The most popular and widely promoted helmet camera as of 2011 was the GoPro HD Helmet Hero. I own one, and I am reasonably satisfied with it. It has proved extremely reliable and I am still using it as of 2018. [See the end of this article for a comparison with newer GoPro cameras]
The Helmet Hero is self-contained, smaller than a pack of cigarettes, larger than a matchbox. In use, it is enclosed in a plastic shell with accessories to attach in a wide variety of ways to a helmet, bicycle, car windshield etc. etc. The camera is sold with two backs for the shell: one is waterproof and the other has openings at the rear to allow better audio pickup.
Thanks to the HD Helmet Hero's simplicity and ruggedness, I can give it very high marks for reliability in use. I have mounted it on a helmet, and also on the rear rack of an unsuspended bicycle, and never lost a shot due to an internal problem.
The best mounting for a bicycle helmet is the supplied helmet platform-and-straps mounting. The platform, on the outside of the helmet, is cinched up tight by means of straps that pass through the helmet's ventilating holes. This mounting is intended for a center-vent helmet, though it can be used with a center-ridge helmet by twisting one strap past the other. There is a pivot in the mounting bracket, so the camera's position on the helmet is not critical. The camera adds noticeable weight to the helmet, and so there is a worse issue with camera shake than with smaller, lighter cameras/.
The mounting hardware supplied with the Helmet Hero is of plastic, and I have some concern that a bracket will break and drop the camera. Still, mine has been trouble-free for six years now. Some mounting hardware uses adhesive pads. I would use one on a car dashboard when the worst calamity would be the camera's falling to the floor of the car, but not on a helmet unless the camera is tethered. It is easy to loop a cord through the slots in the open back; the manufacturer offers a tether that will work with the waterproof back too.
The GoPro's ultra wide-angle fisheye lens is excellent, sharp out to the corners and multi-coated. Color is vivid. Like other helmet cameras, this one has fixed focus (not an issue with the extreme wide angle lens, unless you would like to take extreme close-ups) and automatic exposure control. This can be set either to full-screen or spot mode. The HD Helmet Hero offers four video shooting modes at a 16/9 aspect ratio, from 480 x 848 WVGA up through 1080 x 1920 full HD. A 4/3 aspect ratio is available only at 1280 x 960. Downsampling and cropping in post production are, of course possible. The camera also can take 5 MP still photos, and has a self-timer and sequence timer, so it also can shoot time-lapse video..
The internal battery (when new) and supplied 16GB SDHC memory chip are good for three hours of shooting at the highest resolution. You may use any chip up to 32 GB, maybe more. The battery recharges and the camera may be operated from external power through its USB port. The battery and chip are field-replaceable, however, the camera will lose all its settings including time and date when you switch batteries, unless it is connected to a USB power supply at that time: you could carry an external battery for this purpose. Analog video and USB ports are on the side of the camera. An optional "skeleton" housing is available which allows use of these connections.
Adjusting viewing angle is cut-and-try, even if the camera is within sight -- there is no viewfinder unless you buy an optional viewscreen back. Generally, the camera should be tilted down about 5 degrees when using it on a bicycle, so as to get more road than sky in the picture. When the camera is mounted on a helmet, you might use a mirror or a reflection in a window to align the camera -- or ask a companion to help, or carefully lift the helmet off your head without tilting it, to check the angle. Remember that your head will be tilted forward if you ride in an aerodynamic position.
The front of the shell has a domed glass window which protects the camera's lens. The window is of optical quality but not optically coated, resulting in lens flare when a bright light source (typically the sun) is in front of the camera. Go Pro calls the window a "lens", but it really is only a window.
About audio: If the camera is shaking, as it will when used for action shots, the two small switches on the camera body rattle, spoiling audio quality. Rubber patches which hold the buttons half-depressed may solve this problem. A low-pass filter at 4000 Hz removes most of the rattling noise without degrading typical street noises by much. Wind noise is also a concern when the camera is in motion, and is worse with the open-back shell if it is tilted forward so the openings catch the wind. Foam covering the openings in the shell will reduce wind noise. Audio is monaural and, except for these problems, of rather good quality if the camera is in the open-back or skeleton shell. You will need to use a separate recorder to get stereo or surround-sound audio, and you will have to synchronize the audio in post production. I do this with a hand clap visible to the camera. I clap again at the end of longer shots in case I may need to correct for timing drift, but that is usually unnecessary. There is an article on this site describing in detail how to synchronize cameras and audio recordings.
The GoPro's controls are with only two buttons, to minimize the number of openings in the shell. Adjustments require viewing cryptic codes in a small window on the front of the camera. Setup for different modes is slow. The camera can stay in the shell, but you must remove the camera from the helmet, or remove the helmet, to view the settings and switch modes.
The camera may be left on, with a start-stop button for shooting, but also there is an "on-off" mode which shuts the camera down completely between shots, sparing the battery. Shooting is then controlled by the on-off switch on the front of the camera. It takes a couple of seconds in "on/off" mode for the camera to adjust its exposure. In this mode, your finger will probably be visible at the side of the image as you press the on-off button to start and stop the camera. The camera beeps to indicate starting and stopping, a useful feature when it is mounted out of sight but within reach.
You must open the shell (except if using the skeleton shell) to charge or replace the battery or download data. Downloading the huge files which the camera produces is faster with a card reader than over a USB cable. Files are in MP4 format, which most video editing software packages can read.
It is important to carry an instruction sheet with you, because of the cryptic, abbreviated codes in the display window and the complicated sequences of button pushes needed to change settings. User instructions in several languages are supplied with the HD Helmet Hero, and when folded, they all look the same, with the word “Instructions” in English. When I unfolded the one I brought with me on my first expedition with this camera,, it turned out to be in Spanish. Good thing I read Spanish!
Several accessories have been introduced as as the camera has become popular and users have demanded additional features. One already mentioned is the viewscreen, available as an accessory back for the shell.
Another accessory back is buoyant so the camera will float. GoPro also used to offer a shell for two Helmet Hero cameras side by side, with a linking connector for synchronized 3-D shooting. This required a very fast computer to process the data. GoPro offered only Windows software for this purpose.
A wide range of optional mounting accessories is available -- with a chest strap; to a standard tripod threading; to a car dashboard, and so forth. These are sold as a kit with lots of little pieces, in a cardboard box rather than, as would be better, a foam-lined case with a place for each piece.
Some GoPro mounting hardware including the helmet strap mount has quick-release snap-in clips. When I mount my HD Helmet Hero on a bicycle rack, I use a Manfrotto quick-release head with a bolt-on tripod top, and leave the top part of the quick release attached to the camera -- which still fits into a small belt pouch. It would be nice if the bare camera had a tripod fitting for more convenience of access to connectors, better audio quality and less lens flare when using it without the shell. It shouldn't be too hard for a do-it-yourselfer to cobble up a fitting.
To accommodate the camera's extreme wide-angle lens, the window protrudes from the front of the shell, and so it is vulnerable to scratching if the shell falls on its front. Replacements are available at a moderate price. The camera's lens also protrudes, and so it is vulnerable when the camera is out of its shell. Lay the camera on its back, preferably on a plush surface.
Many of the mounting accessories use embedded stainless steel acorn nuts which can fall out and get lost. Preferably, glue them in place.
If using the open-back shell, you can quickly render it rainproof with a strip of duct tape over the openings on the back. Still, raindrops appear very large in the image because the window is so close in front of the lens. The fully waterproof shell allows the camera to be used underwater -- a feature unavailable with most sports video cameras -- but the window's curvature puts images slightly out of focus. A shell with a larger, flat pane at the front is available to solve this problem, and probably also would reduce the problem with raindrops. Newer GoPro models have a flat window as a standard feature.
The shell and possibly the camera itself would be damaged in a serious impact to the helmet; and as with any helmet camera, there is also the possibility of injury due to the camera's indenting the helmet. The GoPro is very obvious when mounted on a helmet, giving a "Teletubby" appearance to the user. This may contribute somewhat to the commercial success of GoPro cameras, because they advertise themselves in use. The obviousness can be an advantage or a disadvantage depending on whether you want people to know that they are being recorded. Some other helmet cameras are smaller and can be mistaken for helmet-mounted LED bicycle lights.
I've already complained about the packaging of accessories. The packaging of the camera itself was as bad if not worse. It was sold in a hard, transparent plastic jewel case, about 5 inches (12.5 cm) on a side, taped to the top of a cardboard box which holds cables and mounting hardware -- a display case, designed for the sales counter. You tear off the cardboard, and then all you have left is the breakable plastic display case. The manufacturer does not offer any transport case, as far as I know -- a remarkable omission in a camera designed for rugged use, and which comes stock with a number of small accessories.
There was (as of summer 2011) a glut of Helmet Hero accessories at brick-and-mortar camera stores, apparently because the manufacturer prefers to sell the camera through the more profitable direct mail-order channel. So, you may be able to get accessories at a reduced price through a camera shop.
As of November, 2011, a new model, the HD Hero2, became available, with added features including a wider selection of shooting modes, choice of three field-of-view widths, language-based interface, camera control from a computer, tablet or smartphone via wi-fi or Bluetooth, and a stereo microphone jack.
As of June, 2012, a remote control became available, and it also works with the original HD Helmet Hero, though Bluetooth and smartphone control do not. As of 2018, GoPro models have gone as far as the Hero 5, Hero 6, and the Fusion, a 360-degree camera. These have many new features including image stabilization and waterproofness without an external shell. There is a features comparison on the GoPro Web site, and also, GoPro now has many competitors. Some competing cameras are reviewed on this site. I also heartily recommend the Techmoan Web site, linked below, for careful reviews, delivered with a dash of humor.