Active vs Passive 3D Glasses

The vast majority of stereoscopic display methods (primarily 3D TV and cinema) require the viewer to wear some form of "3D glasses". There are many technologies used to display a stereoscopic image, many of which require a certain type of glasses. Broadly speaking, these can be categorised in to Active or Passive 3D glasses. This article will explain the difference between these two technologies and the benefits of each.

Active 3D Glasses

As the name suggests, these types of 3D glasses have an active component to them and can either be Shutter Glasses or Display Glasses:

Shutter Glasses - these are the most common type of active glasses used to create stereo images, as they can be used with most types of existing display technologies. Shutter technology works by creating a pair of glasses with a pair of solid LCDs as the viewing lens. The LCD is not the typical type you see in a TV, but appears as a sheet of glass that when activated will block light.

The shutter glasses use this effect to alternate the image received at the left and right eyes. Meaning that the display can then show alternating angles of the stereoscopic image, so that each frame is seen at one eye, then the other. Using this method will effectively half the refresh rate of a display, as two frames are required to build a stereo image. So, if an LCD TV operates at 100hz, the stereoscopic image is displayed at 50hz.

This process needs to be performed fast enough so that the brain is fooled in to seeing the separate images as a continuous series of motion, rather than a series of stills. Therefore, the higher the refresh rate, the more comfortable the effect will be. 120hz (60hz stereo) is generally considered the minimum.

Shutter glasses are now relatively compact devices, often wireless with rechargeable batteries. However, the image viewing isn't as comfortable as passive glasses for many people. The advantages are that they can often work with existing hardware, reducing the total cost of the solution (even if the glasses component costs more than the passive counterpart).

Display Glasses - are often used in high end applications, usually in the form of Head Mounted Displays (HMDs) in virtual reality environments or military hardware. They consist of two full LCD screens that can display separate images to each eye, often providing a high-resolution and quality image to each eye. This is a much more expensive option, however the result is often a more effective stereo image. A prime advantage is that the display device is built in to the glasses, so they are a much more compact display solution.

Passive 3D Glasses

These type of 3D glasses are more commonly used on specialised stereoscopic hardware, such as high-end 3D monitors and 3D cinemas. There are no active components, just a method of filtering a single image in to two channels (providing a stereo image).

Polarising Glasses - Most people who have seen a 3D film at the cinema will have used some sort of polarising glasses. IMAX and Real3D both use this technology, but implement it in a different way. IMAX uses glasses with linear polarisation and Real3D use circular polarisation. This means that the glasses can filter 2 distinct images from the screen by the direction that the light is travelling in, as the lens in front of each eye is polarised in different directions.

Linear polarisation filters light in to vertical and horizontal lightwave components, which has the disadvantage of requiring the eyes to be almost level. Circular polarisation can filter lightwaves based on the clockwise or counterclockwise direction, meaning that the stereo image is maintained even if the head is tilted.

The disadvantage with these glasses is that the brightness of the source is reduced, as only half of the overall light from the screen reaches each eye. The advantages are that it is each and cheap to display the same stereo image to many users, as passive glasses are cheap to produce.

Each of the 3D display technologies uses a different method of presenting the image, which is beyond the scope of this article. Some use multiple/single projectors, LCD screens and many other display methods.

Anaglyph Glasses - these are the older colour filter 3D glasses that were seen in children's books and early cinemas. Usually red and cyan filters are placed over each eye to allow two images to be formed from a single source. The stereo image doesn't retain the original colours and is generally considered an inferior method of stereo viewing.