How does it work ?

The principle of stereoscopic vision

In order to have a stereoscopic vision it is important that the images seen by each eye are a little bit different. This principle is used in the real life as the image seen by the left eye is slightly different of the image seen by the right eye. Then the brain creates the stereoscopic vision.

The 3D vision in cinema is based on the alternate displaying of an image for left eye and for right eye, each image being shifted and a little bit different. To have a stereoscopic vision, each eye has to see the images intended for that eye to allow the brain to create the 3D effect.

How it works?

Figure 1 shows the principle of operation of a digital projector working in 3D. First, the server sends two frames with a frequency of 24 frames per second corresponding respectively to the right image and the left image. Then the projector divides each native image into 3 images in order to display alternately a left image and a right image etc. Here we speak about triple flash. Each frame lasts 7 ms which corresponds to a frequency of 144Hz for the projector and 72 Hz for each eye.

It is important to notice that the minimum frequency to obtain a good 3D quality is 60 Hz for each eye (120 Hz for the projector or the 3DTV).

Figure 1

The right and left images being shifted and slightly different, the spectator will see a double image when looking at the screen without glasses.

Then, to obtain a stereoscopic vision, it is necessary to use special glasses which can be divided into 2 categories: passive and active. We will see these two technologies in a specific chapter but the main difference is that the glass is “alive” in active system (there is an electronic device inside the glass which allows to shutter successively the right and the left eye: no modification of the images characteristics sent by the projector) and inert in passive system. We can notice that it exists a dark time between two images allowing the transition between the open state and closed state of each LCD.

Nevertheless, even if these two technologies are different, the criteria to have an optimal stereoscopic vision are the same.

1/ The extinction ratio (contrast)

This parameter is really important to avoid ghosting. The ghosting appears when the left eye (or right eye) can see the right image (or left image) due to an insufficient extinction in the closed state. Consequently, the spectator can see double images leading to a very disagreeable vision.

What is the difference between the contrast and the extinction ratio?

Today most of the 3D glasses suppliers talk about contrast which is the ratio between the luminance of the transmitted light through the glass in the clear state Ion and in the blocking state Ioff. If the ratio is equal to 100 the contrast is 1:100.

If this criterion is a good indicator about the quality of the 3D glasses, its value doesn’t allow comparing the different 3D glasses on the market in term of capacity to avoid the ghosting. Indeed, this defect depends essentially on the blocking state and the value of the contrast integrates also the open state. For instance, consider two glasses 1 and 2 with the following characteristics:

Ion1=30 and Ioff1= 0,1 so C1= 300 and Ion2= 40 and Ioff2=0,13 so C2=300

We can notice here that the contrast is the same for the two glasses whereas the transmitted light in the blocking state is different. Consequently, the ghosting can exist on the glass 2 and not on the glass 1 while the contrast is the same. In order to avoid this problem, we prefer to talk about extinction ratio T which is the ratio between the luminance of the incident light I0 (without glasses) and the luminance of the transmitted light in the blocking state Ioff. By using the previous example we have:

I0=100 and Ioff1= 0,1 so T = 1000 and I0= 100 and Ioff2=0,13 so T =769

The extinction ratio gives the capacity of the glass to avoid ghosting. The higher the extinction ratio the better the capacity to avoid ghosting is. Naturally, the notion of contrast is correct. Moreover, the trend in the 3D glasses is to have a very high contrast as for the TV but it is not justified. Indeed, currently we don’t know the minimum value of the contrast to prevent the ghosting so it isn’t necessary to have a contrast of 1:1000 if 1:300 is sufficient (after several tests, 1:300 is largely sufficient).

2/ Residual light and color

The residual light corresponds to the “quantity” of light received by the eye when the 3D glass works. This value is strongly related to the percentage of light transmitted in the clear state. It is important to notice that this criterion is one of the weak points of the 3D glasses which absorbs between 80% and 85% of the incident light.

There is another important parameter which is the color. Indeed, the use of colorless lenses is ideal to obtain a good quality image without denaturing the original colors.

So, the best combination is to have colorless lenses with a residual light as high as possible.

3/ Response times

The response times correspond to the time to switch from the closed state to the open state and vice versa. These switching times have to be as short as possible to ensure the transition during the dark time between two images. The duration of the dark time is between 500 and 1500 µs but the goal is to have a dark time as short as possible to increase the light efficiency.

The figure below shows a graph of transmission vs time for the two LCS (Liquid Crystal Shutter) of a 3D active glasses. We can see for each eye the cross between the two states during the dark time.

Figure 2 : Transmission vs time for the right and the left

4/ Viewing angles

Currently, viewing angles are not the most important criteria in the 3D world. Nevertheless, they can generate defects especially “ghosting” in few configurations. The figure 3 is a scheme explaining what viewing angles are. In few cases the extinction ratio T1 and T2 are inferior to Tincident leading to the appearance of ghosting when the user is looking at the screen following the angles 1 or 2.

Figure 3