How to Spot a Psychopath

A reader asks:

This friend of mine is deathly scared of opening microwaves before they have finished. For example, put something in for a minute, wait about 55 seconds, get impatient and just pull the door open. The microwave stops, and my friend thinks that it takes some time (few seconds) for all the radiation to disappear. So if i ever do this around him, he thinks he might well be losing his ability to reproduce.

Is this true? I would have thought not, but you never know.

His technique is to wait until the microwave fully finishes beeping before it is safe to open.

Peter

The radiation level inside a microwave oven will actually drop to zero pretty much instantaneously after the magnetron is powered down by the safety interlock on the door latch.

Why, one might ask, is this so?

The radiation really is bouncing around in there, after all, reflected by the metal walls and the mesh on the inside of the door (the holes in the door-mesh are much too small to let through the radiation, which has a wavelength of about 12.4 centimetres).

Well, here's an analogy for you. Microwave radiation has a much lower frequency than visible light; 12.4-cm microwaves have a wavelength about 165,000 times that of the reddest light humans can see. But both microwave radiation and visible light travel at the speed of light, 299,792,458 metres per second in vacuum and very marginally slower in air.

Think of the microwave, therefore, as a mirrored box with a light-bulb in one corner lighting it up. Light's bouncing off the walls of the box, just like microwave radiation inside an oven.

If you turn the light bulb off, how long do you think the box would stay lit up?

The reason why the box would go dark pretty much instantaneously, just like the microwave, is that there's no such thing as a perfect mirror, for either wavelength of radiation. Even telescope mirrors only reflect about 95% of the light that hits them. And lightspeed radiation will bounce off the mirrors many, many, MANY times per second. So even a very slight loss of intensity with each reflection will eat all of the radiation in almost no time.

Let's consider radiation bouncing back and forth in the longest axis of a large microwave oven - let's say a whole metre - and assume that 99.9% of it bounces back each time. It'd actually be considerably less, and normal microwave ovens are much smaller than this, but let's presume someone's made a carefully-tuned microwave oven designed to resonate for as long as possible.

(This test microwave is also empty. Obviously if there's food in there then it'll soak up microwaves too, like a non-reflective object would inside a mirror-box.)

In one second, the microwaves in this giant super-reflective oven would have bounced off a wall 299,792,458 times - because that's the speed of light in a vacuum in metres per second - if there wasn't any air in the oven. Since light moves marginally slower in air, light would only have bounced about 299,702,547 times in a second if there were mirrors on each end and air in the oven. Microwaves slow down slightly in air as well, but even less than light.

The first time the radiation bounced, it'd be down to 0.999 of its original intensity. After bounce two, it'd be 0.998. After ten bounces, 0.99. After fifty, 0.951. It's easy to figure this out - it's just the portion of the radiation that bounces off, in this case 0.999, to the power of the number of bounces. 0.999^50 equals 0.951.

As you can see, the intensity is dropping pretty fast, and will be almost zero after a lot fewer than 299.8 million bounces.

After one millionth of a second there would have been almost 300 bounces, and the intensity would be down to 0.74 of its original value. After ten millionths of a second, there would have been almost 2998 bounces, and the intensity would be 0.0498. After a hundred millionths of a second - one ten-thousandth - the intensity would be down to 0.000000000000094.

In a real microwave oven, smaller and with much higher reflection losses, the microwave intensity would actually be functionally zero after less than a millionth of a second, even if there's no food in there soaking up radiation.

So you'd need to open that door pretty darn fast to encounter any microwaves.

(It's possible to jam microwave oven interlocks, or very occasionally for the safety systems to just fail, giving you an oven that can run with the door open. This is indeed hazardous to your health, but not nearly as dangerous as you'd think. In some commercial kitchens, all of the microwaves have, in a huge violation of numerous safety regulations, had their interlocks defeated for faster operation. Hobbyists have done many exceedingly unwise microwave oven experiments, too. But those hobbyists, and people who work in those kitchens, don't seem to come down with ghastly maladies any more often than socio-economically similar people with far less microwave exposure. As long as you don't actually get cooked - you can rapidly lose your eyesight if microwaves cook your eyeballs, for instance - there doesn't seem to be much reason to worry about microwave exposure that's far, far above what you'll ever get from even a half-broken, leaky home microwave oven. This doesn't stop some people from worrying about tiny electromagnetic-radiation exposure, or dastardly microwave-leak conspiracies, or what microwaved food may be doing to their precious bodily fluids.)

This post on the Greater City: Providence blog is excited about LED street lighting. It links to this post on Red Green and Blue, about an LED-street-lighting pilot program in New York, which mentions that they're apparently replacing high-pressure sodium lamps with LEDs.

That doesn't seem like a very good idea to me.

LED street lamps could work very well. But the numbers don't look good yet.

I can believe the part where the Greater City blog quotes ScienceDaily as saying "If all of the world's light bulbs were replaced with LEDs for a period of 10 years...", vast amounts of power could be saved.

But that's talking about replacing incandescent-filament light bulbs, whose luminous efficacy - amount of light produced per watt of power you put into them - is miserable, down around 17 lumens per watt.

Almost no street lights use incandescent bulbs, for exactly this reason. Instead, street lights use fluorescent tubes and gas-discharge lamps of one kind or another - often low-pressure and high-pressure sodium vapour lamps. The NYC pilot program is replacing high-pressure sodium lamps with LEDs.

Low-pressure sodium lamps are highly recognisable, because they output monochromatic orange light. Single-colour light like that only lets you see the world in shades of orange (in other words, its colour rendering index approaches zero), but you get a whole lot of light per watt - up to 200 lumens per watt.

High-pressure sodium lamps give white light with reasonable colour rendering (though their spectrum is still a long way from being smooth). They can have luminous efficacy as good as 150 lumens per watt.

And then there are fluorescents. Fluoro streetlights generally use the highest-efficiency fluorescent tubes in existence, which are the "triphosphor" tubes whose output has a distinctive greenish-white look. (This is why anywhere lit by cheap triphosphor fluoros, like warehouses and public toilets, will make people look zombie-ish.) Triphosphor is close enough to white for government work, though. Triphosphor fluoros manage about 100 lumens per watt.

So existing, common, street-light technologies have luminous efficacy ranging from 100 to about 200 lumens per watt.

Thus far, white LEDs have managed about 100 lumens per watt.

Only a few years ago, the best white LEDs were only achieving about 25 lumens per watt, the same as halogen incandescent lamps. There's been a lot of market pressure to create better white LEDs, and the technology is leaping ahead.

But this doesn't change the fact that if you switch all of your fluorescent street lights to LED now, you'll save no power at all. If you switch discharge-lamp street lights to LED, you'll use more power to get the same illumination.

The one fact about LEDs that everybody latches onto, which leads to things like that Greater City post, is that many LEDs need very little power to operate. A normal 5mm white LED will work very nicely from a twentieth of a watt.

But a 5mm white LED also outputs very little light, by street-light standards. And LEDs are not magic hyper-efficient light sources; they waste energy as heat just like every other kind of lamp. It's just that it's hard to notice that wastage, when the total lamp power is only a twentieth of a watt. So people often seem to think that LEDs waste no power, and must thus be the best light source in the world.

To be fair, LEDs do have one unique advantage over all conventional lamps: They're inherently directional. The light comes from a little metal pit inside the LED, and it comes out of only the top of the pit.

This means that it's quite easy to make an LED lamp that throws light in only the direction you want it to, with no efficiency-sucking reflectors or wasted light shooting up into the night sky to pollute it. So the effective luminous efficacy of an LED lamp, for street-lighting purposes, may be higher than its raw efficacy number might suggest.

I presume it's this fact that makes the NYC pilot program worthwhile. The Red Green and Blue piece mentions that "the light footprints can be tailored for parks, street corners or mid-block", which implies that they're replacing sodium-vapour lamps with an unnecessarily wide throw with LEDs that light up only what needs to be lit. If this is the case, then even replacing 150-lumen-per-watt sodium lamps with 100-lumen-per-watt LEDs could yield a net improvement. Even if you just want the usual round-pool-of-light, a well-designed LED luminaire could work just as well, if not better, than a technically-brighter vapour lamp.

But LEDs are not, yet, the slam-dunk winners that so many people seem to think they are.

Here's another problem: White LEDs wear out.

Nobody's yet made a "native" white LED. All white LEDs so far are actually blue LEDs, with a phosphor layer over the blue die that eats some of the blue and emits the other colours needed to create light that looks white. And the phosphor slowly burns out and becomes opaque, which reduces the LED's brightness.

There's seldom a clear point where a white LED "dies", but you shouldn't expect street-light white-LED lamps to last more than a few years. Fluorescent tubes will probably need replacing more often - and they really do die, not just get dimmer and dimmer - but fluoro tubes are very cheap. I suspect the value-for-money difference between LED and fluorescent in this case would hinge on how much it costs to send people up ladders to change the lamps.

One solution to the white-LED-lifespan problem is to not use white LEDs, but a combination of red, green and blue coloured LEDs. They should last far longer...

...and can decently approximate white light.

They have higher luminous efficacy, as well. Coloured-LED luminous efficacy hasn't been improving nearly as rapidly as white-LED efficacy has, but an array of red, green and blue LEDs should still be highly competitive, in lumens-per-watt, with other street-light lamp types.

(This is also why LED traffic signals work so well. LEDs can natively emit red, amber or green light, and you want a traffic signal to be directional, too. LED traffic lights are just hilariously better, in every important respect, than the old type, which uses low-efficacy incandescent bulbs with coloured filters in front of them that eat most of their output.)

The Greater City: Providence piece dreams of street lights that use so little power that a solar panel on top of each light can charge it up with all the power it'll need to work all night.

That, I'm afraid, is going to remain a dream for some time yet.

Yes, cheap LED garden lights work that way. But if you scale them up and put them on top of a pole, you'll either need an outrageously large solar panel, or have to settle for a very dim street light.

LEDs are not a miracle product for street lighting.