Use sensible precautions when giving your bird sunlight. Avoid temperatures that are too hot or too cold, and always provide a shady spot that your bird can retreat to if it's uncomfortable. There will be some UV exposure even in the shade, since the rays bounce around a lot (CNN, SkinCancer.org).  Guard against hazards like escapes, predators, and exposure to germs and dangerous objects.

It's difficult for many pet owners to give their birds enough exposure to natural sunlight to meet their vitamin D3 needs. But fortunately there is another effective way to provide it. 

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Diet as a source of D3

Poultry industry practices have proved that dietary sources of vitamin D3 are a very effective way to provide this vital nutrient. But there aren't very many natural food sources of vitamin D3 - mostly certain types of fish oil, liver, eggs, and fortified foods like milk. Egg is the only item on that list that is normally offered to birds, but the amount of egg in the diet should be limited to a small amount, to avoid problems from excessive amounts of cholesterol, fat, and protein.

But professionally formulated pellets and Nutriberries are an excellent dietary source of vitamin D3. These manufactured processed foods  are designed to be nutritionally complete, and will provide adequate D3 if your bird is willing to eat a sufficient quantity of them. The Pellet article has charts showing the vitamin D content of several brands. The amount of vitamin D in these brands is two or three times the recommended minimum, so if pellets are 1/3 to 1/2 of your bird's diet they will get enough D from this source. Be aware that Harrisons is the only organic pellet that is professionally formulated.  The other organic or mostly-organic brands like TOPS and Goldenfeast Goldn'Obles do not show a source of Vitamin D3 in their ingredient list.

Vitamin D2 is found in some plant sources, particularly alfalfa. But this form of the vitamin is not utilized well by birds. According to Avian Medicine by Ritchie, Harrison and Harrison, "vitamin D3 is considered to be 30 to 40 times more potent than vitamin D2 as a source of vitamin D activity. Therefore, plant sources of vitamin D are essentially disregarded when providing vitamin D to birds."

You can buy vitamin D3 supplements that you add to the bird's food yourself, but you have to be careful with these. It is impossible to overdose on Vitamin D3 obtained from exposure to light. But supplemental (dietary) D3 is a fat soluble vitamin, which means that amounts that are not used immediately are stored in the body. If fed in excess it can lead to vitamin toxicity and can contribute to hypercalcemia (too much calcium in the blood), which affects the heart, liver, kidneys and other organs, and contributes to gout. If not enough is used, your bird may have a Vitamin D3 deficiency. Pellets and Nutriberries already contain appropriate amounts of D3 and calcium, and if you are feeding these foods you should never give additional vitamin supplements containing D3, calcium, or other overdose-prone nutrients.

It's all right to provide a calcium source such as cuttlebone or mineral block to birds with a varied diet (seeds, vegetables, less than 50% pellets), and to provide bird-safe levels of UVB light in addition to dietary sources of vitamin D.

There are liquid calcium supplements for birds that contain calcium, vitamin D3, and magnesium in balanced amounts.  This form of calcium is very easily absorbed, so caution must be used to avoid an overdose. Some pet shops have liquid calcium on the shelf and others don't, so it pays to check on availability before you make a trip to the store.   Liquid calcium made for humans can also be used, but avoid brands that include iron and/or zinc. Humans require more of these minerals than birds do, and a dose that's appropriate for humans can be too much for a bird.

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Full Spectrum Lighting as a source of D3

Full spectrum lighting is often recommended as a source of ultraviolet light. But this is not a very reliable route to take, and the uncertainty surrounding full spectrum lighting for birds drove me crazy for years, until I finally decided that trying to use artificial lighting as a D3 source just wasn't worthwhile.

"Full spectrum" is a marketing term that isn't very well defined, and it doesn't imply that the bulb delivers ultraviolet radiation or makes a serious effort to replicate natural sunlight. The most basic definition is that the bulb delivers some radiation in all parts of the human visual spectrum (1000 Bulbs), but the spectral distribution could be very different from sunlight.  So you have to be careful what you buy. There are specific wavelengths in the ultraviolet (UV) part of the spectrum that are required for Vitamin D production.  But many full spectrum lights don't even try to deliver any UV because this is not part of the human visual spectrum.  The ones that do try to provide UV aren't always successful. There are potential risks when they do deliver the UV, and possible benefits in addition to the Vitamin D issue.  If FS lighting is used, it's wise do it in conjunction with proven sources of Vitamin D (sunlight and/or diet) to help make sure that your bird's needs are being met.

Ultraviolet light types. Ultraviolet light is divided into three subcategories: lower wavelength UVA (315-400 nm), middle wavelength UVB (280-315 nm), and higher wavelength UVC (180-280 nm). UVA is part of the visual spectrum of many birds (but not humans, whose visual spectrum is wavelengths from 400-700 nm), while UVB is the wavelength involved in vitamin D synthesis. The UVC is not relevant, apart from the fact that its high frequency makes it the most damaging type of ultraviolet radiation.  UVC from the sun is completely filtered out by Earth's atmosphere, so humans and birds are not normally exposed to it.

UVB. Many avian veterinarians recommend full spectrum lighting but there are also many who don't think it's useful, because there are problems in delivering the UVB. For vitamin D3 purposes, a UVB-emitting light must be placed within two feet of the bird since the UVB doesn't travel very far, and the bulb's ability to produce UVB declines rapidly in a fairly short period of time (usually 3 to 6 months), long before there are any problems with the rest of the bulb's output. In addition, tests (both scientific tests and amateur measurements using a solar meter) indicate that some bulbs produce far less UVB than claimed, and the amount is virtually none in some cases. But at least some of the amateurs were not using the right meter for the job.  See the section on Light Meters at the end of the article for information on which meters are suitable.

The nutrition chapter from Clinical Avian Medicine has an interesting table on the output of various FS bulbs versus natural sunlight on page 96 of the book. Click on the thumbnail at left to see it. The UV output of the FS bulbs is miniscule compared to the noonday sun.  The book was published in 2005, so some of this information might be outdated.

Because the bulb has to be placed so close to the bird for UVB purposes, it is important to have an area in the cage where the bird can get out of the glare if it wants to.

UVA. Many bird species can see into the lower part of the UVA spectrum. The peak UVA sensitivity of birds is around 360-373 nm, but it's been shown that parrots can see wavelengths as low as 320 nm (Lind et al) A study of 108 bird species found that 72% of them had UV-reflective markings on their plumage. Many parrots not only have UV-reflective markings, they also have fluorescent markings.  68% of the 51 parrot species that were studied had fluorescent markings, and most of them had ordinary UV-reflective markings as well. (Hausmann et al). Research indicates that all parrots have the ability to see into the UVA (Carvalho et al, Physiological Ecology). Humans can't see their birds' UV markings, but they are very visible to other birds, and are generally used in courtship displays.

Birds can see colors that we can't see, and we can only guess at what these colors might look like to them. Indoor lighting for humans doesn't normally provide this part of the bird's visual spectrum, so an indoor bird can't see any colors in the room that are only visible under UVA light.

But apparently the UVA from a full spectrum UV light doesn't travel too much further than the UVB.  These reptile bulb product sheets show the UVA output dropping to zero at a distance of 40 inches:  ExoTerra Reptile UVB 100 Tropical Terrarium bulb, ExoTerra Reptile Vision bulb. The bulb's UVA output probably lasts longer than the UVB output, but I don't have any specific data for that.

But you may not need a light bulb to get some UVA in the room for visual enrichment. Untinted window glass blocks most of the UVB but lets most of the UVA through. Window glass lets through 90% of the wavelengths above 350nm and blocks 90% of the wavelengths below 300nm (Wikipedia). The UVA range is 315-400nm, and up to 75% of the UVA is coming through the window (Smart Skin Care). So a sunny room may be all you need for UVA purposes. It doesn’t even have to be direct light shining on the window glass.  We know there’s UVB bouncing around in the shade, and there should be plenty of UVA bouncing around there too.

You can also add some UVA with an ordinary blacklight made for home use. Most of the UVA from these blacklights is in the range of 380-400 nm, so it won't go down into the lower parts of your bird's visual spectrum, but it's better than an ordinary light bulb.  I found one blacklight company that advertised an output of 320-400 nm, but when I looked at the spectral output graph, virtually all of it was in the 380-400 nm range. It's possible to buy blacklights with output in a different range, but these are specialty bulbs for commercial use, and they aren't very practical for birdkeepers.

Bright lighting in general can be used as a source of visual enrichment, with or without UV content. Wild birds spend a lot of time experiencing bright sunlight, so having some bright indoor lighting is closer to nature than constantly living in a "dark hole" type of room. Many bird owners have reported improved mood, energy, and feather color after they started using full spectrum lights, which may be due to better visibility in general rather than anything specifically related to ultraviolet lighting..

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The work of Patrick Thrush. Many of the recommendations on lighting for birds originated with Patrick Thrush. Unfortunately many of his statements seem to assumptions that don't have solid scientific evidence behind them, and some are downright erroneous. His background is in sociology and psychology, and he did his work on birds and lighting as an interested amateur not a professional scientist.  He did this work in the 1990s and there have been significant changes in lighting technology since then, so his recommendations on light fixtures may be outdated in some respects. LED technology wasn't a practical lighting option back then so Thrush doesn't talk about it. Thrush no longer has an active website on Birds and Lighting, but an archived copy is available here with a full list of articles.  The articles most relevant to the current discussion are:

The Ultraviolet Myth
Lighting and Your Bird
Why Use Artificial Light
Bird Lighting Hotspots
Light Right/Using FS Lighting With Birds
Too Blue for You 

He rarely lists his information sources, so anyone who wants to check the validity of his statements has to search for the backup themselves. Thrush believes that natural light is important for regulating the bird's general biochemistry and metabolism, so artificial lighting should mimic natural sunlight as closely as possible in all parts of the visual spectrum to provide an appropriate visual environment. Seasonal changes have a very obvious effect on the migratory and breeding behavior of wild birds, and day length is a known breeding trigger for many species (see Hormone Control article). In humans, it's believed that sunlight increases the body's production of serotonin, but it doesn't look like we know which specific component of sunlight causes it - is it the ultraviolet rays or something else? Mead identifies some additional biochemical benefits caused by exposure to  ultraviolet light, but don't forget that UV radiation also has some very well-known hazards.  Apparently there are some less well-known hazards too; Borrodale et al reports that sun exposure can reduce the level of folate (Vitamin B9) in the bloodstream.

Rea et al identified the following factors as reasons that natural light might be better than artificial light for humans; but it hasn't actually been proved that it's better.  (1) Sunlight is a lot brighter than a lot of indoor lighting, (2) the melatonin cycle is dominated by the short-wavelength (blue) portion of the visible spectrum, and daylight has relatively high output in this portion of the spectrum, (3) the lower part of the retina may have greater sensitivity to circadian regulation than the upper part, and the sky tends to selectively illuminate the lower part of the retina, (4) the timing at which we receive certain types of light is important for circadian regulation, and so is the duration.

It seems likely that sunlight has similar effects on birds. But it doesn't look like the effect of lighting on the general biochemistry of birds has actually been studied, and it hasn't been studied all that well in humans either. So we don't know how important it is for artificial lights to mimic the entire spectral output of the sun. Based on Rea et al above, it looks like we should primarily be looking at brightness of the light and emphasizing the blue spectrum. Which is accomplished by going for a high CRI and a high color temperature. And we also need to mimic the timing of the natural light cycle.

The poultry industry considers lighting to be important, but their guidelines are very different (Hyline). They pay very little attention to the factors that Thrush emphasizes. It's possible to find some research on the color temperatures that work well for poultry and the color temperatures that the birds actually prefer (ZooTecnica, Prayitno et al, The Poultry Site), but these are unnatural lighting conditions that are very different from his recommendation to mimic sunlight as closely as possible.  If it really made a difference I’d expect them to be on top of it. Humorous side note:  the ZooTecnica article talks about redefining the technical term "lux" for use with chickens, and calls the new definition "clux".

Thrush feels that providing a consistent visually-balanced light source for birds is more important to their health and well-being than attempting to use UVB as a dietary supplement. The accuracy of this belief is unknown, and there does not appear to be any scientific literature to support it. One of his articles (The Ultraviolet Myth) states that the Harderian gland transmits information about light duration to the pineal gland.  This is not a known function of the Harderian gland, and the scientific literature says that the avian biological clock is regulated by the pineal gland, the retinas, and the "avian homologs of the suprachiasmatic nuclei" (which is something in the brain) (Cassone & Westneat, Underwood et al; see Wikipedia for a more readable explanation of how the pineal gland works).

Thrush believes it's important to provide UVA since it's part of the visual spectrum of birds. It makes sense that being able to use their full visual range will contribute to their well-being, although this hasn't been scientifically verified. UVA is not involved in Vitamin D production. 

UVB is not part of birds' visual spectrum but is used in producing Vitamin D. But Thrush discounts the importance of UVB for vitamin D purposes. He states that "Plants produce a version of Vitamin D, calciferol" and "In native environments, most of the Vitamin D intake of birds is through diet" (The Ultraviolet Myth). But this is incorrect. There are very few plants that contain any form of Vitamin D, and the form in most of these plants (calciferol, aka Vit D2) is not an effective form for birds.  The only plant that's known to produce a useful amount of Vitamin D3 is rimu fruit in New Zealand, which is not available to birds anywhere else. The scientific consensus is that birds mostly get their Vitamin D from an interaction between sunlight and the skin on their legs and not from eating plants.

In any case, natural sunlight is highly variable.  Its intensity and appearance fluctuates throughout the day and throughout the seasons, and is influenced by weather conditions, how reflective the local environment is (it makes a big difference whether there's forest, open field, lakes, sand, snow on the ground, etc), and the position of the individual (in the shade or not, facing toward the sun or away, etc). It's not possible to replicate all that with a light bulb. You have some limited choices about the brightness of the light and its spectral output, and that's about it.

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What are the recommended characteristics for a full spectrum light? 

There is a lot of conflicting information about what kind of full spectrum lighting is best for birds. But it seems to be generally agreed that the following is ideal:

CRI. It should have a CRI (color rendering index) of 90 or more, preferably 95-98. The color rendering index indicates how accurately a light source shows colors compared to how they look under natural sunlight.  Natural sunlight has a CRI of 100, so that's the highest possible score.

Color temperature of 5000-6500K.

Color temperature chart is from Ilumi

"Color temperature" is based on the colors that a black-body object (like an iron pot) would take on as it is heated to different temperatures.  The lower temperatures begin on the red end of the visible spectrum and progress through orange, yellow, white, and blue as the temperature increases. Color temperature charts vary in what temperature they assign to various colors, with the white band usually placed somewhere around 5000-6500.  The chart at left is fairly typical.

Some sources say that a bird bulb should have a color temperature of at least 5000K with no upper limit specified, and others say it should be 5000-5500K. But the 5500K limit seems to be pretty arbitrary. Patrick Thrush argues that temperatures higher than 5500K are too blue and will cause problems (Too Blue for You). But this is apparently based on anecdotal information published by Dr Ott (the inventor of Ott-Lite) that has been at least partly disproved by scientific research (see page 23-28 of Ott's article saying that lighting affects the sex ratio of chinchillas, and Galton's study finding that it doesn't).

A color of around 5500K is considered to be sort of an average daylight temperature, but real daylight is a lot more complex than that. On an overcast day, the color temperature of natural light is 6500K – that’s right, it goes up when it's cloudy! When skies are blue, the estimates of the color temperature range from 8000-27000K. So I don’t really see any problems with using a 6500K light with birds, especially if you're only using it for a few hours at midday. The accepted color temperature range for “daylight” bulbs is roughly 4600-6500K (Westinghouse). Waveform Lighting argues that 6500K (pure white) is actually a better match for natural daylight, but 5000K (which is more yellowish) might be more pleasant and relaxing. 6500K contains more blue light than 5500K, and it was mentioned a few paragraphs back that it may be desirable to emphasize the blue for best circadian function (Rea et al) Around 5500-6500K is considered to be ideal for reptiles, primarily because their colors look best in this range. Reptile vision is very similar to bird vision - both have tetrachromatic vision with spectral peaks at the same wavelengths - so a color temperature that serves reptiles well is also expected to be useful for birds.

Flicker considerations. The light fixture should have an electronic ballast, not magnetic, to avoid flicker problems which are invisible to humans but stressful to birds. Since 2002, fluorescent light fixtures have been manufactured with electronic ballasts because they are much more energy efficient than the old magnetic ballasts. Older fixtures might have a magnetic ballast.

There have been some flicker issues with LEDs, primarily related to the use of an incompatible dimmer switch.  The Washington Post has information on how to correct this. Premier Lighting reports that cheaply made LEDs may also have flicker issues due to inferior components, but a quality product will not have this problem. Some high-quality LEDs are advertised as flicker-free, so look for one of these.

Fixture types. Several types of light fixtures are available: fixtures that hang from the ceiling on a chain (sometimes called shop lights), standing lamps, and fixtures that clamp to the cage or sit on top of the cage. As with any electrical device, your bird should not have access to the cord. Putting the light on a timer and/or dimmer will give you easy control over the amount of artificial light your birds receive.

LED lights. The previous discussion was primarily for fluorescent lighting. There aren't any specific recommendations or warnings about LED lighting for birds, because this wasn't a practical technology in Patrick Thrush's time and no one else has stepped up to the plate to discuss it. So I'll do what I can to fill the gap.

General-purpose LED lights for reptiles with no UV output have been around for a while, and have been generally accepted as safe and useful.  LED reptile bulbs with UVB output reached the market more recently, but it looks like these are off brand products, not the big, well-known names in reptile lighting. As of July 2017, the Chameleon Forums did not believe that an appropriate, affordable LED with UVB output existed.

But LEDs look quite attractive for general-purpose lighting. Full spectrum LEDs are available at a reasonable price (meaning the human visual spectrum only; there is no UV output).  Their CRI and color temperature is as good or better than a fluorescent bulb, and according to the graphs below they come a lot closer to matching the spectral output of the sun. As discussed earlier, trying to match the sun may not actually be necessary; but it can't hurt. The cool white LED is the relevant one, because the warm white has a color temperature of about 3000K which is not what we want. 

It looks like you might get an even closer match for natural daylight if you turned on the LED and an incandescent light at the same time. The LED would provide the blue part of the spectrum, the incandescent would provide the red, and the green and yellow portions would basically combine their resources. If you have a sunny window to let in some UVA and pellets in the diet to provide Vitamin D, then what else do you really need? That sunny window will also let in all the blue-through-red wavelengths of light, so if it's sunny enough you might not need a light bulb at all.

Graphs are from Physics Stack Exchange

Notice how spiky and uneven the output is for the fluorescent bulbs.  This is pretty much what the spectral output looks like for the ExoTerra Reptile UVB 100 Tropical Terrarium bulb and the ExoTerra Reptile Vision bulb, with a few extra spikes for the UV output.  This is what the spectral output is expected to look like for fluorescent bulbs marketed for birds. If the goal is to replicate natural sunlight, this is farther away from it than any other kind of bulb.

There has been talk about alleged eye damage from the "blue light hazard", but Ashdownn says there is no reason to be concerned about blue-rich lighting in typical indoor environments. Exposures that are equivalent to staring at the noonday sun for minutes at a time without blinking could be a problem, but this is not the way that humans or animals look at bright light sources.

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Which FSL bulb should be used for UV purposes?

This is where it gets messy. As mentiioned early, many bulbs labeled as "full spectrum" don't emit any UV. Some bulbs that call themselves daylight, sunlight, natural light, etc will also say full spectrum somewhere on the label, but this isn't meaningful. If you want to provide UV lighting, you should buy a bulb that explicitly promises to deliver an appropriate level of it. 

You need an expensive light meter to accurately determine how much UVB the bulb is emitting, and it needs to be a light meter that's designed for measuring the UV output of household-strength light bulbs, not one for measuring the output of sunlight, tanning beds, and manufacturing equipment. There's a simple but much less reliable way to judge whether the bulb is emitting UVA.  The first time you turn on a full spectrum bulb, your bird may be nervous because they are literally seeing the room in a new light. They can now see colors that they couldn't see before, and it's a little scary at first. But the bird could also be nervous if the room is suddenly a lot brighter without having any UVA.

As mentioned earlier, the effectiveness of FS lighting in providing UVB is questionable, and some sources like the Mickaboo Companion Bird Rescue do not advocate providing UVB at all. But it is generally agreed that a UVB output of 5.0 is safe for birds, and this is the amount claimed to be present in full spectrum bulbs marketed for birds. DO NOT use a UVB bulb with a strength rating of 10.0 or higher.  These may be suitable for reptiles, but they have been known to damage the eyes of birds.  Some lights use a new rating system now;  100 is equivalent to 5.0, 150 is equivalent to 10.0, and 200 is a new extra-strength rating that doesn't have an older equivalent. Mercury vapor lamps and metal halide lamps made for reptiles are inappropriate for birds.

Patrick Thrush states that any fluorescent light with a CRI greater than 90 will meet the need for UVA. This may have been true in the 1990s, but modern high-CRI bulbs are often shielded to prevent UV emissions. Supplemental UVA can be provided with a small black light, which emits in the near UVA but does not emit UVB. Thrush says that ordinary fluorescents are deficient at the red end of the spectrum, so full spectrum fluorescents are needed to fill this need. On the other hand, ExoTerra says

ExoTerra also says that their 100 (5.0) bulb is similar to the UVB in shady environments like the rainforest, where conditions prevent unfiltered sunlight from reaching the reptile’s basking site. So big surprise, the 5.0 bulbs simulate canopy conditions and not direct sunlight. The 150 (10.0) bulb is for reptiles that live in bright-sun environments like the desert, but again: this strength is NOT recommended for birds because it’s hazardous to their eye health.

It is generally agreed that long (18 inches or more) fluorescent full-spectrum tubes designed to provide UVB are effective at doing their job within the useful lifespan for this part of the output. As of 1999, short tubes and compact fluorescents were not effective at providing UVB because the short tube length didn't provide enough space for the necessary technology, but the conversations on reptile forums indicate that they work well enough now. Long tubes don't fit in standard light fixtures; they require a 'shop light' fixture. These are fairly inexpensive at large hardware stores like Home Depot, and many are designed to be hung from the ceiling on chains so you can adjust the height to whatever you want.

The major brands of full spectrum lights marketed specifically for birds are Feather Brite and ZooMed Avian Sun. Feather Brite used to sell an "all in one" compact fluorescent bulb that provided both UV lighting and visible light. But the CFL is history now, and currently they have a two-part system with a 16-watt LED bulb that provides light in the human visual spectrum with a color temperature of 5500K, no UV, and no mention of the CRI, and a separate 1.5-watt UV bulb that provides 8% UVA (at 400nm, same as an ordinary blacklight) and .04% UVB. They also sell expensive light fixtures that accommodate both bulbs.

ZooMed is still selling an all-in-one CFL.  They don't provide specifications online, but other internet sources say that the Avian Sun bulb is 88 CRI, 7500K, no word on UVA, and 5% UVB (which is the industry's alternate way of saying .05% or 5.0).

According to a webpage that is no longer available, the Vital-Lamp CFL (formerly know as Vita-Lite) is 91 CRI, 5500K, UVA 4% and UVB .05%, which are the same specifications as the old Feather Brite CFL. However some scientific papers say that tests indicate this bulb had a minimal UVB output (Ball, Wade), and this information is expected to be more reliable than claims of unknown origin on a sales website. It's 20 watts and the old Feather Brite was 15 watts, so it will deliver more brightness without going into overkill. BTW the Vital-Lamp used to be called Vita-Lite, and had a good reputation under that name. According to another now-defunct page, the old manufacturer went out of business but these bulbs are being made at the same factory (in China, ack!). It is apparently still possible to buy this bulb.

Another avian bulb - Avitech Avilux - had 93 CRI and 5500K. The owner of Avitech passed away in late 2018 and it looks like this product is no longer being made.

Most of the information on specific brands was as of 2014 and may not still be accurate, so check for current information before you buy.

Patrick Thrush advises against using reptile bulbs for birds. A reptile bulb with a UVB rating of 10.0 should definitely not be used for birds because it can burn the eyes and cause other problems. However there are bird owners who report no problems using a reptile bulb with a 5.0 rating.   It is reported but not confirmed that the ZooMed ReptiSun 5.0 bulb is exactly the same as their Avian Sun 5.0 bulb. If reptile lights have been repackaged as bird lights, this might explain why the Avian Sun apparently has a color temperature that is much higher than the 5000-5500K generally recommended for birds.

Thrush advises against aquarium lights because their color temperature of more than 6000K is good for simulating an underwater environment, but it is too blue for birds.  However there are bird owners who report satisfactory results with aquarium lights. As discussed earlier, the whole concept of "too blue" is questionable since the color temperature of a bright blue sky is rated at 8000-27000K by various sources, and a color temperature of 6500K might be better than 5000-5500K.

Here's an interesting quote from the Lighting Research Center:

According to this source, we should specifically seek blue light in our artificial lighting to help regulate our internal functions, not trying to avoid it, and don't need to pay much attention to the longer wavelengths. But we need to pay attention to several other factors, and wavelength is not the only thing that matters.

When considering the suitability of other types of bulbs (for example plant lights), consider the CRI, color temperature, and UV output. An ideal FS bulb for birds will have CRI of 95-98, color temperature of 5000-6500K, UVA of 4-5%, and UVB output not exceeding .05% (aka 5.0).  There are few bulbs if any that will meet all these criteria and really deliver the UVB. Plant lights in general don't provide any UV, because plants don't utilize it for growth and too much UV lighting can damage the leaves.

Standard incandescent bulbs have a CRI of 100 but their color temperature is less than 3000K and their UV output is minimal. Their output is quite different from that of natural sunlight at other frequencies too, skewing toward the red and yellow end of the spectrum and away from green and blue.

Dr. Laura Wade recommends that you aim for a UV Index of 2-4, similar to mid-morning sun (which is apparently what you get with a 5.0 UVB rating; most bird/reptile bulbs don't publish their UV Index so this is probably the only guideline that you'll get). She says,

"Birds have very thin skin and thinner corneas compared to mammals. This is why they appear to be more sensitive to ultraviolet light than mammals and reptiles. This is especially a concern for birds that have bare facial skin, such as African greys and macaws."

Baines et al published an excellent paper on UVB lighting for reptiles, with charts showing the UV Index at different distances for different bulb shapes.  Dr Baines also wrote a shorter, simpler version that's available on Reptiles Magazine.  Keep in mind that they're talking about reptile bulbs, and the only type that's appropriate for birds is a bulb with a UVB rating of 5.0.  But these articles have a great deal of useful information about the UV index in various environments, which can be applicable to birds as well as reptiles.

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My personal choice. Because of all the problems with trying to use artificial lighting for Vitamin D purposes, I decided to not bother with UVB lighting at all. I use pellets and outdoor time to provide Vitamin D, and aim for indoor lighting that provides visual enrichment not nutrition. I chose to go with the Waveform A19 bulb with a color temperature of 6500K.  They also have a 5000K model that is suitable for birds.  These bulbs have better output in the red part of the spectrum than most LEDs, and they're closer to natural sunlight than other types of household light bulbs. I superimposed the spectral output for these bulbs on top of the sunlight spectrum in the graphs below.  It's not a perfect match, but it's pretty good.  The bulb has no UV output to speak of, so I also use a blacklight bulb to provide some UVA.  The black light fills in the left side of the graph where the sunlight spectrum has a nice purple curve and the light bulb spectrum has a flat line. My indoor lighting still peters out on the red end of the spectrum, but this is as close to natural sunlight as I can get. Turning on an incandescent light bulb would add more red, but it might skew the overall light spectrum in other ways so I don't do that. This is close enough for me. 

6500K Waveform A19 bulb

5000K Waveform A19 bulb

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How much full spectrum light should be used?

Full spectrum lighting is less powerful than natural sunlight so more exposure is needed. The recommendations vary considerably (from 1 to 12 hours a day depending on the source and the reason for using the light) but 2-4 hours seems to be most commonly recommended for small birds like cockatiels. Full spectrum bulbs are designed to mimic the noontime sun, and natural sunlight would not have the same intensity for the entire day. 

But we might be just as well off to forget about the UV and just go for good general-purpose lighting. If we forget about the UVB (which probably isn't coming out of the bulb anyway) there's no minimum time that we need to use the bulb, and no specific location or height where we need to hang the bulb. Common sense and personal preference can prevail. A good CRI is still desirable, and a color temperature in the range of 5000-6500K seems close enough to outdoor lighting conditions. Overhead lighting is preferable to something low in the room, since most sunlight comes from above. There's no way you're going to replicate the complexity and variability of natural light with a light bulb, but the sun is usually brightest at midday.  So my own preference would be to use these lights for few hours in the middle of the day and have natural lighting in the room the rest of the time, assuming that you've got enough windows in the room to let in a fair amount of light.

If the room isn't bright enough to use natural lighting, then switching to an incandescent light in the morning and evening might be desirable.  Incandescent lighting is low on blue light and high in red/yellow, and that's what sunlight is like at these times of day too.

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Light meters

Be prepared to spend some money if you'd like to be able to do your own UV readings, because light meters aren't cheap. They're also pretty specialized, and there isn't a meter that measures every kind of light (radiation) from any kind of source.  So you have to decide what you want to measure and buy the one that's made for that task. There are several types of meters available to consumers.  There are visible-light meters, which are mostly used by photographers, artists and other people who need to check the lighting in their work area.  There are electromagnetic frequency (EMF) meters for people who want to go ghost hunting or look for surveillance equipment hidden in their rooms - yes, really. There are UV meters, which are the type we're interested in here. There's also a variety of professional scientific meters that can cost tens of thousands of dollars, which do more or less the same thing as the consumer models and a lot more besides, but with much more precision.

The UV meters that are relevant to us fall into three main categories:  those that measure UVA only, those that measure UVB only, and those that measure combined UVA and UVB.  There are two subtypes within each category: meters designed for use with high-output sources (like the sun, tanning beds, industrial UV lamps used in manufacturing, and light therapy equipment), and meters designed for use with low-output sources like reptile/bird bulbs and other household lighting.  That's six different meters that you need to buy if you want to do it all, because meters for high-output sources are not suitable for low-output sources and vice versa. 

There are also UVB index meters and Vitamin D3 meters that basically take the UVB reading and present it in a different format, so you need two more meters if you want that information. UVC meters are available but they're not relevant to the consumer; this type of radiation doesn't pass through the atmosphere and is not present in ordinary artificial lighting.

Solarmeter is the number one brand for UV meters, and it looks like the current retail price for all of them is $249 each. You may find a used on for less on Ebay. Their model numbers for high output sources all end in ".0" and the ones for low output sources end in a different number, usually ".2" In the past, the one that seemed to be universally recommended on reptile sites for checking light bulbs is model 6.2, which is the UVB-only meter for low-output sources.  It's so popular with reptile keepers that the company has a 6.2R model that is basically the same meter with a picture of a reptile on the front.

The standard (high-input) meters measure milliwatts, and the sensitive (low-input) meters measure microwatts.  One milliwatt equals 1,000 microwatts, so there's quite a difference in scale there.  The sensitive meters show results in whole microwatts, while the standard UVA models can display milliwatts with one decimal place and the UVB models go to two decimal places. There is some overlap in the range that the standard meters and sensitive meters can measure, for example if the sensitive meter shows a result of 1100 microwatts, the standard meter will show a result of 1.1 milliwatts for that same source. But the sensitive meters can't show results higher than 1,999 microwatts (2 milliwatts). The standard meters can't show results for very small numbers, and their ability to measure small numbers accurately is questionable because the meter wasn't really designed for this. 

Nowadays there is also a model 6.5 that measures the UV index (a narrower portion of the UVB spectrum), which is said to be even more useful (Baines et al, Baines). Bird bulbs have the same UV output as the lower-powered reptile bulbs, so these are the appropriate models. The Solarmeter website states very clearly that you need to let the lamp (bulb) warm up for at least 15 minutes before taking a reading.  The low-output (.2) models are also preferred for measuring UV in the shade and for sunlight passing through window glass.

These websites have some information on correlating the "standard" UVB measurement with the UV index, but they don't particularly agree with each other:  Star Tortoises, Tortoise Forum (lilacdragon's post).

If you just want to know what the sun is doing, there are some UV meters on Amazon for under $40 that will probably do a reasonably good job of reading the combined UVA and UVB.  But every time someone asks if these meters are good for checking reptile lights, the manufacturer says "no".

You may or may not get a reading if you use a high-output meter on a low-output source like a bird bulb, but even if you get a reading you shouldn't expect the results to be particularly accurate because the meter wasn't designed for this.  If you look at a specific model on the Solarmeter site, you will find information on what it was made for and what it was not made for. A lot of the information is in the "proper usage" information under the graph and the Application Bulletin (if any).

If you want to measure color temperature and display the spectral output of a light source, be prepared to spend a LOT of money.  The instruments that do these things are a lot more sophisticated than a simple UV meter.

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