Viewing cone

The concept has been introduced as an international standard ISO 13406-2, which defines it as the range of viewing directions that can safely be used for the intended task without "reduced visual performance".

This standard describes a complex procedure which evaluates the viewing cone from measurements of luminance and chromaticity versus direction of observation.

This standard provides a classification for computer monitors with LCDs according to the range of viewing directions that can safely be used for the intended task (here: office work) without "reduced visual performance".

ISO 13406-2 describes a complex procedure according to which the usable viewing cone can be evaluated from measurements of luminance and chromaticity versus direction of observation.

Inside this viewing cone certain physical parameters that are related to the visual performance of the display device must remain within certain (task dependent) limits.

Then the viewing cone is obtained by limiting values of a visual quantity (e.g. contrast) which for a certain application is required to be above e.g. 10 (compare e.g. Vesa FPDM2 307-4 Viewing-cone thresholds).

Comprehensive comparisons between experiments and measurements have been carried out in order to identify the quantities and the corresponding limiting values that define the apparent viewing cone for television screens with LCDs and PDPs.

This is found to be in agreement with other research results that "find a low correlation between contrast ratio and visual assessment value".

Furthermore, "not only the chromaticity coordinates of the primaries, but even more those of the white point play an important role and need to be included in a viewing direction dependent metric".

The authors conclude that "for LCDs, this new metric results in a viewing cone, which is on the order of 70°–90° (subtended angle), and thus, considerably lower than what is usually specified based on a minimum contrast of 10.

Conoscopy, originally proposed and used by Maugin for examination of the state of liquid crystal alignment in 1911 [2] has been used in every LCD-laboratory in the late seventies and throughout the eighties for measurement and evaluation of the optical properties of LCDs and for estimation of LCD-contrast as a function of viewing direction.

In the conoscopic mode of observation, in the old days often realized with a polarizing microscope, a directions image is generated in the rear focal plane of the objective lens.

The first publication of the variation of the contrast of reflective LCDs measured with a motorized mechanically scanning gonioscopic apparatus and represented as a conoscopic directions figure was published in 1979.

Figure 1: Illustration of the variation of the direction of observation (i.e. viewing direction) across the area of the display. All locations on the surface area of the screen are seen from a different direction.
Figure 2: Illustration of an example of a viewing cone centered about the surface-normal of the display. The viewing cone may be tilted and rotated and may be of a less regular shape.
Figure 3: Illustration of the specification of the viewing direction by two polar angles: the angle of inclination (measured from the surface normal of the display) and the azimuth angle, measured in the plane of the display
Figure 4: Illustration of the specification of the range of viewing directions (aka viewing cone) in a polar coordinate system. The pseudo-colors represent the value of a physical quantity (e.g. luminance) for each viewing direction.
Figure 5: Luminance and contrast versus viewing direction in a polar coordinate system. The left column shows the directional luminance distribution of the dark state of the display (IPS LCD), the center column shows the bright state and the right column shows the (luminance) contrast (ratio) resulting from the preceding two luminance distributions. The value is coded by (pseudo) colors. The graphs below the polar coordinate systems each show a cross section in the horizontal plane and indicate the values for luminance and for the contrast. Each borderline between two (shades of) colors represents a line of constant value, in the case of contrast an iso-contrast (contour) line.