The shape most people think of when they hear “light bulb” is a shape referred to as A19. There are many other shapes and sizes light bulbs can come in. See this visual guide.

An acorn lamp is a common style of streetlight often used for main streets to provide a historical or antique look. Acorn lights are typically shaped like an acorn and have a translucent shroud surrounding the bulb. They are typically not fully shielded, so they generate some light pollution.

Adaptive controls refers to any technology which allows you to adjust settings on a light to fit the circumstances. For example, a light that is on a motion detector and only turns on when someone is present is using adaptive controls. More sophisticated tools, like smart lights controllable from an app, allow you to adjust the intensity, color, duration of light, and more to set schedules or activities. Good use of adaptive controls can save energy and increase safety.

The beam angle of a light bulb refers to the direction or angle of the light it emits. An ordinary light bulb has a beam angle of 360 degrees, meaning it emits light in all directions. Many ceiling mounted lights have a beam angle of 120 degrees, so the light is only going broadly in one direction (down). A narrow spotlight may have a beam angle of 30 degrees.

A bulb with a wide beam angle will appear dimmer than a bulb with the same lumen output but a narrow beam angle. Narrow beam angles focus the light whereas broad angles spread it around more.

BUG stands for Backlight, Uplight, and Glare. BUG ratings are a quick guide to the how a fixture distributes light. The higher the BUG rating, generally the worse the performance and lower efficiency. Scores are broken down by each category.

Backlight is the amount of light thrown behind the fixture, potentially falling where it isn’t wanted depending on the context.

Uplight is light that is wasted by allowing light into the sky rather than the target of illumination below.

Glare is how much light falls directly into your eyes as you approach the light.

Look for the lowest possible BUG ratings for the most efficient and safest lights. A night-sky friendly light should have an uplight rating of 0 and a glare rating of no higher than 2. BUG Ratings are usually given for commercial grade fixtures but are less commonly used on consumer products found on store shelves.

A cobrahead fixture is a common style of streetlight, so nicknamed because of its appearance of a cobra’s head in profile.

Some cobraheads are full cut off, meaning they have a flat bottom and do not allow light upwards. Other, typically older models are called sag-lens or drop-lens cobraheads because the lens extends below the housing and allows some light upwards.

CRI is a measure of how well a light source renders the colors of an object. The maximum theoretical value is 100, which is only achieved by direct sunlight. Artificial lights typically have CRIs in the range of 70-90, but some can be as low as 20-30. A low CRI means that objects of different but similar colors would appear difficult to discern from one another. High CRI is important for detailed work where accurate colors are important, such as an artist’s gallery, a busy kitchen, or in a hospital. Generally speaking, higher CRI can also benefit safety and visibility, but very high CRIs are not typically necessary for general use. High CRI fixtures often have other drawbacks, such as higher prices or lower energy efficiencies.

CRI has been criticized by some lighting engineers for being too simplistic and not a good gauge of real-world experience, but it remains the most commonly used metric for color accuracy in lighting.

Compact Fluorescent Lamps were a common lighting choice for many years. These bulbs are easily recognized by their tightly twisted tube design. CFLs are energy efficient compared to older technologies and came in small forms compared to the traditional long, straight tube fluorescent bulbs. Today, CFLs have mostly been surpassed by LEDs which are slightly more energy efficient and come in a greater variety of sizes, colors and intensities.

CCT, often referred to as “color temperature” or simply color, is a measure of how a white light will appear to our eyes. See this chart for a visual guide. White light refers to any light that contains all of the colors of the spectrum: red, green, and blue, but not necessarily equal amounts of each. CCT is measured in the Kelvin temperature scale. 5000K is neutral white with a roughly equal balance between all colors. Below 3000K is more yellow/red, and above 6000K is more blue. Values below 2700K, ideally 2200K, are preferred for outdoor use at night because the more amber appearance is easier on the eyes and does not disrupt wildlife or astronomy as much as bluer lights.

Some lights don’t have a color temperature. For example it would not be possible to determine the color temperature of a purely green or purely red light, since color temperature only applies to white lights where all colors are present to some degree.

Filtered LEDs are a normal white LED with a special coating or filter that blocks blue light to make them more night-sky friendly. This makes them cheaper to produce than more sophisticated technologies like PC-Amber LEDs, but less energy efficient. Filtered LEDs are often marketed as “Bug Lights” in hardware stores because insects are most attracted to blue, which FLEDs don’t emit. Some people criticize FLEDs color as being too green or even appearing “sickly”, but they are a low cost way to be more night-sky friendly.

A full cut-off, or fully shielded light fixtures direct all of its light downward in a cone beneath the fixture. No light is allowed directly into the sky. Full cut-off light fixtures produce little if any glare, significantly reduce skyglow, and reduce energy waste.

A grow light is a light used to stimulate the growth of plants, typically in a greenhouse setting. Grow lights come in a wide variety of forms depending on the plant being grown. Most commonly used are High Pressure Sodium (HPS) lights, but LEDs are becoming more common, or even violet colored lights for hemp. Most plants do not benefit from grow lights at all hours; typically commercial crops bred for rapid growth in a greenhouse environment do best.

Light pollution from greenhouses using grow lights without any shielding is a concern in many areas. The light pollution from glow lights in commercial greenhouses can be seen for dozens of miles, but curtain systems exist that can contain the light at night.

A guard light, sometimes called a “barn light” or “NEMA head” by electricians, is a common style of light found on many homes and businesses (photo). Guard lights are typically mounted on a pole or short arm. The bulb is typically some form of High Intensity Discharge (HID) light. On top of the fixture is a photocell that turns the light on at night and off during the day. Most guard lights are not fully shielded, meaning they emit a lot of light upwards into the sky. The acrylic glare shields often yellow, crack, and eventually fall off with age, exposing the bare bulb.

High Intensity Discharge lamps are any kind of lamp that generates light by an electric arc through a medium of some kind. Examples of HID lamps include Low Pressure Sodium (LPS), High Pressure Sodium (HPS), Mercury Vapor (MV), and more.

High Pressure Sodium lamps are a common lighting technology especially for streetlights. HPS lamps generate light by passing a current through a mixture of sodium and mercury. HPS lamps give off an amber color with a small amount of blue present, and have a color temperature of around 2000K. HPS lights are moderately energy efficient but have short lifespans compared to other options. HPS generates relatively low levels of light pollution when shielded and appropriately intense. HPS fixtures are being replaced by LEDs in many places because of their energy efficiency and lifespan advantages.

The IES is a standards organization for the lighting industry. They have no regulatory power but offer guidelines and recommendations for best practices in lighting. The IES sets standards for safe illumination levels and best practices that lighting designers and policymakers refer to.

Incandescent light bulbs were the first mass-produced light bulbs, made famous by Thomas Edison. Incandescent bulbs generate light by passing electricity through a tungsten filament, heating it up. Eventually the filament becomes hot enough to glow and emit a soft, yellow light. This process is highly inefficient; about 95% of the energy is lost as heat rather than light. Incandescent bulbs are typically too hot to touch while in operation.

Today, incandescent bulbs can still be found but have been largely replaced by other lighting technologies. Modern LEDs are sometimes marketed as “Edison Bulbs” or “Vintage Bulbs” that mimic the look of incandescent lights while being much more efficient and safer.

A Light Emitting Diode, or LED, is a common lighting technology that is now widely used in all manner of applications. LEDs are more efficient than most other forms of lighting and are now more affordable than ever. LEDs can come in almost any color, intensity, or size desired. LEDs are often misused and brighter than necessary, which has given them a reputation as creating more light pollution. However, there are many LEDs which are excellent choices when used appropriately and in accordance with the night-sky friendly lighting principles.

Low Pressure Sodium lamps were once common for streetlight and general area ligthing. LPS lamps generate light by passing a current through sodium and a small amount of neon and argon. The light they generate is a pure yellow/orange color, with zero blue or green emission. LPS lamps are highly energy efficient and generate very little light pollution, making them ideal for use near observatories. LPS were long considered the “gold standard” in night sky friendly lighting. However, they suffer from poor color rendition and short lifespans compared to other options. The last manufacturer of LPS lights halted production in 2019 due to lack of demand. Narrow-Band Amber LEDs have emerged to fill their role.

Lumens are measurement of how bright a light source is. The higher the lumens, the brighter it is. A typical household light bulb may be between 500 and 1000 lumens. A street light on a major intersection could be 8,000 lumens or more. Lumens refers to how bright a source is, but different fixtures may make better or worse use of the light.

Lux is a measurement of how much light is falling on a surface. One lux is one lumen per square meter. A footcandle is the same concept but with imperial units: One footcandle is one lumen per square foot. While lumens tells you how bright the source of the light is, lux tells you how bright the target of illumination appears. The floor in a typical living room may be about 50 lux. The road under a streetlight at night may be 10-20 lux. Direct sunlight is 100,000 lux.

Mercury Vapor lamps are a now-obsolete but common design that can still be found in many places. MV lamps generate light by passing an intense arc of electricity through vaporized mercury. After a roughly 5-minute warm up period, MV lights generate a greenish-white light. Aging models are notorious for flickering or power cycling when they get too hot, or making a constant buzzing sound. MV lights also emit dangerous amounts of ultraviolet light which is contained by a coating on the glass, but over time aging coatings can fail and allow the UV light to escape. Because of the low energy efficiency compared to modern lights, high amount of light pollution they generate, presence of toxic mercury, and safety risk they pose, MV lamps are banned in the European Union and banned for streetlight use in the United States. Many state or local regulations also impose bans on MV lights. Despite this, many remain in use today in residential or commercial businesses.

Narrow-Band Amber lights are a kind of LED that produce a specific color of light that is yellow or amber, and filter out nearly all other colors as much as possible. The resulting “pure” color is ideal for use near observatories because it does not scatter much in the atmosphere, and the specific color can be filtered out by scientific instruments whereas broad spectrum white lights cannot. NBA lights are one of the best possible choices for night-sky friendly lighting and are required in places like Flagstaff, Arizona. NBA lights are also required in many coastal areas, such as much of the Florida coast, to avoid disruption of sea turtles. Because NBA lights only emit one specific color they have very poor color rendition, and are not as energy efficient as other LED choices. For these reasons they are not popular outside of specialty applications. NBA lights are typically available as a custom order option from lighting manufacturers.

NEMA stands for National Electric Manufacturers Association. A NEMA tag or sticker is a label placed on the bottom of most municipal or utility owned streetlights, and is usually big enough to read from the ground. NEMA tags display a number and colored label. The color tells you the kind of light. For example, a white sticker is LED, yellow is HPS, blue is Mercury Vapor.

Adding a zero to the number tells you the wattage. For example, the number 12 on a yellow sticker would mean it is a 120 watt HPS light.

Newer stickers add text to say “LED” or “HPS” rather than rely on color alone. Some utility companies may add additional labels in addition to the NEMA sticker.

PAR stands for Parabolic Aluminum Reflector. PAR lights are shaped like a cone and are typically used as floodlights for home security or as spotlights or track lights for indoor use. The reflector design helps to focus the light onto a given area. PAR lights are often a source of light trespass when aimed poorly. PAR lights can be easily made night-sky friendly by angling them directly downward and allowing the housing to act as a shield. Alternatively, products such as the Parshield can eliminate wasted light and light trespass. The number after PAR, such as “PAR 20” or “PAR 38”, refers to the diameter of the housing in millimeters.

PC-Amber lights use a special phosphor coating technology to convert all of the blue light from an LED into green, yellow, and red light. Thus, PC-Amber lights emit zero blue wavelengths of light and appear overall amber or gold. The absence of blue light makes them a great choice for protecting the night sky and minimizing environmental impact. PC-Amber lights are often marketed as “sleep lights” or “nursery lights” for indoor use because they do not disrupt circadian rhythms as much as other lights. Compared to Narrow Band Amber (NBA) lights they have better color rendition and are more energy efficient, but also create slightly more light pollution.

A photocell is a light-sensitive device used to turn lights on and off according to ambient light levels. They are frequently used in solar powered lighting and to turn outdoor or security lights on and off at dusk and dawn. The photocells of cobrahead streetlights can be seen as small cylinders on top.

Photopic, scotopic, and mesopic vision are the three modes of operation of the human eye.

Photopic vision is the mode of operation used in daytime or brightly lit conditions. Photopic vision is more sensitive to fine details and color. During photopic conditions the human eye is primarily utilizing cones rather than rod cells. The eye is least sensitive to blue and more sensitive to red or green colors of light in these conditions.

Scotopic vision is the mode of operation used at night or dark conditions, when the human eye is primarily using rods cells and the cones are inactive. Humans do not perceive color well under these conditions, but are highly sensitive to light in the peripheral portions of the eye. Under these conditions the eye is least sensitive to red light. This is why many astronomers and submariners use red light: so as to not disrupt their dark adaptation.

Mesopic vision refers to the area between scotopic and photopic conditions. Under mesopic conditions there typically is relatively low light levels but not total darkness. The human eye is typically in mesopic conditions while driving at night, or indoors with only a small lamp on. Under these conditions both rods and cone cells are being used.

Smarting lighting refers to any light that can have settings changed or adaptively controlled, typically through a phone app or other device. Smart lights come in two forms: hub-controlled or hubless.

Hub controlled smart lights communicate with a device that acts as a central controller. The hub may have additional features such as voice controls or connections with security systems. Your phone or other device can communicate with the controller.

Hubless smart light use a local wifi connection or bluetooth to communicate directly with an app on a phone or other device. Hubless smart lights are easier to set up but tend to be more expensive per bulb or have more limited control options.

Violet pump LEDs are a new technology marketed as resolving some concerns of how LEDs affect circadian rhythms, while producing better color rendition. Violet pump LEDs use a phosphor coating technology to convert blue light into violet light while still emitting green and red light. The overall result is a white appearance with excellent color rendition without much blue emission. Blue light exposure has been linked to health effects and eye strain, so violet-pump LEDs hope to rectify this issue while preserving an overall white appearance. Violet-pump LEDs can also be used in anti-germicidal applications by emitting UV light. However, violet light creates even more skyglow than blue light and is highly attractive to insects, so they are not ideal for outdoor use. Violet light also can create more glare than amber or red and could still have health effects. More research is needed to determine how effective violet pump LEDs are at reducing circadian disruption.

A wallpack is any kind of light fixture that is meant to be mounted on a wall and aimed mostly outwards. Wallpacks can commonly be found on the sides of commercial businesses to illuminate parking areas or walkways. Because the light goes directly outwards, wallpacks create a high amount of light pollution, especially glare, and so are not a safe or efficient design as lighting from above. However shielded wallpacks are available that resolve most of these issues.

Rather than using Correlated Color Temperature (CCT), some lighting manufacturers refer to the color of a light with other terms or use them interchangeably, which can be a source of confusion.

Warm Light refers to a light with an incandescent-like yellow color. The color temperature is typically 2700K, but varies with manufacturers.

Soft White refers to a slightly more neutral white that is still slightly yellow. The color temperature is 3000K.

Natural White or Bright White refers to a mostly neutral white color that has only a slight tinge of yellow. The color temperature is 4000K.

Day White refers to a neutral white light that closely resembles sunlight at noon. The color temperature is 5000K.

Cool White refers to a bluish white color. The color temperature is 6000K.

As you can see, these terms can be confusing because the higher color temperatures are described as “cool” whereas the lower temperatures are “warm”, the opposite of what you might expect.

The wattage of a light is how much energy it consumes in watts (W). Because of the wide variety of lighting technologies available, wattage is not always an accurate way to tell how bright a light will be. Manufacturers will often use phrases such as “100W equivalent” on packaging when in reality the light consumes 40W or less. This is because now-obsolete incandescent bulbs would advertise in wattage as a way to portray brightness, and many consumers are still used to this standard. Modern LEDs can produce the same brightness with far lower wattages than incandescent bulbs. Wattage can be used to determine how much a light will cost to operate, but refer to the lumen output for brightness.

0-10V dimming is an industry standard format for dimmable light fixtures. A fixture that supports 0-10v dimming will typically include additional wires to connect to an external dimming controller or supported dimmable switch. Not all light bulbs support dimming, so make sure to check if it is compatible.