Streetlight Optics

Optics for
Streetlights

Most streetlights are designed to control light distribution. There are three types of optical systems that provide different degrees of control. These include the following: Non-cutoff, semi-cutoff and full-cutoff.

Non-cutoff optics, allow light to be emitted in all directions. Many decorative fixtures use non-cutoff fixtures. Non-cutoff optics are the least efficient way to put light on the ground. They are generally effective at throwing light up into trees, and create a large amount of light pollution and glare. These lights are not generally used as prinicpal light sources on thoroughfares, as the glare has the tendency to blind drivers.

Semi-cutoff optics, allow most of the light to be emitted below 90 degres, but some light (up to 5%) to be emitted above 90 degrees. These types of optics are commonly used in cobra-head style streetlighting. They are quite effective at diffusing the light over a large, directed area on the ground. These lights still contribute some direct uplight, although not as much as non-cutoff lights. There is still significant glare from these fixtures, but often, they are mounted on taller poles, which remove them from the driver's field of vision.

Cutoff optics have more controlled lighting than semi-cutoff. Less than 2.5% of the light is allowed to escape the fixture above 90 degrees. Cutoff optics are now common in parking lot applications where greater pole spacing is desired. They offer a wider spread of light than a full-cutoff, yet they generally cannot be spaced as far apart as semi-cutoff fixtures. These lights allow a small amount of direct uplight to escape above the fixture.

Full-cutoff optics put light on the ground below the fixture only. Full-cutoff optics do not emit light above 90 degrees. Cutoff optics are gaining popularity around the USA, to the delight of astronomers. These optics direct light to the ground in a defined, tight pattern. Typically to achieve a uniform lighting job, one would have to use more full-cutoff luminaires or increase the mounting height of those luminaires to achieve the same result as compared with semi-cutoff or cutoff fixtures.

There are five different distribution patterns for streetlights, developed by the Illuminating Engineering Society of North America (IESNA) and the American National Standards Institute (ANSI).

These determine how far light goes across the street, and how far up and down the street light goes. Type V is essentially circular. It is most commonly found with non-cutoff luminaires, whereas type I is more likely to be used with semi-cutoff or cutoff fixtures. Type III is one of the most commonly available in most fixture types.

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	Non-cutoff optics, allow light to be emitted in all directions. Many decorative fixtures use non-cutoff fixtures. Non-cutoff optics are the least efficient way to put light on the ground. They are generally effective at throwing light up into trees, and create a large amount of light pollution and glare. These lights are not generally used as prinicpal light sources on thoroughfares, as the glare has the tendency to blind drivers.
	Semi-cutoff optics, allow most of the light to be emitted below 90 degres, but some light (up to 5%) to be emitted above 90 degrees. These types of optics are commonly used in cobra-head style streetlighting. They are quite effective at diffusing the light over a large, directed area on the ground. These lights still contribute some direct uplight, although not as much as non-cutoff lights. There is still significant glare from these fixtures, but often, they are mounted on taller poles, which remove them from the driver's field of vision.
	Cutoff optics have more controlled lighting than semi-cutoff. Less than 2.5% of the light is allowed to escape the fixture above 90 degrees. Cutoff optics are now common in parking lot applications where greater pole spacing is desired. They offer a wider spread of light than a full-cutoff, yet they generally cannot be spaced as far apart as semi-cutoff fixtures. These lights allow a small amount of direct uplight to escape above the fixture.
	Full-cutoff optics put light on the ground below the fixture only. Full-cutoff optics do not emit light above 90 degrees. Cutoff optics are gaining popularity around the USA, to the delight of astronomers. These optics direct light to the ground in a defined, tight pattern. Typically to achieve a uniform lighting job, one would have to use more full-cutoff luminaires or increase the mounting height of those luminaires to achieve the same result as compared with semi-cutoff or cutoff fixtures.