SPEED CAMERA DETECTOR - TECHNICAL INFO

Before answering the above question, we first need to talk about how does police measure the speed of a moving vehicle. There are several methods that are used for this task by the law enforcement agencies. According to Wikipedia, there are stationary radars, radars that can be used in a moving police patrol car, stationary speed cameras, and finally multiple cameras with number-plate recognition software, which check and lock the average speed of a moving vehicle between point A and point B.

Radars

Speed measuring radars can be used both from a moving patrol car or set on a stand (or held in officer's hand) at a chosen location (often referred to as "speed trap").
These radars transmit either radio waves or invisible electromagnetic waves at a specific frequency and wavelength, which reflect off most metallic objects, concrete, trees, etc.

For United States the following bands are used:

S at 2.455GHz (not used anymore due to its obsolescence)
X at 10.525 GHz ± 25MHz
K at 24.125 GHz ± 100MHz
K at 24.150 GHz ± 100MHz
Ka at 33.4-36.0 GHz
Ku at 13.425 GHz ± 25 MHz (very rare in US)
IR infrared THz (for laser technologies)
33 MHz bandwidth

A portion of that transmitted wavelength is then reflected from a vehicle at which the radar is pointed. When this vehicle is moving, so-called Doppler effect (change in the reflected wave's frequency) is measured by the police radar. The change is calculated by the microcomputer inside the police radar and based on that data the speed of a vehicle can be determined.

The radar beam can be as wide as 100 feet in diameter; however, the speed of a vehicle cannot be determined when the target is traveling perpendicular to the radar beam.

Lasers

Laser guns (ladars) use different technology than regular radars. Instead of projecting radio frequencies or continuous microwaves, they project a tiny beam of pulsed infrared light to sense a target. This beam is so small, that for most radars detectors that are on the market today, it is virtually impossible to detect it. The speed is computed by measuring the time it takes for a laser light pulse to travel (at the speed of light) from the ladar to the target and back. Laser guns must send out a minimum of 2 pulses to get at least 2 measurements at 2 different times to compute speed. But no worries, they are actually able to transmit tens to hundreds of pulses per second.

Laser radars are always stationary; hence you will be able to see them more in cities as oppose to highways.

Speed camera detectors/anti-radars/fuzz-busters

According to Wikipedia, radar detectors use something called super-duper-stuff (just kidding), they used technology called superheterodyne receiver, which allows the device to detect electromagnetic emissions coming from a radar gun and set the alarm off when that happens. The only problem with that technology is the fact that many other electronic devices operate on the similar frequencies as police radar guns (garage openers are a good example of that). Those devices than have a negative impact on the radar detector, as you get tons of false alerts, which can be very disturbing while driving and can diminish your alertness and hence the reaction to a real police radar gun.

GPS technology

The attempts to solve this unfavorable situation is currently under way with the new gps technology being implemented into the most up to date speed camera detectors.

There are two ways this technology is used to your advantage.
First, there is a continuously built database that stores data on the locations of known speed traps around the United States, so you are always protected and your information is always up to date.
Second, you can manually store false alarms mentioned above. So, when you, for example, drive to work and use the same road over and over, you store the false alarms into your unit by a simple pressing of a button, and the next time you are passing by the same area, this false alarm will not be there, as your unit will know that it was a false alarm the last time you drove by and it will ignore it.
There also exists an option with some units (such as Escort Passport 9500xi) to download locations for red-light and speed cameras from the Internet.

Radars in rain

Yes, they do work although in a heavy rain the range is reduced. However, in this kind of weather you will slow down naturally anyway, and most importantly, it is very unlikely you will see a police officer to get out of his/her car.