LED lights won't kill you

 kw: risk analysis, lighting, light sensitivity, color vision, hype

Perhaps you've seen online warnings, that we must discard all our LED lamps Right Now! Fear not, they are no more dangerous than old-fashioned incandescent bulbs, and frequently much safer. The ones to be wary of are fluorescent bulbs, whether spiral or straight tubes, because they contain mercury.

Two sorts of fears are expressed about LED lamps. Firstly, they are said to have lots of blue light, which can damage eyes and cause loss of sleep. Secondly, cheaper ones may flicker, which can cause eye strain and even nausea for some people.

I studied color vision professionally, and I've been a spectroscopist. So let's tackle the color issue first: Do LED lamps produce more blue light than other types? I will show next that the answer is Yes for some and No for others. This has to do with the concept of Color Temperature.

Most of us are familiar with the terms "red hot", "yellow hot" and "white hot", in order of increasing temperature. Not many people know there is also "blue hot", which is even hotter. If you use metal tongs to hold a small nail in the fire in a fireplace, it can be heated red hot. It takes a hotter flame to make it glow more brightly with an orange light. And we tend to think of "white hot" as the color of molten iron as seen in a steel plant. Actually, the light from the filament of a 100-watt incandescent light bulb is even whiter than the light from liquid iron, because it is several hundred degrees hotter.

Most incandescent lamps use a filament temperature near 2,700 K, where "K" refers to Kelvins, the temperature standard used in physics. In everyday terms, 2,700 K = 2,427°C = 4,400°F. The light from incandescent lamps is actually orange, which is why when you are outside on an overcast day, particularly in the early evening, the light coming out the windows of houses looks yellow or orange, compared to the gray or blue-gray sky. Your eye is actually tuned to see a cloudy sky as white. That "gray sky" color is the true color of the Sun.

The color temperature of the Sun is close to 6,500 K (~6,230°C or 11,250°F). This high temperature produces much more blue light, so sunlight is bluer than the light from a light bulb.

Before compact fluorescent lamps were invented, to get a brighter, slightly bluer light, a couple of technologies were invented to use lamps with a filament temperature of 3,000 K. This is called "cool white", and it is bluer than the "warm white" of an ordinary incandescent bulb. To get even bluer light required a filter.

Both compact fluorescent lamps (CFL's) and LED lamps use fluorescence to create broad-spectrum light. In a CFL, mercury in a glass tube is stimulated by an electric arc to emit strong ultraviolet light. Phosphors (fluorescent powders) coating the inside of the tube convert nearly all of this UV light to the many wavelengths of "white" light. It happens that mercury also emits a deep blue wavelength, plus green, yellow, and red, but there are sharp "lines" with narrow bandwidths. The phosphors (there are several) add more blue, green, yellow and red in wider bands that make a more pleasant light. The mix of phosphors can be varied to make CFL's that have "warm" or "white" or "cool white" colors—although the "cool white" actually matches a super-hot object, hotter than any earthly material—, and "daylight" which is even more bluish.

LED's are even more flexible. They all work by producing a primary blue color and having phosphors set to intercept some of this and convert it to the rest of the colors. In contrast to CFL's, which can produce only one color temperature at a time, LED's can be color-variable. The light-emitting elements are very small (a few millimeters or smaller), so many are placed side by side. If some of these are blue-only, then extra blue light can be added to shift the visual appearance from warm white to daylight and all points in between. Also, when you buy a single-color CFL or LED lamp, it will be described according to both a term like "cool white" and a color temperature like 3,000 K. A "daylight" lamp will be reported as a color temperature of 6,500 K, and a "bright white" lamp will be in between, usually 5,000 K. 

Our psychology associates yellower light with early morning or late evening or firelight, and makes us feel restful. "Warm white" light promotes sleep. Bluer light is associated with daylight and makes us alert and wakeful. For this reason most office spaces are lighted with "bright white" 5,000 K lamps. Daylight colored lamps were tried in the past, but the light is considered too harsh and it makes people irritable. "5K" is a good compromise.

The spectrum of my shop light (it is variable, and I have it set to 5,000 K) is shown here:

This image's colors are a composite of the lamp's spectrum and the camera's sensors, which are not the same as the color sensors in our eye. Thus, one of the features of this image is the narrow, rather dim yellow section; to the eye, it looks wider and brighter. But another feature is very important for understanding how these lamps work, and why some folks are alarmed. Between the green band and the blue band there is a much dimmer section where "sky blue" should be. There is actually some, but not much. The narrow, deep blue band is the original light from the blue-emitting element. All the other colors are from the phosphors.

The main way that color temperature is varied in LED lamps is by setting the percentage of the blue light that the phosphor section intercepts. In a variable LED, which may contain a few dozen emitters, some of them are blue-only and the variability is produced by increasing or reducing the current to these.

Therefore, a "warm white" LED produces only a little blue light, while a "bright white" LED will produce much more blue by comparison. I use lamps specified as "warm white", or 2,700 K, in my bedside lamp and in the bathroom. I tested their spectra as compared to an incandescent bulb (I still have a few squirrelled away). In this image the incandescent lamp's spectrum is at the top, and spectra for two versions of LED are second and third:

The color temperature of the incandescent bulb is 2,700 K. The two warm white LED's match this very well, although they have a little less of the deepest blue. The phosphors are better than those in my shop light; they even show sky blue (cyan) colors.

The "LED Danger" hype is firstly based on blue light from LED's making it hard to go to sleep. This is simply remedied by using warm white lamps in the bedroom and bathroom. If you get fixtures with color-variable lamps for the rest of the house, you can set them to be bluer (4,000 K or 5,000 K) in the daytime and yellower (2,700 K) after sundown. If your cell phone has an "evening" or "night" setting, use that appropriately. Phones by default are close to daylight color on average (6,500 K). Going "orange" in the evening entirely solves the "blue white at night" issue!

For those who may have technical questions about taking spectra, I use an "inside out spectrograph" I made. This picture shows the three lamps used to make the triple spectrum above. This 3-socket fixture is covered by a black hood with a long slit. My camera is mounted ten feet down a darkened hall. I have a little holder mounted on the camera with a grating sold by Rainbow Symphony; you can buy them for a few dollars each. I just point the camera where the spectrum is and zoom in on it.

Now, about flickering. The LED lamps shown in this picture are of different ages. The older one is at the bottom, and it flickers in two ways. Firstly, it pulses at the rate of 120 Hz the same way old fluorescent tubes do. This is not as strong an effect as in the old tubes, but I can notice it if I sweep my eyes past the lighted lamp; a little banding is evident. This shouldn't bother most people, but some people might notice the pulsing in peripheral vision, with is more motion sensitive.

The other kind of flickering is more evident. These "pseudo-filament" bulbs look nice, but older ones can have momentary disconnections inside as they heat up. This causes a brief dimming that looks like a power interruption. It is, but inside the lamp itself. Further, some really cheap lamps have poor circuitry inside the base of the lamp that can also flicker like that. If you notice this kind of flickering, try putting a better quality lamp in the fixture and see if it continues. I replaced the lower lamp with one of the other kind (with yellow-colored filaments), and both kinds of flickering no longer happen. The 120 Hz pulsing is dealt with by using phosphors that have a little "dwell"; they emit light for a fraction of a second longer than the flash from the blue LED element.

There you have it. LED's are at least as safe as incandescent bulbs, and they produce much less heat. Use a comment to ask follow-up questions.