At the risk of sounding click-bait-y, your front door’s light may be just as useful to a criminal as it is to you. By way of explanation, let us consider a few of the schools of thought when it comes to your front (or garage, or back, or whatever) door lights.
Always leaving the lights off. By far and away, this is the greenest option and it tells criminals absolutely nothing as they drive by your house, but it also does not help you at all, whether it is identifying people on your front step or fumbling for the keyhole a night.
Only turning the light on when someone knocks. Still not tremendously helpful when you are trying to unlock your door from the outside, but it does at least allow you to identify knockers. However, what if a criminal – or group of criminals – pose as pollsters or someone who lost their dog or whatnot, and know that you only turn on your light when someone knocks? Nominally, that is not a huge concern… until you go out of town, they knock, and no light comes on.
Manually turning on the lights at night. Unfortunately, while this option strikes probably the best balance between green-ness and utility, it also has the same shortcoming as the previous one – what if you go out of town? A criminal who drives by every night and sees your lights on will then see your lights off, and if they are off for more than a day or two, he will probably come to the correct conclusion.
Putting a timer on the lights. There are no shortage of options in this particular arena: some compensate for Daylight Savings Time, some do not; some can adjust for changing sunrise/set times, some cannot; most require replacing the actual switch for your light (always turn off your circuit breaker before working on switches!), some do not; etc. etc. These provide a very “human” appearance for your house, even when you are not there, but can be finicky about programming, and run about $20 a switch.
One word of caution: while they are tempting, please avoid the wifi-enabled and other “smart” timers and switches. They may be quite easy to use and adjust, but as the recent botnet of IP cameras demonstrates, the security features baked into internet-of-things devices leaves a lot to be desired.
Putting a photocell on the lights. The two big variations in this case is you can either get a device that plugs into the socket between your bulb and the socket proper, or you can get bulbs that come pre-equipped with sensors; both cost about $10. In either case, the premise is that the light comes on automatically when it gets dim/dark out, and turn off either a set time later or when the sun comes up. The reality seems to be that the sensor frequently gets confused by the light reflecting off the fixture, it can significantly decrease the output of the bulbs, and it can completely remove your manual control over turning on the light when you like.
Using a motion-sensitive light fixture. This will likely require replacing your entire light fixture at a cost of about $40 (remember to turn off your circuit breaker), but gives you a system that will automatically turn on the lights when they detect motion at night. There are cheaper alternatives that can screw into the existing light socket, but they will need to “see” out of your fixture, and cannot detect motion through glass.
Always leaving the lights on. This provides criminals the same amount of information as always leaving them off – nothing – while still allowing you to see around outside your door. This is, of course, the least-green option, but that can be offset by installing LED bulbs. Light switch covers make it easier to remember which switch controls your exterior lights, but a sharpie can do the trick too.
Your door light arrangement will obviously have to depend on your family’s own personal preferences, as well as what you can afford / remember to do, but it is important to remember the kind of information you could be providing potential aggressors. External lights seem innocuous, but every little thing adds up.
(The colloquial term for criminals scouting and sizing up potential targets is “casing” and the military generally refers to denying enemies operationally-significant information – such as ship movements, or, more topically, when you are out of town – as “operational security” or “OPSEC”. You will probably see both terms in the future.)