determine than the IESNA photometrically defined cutoff terms (below). It has the distinct advantage that laypersons can quite reliably determine if a fixture is Fully Shielded by simply examining a picture of the fixture: if the lamp or any refractive lens or reflective surface is visible from the side of the fixture, the fixture will not be fully shielded. Your eyeballs are the only photometric equipment you need. This is a huge advantage for lighting codes that are used by a wide audience of lighting users, not just professionally trained engineers and lighting designers.

IESNA Cutoff Terms: A fixture conforming to the Illuminating Engineering Society of North America (IESNA) definiton of Full Cutoff is also Fully Shielded. The converse is not necessarily true - i.e. a fully shielded fixture is not necessarily full cutoff. Conformance to either specification, however, is sufficient to guarantee no direct uplight from the fixture. (For a discussion and comparison of shielding terms such as fully shielded and full cutoff, see the the IDA Outdoor Lighting Code Handbook Section 9.16)

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